VACUUM FURNACES TECHNICAL CONTENT Archives

Heat Treat Radio #133: Process Qualification & Recipe Development in Vacuum Carburizing

May 28, 2026 / HARDENING TECHNICAL CONTENT, HEAT TREAT RADIO, MANUFACTURING HEAT TREAT TECH, VACUUM FURNACES TECHNICAL CONTENT

Heat Treat Radio host Heather Falcone and guest Vincent Lelong, Senior Synergy Center Manager and Metallurgist at ECM USA, explore the realities of process qualification and recipe development in modern heat treating. Vincent shares decades of experience developing vacuum carburizing processes for automotive, aerospace, and high-volume manufacturing applications. Together, they discuss how heat treaters can balance metallurgy, fixturing, quench strategy, and production demands to achieve repeatable results. From practical troubleshooting insights to the evolution of vacuum carburizing technology, this conversation offers a grounded look at what it takes to optimize heat treating.

Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.

Heat Treat Today · Heat Treat Radio #133: Process Qualification & Recipe Development in Vacuum Carburizing

The following transcript has been edited for your reading enjoyment.

Introduction (00:05)

Heather Falcone: Hi, I’m Heather Falcone, and welcome to Heat Treat Radio. Today, we are asking a metallurgist, and we are talking about the ins and outs of process qualification and recipe development. We are on-site with the sponsor of our episode, ECM USA, in beautiful Pleasant Prairie, Wisconsin. It’s a real treat to be able to record on site. We hardly ever get that chance. Joining me today is Vincent Lelong, the Senior Synergy Center Manager and Metallurgist. Thanks for joining me today, Vincent.

Vincent Lelong: Thank you, Heather.

Heather Falcone: So you have the luxury, since we are in your beautiful facility, to tell me not just about yourself, but also about ECM. Start off with your background because it is extensive.

Vincent Lelong: I am a metallurgist, I went to college in France, and I started with ECM in 1999. Since then, I’ve been across the world in the U.S. for 20 years. I do presentation and testing for the furnace behind us and our larger furnace. I go on-site. This is our Synergy Center; it’s a nice environment, clean — quiet today because we’re filming.

Heather Falcone: Tell me a little bit about what ECM does.

Vincent Lelong: At ECM, we manufacture and install heat treatment vacuum furnaces. Our main focus is on low-pressure carburizing modular furnaces, large and small. We integrate not just heat treatment itself, but the pre-treatment, like washing, storage, preparation of the load with robots, heat treatment, temper, cryo… Everything is set up together, and that is our main goal: integration of a fully automatic installation.

Heather Falcone: A one-stop shop. You go to one place, and you’re going to make it all work.

Vincent Lelong: There’s a part and everything will be ready in few hours later, sometimes days, depending on the treatment.

Heather Falcone: Hopefully it all works.

ECM Nano vacuum furnace used for client cycle development and qualification.

Vincent Lelong: Well, it always works. It’s a really repeatable modular furnace. In the U.S., we are focusing on vacuum carburizing, but we also manufacture other types of furnaces for crystal growth, silicon heat treatment, melting silicon for solar panel, and others. Within ECM Group, there’s a range of heat treatment processes that we manufacture for. And not just steel; it can be other materials. I’m specialized in steel and very restricted heat treatment with vacuum carburizing, but maybe one day we can do other materials.

Heather Falcone: Steel makes the world run.

Vincent Lelong: With our modular furnace, we can do hardening, gas quench, oil quench, carburizing, gas quench, oil quench, and now we do vacuum carbonitriding and we can do nitriding in the furnace. So you can have one installation for multitask heat treatment. It’s the purpose of the modular furnace, the beauty of it.

Heather Falcone: More flexibility, more capabilities.

Biggest Challenges of Process Qualification and Recipe Development (4:08)

Heather Falcone: That brings us into the first core question that I want to ask, because when you’re evaluating bringing in another piece of equipment or if you’re trying to bring on a new process, it can be a little challenging. What do you see are the biggest pitfalls or what do you see people struggling with most when it comes to process qualification and recipe development?

Vincent Lelong: Most customers say, “I would like a good metallurgy. I would like mechanical properties.”

Heather Falcone: Right, make good parts. That’s number one.

Vincent Lelong: But you need to look at what kind of furnace you would like to use, as well as the size, the type of part, and the process. If you have a small part, do you want a bulk load? Do you want special fixtures because the main target is distortion-free? Everybody would like everything…

Heather Falcone: …distortion-free!

Vincent Lelong: Always. No problem. So, as a metallurgist, you need to think not just about heat treatment because you’ve been asked to heat treat a part. I could heat and quench in oil and say, “You have the metallurgy. My job is done.”

Heather Falcone: Right. It hit hardness.

Vincent Lelong: Exactly. Hardness, good. Vacuum carburizing, no oxidation. Good. How about the distortion? Okay, let’s speak about what we can do to reduce the power of the heat treatment itself. Then, let’s do a gas quench, but let’s work on fixtures also. It’s working with the supplier for fixtures and working with the customer with the machinists.

Sometimes, because we propose gas quench, people say, “Oh, gas quench means no distortion.” Well, the first step is to get the metallurgy right. If you need 20 bar gas quench, you might have some distortion. From experience, we know that the fixtures are also important. So we can work with the customer and supplier to propose the right fixtures and test the fixtures to target the best cooling rate and other properties.

But we also need to work with the machinists. If you have a challenging part, machinists may say, “It’s the fault of the metallurgist.” And the heat treater will say, “No, let’s work together at the table. We’ll sit, and we do the testing.” At the Synergy Center we have also the CMM, so we can measure before heat treatment and after heat treatment. It’s the best feeling when you bring not just the metallurgists together, but also the people who make the part. We work together to have the best of the best. It’s a lot of work, but we have years of experience, so we can reach the target faster than 20 years ago.

Technology has changed. As such, you also need to work with the steel manufacturer, the way the steel is made and the composition. Research has shown that if you improve the steel, you can reduce the quench pressure. If you need oil quench, then you already know that distortion will be potentially higher. If you need 20 bar, it’s one thing. Well, I did a presentation not long ago, and I found that the distortion with 10 bar or 20 bar in a Nano furnace was much better than in a larger load. For one part, that is. This might not be true if it’s other parts. There’s always that phrase in heat treatment, “that depends.” I don’t like this phrase.

Heather Falcone: It always depends.

Vincent Lelong: But it’s true, unfortunately. So, when you have worked all that together, you can bring the best analysis. I’m working before that to sell a furnace.

I need to choose whether it’s better for the customer to have a smaller furnace or a larger installation. That really depends on how many parts, the diversity of parts. If the customer has mostly small parts, the Nano may be the better choice. You have a faster answer because it’s in and out. You don’t need to buy so many fixtures. But if you have a larger part, a larger furnace is better. It’s not whether one furnace is better than the other. You need to choose which one will achieve your production target.

A customer in facility number A may need a larger furnace, but in facility B, there are other types of part, so that facility may need a smaller furnace. That’s how we work with the customer to target what is important.

Heather Falcone: Pick the right atmosphere, pick the right hot zone size, pick the right fixturing, raw material specs. All of those are going to influence how the runs going to go. I bet you have some stories about TCE from fixturing and eutectic and inadvertent bonding.

Vincent Lelong: I certainly have had a few mistakes in testing. I used a higher temperature because with vacuum carburizing we say we can go to a higher temperature. But then I used CFC fixtures at the wrong temperature!

Today however, the mixtures of fixtures can work and reduce the weight. As a metallurgist and a heat treater, I prefer to heat treat the part rather than the fixtures. When I see a customer running a load with almost more fixtures than parts, I’m asking, “What do you heat treat?” Are you losing money on heat treating fixtures more than parts?”

Heather Falcone: There is such a big weight differential between fixturing and the parts. How much lag time on your heat up and cool down are you wasting on having too much fixturing?

Vincent Lelong: It’s true.

Balancing Technical Requirements with Production (11:30)

Heather Falcone: Once you’re in recipe development, how do you balance technical requirements, the repeatability, and the realities of production?

Vincent Lelong: Repeatability is firstly about how you design the load. Then you need to know the quantity of parts. You also may know the surface of your load, but it may always be necessary, I will say. Most of the time we don’t know that and it still runs correctly.

Heather Falcone: Still a black box. It’s an art what we do.

Vincent Lelong: Technically, it’s a good thing because when we put something inside the furnace, it’s coming back, and we don’t see much difference. But the mechanical properties are completely different, so it is kind of like magic.

So you define your load, you define your heating time to get temperature, and when you’re sure you’re at temperature, you start to carburize. I will speak about vacuum carburizing. First, we have software for the carburizing boost and diffusion; you input your parameters and then you have your recipe. You can run it generic, or you can go into detail and improve it. But as built, the software will give you some set parameters and will work.

Heather Falcone: Technology is so cool.

Vincent Lelong: It’s getting better and better. You will also need to select the type of furnace. When you have one part, it’s not the same as when you have 3,000 parts. The density of the load will influence your gas flow and the capacity of your installation.

Most of our larger furnaces have a maximum of acetylene of 4,000 liters. But you can play with the gas and the duration of the boost and diffusion so it goes inside the part; when you have a blind hole or you need 3 millimeters, you carburize. (Acetylene is beautiful, but molecules can go everywhere, sometimes where we don’t want acetylene, which is why we have the stop off.) But you define your load, you define your recipe, your gas flow, and then you run a test and analyze the metallurgy. If it’s good, then you’re done and you don’t move from that.

Heather Falcone: That’s what production wants to hear.

Vincent Lelong: When you are a heat treater, you may receive 10 parts to heat treat. Tomorrow you may receive 20 parts, and maybe you need to run the same recipe. In general, you can run the same recipe for 10 parts or 20 parts. You would use the parameters of the larger load for the smaller, if possible. The result will be mostly the same, but the cost will not be the same.

This is where you can optimize your recipe for different types of load, like half of the load versus a full load. You can change the flow of gas, and then reduce your heating time because why set for two hours when in one hour, it is at temperature. One hour is money.

Heather Falcone: Got to turn and burn.

Vincent Lelong: This is also where you define your database and your repeatability. I once had a customer that had me create a recipe for a specific quantity of parts and a design of load. A few years later, that customer called and said, “It doesn’t work.” With modern heat treating controls, we record everything, so you have a database, the curves, and you can go back and see what was wrong.

Heather Falcone: Right. What changed?

Vincent Lelong: In this case, I discovered the customer was running the double quantity of parts in the same load without changing the recipe at all.

Heather Falcone: Makes sense. Start there.

Vincent Lelong: Heating time was not long enough. Gas flow was not long enough. And they were not working at the right pressure in the furnace, so failure occurred. I said, “Change that.” No news is good news usually. If the customer doesn’t call you, it’s because…

Heather Falcone: …Everything works.

Vincent Lelong: That’s the beauty of this furnace.

With repeatability, you always need to look at the curves. If you have an issue of temperature out of the range, there will be an alarm. So, you can check if the part is good without checking the metallurgy. If you heat treat a big part, usually you won’t cut the part. If you have six parts in a load or 10, you have samples. You have to validate your sample and your real part at the beginning.

Heather Falcone: To make sure it’s representative.

Vincent Lelong: Exactly, or if there are differences because you cannot find exactly the same material, you know the difference, and the difference would be always the same.

Heather Falcone: Right. Make it predictable.

Vincent Lelong: We have some customers that would check one load per day or per shift and not every load. If you have 3,000 parts in a load, you can check a part. When you have six parts, you will likely not cut a part. If the furnace tells you it’s good, why check? When you check repeatability, you still need a lab. Not checking the metallurgy is difficult. You should always check again. Repeatability shouldn’t be left to chance or statistics. Take a part and check. Is it good? Then continue.

Heather Falcone: That’s kind of the target of qualification, right? To get those parameters that you predefined.

Vincent Lelong: In time you need to be sure nothing changes.

Heather Falcone: What does re-qualifying look like?

Vincent Lelong: You want to be sure that nothing changes, material-wise. Sometimes, you run the same material, and you achieve 35 HRC. But at one point in time, you achieved only 25 HRC. This is the same material on the paper, but something has happened. So you need to go back in time and figure out what was originally going on. Is it the heat treatment? Is the installation of something around it?

Heather Falcone: Use all the data that’s available to you.

Vincent Lelong: This is where you check the productivity.

Quench Media (19:20)

Heather Falcone: How about the quench media?

Vincent Lelong: When you develop a recipe, if you do oil quenching you will always do austenitizing. You don’t want a crack. You will carburize, austenitize, and go to oil quench. It’s pretty easy to switch from atmosphere oil quench to vacuum oil quench because technically the recipe is pretty much the same, and we’re going to cover that ground extensively because I know it’s kind of scary to even consider that possibility.

When you go with gas quench, if you don’t know the target or are unsure, you select 20 bar.

Heather Falcone: Sure, 20 bar. That’s the easiest in the world.

Vincent Lelong: Exactly. You do 20 bar and you get what you get. There is an advantage of the gas quench. Many customers will ask, “Do I need to do a direct quench? Do I need to do austenitizing?” Most of the time we say direct quench, and we found that you don’t crack with gas quench. Whatever the pressure, we never really have much cracking. Or a customer may ask, “Will the part break due to the gas quench?” That will never happen. I once asked the competition if they ever saw a crack with gas quenching and they said no. It’s the way the gas quench quenches and cools the part down, then it’s less powerful than the liquid, so you don’t have this potential issue.

But if you don’t know, you stay at 20 bar. If you would like to optimize, you can reduce, but you need to achieve the target metallurgy.

Heather Falcone: Right. You’ve got to get your core hardness.

Vincent Lelong: Your limit is when your metallurgy is not right. When you reduce, you reduce the cost. If you reduce the pressure and the speed, you will reduce the distortion potential.

Heather Falcone: Which is always a good thing.

Vincent Lelong: If you have a shaft, you should not place it horizontal, because whenever you quench that, it will not be straight.

Heather Falcone: We’ve potato chipped a few parts over the years, yes.

Vincent Lelong: Me too. I have tried horizontal in some cases. It’s interesting, but you need more support. I think it’s possible, but nobody wants to try it.

Heather Falcone: Why bother?

Vincent Lelong: Vertical is easier.

Heather Falcone: If you’re qualifying, just make the fixture that’s going to support it.

Vincent Lelong: Yes. Why should I change? It’s always a big question; we just quench it vertically this way. We do it this way. Now for gears, in heat treat sites I see a lot of vertical positions for gears strung on a rod. As an operator, I don’t like that.

Heather Falcone: Tell me why.

Vincent Lelong: Because it’s heavy. We already have difficulty finding operators in heat treatment. If it’s heavy, nobody wants to do it.

From a robotic point of view, it’s more difficult, too. Today, with most vacuum carburized and gas quenched gears are heat treated horizontally on fixtures and most of the time with offset position. That will give you the best metallurgy, but also the least distortion overall. Vertically for machinists, it’s very difficult to re-machine something round to oval. When you place it horizontally, you can do potato chips but machinists can grind and reshape the part easily, if it’s possible.

Heather Falcone: If you’re already near net, it’s going to be a different story.

Vincent Lelong: Exactly. That’s where when you check your distortion and repeatability of process — it’s the fixtures.

Heather Falcone: I would think working with the customer as much as you can to see if material can be left on the part too, if we do need to have grinding, if we do need to have repair or recovery for any possible distortion.

Vincent Lelong: So, let’s say we are working on a six-speed. For the six-speed, everybody wants to vacuum carburizing gas quench. The objective: zero distortion, heat treat, and assemble.

Heather Falcone: In theory.

Vincent Lelong: No, in truth.

Heather Falcone: Really? Okay.

Vincent Lelong: That was the target.

Heather Falcone: Ambitious, I like it.

Vincent Lelong: Yes, but when you have a new product, you can use new technology, you can work on the fixtures, you can work on everything. We worked a lot with car manufacturers to do the best heat treatment, the best fixtures, the maximum of parts, of course, and repeatability. This furnace is running millions of parts.

That is why we know vacuum carburizing works and it’s repeatable. For this high volume, we had to work on zero distortion. But the specification then didn’t change, metallurgically speaking. Most of the time it was 0.3 to 0.7 millimeters. It’s a large gap. No problem. Then we went to the 10-speed for most of the automotive, and then the distortion was the target because of the experience we gained with the six-speed, which was the noise. People don’t want the noise. Today, with the 10-speed, we start to grind at 0.1 millimeter.

You have to compensate for your carburizing process so that it is longer and deeper, but also most customers will reduce the metallurgical requirements. From 0.3 to 0.7, they want 0.5 plus or minus 0.05, which is much thinner. With electrical applications today, you want zero noise, because you can hear everything. There’s a lot of grinding, and when I say a lot, I mean exact, 0.2 millimeter.

Heather Falcone: That is a lot of post-process work.

Vincent Lelong: You have the perfect teeth. You need to anticipate longer carburizing, and it’s great! Also, what I see with metallurgy, it’s not that you don’t have a general metallurgical specification, but each area of a part will have its own metallurgy. That means you have the pitch of the gear, the roots, a minimum, but sometimes you have a specification of the tip.

A customer may specify, “I don’t want more than 0.8.”

Heather Falcone: Just for that area.

Vincent Lelong: Yes. As the parts get more and more complex, they have more than just one application or function. You have the spline. You have double teeth. Each one will have its own requirement. With an atmospheric furnace, to get that, it doesn’t work very well. But with vacuum carburizing, you can achieve very precise requirements.

Switching from Atmospheric to Vacuum Carburizing (28:11)

Heather Falcone: To that point, is that one of the things that stops people from considering the change from atmospheric to vacuum carburizing, or is it part complexity?

Considering the switch? See how different carburizing technologies and furnace features stack up when you click on the image above.

Vincent Lelong: The larger companies do not seem to be afraid, because they know what they want and they already have experience. For the heat treaters, the smaller companies, it’s very difficult to switch with the requirements today. When you see the requirement on the drawing, it’s funny because before there was just heat treatment.

Heather Falcone: Yeah, it’s on this process sheet.

Vincent Lelong: One line. Surface furnace ECD, core

Heather Falcone: Right.

Vincent Lelong: Today there are different requirements, and there are several requirements: before heat treatment, after, and final. You know how much you need to take off, and not every area you take off. You said keep more material. The advantage of that is more material, less distortion. But you will have to carburize more.

Heather Falcone: Ultimately it may be more expensive for everybody.

Vincent Lelong: Exactly. The machining behind the grinding is also costly. When we develop a recipe, we have the customer machine to the final dimension, do the heat treatment, and then we will see where we are in terms of metallurgy and distortion. If we are not where we need to be, don’t take off too much. Then you adjust.

One other story is about a thread.

Heather Falcone: Oh, God, threads. The bane of every heat treater’s existence, threads.

Vincent Lelong: I get a lot of questions about threads. Do I need to make them before heat treatment? Do I need to put a mask on, paint, or make the fixture?

Heather Falcone: Texture or mechanical.

Vincent Lelong: Or to not do them and do them after?

Heather Falcone: That is also expensive.

Vincent Lelong: Yes, also expensive; but I think it could be a robotic application.

Heather Falcone: Oh true. Very good point

Vincent Lelong: It’s the way I would go.

Heather Falcone: How interesting..

Vincent Lelong: To not do the thread before.

Heather Falcone: Lower risk.

Vincent Lelong: First, when you carburize, you can create a brittleness of the thread. But also operator movement of the thread from crate to the fixtures can cause damage to the thread. What do you do? Can you save the thread after heat treatment? Not always. Then that is garbage. You had to manufacture the part, heat treat the part, just to put in the garbage, which is a cost.

Heather Falcone: Probably 80 or 90% of your whole cost, gone in an instant.

Vincent Lelong: In my opinion, you could have a robot preparing the load. You would have a robot take off, and every time it’s the same movement. Then, thread or not, it’s easier.

Heather Falcone: More predictable.

Vincent Lelong: Then the robot, if you don’t do the thread, can put the shaft, usually it’s a shaft with a right connection on the thread on the end, put to a little induction machine, reheat, and then put in a crate and go back to machining. Then it’s all done.

In that instance, I think it’s my preference not to do the thread first. I have customers who ask me for paint or for a mask. Paint is not, I would say, 100% safe. You need a specialist to put on the paint. There are some tricks, as I know heat treaters know. They have been doing this for years and they know their stuff.

Heather Falcone: That’s their secret.

Vincent Lelong: I think heat treaters have more secrets about painting and protection of the part than the big companies do. They know it better.

Heather Falcone: Their lives are on the line. That’s all they do, so they have to make it perfect.

Vincent Lelong: You can learn a lot from heat treaters. They know their work.

Heather Falcone: That’s what I always recommend to the captives. Get out to as many heat treat shops as you can, because it’s going to make your in-house heat treat better.

Vincent Lelong: They have years of experience to learn from. Heat treatment is tricks after tricks. Some customers are afraid to go to in-house heat treatment. These heat treaters hold a variety of information that is helpful for these customers.

Heather Falcone: It’s an interesting point that you brought up about being afraid, because vacuum carburizing has gotten that reputation over the years that it’s difficult. It’s tough to figure out.

Vincent Lelong: Because we did a good job. We did a good job to say you can optimize. In reality, if you understand what you want to heat treat, the carburizing process is all made by software. Every company has their own software. So it is pretty simple, and though it can still be scary for the heat treater.

Heather Falcone: Process change is scary.

Vincent Lelong: There are some companies that develop vacuum carburizing software where you need to know everything about the steel, the chemistry, all the parameters. We work with carbon. For our software, you enter the carbon content originally, roughly the temperature you would like to heat treat, and what you would like on the final. The software will give you something, and it will be 95% of what you’re looking for.

Now you need to quench. Like I said, with oil quenching, it’s no problem. Gas quenching, 20 bar, no problem, very easy and straightforward. Optimization can be where it’s trickier. But it’s just like an atmospheric furnace. I work with atmospheric furnaces, and they all have their cheat sheets.

You need carbon potential, temperature, time, and in vacuum carburizing, it’s the same thing. The temperature, your carbon potential, or what you expect for carbon, the time, the case depth you would like to achieve, and here is your process.

Heather Falcone: Then you don’t need the cheat sheet. It’s gone. Then it’s documented and repeatable.

Vincent Lelong: Right, then you have to heat up and quench. So, straight heat treatment: heat it up to be sure you’re at temperature, then carburize, and you quench.

Heather Falcone: One test?

Vincent Lelong: Yes, I often do just one test.

Customers may ask me to do a test for a Flex system, a larger system. I do it here at the Synergy Center first because it’s cheaper for me because it just value add and it’s here. I can mix a different part, different design, and run. “Oh, it’s a 8620? No problem.” Usually, it’s good.

For a Nano load, it’s like a one-fifth or it’s a one fixture and the bigger load, it’s like two column of fixtures, then you stack. So, it’s not much different. You start on a smaller scale and you go bigger, and you just add a little heat up time.

And if you ask me, “What do you need?” Put everything to the maximum!! (We are here to sell you a spare part, so, you know.) But really, if you put everything to the maximum you will be good. Maybe too good, and you might have a more maintenance, but we’d be able to provide a quote to reduce the maintenance. (*joking laughter*)

Closing Thoughts (37:36)

Heather Falcone: As we finish up, if there’s one thing that you’d want our listeners to take away, rethinking about their approach to process qualification and recipe development, what do you want them to know?

Vincent Lelong: It’s not difficult. It’s like every other heat treatment; you have to test it and you will quickly see that it’s easy. At ECM USA, we provide training and testing to show you what can be achieved and answer your questions. If you’re worried, we will show you how easy it is, how clean it is. There’s no flame. If you have oil quench, there’s no flame because everything is protected. So it’s not difficult, it’s just one step. Do not be afraid.

Heather Falcone: Take that first step, and explore the process.

Vincent Lelong: I think there’s enough research and evidence in the last 25 years in the U.S., especially with large automotive and aerospace companies to know that it’s not a big deal. Most people — in heat treating or not — don’t like change.

Heather Falcone: But they can partner with you, Vincent, who has decades of experience. Reach out to ECM. Get in touch. Start exploring.

Vincent Lelong: The funny thing is, with decades of experience, what we are capable of heat treating 20 years ago we can do way better now. The technology is better. Gas quenching is made to quench, not to cool. It’s quenching. It’s hard. It’s almost as hard of a quench as an oil quench. You can do bulk load carburizing. I did carbonitriding in bulk load not that long ago. If you had asked me to do that 20 years ago, I would say, “No way that works.” But today that works. Just contact us.

Heather Falcone: Start the conversation, right?

Vincent Lelong: And I would be happy to show you. I like my job.

Heather Falcone: You love your job. I’m going to say it. You’re very passionate.

Vincent Lelong: I like testing because it’s a challenge every day. It’s pushing the limit. It’s like a movie. Is it possible? If you follow the book or internet, they will say no. I would say, “Let’s try.” I’m testing on materials other than steel as well that I would not have expected to work. Modular furnaces can be a very versatile.

Heather Falcone: Well it sounds like production is getting ready to get things done. Thanks so much, Vincent. It was great spending time with you.

About the Guest

Vincent Lelong
Synergy Center Manager / Sr. Metallurgist
ECM USA, INC.

Vincent Lelong, ECM USA Synergy Center Manager, transferred to ECM USA in 2005 to manage the North American Testing Program in Wisconsin after 6 years’ experience with production/testing furnaces at ECM Technologies headquarters in Grenoble, France. Vincent has degrees in Chemistry & Physics from the University of Reims, and Treatment of Materials (specializing in Heat Treatment) from BTS Roosevelt, also located in Reims, France. He began his career as a production and laboratory technician for a commercial heat treater, and joined the ECM Group in 1999 as an ECM Technician running LPC testing/metallurgical analysis within ECM vacuum furnace systems.

For more information: Contact Vincent Lelong at vincentlelong@ecm-usa.com.

Heat Treat Radio #133: Process Qualification & Recipe Development in Vacuum Carburizing Read More »

Utilization: The Hidden Sustainability Metric in Heat Treatment

May 26, 2026 / MANUFACTURING HEAT TREAT TECH, VACUUM FURNACES TECHNICAL CONTENT

As manufacturers push toward ambitious sustainability targets, heat treatment remains both essential and energy intensive, making efficiency gains critical. In this Technical Tuesday installment, Myles McCarthy, a senior sustainability and climate leader at Bodycote, highlights utilization as a powerful, often overlooked lever, showing how maximizing furnace loading and focusing on energy per component can significantly reduce emissions and improve overall process performance.

This informative piece was first released in Heat Treat Today’s May 2026 Sustainable Heat Treat Technologies print edition.

The drive toward more sustainable manufacturing continues to gather momentum across sectors like aerospace, automotive, and advanced engineering. While political priorities may fluctuate, the direction is clear: manufacturers are under increasing pressure to reduce emissions, improve energy efficiency, and demonstrate measurable progress against ambitious environmental targets. For many organizations, thermal processing sits at the center of this challenge.

Heat treatment, hot isostatic pressing (HIP), and specialist surface technologies are essential to the performance, safety, and longevity of critical components. Without them, components would not perform as designed, leading to higher raw material consumption, increased emissions, and excessive waste. Yet, thermal processing is also one of the most energy-intensive stages of manufacturing. In some industries, thermal processing can account for 25–35% of a component’s carbon footprint.

As a result, attention is increasingly drawn not just to what processes are used, but how efficiently they are delivered.

Beyond Furnace Efficiency

Much of the conversation around sustainable heat treatment has focused on equipment: furnace design, insulation, electrification, and the transition to renewable energy sources. These are all important developments, and they continue to play a key role in reducing emissions.

However, an equally important and often overlooked factor is utilization. The energy consumed during a heat treatment cycle is largely fixed, regardless of whether a furnace is fully loaded or only partially utilized. As a result, the true energy intensity of heat treatment is not simply a function of furnace efficiency, but of energy consumed per component processed.

In practice, this means that two identical furnaces operating under different loading conditions can produce significantly different carbon outcomes.

The Impact of Underutilization

In many in-house environments, heat treatment is one step within a broader manufacturing process. Production variability, batch sizes, scheduling constraints, and part mix can all lead to suboptimal furnace loading. Partial loads, idle time between cycles, and non-continuous operation are common realities.

From an operational perspective, these challenges are often unavoidable. From a sustainability perspective, however, they have a direct impact, increasing energy consumption per component, raising associated carbon emissions and reducing overall process efficiency.

In this context, even highly efficient equipment may not deliver optimal environmental performance if it is not consistently utilized to capacity.

Utilization as a Sustainability Lever

True energy intensity needs to measure the energy consumed per component heat treated. | Image Credit: Bodycote

This raises an important question for manufacturing leaders and heat treatment engineers: What is the true energy cost per treated component, and how much of that is driven by utilization rather than technology?

Increasingly, improving sustainability outcomes is less about incremental gains in furnace design and more about maximizing throughput efficiency. Higher and more consistent utilization levels enable lower energy consumption per unit processed, improved process stability and repeatability, and reduced waste associated with inefficient batch cycles. In some cases, higher utilization has been shown to reduce carbon per component by up to 60%. In simple terms, a well-utilized process is often a more sustainable process.

Rethinking Traditional Boundaries

Achieving consistently high utilization is not always straightforward within a single manufacturing site. Demand variability, product diversity, and production scheduling can all limit the ability to fully optimize furnace loading.

As sustainability targets become more demanding, some organizations are beginning to explore how these constraints can be addressed more strategically. In particular, there is growing recognition that where heat treatment takes place can influence overall efficiency outcomes, especially when greater consistency of loading and throughput can be achieved.

In environments where demand from multiple sources can be aggregated, including across organizational boundaries, it becomes possible to operate equipment closer to optimal utilization levels on a sustained basis. This can improve energy efficiency per component while maintaining process control and quality standards.

At the same time, continued advances in process technology, such as vacuum processing and low-pressure carburizing, are enabling more efficient and repeatable outcomes, particularly when combined with modern, well-utilized infrastructure.

The Role of Data in Decision Making

As expectations around sustainability reporting increase, decisions related to thermal processing are also becoming more data driven. Manufacturers are increasingly required to understand and report the carbon footprint of individual components, not just emissions at site level. This shift is placing greater emphasis on measuring energy consumption and emissions at process level, including the impact of utilization.

Tools and methodologies aligned with recognized standards are enabling more accurate modeling of energy consumption per cycle and per component, emissions associated with different processing routes, and the comparative impact of alternative operating models. This data allows engineers and decision makers to move beyond assumptions and evaluate thermal processing strategies based on measurable environmental performance.

Balancing Control, Efficiency, and Sustainability

For decades, the benchmark of a well-run heat treatment operation was control, over equipment, processes, and supply. That principle remains important. However, the definition of control is evolving. Today, control increasingly includes visibility of process performance, confidence in quality and repeatability, and the ability to meet sustainability targets alongside production requirements. In this context, improving utilization is emerging as a key consideration. It offers a practical and measurable way to reduce energy intensity without compromising technical outcomes.

A Shift in Perspective

Sustainability in thermal processing is often framed in terms of new technologies or alternative energy sources. While these remain critical, utilization highlights a broader point. Efficiency is not just designed into equipment; it is achieved through how that equipment is used.

As manufacturers continue to navigate the complexities of decarbonization, focusing on energy per component rather than energy per cycle provides a more complete picture of performance. This shift in perspective does not prescribe a single solution. Instead, it encourages a more holistic evaluation of thermal processing, one that considers utilization, technology, data, and operational context together.

Successful Examples of High Utilization, Advanced Heat Treatment

Future heat treatment facilities must deliver the reliability, quality, and flexibility demanded by leading OEMs and their suppliers, while meeting efficiency and sustainability challenges in global markets, such as aerospace and automotive. An outsourced approach, supported by local and dedicated specialist capacity, can meet these needs.

Bodycote’s heat treatment plants in Derby and Rotherham — combining advanced heat treatment and densification services — are examples of a co-located outsourced model. The aerospace partnership behind these plants demonstrates three decades of reliable, dedicated, and flexible capacity, aligned to core customer requirements.

A key advantage is utilization. By complementing core aerospace demand with additional volumes from other clients and markets, these facilities maximize utilization, driving higher efficiency, lower cost per part, and improved sustainability performance.

Both sites operate highly utilized, fully electric furnaces powered by 100% renewable electricity, enabling zero-emission thermal processing. Alongside electrification, ongoing investment in energy efficiency continues to reduce consumption. The Derby site (opened in 1999) recently installed a closed-circuit adiabatic cooling system, replacing evaporative towers and delivering electricity savings of 73%, reducing peak load and associated emissions, and cutting water use by over 85%, while eliminating chemical dosing and cleaning.

These examples demonstrate how specialist providers can deliver both advanced technical capability and low-carbon infrastructure for modern aerospace manufacturing.

Similar approaches are emerging across the aerospace industry, as manufacturers replace legacy fossil-fuel-based heat treatment with more efficient outsourced solutions. These partnerships support ambitious Scope 1 and 2 emissions reductions while ensuring long-term access to modern, lower-carbon processing capacity operated at consistently high utilization.

From Energy Per Cycle to Energy Per Component

Thermal processing will remain an essential part of advanced manufacturing. Its energy intensity makes it a natural focus for sustainability efforts, but also a significant opportunity for improvement.

Focusing on utilization shifts the conversation from how much energy a furnace consumes to how effectively that energy is used. This highlights a more meaningful measure of performance: not energy per cycle, but energy per component.

As sustainability expectations continue to rise, engineers and manufacturing leaders are being asked not only to ensure process integrity, but to demonstrate measurable efficiency and carbon performance.

In that context, the most effective improvements may not come from new equipment alone, but from rethinking how processes are operated, optimized, and where appropriate, configured.

Because ultimately, sustainable heat treatment is not just about using less energy — it is about using energy more effectively.

About The Author:

Myles McCarthy
VP, Group Sustainability
Bodycote plc

Myles McCarthy is a senior sustainability and climate leader within Bodycote’s sustainability team, focused on driving and delivering corporate strategies that support the transition to more sustainable businesses. He has 25 years of experience working with boards and senior management of global businesses, both as an external climate advisor and as an in-house sustainability lead.

For more information: Contact Myles McCarthy at Myles.McCarthy@bodycote.com.

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Optimized Heat Treat Results Start with Optimized Cleaning

May 19, 2026 / CARBURIZING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, PARTS CLEANING TECHNICAL-CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Optimized heat treat performance starts long before parts reach the furnace. In this Technical Tuesday installment, Chris Tivnan of SAFECHEM North America Inc. highlights how SEW-EURODRIVE‘s switch to solvent-based cleaning enabled faster cycles, reliable residue removal, and consistent results.

This informative piece was first released in Heat Treat Today’s April 2026 Annual Induction Heating & Melting print edition.

In the world of industrial motion systems, precision, durability, and efficiency are non-negotiable. SEW-EURODRIVE, a manufacturer of advanced drive solutions, focuses on delivering performance-driven gearboxes and industrial drives that power everything from airport walkways and roller coasters to heavy-duty conveyors in manufacturing plants. At the heart of this capability lies the careful heat treatment of steel components, specifically gears and pinions, processed to exacting standards for strength and longevity.

From Atmospheric Carburizing to New Demands

Since 2002, SEW-EURODRIVE had relied on a well-established process: aqueous cleaning, followed by atmospheric carburizing, oil quenching, and a second aqueous cleaning process. The approach was reliable but not without limitations.

Their gas-fired furnaces demanded costly maintenance, such as re-bricking the hot zone, replacing furnace rails, and frequently tuning the burners to ensure safety. Oil quenching created a messy environment and required an additional post-quench wash. For smaller parts, the process was also highly labor-intensive. Operators had to manually build furnace loads, then shot blast parts after heat treatment. Processing several hundred thousand gears and pinions per year in this way translated into significant time and manpower.

Figure 1. Advanced robotics drive SEW-EURODRIVE’s fully automated cleaning and vacuum carburizing line — delivering higher throughput, consistency, and precision. Image Credit: ECM & SEW-EURODRIVE

SEW-EURODRIVE maintained five atmospheric furnaces on site, but to improve efficiency they envisioned a new setup: continuing to run large parts in the existing furnaces while shifting smaller, higher-volume gears and pinions to a vacuum carburizing line with robotic automation.

Why Vacuum Carburizing and Why Cleaning Matters

The ECM NANO vacuum carburizing system, designed for small batch sizes, allowed SEW-EURODRIVE to integrate robotic loading and unloading, a crucial step toward automation. Vacuum carburizing also offered tighter process control, reduced distortion, and more consistent results than atmospheric methods.

However, vacuum carburizing is unforgiving when it comes to cleanliness. Unlike atmospheric furnaces, which can tolerate some surface contamination, vacuum furnaces demand perfectly clean parts. Any residue from machining oils, coolants, or metal shavings risks compromising part quality and furnace integrity.

This is where cleaning — often treated as a secondary or preparatory step — became the cornerstone of SEW-EURODRIVE’s process reengineering. The HEMO hybrid cleaning machine, capable of running both aqueous and solvent programs, was selected to provide maximum flexibility. The system runs on the modified alcohol solvent DOWCLENE™ 1601.

Overcoming Initial Concerns

For a company committed to environmental responsibility, introducing a solvent-based process was not taken lightly. Concerns about waste disposal, flammability, and worker exposure were thoroughly evaluated. However, the hermetically sealed HEMO cleaning system, designed for safe solvent handling and minimal emissions, provided the reassurance the Environmental Health and Safety (EHS) team required.

Beyond the demands of vacuum carburizing itself, another decisive factor for solvent cleaning is the use of carbon fiber composite (CFC) fixtures in the cleaning and heat treat line. Lightweight yet highly durable, these fixtures make automated handling of smaller batch sizes possible. However, their porous structure tends to absorb liquids during cleaning. Any residual moisture or oils can later release in the furnace, risking damage to the hot zone and compromising part quality.

Compared with aqueous cleaning, solvent cleaning evaporates completely and removes absorbed residues far more effectively, leaving both parts and fixtures perfectly dry. In this way, solvent cleaning makes automation with CFC not only feasible but reliable. Multiple test cycles, conducted both at HEMO’s and ECM’s facilities, confirmed the performance: only solvent cleaning reliably removed the oils and coolants that could otherwise lead to furnace fouling or part discoloration.

A Technical and Operational Leap Forward

By March 2025, the fully integrated cleaning and vacuum carburizing line was in full production. The new process — solvent cleaning, vacuum carburizing, gas quenching, and tempering — represented a dramatic leap forward, both technically and operationally.

Figure 2. Full integration of HEMO cleaning and ECM vacuum technology enables a streamlined, automated workflow. | Image Credit: ECM & SEW-EURODRIVE

Parts now exit the furnace bright and clean, with no spotting or discoloration. The smaller batch sizes of the vacuum furnace system enable robotic loading, helping to achieve a streamlined, automated heat treat flow, especially critical for high-volume parts.

Manual processes once needed to build and break down furnace loads, as well as to shot blast parts post-treatment, have been fully eliminated for small components. This shift has not only freed up significant labor hours for larger parts that still require traditional handling but has also eliminated roughly $6,000 per month in consumable abrasive costs.

“In the past, it would take us two weeks to process an order of 25,000 gears and 25,000 pinions through the manual steps. That manpower is no longer needed on a very large section of our product family,” explained Chris Rollins, SEW-EURODRIVE’s Heat Treat Supervisor.

The hybrid cleaning system, equipped with aqueous and solvent cleaning technologies, was selected to provide maximum flexibility in removing different types of machining soils. This versatility ensured that the system could adapt to any future cleaning requirements. In practice, after extensive testing, SEW-EURODRIVE determined that solvent-only cycles best matched the needs of their vacuum carburizing line, offering the shortest cycle times and most consistent cleaning results.

While hybrid programs run in about 30 minutes and aqueous cycles in around 50 minutes, solvent-only cycles achieve the same high cleanliness in just 18 to 22 minutes — fast enough to keep pace with furnace loading and optimize overall throughput.

Gas quenching has also replaced oil quenching, eliminating the need for a second aqueous wash and the associated challenges of soap concentrations, rinses, and tank maintenance. Beyond weekly solvent checks and routine discharges, maintenance requirements for the cleaning machine remain low.

“With aqueous cleaning, it’s always a delicate balance to get the right amount of soap for cleaning without leaving spots,” explained Rollins. “With solvent cleaning, we don’t see spotting, rust, or any contaminants. The vacuum process also helps reduce distortion, so we have more consistent parts.”

Cleaner Start, Cleaner Finish

Optimizing heat treat results meant looking beyond the furnace for SEW-EURODRIVE. With vacuum carburizing, cleanliness is no longer optional — it’s critical. The integration of the hybrid cleaning technology unlocked the full advantages of the vacuum carburizing furnace system: automation, speed, quality, and consistency.

This process reengineering experience demonstrated that heat treat success starts far earlier, in the cleaning phase, and that true optimization comes from understanding how each part of the system supports the others. In this case, the cleaner the start, the cleaner the finish. “The new system has made us faster, leaner, and more confident in every part that leaves the line. Solvent cleaning wasn’t just a switch — it was the key to making vacuum carburizing work,” concluded Rollins.

About The Author:

Chris Tivnan
Sales Manager
SAFECHEM North America Inc.

With two decades of experience in the chemical industry, Chris Tivnan of SAFECHEM North America Inc. counsels manufacturers on the right choice of cleaning agent and their parts cleaning operation. He also manages relationships with regional distributors as well as local OEMs/OEAs.

For more information: Contact Chris Tivnan at c.tivnan@safechem.com.

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Robotic Material Handling in Heat Treatment

May 5, 2026 / CARBURIZING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, VACUUM FURNACES TECHNICAL CONTENT

Manual loading and batch transfers are giving way to robotic material handling in modern heat treat operations. In this Technical Tuesday installment, Dennis Beauchesne, general manager of ECM USA, examines how automation improves repeatability, boosts productivity, and reduces operator exposure to hazardous conditions near furnace hot zones — and how robotics, vision systems, and mobile transport technologies are helping heat treat facilities build safer and more efficient production environments.

This informative piece was first released in Heat Treat Today’s April 2026 Annual Induction Heating & Melting print edition.

Robotic material handling is rapidly transforming modern heat treat operations traditionally dependent on manual loading and batch transfer. As heat treaters face increased pressure to improve throughput and working conditions while maintaining strict quality standards, automation has become a strategic investment.

Figure 1. SEW-EURODRIVE (Lyman, SC) robotic integration by ECM Robotics features a rear robot and pallets on the left and open area on the right for dunnage storage and management | Image Credit: ECM USA

Heat treat material handling is more than simple part movement. Parts must first be positioned onto fixtures or loaded into bins which are transferred, placed into the furnace, and then moved again for quenching and/or tempering — sometimes under undesirable conditions depending on the installed technology. Additionally, a robot needs to store dunnage in the designated robot area during the processing of the parts in the furnace and then reuse it when the parts are unloaded from the furnace. Dunnage can also be stored in the heat treat area and handled by automation (Figure 1). Robotics and automation promote efficiency and repeatability in this process, which is difficult to achieve with manual operations.

Robotic Advantages

The most significant advantages of robotic material handling are repeatability, consistency, and reduction of work force. Robots execute the same motions cycle after cycle, which ensures uniform loading and proper spacing between parts within fixtures or baskets. For example, in vacuum furnaces, correct part placement is essential to achieving even heat distribution and minimizing distortion. Automated loading eliminates error caused by human fatigue or procedure changes, leading to more consistent and desirable metallurgical results and reduced scrap/re-work.

Improved throughput and increased productivity are other major justifications for robotic integration. Heat treatment can hold-up manufacturing due to cycle times and variable material flow. Robotic systems streamline loading and unloading, reduce wait time between cycles, and allow furnaces to operate at optimal capacity. In high-volume environments, robotics can be managed with upstream machining and downstream finishing processes to create a continuous, automated production line. This level of integration shortens lead times and supports just-in-time manufacturing.

Safety is equally if not more important, as handling baskets or fixtures near hot zones increases operator risk of burns and injuries. Integrating robotics improves workplace safety by removing operators from direct exposure to these hazards. This solution also addresses labor shortages by allowing skilled personnel to focus more on process optimization and quality control rather than repetitive physical tasks.

Specifically in vacuum heat treatment, robotic systems are particularly beneficial. Vacuum furnaces require precise loading to maintain thermal uniformity and protect sensitive components. Automated loaders can transfer loads between heating chambers, quench cells, and temper furnaces in a continuous process flow that minimizes temperature loss and handling delays. Metallurgical results (e.g., hardness, case depth, distortion) are also directly influenced. This is especially helpful for critical and sensitive applications, such as aerospace components and medical devices.

Robotic Components Explained

For manufacturers with in-house heat treat or commercial shops processing multiple part types, the flexibility to program and handle a wide range of part geometries, weights, and batch sizes is vital for efficient operations. Quick-change grippers, adaptive tooling, mobile transport, and vision systems are key robotic components to achieve this goal (Figure 2). Vision systems of today are far more advanced in assisting with the programming phase than those from just a few years ago.

Figure 2. ECM Robotics manipulating parts | Image Credit: ECM USA

Figure 3. AGV (automated guided vehicle), a portable robot that follows a path, delineated physically (e.g. lines on the floor) or through other guide posts (e.g. radio waves, magnets, lasers) | Image Credit: ECM USA

After the load building, automated mobile robots (AMR) or automated guided vehicles (AGV) can also be used to transport loads to and from the furnace. These mobile robots are integrated into factories to automate the transport of loads between different areas without requiring fixed infrastructure (rails or magnetic strips). This system coexists easily with operators and other equipment and adapts well to production floor changes. Integration of AMRs and AGVs frees up operators for more value-added tasks and reduces manual labor time (Figure 3).

Quick-change grippers or end effectors are tailored to the specific application and conditions when in use. Their design focuses on optimizing part clamping, friction, and contact while considering part geometry, cycle constraints, and precision requirements. Gripping technologies are available as pneumatic, electric, magnetic, or vacuum and can handle even the most delicate or fragile components in soft (flimsy) or hard state. Heat treat specific robotics companies, like ECM Robotics, also provide robotic machine vision systems. Integration of these vision systems improves precision and handling to optimize pick & place, palletizing, bulk unloading, and assembly.

For example, by identifying parts based on the diameter or number of teeth on the gear, these systems can then sort and track them within a heat treatment cell through part marking, tray/fixture encoding (QR codes), and weight scenarios or simply virtually through software, which removes the need to use any hardware tracking. Vision systems go beyond the physical movement of parts; by checking for surface imperfections and integrity, they are advantageous for quality assurance purposes.

The most common issue in the heat treating industry when integrating with robots has been fixture warpage. Modern 3D cameras can detect bent or warped pins and alloy trays to allow for movement to a new position. This capability allows for much more robust loading and unloading using moderately warped fixturing, which is common in heat treat operations. While the best consistency typically comes with the use of carbon fiber composite (CFC) trays, it is not necessary to upgrade to all CFC fixtures to get consistent loading and unloading as the system can be designed to handle either alloy trays or CFC as well as some systems with both.

In a recent vacuum furnace installation, a heat treater automated their gear cutting operation to prepare the dunnage before low pressure carburizing. The robotics integration simplified part storage by specific location to allow the robot to “see” with its vision system. Parts were then scanned using QR coding by laser marking and automatically connected to the part’s recipe as stored in the system. Typically, in a modular system using low pressure carburizing, individual cells are utilized and production is recipe driven. In this case, after a part was scanned, the recipe was uploaded into the next available cell, and the scanned parts and heat treat fixture were moved to the cell.

Capital Investment

While the initial capital investment in robotics can be significant, long-term returns are quickly realized through process optimization, better working conditions, reduced re-work, higher up-time, improved quality, and reduced labor hours. Predictive maintenance features and diagnostic monitoring further reduce unscheduled downtime. As manufacturers evaluate total cost of ownership, robotic material handling often proves to be a strategic solution that supports both operational efficiency and competitive positioning.

Future Impact on the Industry

In an industry where precision, repeatability, and reliability are essential, robotic material handling is increasingly valuable for modernizing heat treatment operations. By combining automation with advanced furnace technology or upgrading material handling of older furnace equipment, manufacturers can achieve safer workplace conditions, higher metallurgical quality, and greater overall process efficiency.

Looking ahead, the role of robotics in heat treatment will continue to expand alongside industry trends. Data-driven automation, AI-assisted scheduling, and collaborative robots are opening new possibilities for smarter, more connected facilities. Rather than replacing human expertise, robotics complement it by providing process precision and efficiency to allow heat treat professionals to focus on process innovation and more value-added responsibilities.

References

International Federation of Robotics. 2023. World Robotics Report.

Beauchesne, D. 2025. Heat Treat Robotic Paradigm Shift. Heat Treat Today, January.

About The Author:

Dennis Beauchesne
General Manager
ECM USA

Dennis Beauchesne brings experience of over 200 vacuum carburizing cells installed on high pressure gas quenching and oil quenching installations. He has worked in the thermal transfer equipment supply industry for over 30 years, 24 of which have been with ECM USA where he is the General Manager.

For more information: Contact Dennis Beauchesne at DB@ECM-USA.com.

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Improve Vacuum Quench, Maximize Tool Life

April 21, 2026 / HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, QUENCHING TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Vacuum furnaces performing hardening have been in use for over 50 years, yet many heat treaters may not be taking full advantage of newer, more advanced analysis tools and methods. Controlling the cooling pressure can dramatically improve toughness and tool life, but only if applied with precision. In this Technical Tuesday installment, Paulo Duarte, technical director at Treatnorte, explores the science behind gas quenching, the role of step cooling, and why measuring and adjusting cooling curves is critical for consistent, high-performance results.

This informative piece was first released in Heat Treat Today’s March 2026 Annual Aerospace Heat Treating print edition.

Introduction

It has been a long time since the invention of the vacuum hardening process, yet innovation in this field continues. In recent years, industrial furnaces capable of operating with higher cooling gas pressures — up to 15 bar now commonly offered on the market — have become standard. But do we truly know how to make the best use of such high pressures?

Pressures up to 10 bar were first applied to cool small parts made from cold-work tool steels, such as sheet metal stamping tools. However, such high pressures can lead to cracking in larger hot-work steel dies when cooled directly. Step cooling was introduced as a solution: start with a fast initial cooling at higher temperatures to avoid carbide formation, then gradually lower pressure stages during the final cooling phase to reduce distortion and minimize the risk of crack appearance.

Despite this empirical knowledge, the question remains: do we really understand what we are doing? Are we routinely measuring cooling rates to determine where they stand on the CCT diagram, predicting microstructure and properties, and adjusting quenching parameters accordingly? And are we certain about which pressures to use for producing high-performance, demanding tools?

Cooling in Vacuum Furnaces

Quenching is one of the most critical steps in the hardening cycle. It transforms austenite into the optimal final microstructure, avoiding the formation of coarse carbides and pearlitic constituents during cooling. This ensures the finest possible microstructure.

Figure 1. Gas quenching in a vacuum hardening furnace | Image Credit: SECO/WARWICK

Figure 2. Surface cooling rates region on systematic analysis of parts quenching in a 600 mm x 600 mm x 900 mm furnace. Parts comprising weights from 500 up to 1,000 kg. Cooling pressures varies from 4 to 5 bar. Hot work tool steel. | Image Credit: Metaltec Solutions

In vacuum furnaces, this is typically achieved by injecting cooling gas through nozzles directed at the surface of the parts located in the furnace hot zone. During cooling, the gas circulates through the chamber, being drawn through furnace ports into contact with the heat exchanger tubes. A turbine then blows the cooled gas back into the hot zone where the load is located (Figure 1).

The higher the programmed cooling pressure, the greater the volume of gas passing through the nozzles over the same period of time. This increases the heat transfer from the parts to the cooling gas, resulting in a faster cooling rate.

By measuring successive cooling curves for different loads, specifically for single hot-work steel tools weighing over 500 kg, surface cooling rates pass through the bainitic–martensitic domain (the green area of the CCT diagram shown in Figure 2). Thinner parts tend to cool closer to the martensitic end at the Ms-Bs intersection, while larger tools tend to approach the pearlitic nose.

These observations highlight the importance of adjusting cooling pressure to produce the desired microstructure and account for the different cooling behaviors of large, medium, and small parts.

Investigative Approach: Testing Furnace Data Against CCT Diagrams

Measuring part temperatures during cooling began over 20 years ago, using thermocouples and data loggers, and comparing the results to steel continuous cooling transformation (CCT) diagrams. Most vacuum furnaces do not include this capability as standard, and when available as optional software, many companies choose not to invest in it. In 2005, it was discovered what few in the industry knew at the time: hardening hot-work tool steels in industrial vacuum furnaces often results in a bainitic–martensitic microstructure. This phenomenon is now more widely recognized, with published cooling curves overlaid on CCT diagrams for larger tools becoming more available.

Even so, open discussion remains rare, partly because many heat treaters are reluctant to present this evidence to academia, fearing criticism that their results do not match the fully martensitic microstructure taught at universities. This is not a debate about right or wrong, but rather an opportunity for research and improvement in heat treatment practices worldwide.

After initial testing with a 600 mm × 600 mm × 900 mm French-made single-chamber furnace, trials continued with a larger 900 mm × 900 mm × 1,800 mm German-made vacuum furnace. These tests began by measuring both surface and core temperatures for repeated cycles with small and large charges ranging from small cold-work tools to hot-work tool steel parts weighing 500–1,500 kg. Leading vacuum furnace manufacturers in North America and Europe have developed technologies capable of successfully heat treating small, medium, and large tools, resulting in microstructures that often contain both bainite and martensite. This is, in fact, an inherent characteristic of the technology. Such tools have performed well in service for decades. That said, heat treaters using higher cooling pressures have seen improved tool life significantly, while also increasing the risk of treatment failures if the pressure is too high.

In the last 10 years, properties and microstructure analyses have shown that variations in cooling rate can significantly change the microstructure and toughness of the part even within the same bainitic–martensitic domain of the CCT diagram.

With the emergence of Industry 4.0 and 5.0, along with digitalization and AI, systematic research into heat treatment processes combined with quenching deformation simulation can lead to better selection of cooling pressures. This is a critical parameter in controlling the hardening process, and it has a direct impact on part toughness and service performance. Metaltec Solutions introduced one of the first software tools aimed at improving vacuum heat treatment through Industry 4.0 concepts in 2017. This technology represents a step toward greater awareness and precision in tool steel hardening, helping heat treaters program their cycles for optimal performance in demanding applications.

Regulating Pressure in Vacuum Hardening Furnaces

To obtain the best possible microstructures, gas quenching must be programmed in the furnace so that the cooling rate is kept as close as possible to the martensitic end, i.e., at the Ms-Bs intersection, of the CCT diagram, avoiding the formation of coarse and undesirable microconstituents in the steel. This is achieved by selecting the highest permissible cooling pressure that still prevents cracking or excessive deformation. While small parts can withstand direct high-pressure cooling, larger tools require a reduction in cooling pressure.

Preliminary Pressure Comparison

For optimal quenching of large parts, the cooling pressure should not remain constant throughout the entire cooling cycle. Instead, high pressure should be applied during the initial cooling stage to prevent coarse carbides and pearlite formation and then reduced when the surface temperature reaches approximately 550°C (1022°F). This creates a martempering stage at lower pressures, reducing the risk of distortion and cracking.

Figure 3a. Cooling pressure effect on Vidar Superior (an H11 steel grade
variation) part surface toughness | Image Credit: Metaltec Solutions

Figure 3b. Cooling pressure effect on 400 mm x 400 mm x 400 mm
block surface toughness | Image Credit: Metaltec Solutions

If we measure the toughness of steel pieces quenched at different cooling pressures, then tempered together to achieve a typical 46–48 HRC hardness (in hot work tool steel), we find that higher cooling pressures result in greater toughness. Using older furnace pressures (around 3 bar) yields lower toughness, whereas increasing cooling pressure can improve toughness by approximately 60% (Figure 3a). This translates into longer tool life, since high-pressure-quenched tools better absorb stress, delaying the initiation and propagation of cracks. These benefits result from higher cooling rates (Figure 3b) and the corresponding finer microstructures achieved.

Although quenching at 3, 6, and 9 bar passes through the same transformation domain on the CCT curve, differences in the resulting internal steel structure, whether coarser or finer, are clearly observable.

True Toughness and Speed

Looking in more detail at the above findings, we can observe that when parts are cooled in a 900 mm × 900 mm × 1,800 mm vacuum furnace, the gas temperature drops below the Ms temperature (for typical hot work tool steels) in less than one minute. The gas temperature then remains near room temperature during the subsequent cooling of the parts (Figure 4a).

Figure 4a. Cooling NADCA block in a large vacuum hardening furnace; gas cooling rate according to gas pressure used | Image Credit: Metaltec Solutions

Figure 4b. Cooling NADCA block in a large vacuum hardening furnace; surface cooling curves and its respective toughness after tempering, with the alteration of the cooling curve behavior provided by the martempering (final hardness level 46–48HRC hot work tool steel | Image Credit: Metaltec Solutions

The parts, however, take considerably longer to cool down to the furnace unloading temperature, depending on the cooling pressure applied. When analyzing the cooling of large dies using the NADCA block as the standard size for comparison, the surface cooling curves vary according to the applied pressure, falling into the bainitic–martensitic domain for 3, 6, and 9 bar cooling pressures.

From this data, it can be seen that hardness is not significantly affected by using 3, 6, or 9 bar cooling pressures, even though the higher pressures produce cooling rates up to twice as fast as the slower ones. Toughness, however, is largely influenced by the way the cooling curves pass through the bainitic–martensitic domain, whether crossing the Bs and Ms intersection closer to the martensitic end (9 bar), near the center (6 bar), or closer to the pearlitic nose (3 bar).

Tuning Pressure and Time

These results show that, within the typical cooling rates of vacuum hardening (Figure 2), toughness varies significantly with cooling pressure, corresponding to finely tuned cooling speeds ranging from approximately 9 to 16°C/min (48 to 61°F/min) between 800°C and 500°C (932°F and 1472°F). This highlights the need to use the highest possible cooling pressures to achieve excellent properties while avoiding direct high-pressure cooling of large parts by applying step cooling with an initial fast cooling phase, followed by reduced pressure.

How Microstructure Drives Toughness

The reason for achieving better properties at higher cooling pressures lies in the resulting microstructure, as shown in Figure 5. Fine bainite and martensitic needles, formed through faster cooling rates, are responsible for the higher toughness observed. When lower cooling pressures are used, the cooling rate decreases, leading to coarser needle sizes (Figres 5a–c) and, consequently, lower toughness values.

Figure 5a-c. Microstructures obtained after quenching Orvar Supreme (premium H13 steel): a) 100°C/min; b) 12°C/min; c) 3°C/min (or, a) 180°F/min; b) 22°F/min; c) 5°F/min) | Image Credit: Metaltec Solutions

Figure 6. Toughness model | Image Credit: Metaltec Solutions

This can be explained by Figure 6. In a coarser microstructure, cracks can propagate more easily because there are fewer obstacles to their advance. In finer microstructures, the higher density of needles forces cracks to deviate repeatedly from their path due to the branching effect, altering the directions of crack propagation. This “shock absorber” effect — caused by the frequent detours a crack experiences when traveling through a greater number of fine needles — is the reason for the toughness improvement observed when higher cooling pressures are used to achieve faster cooling rates.

Figure 7. Convection coefficients for a 900 mm × 900 mm × 1,800 mm vacuum hardening furnace according to the pressure being used | Image Credit: Metaltec Solutions

Each furnace behaves differently, from one furnace builder to another and also depending on the level of maintenance of a furnace. So a similar furnace to the one used for obtaining cooling curves and corresponding toughness values (Figure 4b) was used to obtain the convection coefficients (Figure 7). We can see a strong correlation between convection coefficient, pressure, and final toughness obtained, indicating that these features must be carefully adjusted to reach optimal part properties and longer service life.

Conclusion

Properly applying cooling pressures, through direct high-pressure cooling for small loads or step cooling for larger tools, can significantly increase part toughness and extend tool life. The key lies in understanding how cooling curves interact with the bainitic–martensitic microstructure and adjusting pressure according to part size, geometry, and furnace characteristics.

By measuring temperatures, analyzing microstructures, and fine-tuning cooling cycles, heat treat operators can achieve consistent, high-performance results, as demonstrated with the above studies on tool steels. Faster, well-controlled cooling typically produces finer bainitic–martensitic microstructures which results in a part with “shock absorber” qualities.

Ultimately, maximizing cooling pressure, not just for minimal distortion, creates more durable tools, reduces downtime, and strengthens competitiveness through part performance.

About The Author:

Paulo Duarte
Technical Director
Treatnorte

Paulo Duarte is an independent researcher and consultant on heat treat technologies, also working as technical director at Treatnorte. His education and expertise in metallurgy have culminated in several articles and patents. Previously, he was the project manager at Metalsolvus and also had been the technical manager and heat treatment manager within bohler-uddeholm group for the Portuguese market. Currently, Paulo focuses on helping heat treaters by providing innovative, more efficient, and profitable heat treatment services to companies.

For more information: Contact Paulo Duarte at pauloduarte@treatnote.pt.

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Hypersonics Come Alive with Vacuum and Controlled Atmosphere Furnaces

April 14, 2026 / AEROSPACE HEAT TREAT TECHNICAL CONTENT, ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, SINTERING POWDER/METAL TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Hypersonic vehicles and missiles operating at Mach 5 and beyond place unprecedented thermal and environmental demands on aerospace materials. In this Technical Tuesday installment, Scott Robinson, product manager of ceramics and powder metallurgy at Centorr Vacuum Industries, examines how vacuum and controlled-atmosphere furnaces support the research, prototyping, and production of ultrahigh-temperature ceramics, carbon–carbon composites, and other advanced materials used in hypersonic applications.

This informative piece was first released in Heat Treat Today’s March 2026 Annual Aerospace Heat Treating print edition.

Introduction

Hypersonic missiles and vehicles are an emerging class of aerospace technology that is developing rapidly toward active use in military and potentially commercial applications. These machines can achieve sustained speeds of Mach 5 or greater within the Earth’s atmosphere (i.e., at altitudes below about 90 km). While conventional intercontinental ballistic missiles can also achieve hypersonic speeds during atmospheric reentry, they follow a high-arching ballistic trajectory with limited maneuverability, in contrast to the real-time in-flight maneuverability offered by hypersonic systems. As such, military actors prefer hypersonic missiles for precision strikes (Mesa 2024), while in the commercial realm, airliners are excited by the possibility of drastically shortened journey durations with hypersonic vehicles (TomorrowDesk 2025).

Because hypersonic missiles and vehicles move at extreme speeds within Earth’s atmosphere, they are subject to significant atmospheric compression and friction effects (Smith 2021). These effects result in considerable aerodynamic heating of the leading edges, nose tips, and exhaust-washed structures, from 1800°C (3200°F) to more than 3000°C (5400°F).

Traditional aerospace materials such as aluminum, stainless steel, and titanium cannot be used at these elevated temperatures without thermal protection engineering. In contrast, an emerging portfolio of materials including refractory metals, carbon-carbon composites, ultrahigh-temperature ceramics (UHTCs), and ceramic matrix composites (CMCs) can more easily deal with this extreme heat.

UHTCs and CMC materials typically are composed of metal carbides, borides, and nitrides, which means they are traditionally processed at very high temperatures. Currently, the leading candidate materials are silicon carbide (melting/decomposition point: 2730°C, or 4945°F) and zirconium diboride (melting point: ~3246°C, or 5875°F) due in part to their reasonable raw material costs.

Processing of UHTCs, CMCs, and other advanced materials for aerospace applications includes one or more of the following high-temperature processing steps, often using vacuum and controlled atmosphere furnace technology:

Chemical vapor infiltration
Chemical vapor deposition
High-temperature sintering
Graphitization
Silicon melt infiltration of carbon-carbon composites

Each stage of the product development cycle — from laboratory-scale research and development to prototype development to production-scale manufacturing — requires a portfolio of specialized furnaces to achieve the goals of each stage.

This article takes a closer look at the types of furnace solutions available to develop, process, and commercialize these high-performance materials.

Laboratory-Scale Research and Development

Figure 1. a) Centorr Vacuum Industries’ LF 3000°C (5400°F) graphite vacuum furnace and b) top view of hot zone; 3” x 4” (75 x 100 mm; Ø x h) hot zone. | Image Credit: Centorr Vacuum Industries

Laboratory-scale R&D activities focus mostly on the development, fabrication, and testing of small-scale parts, which require a small, adaptable furnace.

The LF graphite vacuum furnace is an example of the type of furnaces needed for small-scale parts (Figure 1). First designed in 2012, it is a robust, low-cost development furnace with temperature capability up to 3000°C (5400°F) in vacuum or inert gas. This temperature range covers most hypersonic, UHTC, and other applications. For example, current users fit the small 3″ x 4″ (75 x 100 mm; Ø x h) hot zone with small graphite crucibles to fire graphite-based powders for applications in battery and electric vehicle technology.

In another case, Dalhousie University in Nova Scotia, Canada, a research-based university, modified the base LF system by adding a small binder/off-gassing trap and positive pressure exhaust tower for processing of non-oxide ceramics produced by additive manufacturing. These samples include silicon-based ceramics (silicon carbide and silicon nitride), high-entropy ceramics, and cermet systems.

Subsequent laboratory applications require a larger hot zone furnace for processing bigger samples. One example of this type of furnace is the Series 10 graphite tube furnace (Figure 2). This tube furnace is based on a more than 50-year-old furnace design, although the traditional alumina or quartz tube has since been replaced with a solid graphite tube. Operating in vacuum or partial/positive pressures of argon, R&D centers use this furnace to process carbon powder formulations to maximize the percent conversion to graphite, as not all carbon-based starting materials will convert to crystalline graphite.

Figure 2. Series 10 3000°C (5400°F) graphite tube furnace; 4″ x 16″ (100 mm x 400 mm) hot zone diameter and height. Image Credit: Centorr Vacuum Industries

Figure 3. a) Series 45 graphite top-loading furnace and b) top view of hot zone. Used for carbon/graphite work, this model offers a larger useable firing footprint at higher temperatures than the Series 10 furnace. The hot zone diameter and height dimensions approximate 6″ x 6″ (150 mm x 150 mm), and temperature is rated for 3200°C (5790°F). | Image Credit: Centorr Vacuum Industries

As R&D activities begin to focus on particular material compositions, larger furnaces are needed to synthesize meaningful sizes and quantities of candidate materials prior to scaling up for manufacture, like the Series 45 graphite top-loading furnace (Figure 3).

Characterization and Prototyping Stage

Figure 4. Front view of the Series TT Testorr graphite hot zone rated for 2700°C (4890°F) processing temperatures | Image Credit: Centorr Vacuum Industries

Once the final candidate materials are processed, aerospace design engineers need to test meaningfully sized samples of the materials at high temperature under mechanical loading. It is best to have a furnace that can be combined with mechanical test stands to take measurements of mechanical properties. This is the case for Wichita State University’s National Institute for Aviation Research, which leverages multiple Testorr® furnace units to measure tension, compression, and shear properties of ceramic matrix composites, refractory metals, and other materials at high temperature. Rated for temperatures up to 2700°C (4890°F) in vacuum or inert gas, the furnace can better simulate some aspects of hypersonic service environments (Figure 4).

An important task of the R&D and prototyping stages is to work out processing parameters that will be translated to production-scale manufacturing processes. For example, simple carbon structures will react with air during reentry and suffer damaging effects at temperatures as low as 500°C (930°F). Therefore, any carbon-carbon materials or solid carbon shapes used in hypersonic applications must be protected with advanced ceramic coatings for durability and oxidation resistance.

Chemical vapor deposition is one such coating deposition process, and one of the most popular protective coatings is silicon carbide. The coating is deposited on substrate parts by flowing hydrogen gas through a bubbler of liquid methyltrichlorosilane (MTS; CH₃SiCl₃) gas. Newer systems use a heated evaporator to vaporize the MTS liquid in a hydrogen carrier gas stream. The combination of hydrogen and MTS is introduced at partial pressures into the furnace hot zone inside a graphite retort, where the gases “crack” or decompose, depositing microns-thick coatings of silicon carbide onto the part’s surface.

Production Stage

Once the advanced materials are properly characterized and prototyped, it is time to look at equipment for full-scale production manufacturing. The furnace configurations for these processes can be conventional front-loading designs or may be oriented in vertical top- or bottom-loading designs for floor space savings and gas flow dynamics.

Figure 5. Production-size Sintervac vacuum furnace for processing carbon-carbon melt infiltration composite materials | Image Credit: Centorr Vacuum Industries

The Sintervac® front-loading graphite furnace (Figure 5) has integral graphite retort and dual gas flow to the main chamber and retort. These furnace systems include durable rotary piston pumping systems with inline binder traps and particulate filters to protect the pumping systems from damage from abrasive ceramic particulates. The internal graphite retort compartmentalizes the off-gassing that takes place and prevents it from escaping into the hot zone, where the oxide byproducts can attack and degrade the graphite heating elements and rigid graphite board insulation.

One common application for this type of furnace is melt infiltration of carbon-carbon composites to improve the physical properties and oxidation resistance of the composite. When processed in partial pressures (or even at positive pressures) of argon, silicon will melt at approximately 1450°C (2640°F). The silicon liquid and vapor infiltrate into the void spaces of the porous carbon-carbon composite via capillary action. The infiltrated silicon reacts with the free carbon in the carbon-carbon fiber structure, forming a silicon carbide matrix around the carbon-carbon fiber structure.

Firms like Exothermics (Amherst, NH) use this process for missile and aerospace applications. The silicon carbide matrix structure provides an environmental barrier to oxidation during reentry into Earth’s atmosphere and improves the matrix’s temperature performance to approximately 1600°C (2910°F) in air.

Smaller production units were also developed for carbon-carbon work at temperatures from 2450°C and 2600°C (4440°F and 4710°F). The addition of dedicated water-cooled filtration traps and 10-μ particulate filters helps deal with the heavy off-gassing expected from processing of carbon-carbon materials.

In contrast to melt infiltration, chemical vapor infiltration drives gaseous reactants into the porous matrix where the gas reacts with the porous structure to form a dense matrix. The chemical vapor infiltration process is used to fabricate larger parts for hypersonic applications, such as rocket motors and missile components, and carbon-carbon aircraft brakes. Vertical top- and bottom-loading chemical vapor infiltration units like the example in Figure 6 can be used for these types of applications.

Figure 6. Series 4300 vacuum furnace for chemical vapor infiltration and graphitization. The furnace may be built in a top-loading or bottom-loading configuration; the unit scales from 52″ to 80″ (1,320 mm to 2,000 mm) in diameter and heights from 80″ up to 120″ (2.0 to 3.0 meters). | Image Credit: Centorr Vacuum Industries

In the chemical vapor infiltration process, gases, including hydrogen, methane, and propane, are fed into the furnace chamber at high flow rates and at temperatures approaching 1000°C–1100°C (1830°F–2010°F). The methane and propane gases break down and deposit carbon deep into the matrix of the carbon-carbon fibrous parts. These cycles can be very long, approaching seven to ten days, for the material to fully densify, and multiple cycles are usually necessary.

Low operating pressures require extremely large mechanical pumping systems with large vacuum blowers or boosters. These furnaces include water-cooled “tar” traps (with a heated stripping system) and large Dollinger particulate filters for handling the resin off-gas byproducts.

These furnaces are almost always induction heated, using multizone induction coils and large, thick-wall graphite susceptors for optimal temperature uniformity. The insulation design uses carbon black powder, which is economical and highly efficient for temperature reduction.

While more conventional rigid or flexible graphite board or felt materials can be used, Centorr’s experience has shown that the degree of infiltration of carbon resins over time will affect the density and porosity of the insulation pack (as it does the load material), causing degradation and densification of the insulation. The denser insulation results in high coil water temperatures, which compromises hot zone life. Specialized carbon black installation and removal equipment is required by the end-user to maintain the insulation efficiency of the furnace hot zone. Because gas flow in the furnace is critically important, special diffusor plates or plenums are used to uniformly direct gas flow across the entire geometry of the parts.

Once the advanced materials undergo chemical vapor infiltration, they are still composed of a carbon base material, which needs to be converted to a more orderly crystalline graphite structure to impart the durability and strength required in aerospace applications. To accomplish this conversion, the material needs to be heated at temperatures greater than 2300°C (4170°F), a process called graphitization.

The graphitization process employs similar furnace designs to the chemical vapor infiltration process, but the induction heating power supply is changed to the more conventional single zone coil, and the vacuum pumping systems are smaller with no tar traps needed. Load sizes of 3,000–5,000 lb. (1,360–2,268 kgs) are possible. Both the smaller and larger chemical vapor infiltration and graphitization units have large, water-cooled heat exchangers inline with large cooling fans, which reduce cooling times from ten or more days to less than 175 hours.

Figure 7. a) Series 3800 bottom-loading silicon carbide chemical vapor deposition furnace. b) Series 3800 chemical vapor deposition furnace hot zone with multizone control; 53″ diameter x 83″ height (1,350 mm x 2,108 mm) graphite hot zone furnace rated for 1600°C (2910°F) operation. | Image Credit: Centorr Vacuum Industries

A smaller graphitization unit was also developed in a 30″ diameter x 40″ height (76 mm x 1,000 mm) size rated to 2900°C (5250°F) maximum temperature in a vertical bottom-loading configuration for processing smaller parts in lower volumes for aerospace brakes.

The silicon carbide chemical vapor deposition units for laboratory applications discussed previously are also needed for production-size volumes (Figure 7). Due to tight temperature uniformity requirements, these units are multizone control with graphite hot zones constructed of rigid graphite board for process durability. The pumping systems can be either “dry” or “liquid ring” designs for processing the acidic off-gas materials. A post-exhaust chemical scrubber system is required to safely neutralize the hydrogen chloride off-gases.

Enabling the Next Generation of Aerospace Materials

The difficult design requirements of next-generation aerospace technologies will continue to push the existing limits of material performance. As characterization and development of new materials will be critical to the success of these aerospace programs, vacuum and controlled atmosphere furnaces will play an essential role in the production of such materials.

References

American Elements. n.d.a “Silicon Carbide Data Sheet.” https://www.americanelements.com/silicon-carbide-409-21-2.

American Elements. n.d.b “Zirconium Diboride Data Sheet.” https://www.americanelements.com/zirconium-diboride-12045-64-6.

Mesa, J. 2024. “What’s the Difference Between a Hypersonic Missile and ICBM?” Newsweek, November 21, 2024. https://www.newsweek.com/difference-between-icbm-irbm-missiles-1989780.

Smith, C. R. 2021. “Aerodynamic Heating in Hypersonic Flows.” Physics Today 74 (11): 66–67.

TomorrowDesk. 2025. “Hyperian Aerospace and the Dawn of Hypersonic Flight.” TomorrowDesk, March 29, 2025. https://tomorrowdesk.com/evolution/hyperian-aerospace-hypersonic-flight.

Heat Treat Today thanks the American Ceramic Society for allowing us to print this piece. This article was originally published in ACerS Bulletin, September 2025.

About The Author:

Scott K. Robinson
Product Manager of Ceramics and Powder Metallurgy
Centorr Vacuum Industries

Scott K. Robinson is product manager of ceramics and powder metallurgy at Centorr Vacuum Industries (Nashua, NH).

For more information: Contact Scott Robinson at srobinson@centorr.com.

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Ask The Heat Treat Doctor®: Why and How Do We Heat Treat Gears? Part Two

April 9, 2026 / AEROSPACE HEAT TREAT TECHNICAL CONTENT, CARBURIZING TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, INDUCTION HEATING TECHNICAL CONTENT, OP-ED, QUENCHING TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers and to answer your questions about heat treating, brazing, sintering, and other types of thermal treatments as well as questions on metallurgy, equipment, and process-related issues. In this installment, Dan Herring continues his discussion on gear heat treatment, exploring vacuum and induction hardening methods for gears — from low-pressure carburizing for advanced materials to single shot and tooth-by-tooth induction techniques — and how each can be matched to the specific demands of any gear application.

This informative piece was first released in Heat Treat Today’s March 2026 Annual Aerospace Heat Treating print edition.

In Part One of this discussion (Air & Atmospheres Heat Treating, February 2026), we discussed various gear types, materials, and how they can be atmosphere heat treated. This month, we are focusing on vacuum and induction heat treating methods. Let’s learn more.

Vacuum Heat Treatment Processing Methods

Table A. Advanced Materials Processed by LPC

Vacuum processing can be used for most of the atmosphere treatments mentioned in Part One including carburizing (Figure 1). Low pressure carburizing (LPC) is a proven technology and the choice for many advanced applications in aerospace, automotive, off-highway, and motorsports markets, as well as the development of carburizing cycles for high-performance materials (Table A).

Figure 1. Typical commercial heat treat load of gears for vacuum carburizing (Otto and Herring 2007) | Image Credit: Photo courtesy of Midwest Thermal-Vac

Figure 2. Pyrowear 675 – LPC – anneal – double normalize – harden – anneal – deep freeze – double temper | Image Credit: The HERRING GROUP, Inc.

The range of effective case depths for most of these grades can range up to 2.0–3.0 mm (0.080–0.120 inches) without significant sacrifice of microstructure (Figure 2). Furnace variables, such as temperature uniformity (± 3°C or ± 5°F), control of cycle parameters (boost/diffuse times, gas flow rate, pressure, hydrocarbon type) and surface carbon optimize the microstructure, producing case uniformities of ± 0.05 mm (± 0.002 inches). Where permitted, the range of carburizing temperatures now includes the use of high temperature (> 980°C, or 1800°F) techniques.

All these advanced materials required extensive development testing to produce custom designed recipes to optimize cycle parameters. Also, quenching methods (Otto and Herring 2002) have improved, allowing us to achieve desired core properties with quenching parameter selection (high-pressure gas or oil) for distortion-sensitive and distortion-prone part geometries (Otto and Herring 2005, 2008).

Induction Hardening Methods

Various methods of hardening via applied energy are used in manufacturing gears, including flame hardening, laser surface hardening, and induction hardening.

Of the various types of applied energy processing, induction hardening is the most common. Induction heating is a process that uses alternating electrical current that induces a magnetic field, causing the surface of the gear teeth to heat. The area is then quenched resulting in an increase in hardness within the heated area. This process is typically accomplished in a relatively short time. The final desired gear performance characteristics are determined not only by the hardness profile and stresses but also by the steel composition and prior microstructure. External spur and helical gears, bevel and worm gears, racks, and sprockets are commonly induction hardened. Typical gear steels include AISI/SAE grades 1050, 1060, 1144, 4140, 4150, 4350, 5150, and 8650.

Figure 3. Patterns produced by induction hardening (Rudnev 2000)

The hardness pattern produced by induction heating (Figure 3) is a function of the type and shape of inductor used, as well as the heating method. Quenching or rapidly cooling the workpiece can be accomplished by spray or submerged quench. The media typically used for the quench is a water-based polymer. The severity of this quenchant can be controlled by the polymer’s concentration. Cooling rates are usually somewhere in between what would be obtained from pure water and oil. In some unusual situations compressed air or nitrogen is used to quench the part.

The most common methods for hardening gears and sprockets are by single shot (Figure 4) or the tooth-by-tooth method (Figure 5). Single shot often requires large kW power supplies but results in short heat/quench times and higher production rates. This technique uses a circumferential copper inductor, which will harden the teeth from the tips downward.

Figure 4. Typical single shot induction hardening operation | Image Credit: Photo courtesy of Ajax-Tocco-Magnethermic

Figure 5. Tooth-by-tooth induction hardening of a helical gear | Image Credit: Photo courtesy of Ajax-Tocco-Magnethermic

The larger and heavier loaded gears (where pitting, spalling, tooth fatigue, and endurance are issues) need a hardness pattern that is more profiled like those produced by carburizing, which can be obtained by tooth-by-tooth hardening. This method is limited to gear tooth sizes with modulus 4.23–5.08 (6 or 5 DP) using frequencies from 2 to 10 kHz and about 2.54 (10 DP) using a range of 25 to 50 kHz.

The lower the frequency, the deeper the case depth. Tooth-by-tooth hardening is a slow process and usually reserved for gears and sprockets that are too large to single shot due to power constraints. The process involves heating the root area and side flanks simultaneously, while cooling each side of the adjacent tooth to prevent temper-back on the backside of each tooth. The induction system moves the coil at a pre-programmed rate along the length of the gear. The coil progressively heats the entire length of the gear segment while a quench follower immediately cools the previously heated area. The distance from the coil to the tooth is known as coupling or air gap. Any changes in this distance can yield variation in case depth, hardness, and tooth distortion. The gear is indexed after each tooth has been hardened, often skipping a tooth. This requires at least two full revolutions in the process to complete the hardening of all teeth. Straight, spur, and helical gears up to 5.5 m (210 inches) weighing 6,800 kg (15,000 lb) have been processed with this method. The entire process yields a repeatable soft tip of the tooth with hard root and flank. In other applications, the tip and both flanks can be hardened simultaneously and yield a soft root.

In Summary

Today’s design engineer has the good fortune of being able to choose from a number of heat treatment technologies for any given type of gear material and design. When selecting a gear hardening method, it is essential to specify not only the desired mechanical and metallurgical properties, but the critical dimensions that must be held and even the desired stress state of the gears themselves. The secret to success is understanding the advantages and limitations of each technology and taking these into consideration when determining the overall cost of gear manufacturing.

References

Herring, Daniel H. 2004a. “Gear Heat Treatment: The Influence of Materials and Geometry.” Gear Technology, March/April.

Herring, Daniel H. 2004b. “Reducing Distortion in Heat-Treated Gears.” Gear Solutions, June.

Herring, Daniel H. 2007a. “Oil Quenching Technologies for Gears.” With Steven D. Balme. Gear Solutions, July.

Herring, Daniel H. 2007b. “Heat Treating Heavy Duty Gears.” With Gerald D. Lindell. Gear Solutions, October.

Herring, Daniel H. 2012–2016. Vacuum Heat Treatment. Vols. 1–2. BNP Media Group.

Herring, Daniel H. 2014–2015. Atmosphere Heat Treatment. Vols. 1–2. BNP Media Group.

Herring, Daniel H., Gerald D. Lindell, D. J. Breuer, and B. Matlock. 2001. “Atmosphere vs. Vacuum Carburizing.” Heat Treating Progress, November.

Herring, Daniel H., Gerald D. Lindell, D. J. Breuer, and B. Matlock. 2002. “An Evaluation of Atmosphere and Vacuum Carburizing Methods for the Heat Treatment of Gears.” In Off-Highway Conference Proceedings. SAE International.

Otto, Frederick J., and Daniel H. Herring. 2002a. “Gear Heat Treatment: Today and Tomorrow, Part 1.” Heat Treating Progress, June.

Otto, Frederick J., and Daniel H. Herring. 2002b. “Gear Heat Treatment: Today and Tomorrow, Part 2.” Heat Treating Progress, July/August.

Otto, Frederick J., and Daniel H. Herring. 2005. “Vacuum Carburizing of Aerospace and Automotive Materials.” Heat Treating Progress, January/February.

Otto, Frederick J., and Daniel H. Herring. 2007. “Advancements in Precision Carburizing of Aerospace and Motorsports Materials.” Heat Treating Progress, May/June.

Otto, Frederick J., and Daniel H. Herring. 2008. “Improvements in Dimensional Control of Heat Treated Gears.” Gear Solutions, June.

Rudnev, V. 2000. “Gear Heat Treating by Induction.” Gear Technology, March/April.

About the Author

Dan Herring
“The Heat Treat Doctor”
The HERRING GROUP, Inc.

Dan Herring has been in the industry for over 50 years and has gained vast experience in fields that include materials science, engineering, metallurgy, new product research, and many other areas. He is the author of six books and over 700 technical articles.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

For more information about Dan’s books: see his page at the Heat Treat Store.

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Heat Treatment of Carbon and Graphite-Based Materials for Space Travel and Exploration

April 7, 2026 / AEROSPACE HEAT TREAT TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Carbon and graphite-based materials are among the few engineered materials capable of withstanding the extreme thermal, mechanical, and environmental demands of space travel. In this Technical Tuesday installment, Kimberly Thompson, technology manager at Morgan Advanced Materials, examines how carefully controlled heat treatment directly governs the structure, purity, and performance of these materials and emphasizes the importance of furnace design, atmosphere control, and temperature precision required to achieve reliable properties for aerospace and space exploration applications.

This informative piece was first released in Heat Treat Today’s March 2026 Annual Aerospace Heat Treating print edition.

Space travel presents one of the most extreme operating environments encountered by engineered materials. Launch, orbital operation, atmospheric reentry, and deep space exposure all impose combinations of extreme temperature, rapid thermal cycling, oxidative attack, mechanical stress, and radiation. Among the limited class of materials capable of performing in these challenging conditions, carbon and graphite-based materials have played a critical role for decades, continuing to enable and innovate modern space exploration.

From carbon ablatives, which play a crucial role in heat shields and propulsion systems, to structural graphite components and ultra-pure graphitic elements, carbon materials are uniquely suited to survive and perform in extreme temperature regimes that exceed the limits of metals and ceramics. The performance of these graphite/graphite-like materials is directly linked to the heat treatment (aka graphitization) and processing steps used during their manufacture. The relationship between processing conditions and final product performance is driven by the technical foundations for heat treating carbon and graphite materials for space travel through the conversion, graphitization, and purification steps.

Carbon Materials in Space Applications

Carbon-based materials, such as those produced by the Performance Carbon division of Morgan Advanced Materials (see Figures 1-3), have a long history of being utilized in spaceflight systems due to their exceptional thermal stability, low density and mass, and resistance to thermal shock. Common applications include:

Ablative thermal protection systems for atmospheric re-entry
Nozzle throats and rocket motor components
High temperature insulation and support
Structural graphite parts
Ultra-pure graphitic components for sensitive electronic or propulsion systems

The performance outcomes required in these applications are diverse but share common demands: survivability at extreme temperature exceeding 2000°C (3630°F) and down to -270°C (-450°F), predictable thermal behavior, low outgassing, and controlled erosion or sublimation rates. Achieving these characteristics relies heavily on heat treatment processing conducted through both low and high temperatures.

Figure 1. Carbon cloth | Image Credit: Morgan Advanced Materials

Figure 2. Felted rayon billets | Image Credit: Morgan Advanced Materials

Figure 3. Rayon felt | Image Credit: Morgan Advanced Materials

As the material is heat treated and temperatures increase, the carbon structure will undergo progressive stabilization. Precursor conversion or pre-carbonization will occur during low temperature thermal processing. Fundamental transformation will occur as the disordered carbon structure reorganizes into graphitic planes through realignment of aromatic carbon layers. The planes become progressively ordered in the presence of elevated temperatures, increasing crystallographic alignment. The transition from carbon to graphite is gradual and highly reliant on heat treatment process capability to maintain thermal uniformity throughout all process stages.

The extent of graphitization is typically tailored to meet specific application requirements, as carbon and graphite structures each offer distinct performance advantages. In certain applications, a predominately carbon structure is preferred due to its higher bulk density and superior mechanical strength as compared to graphite. These characteristics can be advantageous in space applications where structural integrity, load bearing capability, or erosion resistance under mechanical stress are primary concerns. However, carbon materials generally exhibit slightly lower carbon yield and may experience higher erosion or mass loss when exposed to extreme thermal flux or oxidative stress.

In contrast, materials processed to a graphitic structure offer enhanced thermal stability at elevated temperatures, improved resistance to thermal shock, and reduced impurity content due to high temperature volatilization of contamination. Graphite materials typically demonstrate superior performance in applications requiring sustained exposure to ultra high temperature or rapid thermal cycling. This benefit, however, is often accompanied by lower density and reduced mechanical strength relative to their carbon counterparts, which can limit their use in mechanically demanding roles.

Ultimately, the selection of a carbon- or graphite-based structure involves balancing thermal performance, mechanical requirements, erosion behavior, and environmental exposure. Heat treatment parameters must be carefully engineered to achieve the desired degree of structural ordering, ensuring that the final material delivers optimal performance for its intended spaceflight application.

Low Temperature Thermal Processing

Most engineered carbon precursors, such as cellulose, phenolic resin, acrylic (polyacrylonitrile), or pitch-based materials, require conversion into carbon through a sequence of controlled thermal processes. The first conversion process is considered low temperature in terms of graphite processing. During this process, hydrogen, oxygen, and nitrogen content decreases, volatile species are driven off, and the aromatic carbon structures begin to form. The thermal processing profile through this low temperature conversion is critical. Careful control of heat treatment parameters, tailored to the material system, supports uniform carbon conversion and minimizes defects as the foundational microstructure is established.

At these processing temperatures, a wide range of conventional heat treatment equipment can be effectively used. Because the operating temperatures remain below the limits of most metallic alloys, furnaces equipped with metal-based heating elements, structural components, and containment systems are generally suitable for this phase of processing. Additionally, these furnaces may be configured as batch or continuous systems, depending on part geometry, production volume, and process control requirements. Heat input is achieved through established industrial thermal processing approaches, with system selection guided by requirements for temperature uniformity, process control, and operational considerations. Regardless of the configuration, the selected furnace must be capable of maintaining stable inert or reducing atmospheres to prevent oxidation of the carbon precursor during thermal decomposition.

Successful low temperature processing demands a strong foundational understanding of carbonization mechanisms combined with sound materials science principles. As organic precursors are heated, complex chemical reactions occur that result in the formation of increasingly ordered carbon structures. The heating rate and soak durations must be carefully engineered to accommodate these reactions while minimizing internal stresses, distortion, or excessive porosity. Improper ramp rates or insufficient soak times can result in non-uniform shrinkage and irreversible defects that propagate through later processing stages.

Equally critical is the furnace’s ability to execute the programmed temperature profile with a high degree of precision and repeatability. Accurate control of heating ramps, dwell temperatures, and cooling rates is essential, as even modest deviations can alter the evolving microstructure of the material. Temperature overshoot, uncontrolled gradients, or localized hot spots can lead to uneven carbon yield, variations in density, and inconsistent mechanical or thermal properties in the final product. Consistency of temperature accuracy within the furnace ensures that the entire product load processes uniformly, reducing variation in the material to allow for uncompromised performance in demanding aerospace applications.

High Temperature Thermal Processing

Materials that have completed precursor conversion or pre-carbonization are subsequently subjected to high temperature thermal processing to complete carbonization or to initiate and advance graphitization, thereby establishing the final material structure and properties. Although the carbon microstructure continues to evolve significantly during this stage, the material experiences minimal additional mass loss, shrinkage, or chemical decomposition compared to earlier processing stages. Most volatile species have already been removed, resulting in a comparatively stable structure that is less susceptible to distortion, cracking, or dimensional change. As a result, parts processed in this temperature regime typically exhibit improved dimensional stability and reduced sensitivity to heating rates when compared to low-temperature carbonization operations.

High-temperature processing can be conducted at temperatures well above the lower temperature processing. At these elevated temperatures, the available furnace technologies become significantly limited. Conventional metal or ceramic based furnace systems are no longer suitable due to material degradation, contamination risk, and structural instability under these conditions. Instead, furnaces designed for high-temperature carbon and graphite processing are typically induction or resistance (i.e., vacuum furnace) heated and constructed primarily from graphite-based components.

These furnace systems are specifically engineered to withstand extreme temperatures while maintaining thermal uniformity and chemical compatibility with the carbon materials being processed. The use of graphite heating elements, insulation, and structural components minimizes contamination and allows operation in inert or controlled atmospheres required for carbon and graphite processing.

As relatively little chemical decomposition occurs during this phase, high-temperature processing cycles can often be completed more rapidly than low-temperature carbonization cycles. Additionally, moderate temperature variations within the furnace are less likely to produce significant variability in final material properties. However, precise temperature control remains critical, as the peak temperature achieved during processing largely determines the degree of carbonization or graphitization and, consequently, the final microstructure and performance characteristics of the material.

Continuous monitoring and accurate measurement of operating temperatures are therefore essential. Even small deviations in maximum temperature can lead to meaningful differences in crystallinity, density, thermal conductivity, and mechanical behavior. For space applications, where consistency and reliability are paramount, ensuring that each component reaches the intended peak temperature is a defining requirement of high-temperature thermal processing. As space missions continue to demand materials capable of performing in the most extreme environments, advanced heat treatment remains a critical enabler of reliability and innovation.

About The Author:

Kimberly Thompson
Technology Manager
Morgan Advanced Materials

Kimberly Thompson holds a bachelor’s degree in chemical engineering from Purdue University and a master’s degree in materials engineering from Auburn University. With nearly nine years with Morgan Advanced Materials, she currently serves as technology manager leading new product development and has spent six years as the technical resource for rayon-based carbon and graphite materials supporting space industry applications.

For more information: Contact Kimberly Thompson at Kimberly.Thompson@morganplc.com.

Heat Treatment of Carbon and Graphite-Based Materials for Space Travel and Exploration Read More »

IN 718 Part 2: Heat Treatment

March 31, 2026 / AEROSPACE HEAT TREAT TECHNICAL CONTENT, ANNEALING TECHNICAL CONTENT, HIPING TECHNICAL CONTENT, VACUUM FURNACES TECHNICAL CONTENT

Today’s Technical Tuesday highlights the second installment in a multi-part series by Nikolai Alexander and The Heat Treat Doctor® Daniel H. Herring, diving into the controlled heat treatment strategies required to unlock IN 718’s exceptional high-temperature strength, toughness, and corrosion resistance. From solution annealing and duplex aging to hot isostatic pressing and additive manufacturing considerations, the authors explore how precise process control and equipment selection directly shape microstructure and performance in critical applications.

This informative piece is from Heat Treat Today’s March 2026 Annual Aerospace Heat Treating print edition. For part 1 on the history, production, and general applications, read Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

Introduction

IN 718 was developed for and is extensively used in the aerospace industry. Today, the superalloy and its modifications are heavily relied upon, including IN 718Plus® (US Patent No. 6.730.264 B2), which is designed for operating service temperature to 705°C (1300°F), approximately 55°C (100°F) above that of IN 718. (IN 718Plus will be the subject of a future article by the authors). This article reviews the heat treatment of IN 718 and the need to control both equipment and process variability. Also discussed is the method of additive manufacturing (AM) to produce component parts and the heat treat challenges it poses, including the need to HIP (hot isostatically press) the material to achieve maximum property benefits.

Heat Treatment of IN 718

Figure 1. Typical vacuum furnace used for heat treating IN 718 | Image Credit: Solar Manufacturing

IN 718 is typically heat treated in a vacuum furnace given that it is a sensitive alloy and easily oxidized. Processing in an all-metal shielded furnace (Figure 1) offers advantages for keeping the parts bright after the aging process, without the need to wrap them.

Graphite-lined vacuum furnaces, often with molybdenum heating elements, can also be used provided appropriate precautions are taken. The furnace must be extremely leak tight with a rate of rise under 5 microns Hg per hour. Processing in vacuum is typically done in the 10⁻⁵ torr range. Argon as a partial pressure or cooling gas is necessary to avoid nitriding or oxidation. An alternative, albeit older technology, approach is the use of a vacuum-purged argon atmosphere box furnace with a retort.

From a metallurgical perspective, the amount, morphology, and distribution of the delta (δ) phase have a great influence on the properties of IN 718. During heat treatment, delta phase is extremely important for optimizing mechanical properties, particularly at high temperatures to control migration and precipitation in IN 718. The delta phase inhibits grain growth (by pinning the grain boundaries) and enhances creep and fatigue performance. However, excessive or poorly controlled precipitation is detrimental to other properties like ductility and fracture toughness.

Optimization of delta phase distribution includes selecting the proper solutionizing temperature, between 980–1040°C (1800–1900°F) depending primarily on nickel content, where the delta phase is stable (and thus precipitates out). Thermomechanical working can also achieve this effect by forming more globular-shaped particles rather than acicular (needle-like) ones (Guan, et al. 2023).

There are a number of heat treatments that can be performed on IN 718, including stress relief, homogenizing, solution annealing, precipitation hardening (aka aging), and HIP.

Stress Relief

Stress relief is typically performed at the mill and is a compromise between the amount of residual stress one would like to remove and the possibly harmful effects to both high temperature properties and corrosion resistance. For wrought alloys, stress relief at full annealing temperature is recommended since intermediate temperatures might cause aging. Hold times are one hour per inch of section thickness. For castings, stress relief is especially important when dealing with complex shapes, which may be prone to cracking in subsequent operations or when dimensional control is important.

Homogenization

Homogenization heat treatment is applied to IN 718 for the uniform distribution of alloying elements and dissolution of detrimental phases after its processing through casting and additive manufacturing (AM) routes. There is a definite relationship between laves phase fraction (i.e., the brittle intermetallic compound formed due to niobium segregation during solidification) and homogenization time at various temperatures 1140–1170°C (2085–2140°F). With an increase in homogenization temperature, the time required for dissolution of laves phase and reduction in laves phase fraction reduces drastically. Also, at a given temperature the reduction in laves phase fraction has been shown to occur with the increase of time (Eliasen and Somers 2010).

Full Annealing

The process of full annealing involves complete recrystallization and dilution of all or most of the secondary phases to reach maximum softness (Figure 2).

The process is typically run at 955°C (1750°F) holding one hour per inch of cross-sectional area. If welding is to be performed on the component, annealing should be performed immediately after the welding operation. It is noteworthy that niobium additions help overcome cracking problems during welding.

Solution Annealing

Solution annealing (aka solution heat treating) is designed to dissolve secondary phases to prepare the alloy for age hardening and produce maximum corrosion resistance. An added benefit is homogenization of the microstructure.

Figure 3. Standard heat treatment cycle of IN 718 | Image Credit: Polasani and Dabhade 2024

A typical heat treatment of IN 718 involves a two-step process — solution heat treating and then age hardening — to control the mechanical property response of the material (Figure 3).

For bar stock, a typical cycle might involve solution annealing at 955°C (1750°F) followed by a 2-bar quench under argon or nitrogen (which can be used if post machining will be performed). This is followed by duplex aging at 730°C (1350°F) for eight hours followed by a vacuum or rapid cool to avoid surface reactions (such as oxidation) and (depending on whether further precipitation is needed) to 650°C (1150°F) and another hold for eight hours followed by a gas fan quench.

Solution annealing at 925–1010°C (1700–1850°F) with its corresponding aging treatment is considered the optimum heat treatment for IN 718, where a combination of rupture life, notch rupture life, and rupture ductility is of greatest concern. The highest room-temperature tensile and yield strengths are also associated with this treatment. In addition, because of the fine grain developed, it produces the highest fatigue strength (Herring 2019).

By contrast, solution annealing at 1040–1065°C (1900–1950°F) with its corresponding aging treatment is the treatment preferred in tensile-limited applications because it produces the best transverse ductility in heavy sections, impact strength, and low-temperature notch tensile strength. However, this treatment tends to produce notch brittleness in stress rupture (Herring 2019).

Aging/Duplex Aging

Figure 4. Duplex aging of IN 718 land-based turbine rods | Image Credit: Solar Atmospheres, Inc.

The aging process is designed to strengthen the material, forming precipitates from the supersaturated solid solution mastic from the solution annealing step.

Duplex aging (Figure 4) involves a two-step heat treatment process and on IN 718 is performed around 730°C (1350°F) for eight hours followed by a vacuum cool or in some cases a rapid cool to avoid surface reactions (such as oxidation) and (depending on whether further precipitation is needed) down to 620°C (1150°F) and another hold for eight hours. This is followed by a gas fan quench. The first soak temperature is intended to initiate precipitation of phases influencing strength and hardness properties. The second soak temperature further refines the microstructure and optimizes the material’s properties based on the phases developed in the initial aging and cooling stages.

Hot Isostatic Pressing

Figure 5. Typical HIP furnace capable of high temperature/pressure | Image Credit: AVS Inc.

Hot isostatic pressing (HIP) combines high pressure and high temperature to influence the density and microstructure of IN 718 (Figure 5). It is critically important to improve the mechanical strength of shape cast and additive manufactured components to homogenize the as-built microstructure and minimize variation in mechanical properties. It helps to eliminate residual stresses, close pores, close cracks and ensures the material is properly fused (Shipley 2023).

For example, it has been reported (Lee, et al. 2006) that four hours at 2155°F (1180°C) under a pressure of 25.5 ksi (175 MPa) is optimal to improve the microstructure (grain size and segregation) along with tensile properties of IN 718 castings.

Future Outlook

Additive manufacturing (AM) of IN 718 (and superalloys in general) is becoming an increasingly important method for component part manufacturing. It allows complex 3D shapes to be formed without the difficulties inherent in casting, forming, and machining of these alloys.

Electron beam-powder bed fusion (E-PBF) and laser-beam powder bed fusion (L-PBF) have shown great promise for processing IN 718 and other nickel-based superalloys. An absolutely necessary, if not critical, step in the process is post-HIP to heal cracks and homogenize the microstructure.

Heat treating will continue to play an important role in enhancing the properties of IN 718. It will be necessary to update the standard heat treatment requirements (e.g., AMS5662 and AMS5663) to incorporate powder metallurgy (PM) and AM technologies to optimize properties for components made by these methods.

More investigation is needed to optimize solutionizing and aging temperatures for modified IN 718 chemistries. For example, the effect of the cooling rate after aging treatments on the precipitate size and morphology and subsequent mechanical properties of the alloy must be explored in more detail (Eliasen and Somers 2010). And from a heat treatment perspective there is interest in case hardening (nitriding, low-temperature carburizing) of IN 718 (Sharghi-Moshtaghin, et al. 2010, Eliasen and Somers 2010).

Finally, AM processes rely on layer-upon-layer melting. As such, modeling, sensor technology, process temperature monitoring and control of surface displacement improve the build. Emerging trends suggest that the integration of machine learning and artificial intelligence for real-time quality control and process optimization will be a key part of the manufacturing strategy moving forward (Babu, et al. 2018).

References

Akca, Enes, and Gursel, Ali. 2015. “A Review on Superalloys and IN718 Nickel-Based INCONEL Superalloy.” Periodicals of Engineering and Natural Sciences 3 (1): 15–27.

ASM International. 2016. ASM Handbook, Volume 4E: Heat Treating of Nonferrous Alloys. ASM International.

Babu, S. S., N. Raghavan, J. Raplee, S. J. Foster, C. Frederick, M. Haines, R. Dinwiddie, M. K. Kirka, A. Plotkowski, Y. Lee, and R. R. Dehoff. 2018. “Additive Manufacturing of Nickel Superalloys: Opportunities for Innovation and Challenges Related to Qualification.” The Minerals, Metals & Materials Society and ASM International: 3764–3780.

Bradley, Elihu F., ed. 1988. Superalloys: A Technical Guide. ASM International.

del Bosque, Antonio, Fernández-Arias, Pablo, and Vergara, Diego. 2025. “Advances in the Additive Manufacturing of Superalloys.” Journal of Manufacturing and Materials Processing 9 (215): 1–31.

Chandler, Harry, ed. 1996. Heat Treater’s Guide: Practices and Procedures for Nonferrous Alloys. ASM International.

Croft Systems. n.d. “The Difference between a Wellhead & Christmas Tree.” https://www.croftsystems.net/oil-gas-blog/the-difference-between-a-wellhead-christmas-tree/.

Decker, R. F. 2006. “The Evolution of Wrought Age-Hardenable Superalloy.” Journal of The Minerals, Metals & Materials Society, September: 32–36.

Eliasen, K. M., T. L. Christiansen, and M. A. J. Somers. 2010. “Low-Temperature Gaseous Nitriding of Ni-Based Superalloys.” Surface Engineering 26 (4): 248–255.

Guan, Hao, Wenxiang Jiang, Junxia Lu, Yuefie Zhang, and Ze Zhang. 2023. “Precipitation of δ Phase in Inconel 718 Superalloy: The Role of Grain Boundary and Plastic Deformation.” Materials Today Communications 36 (August).

Herring, Daniel H. 2011. “Stress Corrosion Cracking.” Industrial Heating, October: 22–24.

Herring, Daniel H. 2012. Vacuum Heat Treating: Principles, Practices, Applications. BNP Media II, LLC.

Herring, Daniel H. 2019. “The Heat Treatment of Inconel 718.” Industrial Heating, June: 12–14.

Lee, Gang Ho, Ang Ho, Minha Park, Byoungkoo Kim, Jong Bae Jeon, Sanghoon Noh, and Byung Jun Kim. 2023. “Evaluation of Precipitation Phase and Mechanical Properties According to Aging Heat Treatment Temperature of Inconel 718.” Journal of Materials Research and Technology 27 (Nov–Dec): 4157–4168. https://doi.org/10.1016/j.jmrt.2023.10.196.

Lee, Shin-Chin, Shih-Hsien Chang, Tzu-Piao Tang, Hsin-Hung Ho, and Jhewn-Kuang Chen. 2006. “Improvements in the Microstructure and Tensile Properties of Inconel 718 Superalloy by HIP Treatment.” Materials Transactions 47 (11): 2877–2881.

Loria, Edward A. 1988. “The Status and Prospects of Alloy 718.” Journal of Materials, July: 36–41.

Polasani, Ajay, and Vikram V. Dabhade. 2024. “Heat Treatments of Inconel 718 Nickel-Based Superalloy: A Review.” Metals and Materials International: 1204–1231.

Sharghi-Moshtaghin, Reza, Harold Kahn, Yindong Ge, Xiaoting Gu, Farrel J. Martin, Paul M. Natishan, Arrell J. Martin, Roy J. Rayne, Gary M. Michal, Frank Ernst, and Arthur H. Heuer. 2010. “Low-Temperature Carburization of the Ni-Base Superalloy IN718: Improvements in Surface Hardness and Crevice Corrosion Resistance.” Metallurgical and Materials Transactions A 41A (August): 2022–2032.

Shipley, Jim. 2023. “Hot Isostatic Pressing and AM: How to Improve Product Quality and Productivity for Critical Applications.” Metal AM 9 (3).

U.S. Patent No. 3,046,108.

Acknowledgments: This paper would not have been possible without discussions, guidance and contributions from a number of individuals in both the heat treat industry and academia.

Special Note: Inconel® is a registered trademark of Special Metals Corporation group of companies.

About the Authors:

Dan Herring
“The Heat Treat Doctor®”
The HERRING GROUP

Dan Herring, who is most well known as The Heat Treat Doctor®, has been in the industry for over 50 years. He spent the first 25 years in heat treating prior to launching his consulting business, The HERRING GROUP, in 1995. His vast experience in the field includes materials science, engineering, metallurgy, equipment design, process and application specialist, and new product research. He is the author of six books and over 700 technical articles.

Nikolai Alexander Hurley
Intern
The Heat Treat Doctor®

Nikolai Alexander Hurley is a young academic, interning with The Heat Treat Doctor®.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

IN 718 Part 2: Heat Treatment Read More »

IN 718 Part 1: History, Applications, and Production

February 24, 2026 / ENERGY HEAT TREAT TECHNICAL CONTENT, FOUNDRY CASTING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, VACUUM FURNACES TECHNICAL CONTENT

Today’s Technical Tuesday highlights this first installment in a multi-part series by Nikolai Alexander and The Heat Treat Doctor® Daniel H. Herring, which introduces Inconel® Alloy 718, one of the most widely used nickel-based superalloys, tracing its history, applications, and production fundamentals. Understanding why this alloy performs so well in extreme environments is critical as manufacturers consider material choices available for demanding components, especially alloys more typically sourced outside of one’s own industry. As demanding performance capabilities are being required of new engineered solutions, selecting the right alloy becomes a strategic decision to meet the need for higher temperatures, pressures, and corrosive environments.

This informative piece is from Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

History

Inconel® Alloy 718 (IN 718) is a nickel-iron base superalloy known for its exceptional strength, resistance to high temperatures and ability to withstand harsh environments, where oxidation, creep, and corrosion resistance are paramount. The alloy was created by Dr. Herbert L. Eiselstein, who began his research in 1958, culminating in a patent assigned to The International Nickel Company in 1962 (U.S. Patent No. 3,046,108). In the many years since its creation, IN 718 remains the most widely used of all superalloys due to its availability in both wrought and cast products with high strength and stress-rupture life up to 650°C (1200°F), good hot working characteristics, castability, weldability, and cost effectiveness — all in an alloy with nominally 18% iron! The alloy’s superior performance is due in large part to its unique strengthening mechanisms.

There are different classifications of a superalloy, all based around the predominant metal present in the alloy. These categories include (Akca and Gursel 2015):

Nickel-based
Iron-based
Cobalt-based

The microstructural design makes IN 718 one of the best alloys for service applications below 650°C (1200°F) (Loria 1988, Herring 2011). It is widely used in extreme environments where components are subjected to high temperature, pressure, and/or mechanical loads. When heated, IN 718 forms a thick, stable, passivating oxide layer that protects the surface from further attack.

The alloy retains strength over a wide temperature range, making it attractive for high-temperature applications where materials like aluminum and steel would fail due to creep caused by thermally induced crystal vacancies. Inconel’s high-temperature strength is developed through heat treatment by solutionizing and precipitation hardening.

IN 718 is an alloy used around the world, but you might know it better by one of a variety of trade names (see sidebar).

The alloy has been modified numerous times to extend its operating temperature and service life. The alloy is readily available in all of these modified variations, each having slight differences in chemistry, cast and wrought processing methods, and heat treatments.

Applications

There is a wide variety of IN 718 applications across many industries, including aerospace, nuclear, oil and gas, automotive, motorsport, chemical processing, non-nuclear power generation, medical, tooling and molds, and fire protection systems.

In the automotive and motorsport industry, IN 718 is used for turbocharger rotors, exhaust manifolds, and valve springs in high-performance engines, such as those found in Formula 1 or the 24 Hours of Le Mans race cars. Naval warships are also purported to use IN 718 for components in their nuclear reactors (Table A).

Table A. Possible Uses of IN 718 in Naval Warship Nuclear Reactors

Table B. Oil & Gas Industry Use Examples for IN 718

Figure 1. A “Christmas tree”: the complex assembly of valves, gauges, and controls installed at the surface of a completed oil or gas well which has the primary function of regulating and controlling the flow of oil from the well. | Image Credit: Croft Systems

Perhaps surprisingly, IN 718 is also widely used in the oil and gas industry, which in addition to its many other benefits has remarkable resistance to sulfide and chlorine stress corrosion cracking at both high and low temperatures (Table B). Stress corrosion cracking is a failure mechanism that is caused by a combination of environment, a susceptible material, and the presence of tensile stress. Oil and gas applications like downhole tools, wellhead components, and subsea equipment benefit from IN 718’s other valuable properties as well, some of which include:

High strength and toughness at temperatures up to 700°C (1290°F)
Excellent resistance to pitting, crevice corrosion, and stress corrosion cracking
Sustained strength in hydrogen sulfide (H₂S) and CO₂-rich environments
Good weldability and fabrication

Continuous innovations in processing and material chemistry have enhanced superalloy properties resulting in the extension of its use into other industries, such as the energy and more conventional transportation sectors (Loria 1988).

Production Methods

IN 718 is available in cast and wrought alloy form and follows a stringent production process (Figure 2). Basic melt practices are used, such as vacuum induction melting (VIM), vacuum arc remelting (VAR), and electro-slag remelting (ESR).

Figure 2. Flow diagram of processes widely used to produce superalloys (Data reference: Akca and Gursel 2015)

VIM

The VIM process produces liquid metal under vacuum in an induction-heated crucible. It is used as a primary melting step in the route to producing wrought and cast products. Before being melted, the raw material can be refined and purified, and its composition can be controlled. VIM has been widely used in the manufacture of all types of superalloys, which must be melted under vacuum or in an inert gas atmosphere because of their reactivity with atmospheric oxygen and nitrogen.

VAR

The VAR process, a secondary melting technique, converts VIM-processed electrodes into ingots whose chemical and physical homogeneity have been significantly improved. In this process, a stub is welded to one end of an electrode, which is then suspended over a water-cooled copper crucible. Next, an arc is struck between the end of the electrode and the crucible bottom. Maintaining the arc generates the heat required to melt the electrode, which drips into the crucible and can subsequently be poured into molds. Many inclusions can be removed by flotation or chemical and physical processes before the molten material solidifies.

ESR

The ESR process, another secondary melting technique, is similar to the VAR process, but with notable differences. Remelting does not occur by striking an arc under vacuum. Instead, an ingot is built up in a water-cooled mold by melting a consumable electrode that is immersed in a slag, which is superheated by means of resistance heating. Rather than operating in a vacuum, the process is conducted in air under the molten slag. During melting, metal droplets fall through the molten slag, and chemical reactions reduce sulfur and nonmetallic inclusions. Both ESR and VAR processes allow directional solidification of an ingot from bottom to top, yielding high density and homogeneity in its macrostructure, as well as an absence of segregation and shrinkage cavities.

Casting Methods

IN 718 can also be produced by several casting methods. The most common of these are investment casting and (vacuum) die casting:

Investment casting: This process involves creating a wax pattern, coating it with a ceramic shell, melting out the wax, and then pouring molten IN 718 into the ceramic mold.
Vacuum die casting: This method uses a vacuum to fill the mold, resulting in a refined grain structure, minimal porosity, and good dimensional reproducibility, making it suitable for components like airfoils.
Sand casting: This method is far less common due to its inherent limitations in precision and surface finish, but the technology has been used for large castings.

A Metallurgical Perspective: The Role of Gamma Prime and Double Prime

IN 718 is a precipitation hardening superalloy. Its principle strengthening phases are gamma prime (γ′) or Ni3Al and gamma double prime (γ″) or Ni3Nb. The relationship between these precipitates (and others) and the gamma (γ) nickel matrix is critically important. For example, the coherency strain (i.e., the elastic deformation that occurs between two phases when their lattice structures do not perfectly match) is due to the fact that γ′ is face-centered cubic and γ″ is body centered tetragonal. In the case of IN 718, these strengthening effects are influenced more by γ″ than γ′ (ASM International 2016, Lee et al. 2023).

In addition, IN 718 has a natural tendency to precipitate rapidly by homogeneous nucleation in the noncompressible γ matrix. Depending on chemistry, γ′ volume percentage can vary over a wide range (3%–65%). Practically speaking, creep strength is proportional to volume percent over this range at temperatures between 700–980°C (1290–1800°F). As a result, the ratio of titanium to niobium/aluminum is key to hardening. High ratios imparted by niobium assure high strength at intermediate service temperatures around 600°C (1110°F). For higher service temperatures, higher aluminum content and molybdenum additions minimize the γ and γ′ mismatch, thus contributing to more stable alloys (Decker 2006, Guan et al. 2023).

Finally, the size and shape of these precipitates is important; larger precipitates enhance the strengthening effect. Key to the formation of these two precipitates is the aging treatment temperature, time, and alloy composition. According to existing research, higher aging treatment temperatures and longer times can lead to an increased amount of γ″ while extended aging coarsens the γ′ and γ″ particles, potentially leading to a reduction in strength and creep resistance. Furthermore, the composition ratios of Al, Ti, and Nb in the alloy influence the shapes of γ′ and γ″ precipitates, forming so-called co-precipitates that also affect the properties (Table C).

*SS = solid solution; + = enhancement; — = negative effect
Table C. Effect of Various Alloying Elements (Data Reference: Decker 2006)

The highest strength and hardness, coupled with reduced impact toughness, have been observed after heat treatment at 718°C (1325°F), due to an increase in the size and quantity of γ′ and γ″ precipitates.

In addition, as a result of surface analysis of Charpy bars, intergranular fracture occurs due to abundant small-sized precipitates formed within the boundary. In the case of the Charpy impact test, the absorbed energy decreases as the aging temperature increases. The formation of carbide, γ′ and γ″ precipitates can reduce the impact toughness of materials because precipitates may cause more obstacles to dislocation movement and promote crack initiation and propagation (Lee et al. 2023).

This article’s discussion continues in Heat Treat Today’s Annual Aerospace Heat Treat (March 2026) print edition to address heat treatment methods for this superalloy.

References

Akca, Enes, and Gursel, Ali. 2015. “A Review on Superalloys and IN718 Nickel-Based INCONEL Superalloy.” Periodicals of Engineering and Natural Sciences 3 (1): 15–27.

ASM International. 2016. ASM Handbook, Volume 4E: Heat Treating of Nonferrous Alloys. ASM International.

Babu, S. S., N. Raghavan, J. Raplee, S. J. Foster, C. Frederick, M. Haines, R. Dinwiddie, M. K. Kirka, A. Plotkowski, Y. Lee, and R. R. Dehoff. 2018. “Additive Manufacturing of Nickel Superalloys: Opportunities for Innovation and Challenges Related to Qualification.” The Minerals, Metals & Materials Society and ASM International: 3764–3780.

Bradley, Elihu F., ed. 1988. Superalloys: A Technical Guide. ASM International.

Chandler, Harry, ed. 1996. Heat Treater’s Guide: Practices and Procedures for Nonferrous Alloys. ASM International.

Croft Systems. n.d. “The Difference between a Wellhead & Christmas Tree.” https://www.croftsystems.net/oil-gas-blog/the-difference-between-a-wellhead-christmas-tree/

Decker, R. F. 2006. “The Evolution of Wrought Age-Hardenable Superalloy.” Journal of The Minerals, Metals & Materials Society, September: 32–36.

del Bosque, Antonio, Fernández-Arias, Pablo, and Vergara, Diego. 2025. “Advances in the Additive Manufacturing of Superalloys.” Journal of Manufacturing and Materials Processing 9 (215): 1–31.

Eliasen, K. M., T. L. Christiansen, and M. A. J. Somers. 2010. “Low-Temperature Gaseous Nitriding of Ni-Based Superalloys.” Surface Engineering 26 (4): 248–255.

Herring, Daniel H. 2011. “Stress Corrosion Cracking.” Industrial Heating, October: 22–24.

Herring, Daniel H. 2012. Vacuum Heat Treating: Principles, Practices, Applications. BNP Media II, LLC.

Herring, Daniel H. 2019. “The Heat Treatment of Inconel 718.” Industrial Heating, June: 12–14.

Lee, Gang Ho, Ang Ho, Minha Park, Byoungkoo Kim, Jong Bae Jeon, Sanghoon Noh, and Byung Jun Kim. 2023. “Evaluation of Precipitation Phase and Mechanical Properties According to Aging Heat Treatment Temperature of Inconel 718.” Journal of Materials Research and Technology 27 (Nov–Dec): 4157–4168. https://doi.org/10.1016/j.jmrt.2023.10.196

Loria, Edward A. 1988. “The Status and Prospects of Alloy 718.” Journal of Materials, July: 36–41.

Polasani, Ajay, and Vikram V. Dabhade. 2024. “Heat Treatments of Inconel 718 Nickel-Based Superalloy: A Review.” Metals and Materials International: 1204–1231.

Sharghi-Moshtaghin, Reza, Harold Kahn, Yindong Ge, Xiaoting Gu, Farrel J. Martin, Paul M. Natishan, Arrell J. Martin, Roy J. Rayne, Gary M. Michal, Frank Ernst, and Arthur H. Heuer. 2010. “Low-Temperature Carburization of the Ni-Base Superalloy IN718: Improvements in Surface Hardness and Crevice Corrosion Resistance.” Metallurgical and Materials Transactions A 41A (August): 2022–2032. https://doi.org/10.1007/s11661-010-0299-y

Shipley, Jim. 2023. “Hot Isostatic Pressing and AM: How to Improve Product Quality and Productivity for Critical Applications.” Metal AM 9 (3).

U.S. Patent No. 3,046,108.

Acknowledgments: This paper would not have been possible without discussions, guidance and contributions from a number of individuals in both the heat treat industry and academia.

Special Note: Inconel® is a registered trademark of Special Metals Corporation group of companies.

About the Authors:

Nikolai Alexander Hurley
Intern
The Heat Treat Doctor®

Nikolai Alexander Hurley is a young academic, interning with The Heat Treat Doctor®.

For more information: Contact Dan at dherring@heat-treat-doctor.com.

IN 718 Part 1: History, Applications, and Production Read More »

VACUUM FURNACES TECHNICAL CONTENT

Introduction (00:05)

Biggest Challenges of Process Qualification and Recipe Development (4:08)

Balancing Technical Requirements with Production (11:30)

Quench Media (19:20)

Switching from Atmospheric to Vacuum Carburizing (28:11)

Closing Thoughts (37:36)

About the Guest

Beyond Furnace Efficiency

The Impact of Underutilization

Utilization as a Sustainability Lever

Rethinking Traditional Boundaries

The Role of Data in Decision Making

Balancing Control, Efficiency, and Sustainability

A Shift in Perspective

Successful Examples of High Utilization, Advanced Heat Treatment

From Energy Per Cycle to Energy Per Component

About The Author:

From Atmospheric Carburizing to New Demands

Why Vacuum Carburizing and Why Cleaning Matters

Overcoming Initial Concerns

A Technical and Operational Leap Forward

Cleaner Start, Cleaner Finish

About The Author:

Robotic Advantages

Robotic Components Explained

Capital Investment

Future Impact on the Industry

References

About The Author:

Introduction

Cooling in Vacuum Furnaces

Regulating Pressure in Vacuum Hardening Furnaces

Preliminary Pressure Comparison

True Toughness and Speed

Tuning Pressure and Time

How Microstructure Drives Toughness

Conclusion

About The Author:

Introduction

Laboratory-Scale Research and Development

Characterization and Prototyping Stage

Production Stage

Enabling the Next Generation of Aerospace Materials

References

About The Author:

Vacuum Heat Treatment Processing Methods

Induction Hardening Methods

In Summary

References

About the Author

Carbon Materials in Space Applications

Low Temperature Thermal Processing

High Temperature Thermal Processing

About The Author:

Introduction

Heat Treatment of IN 718

Stress Relief

Homogenization

Full Annealing

Solution Annealing

Aging/Duplex Aging

Hot Isostatic Pressing

Future Outlook

References

About the Authors:

History

Applications

Production Methods

VIM

VAR

ESR

Casting Methods

A Metallurgical Perspective: The Role of Gamma Prime and Double Prime

References

About the Authors:

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