HARDENING 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 condition, effective case depth, core hardness.

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: Fixture 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 »

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

March 19, 2026 / ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, CARBURIZING TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH, NITRIDING TECHNICAL CONTENT, NITROCARBURIZING TECHNICAL CONTENT, OP-ED

Ask The Heat Treat Doctor® has returned to bring sage advice to Heat Treat Today readers, answer questions about heat treating, brazing, sintering, and other types of thermal treatments, as well as metallurgy, equipment, and process-related issues. In this installment, Dan Herring examines the essential role of heat treatment in gear performance: exploring the key material and design considerations for power transmission gears, the difference between through hardening and case hardening, and the atmosphere heat treatment processes — from carburizing and carbonitriding to nitriding and nitrocarburizing — that determine how well a gear handles load, wear, and fatigue in heavy-duty applications.

This informative piece was first released in Heat Treat Today’s February 2026 Annual Air & Atmosphere Heat Treating print edition.

Have questions or feedback? We’d love to hear from you — reach out to our editorial team at editor@heattreattoday.com.

Gears play an essential role in the performance of many products that we rely on in our everyday lives. When we think about gears, we generally separate them into two categories: motion-carrying and power transmission. Motion-carrying gears are generally nonferrous alloys or plastics, while load bearing power transmission gears (Figure 1) are usually manufactured from ferrous alloys and are intended for heavy-duty service applications.

Figure 1. Typical off-highway truck power transmission gears | Image Credit: The Heat Treat Doctor®

Gear Materials & Engineering

Power transmission gears involve a wide variety of steels and cast irons. In all gears, the choice of material must be made only after careful consideration of the performance demanded by the application end-use and total manufactured cost, taking into consideration such issues as pre- and post-machining economics.

Key design considerations require an analysis of the type of applied load, whether gradual or instantaneous, and the desired mechanical properties, such as bending fatigue strength or wear resistance — all of which will define core strength and heat treating requirements.

Figure 2. Stress profile in a heavy-duty transmission gear | Image Credit: The Heat Treat Doctor®

It is important for the designer to understand that each area in the gear tooth profile sees different service demands (Figure 2). Consideration must be given to the forces that will act on the gear teeth with tooth bending and contact stress, resistance to scoring and wear, and fatigue issues being paramount. For example, in the root area, good surface hardness and high residual compressive stress are desired to improve endurance or bending fatigue life. At the pitch diameter, a combination of high hardness and adequate subsurface strength are necessary to handle contract stress and wear and to prevent spalling.

Some of the factors that influence fatigue strength are:

Hardness distribution, a function of:
- Case hardness
- Case depth
- Core hardness
Microstructure, a function of:
- Retained austenite percentage
- Grain size
- Carbide size, type, and distribution
- Non-martensitic phases
Defect control, a function of:
- Residual compressive stress
- Surface finish and geometry
- Intergranular toughness

In the total manufacturing scheme, a synergistic relationship must exist between the material selection process, engineering design, and manufacturing (including heat treatment). A balance of the priorities in each discipline must be reached to achieve the optimization necessary for the ultimate performance of the gear design. This is often not an easy task.

Various atmosphere heat treatment methods are used for most types of gears including pre-hardening steps (e.g., annealing, normalizing, stress relief) and hardening processes (e.g., neutral hardening and case hardening).

Hardening

Neutral (aka through hardening) refers to heat treatment methods that do not produce a case. Examples of commonly through-hardened gear steels are AISI/SAE grades 1045, 4130, 4140, 4145, 4340, and 8640. It is important to note that hardness uniformity should not be assumed throughout the gear tooth. Since the outside of a gear is cooled faster than the inside, there will be a hardness gradient developed. The final hardness is dependent on the amount of carbon in the steel. The depth of hardness depends on the hardenability of the steel.

Through hardening can be performed either before or after the gear teeth are cut. When gear teeth will be cut after the part has been hardened, machinability becomes an important factor based on final hardness. The hardness is achieved by heating the material into the austenitic range, typically 815°C–875°C (1500°F–1600°F), followed by quenching and tempering.

Case Hardening

By contrast, case hardening is used to produce a hard, wear resistant case (surface layer) on top of a ductile, shock resistant interior (core). The idea behind case hardening is to keep the core of the gear tooth at a level under 40 HRC to avoid tooth breakage while hardening the outer surface to increase pitting resistance.

Carburizing

Atmosphere carburizing is the most common of the case hardening methods in use today and can handle a diverse range of part sizes and load configurations (Figure 3). In general, a properly carburized gear will be able to handle somewhere between 30–50% more load than a through-hardened gear. Examples of commonly carburized gear steels include AISI/SAE grades 1018, 4320, 5120, 8620, and 9310, as well as international grades, such as 20MnCr5, 17CrNiMo6, 18CrNiMo7-6, and 20MoCr4.

Atmosphere carburizing is typically performed in the temperature range of 870°C–955°C (1600°F–1750°F) although temperatures up to 1010°C (1800°F) are used for deep case work. Carburizing case depths can vary over a broad range, typically 0.13–8.25 mm (0.005–0.325 inches).

Carbonitriding

Carbonitriding is a modification of the carburizing process, not a form of nitriding. This modification consists of introducing ammonia into the carburizing atmosphere to add nitrogen to the carburized case as it is being produced. Examples of gear steels that are commonly carbonitrided include AISI/SAE 1018, 1117, and 12L14.

Carbonitriding is done at a lower temperature than carburizing, typically between 790°C–900°C (1450°F–1650°F), and for a shorter time. Combine this with the fact that nitrogen inhibits the diffusion of carbon, and what generally results is a shallower case than is typical for carburized parts. A carbonitrided case is usually between 0.075–0.75 mm (0.003–0.030 inches) deep.

Nitriding

Nitriding is another surface treatment process that has as its objective increasing surface hardness. One of the appeals of this process is that rapid quenching is not required, hence dimensional changes are kept to a minimum. It is not suitable for all gear applications; one of its limitations is that the extremely high surface hardness case produced has a more brittle nature than say that produced by the carburizing process. Despite this fact, in a number of applications, nitriding has proved to be a viable alternative. Examples of commonly nitrided gear steels include AISI/SAE 4140, 4150, 4340, and Nitralloy® 135M.

Nitriding is typically done in the range of 495°C–565°C (925°F–1050°F). Case depth and case hardness properties vary not only with the duration and type of nitriding being performed but also with steel composition, prior structure, and core hardness. Typically, case depths are between 0.20–0.65 mm (0.008–0.025 inches) and take from 10 to 80 hours to produce.

Nitrocarburizing (Ferritic or Austenitic)

Nitrocarburizing is a modification of nitriding, not a form of carburizing. In the process, nitrogen and carbon are simultaneously introduced into the steel while it is in a ferritic or at times an austenitic condition. A very thin “white” or “compound” layer is formed during the process, as well as an underlying “diffusion” zone. Like nitriding, rapid quenching is not required. Examples of gear steels that are commonly nitrocarburized include AISI/SAE grades 4140, 5160, 8620, and certain tool steels, such as H11 and H13.

Nitrocarburizing is normally performed at 550°C–600°C (1025°F–1110°F) and can be used to produce a 58 HRC minimum hardness, with this value increasing dependent on the base material. White layer depths range from 0.0013–0.056 mm (0.00005–0.0022 inches) with diffusion zones from 0.03–0.80 mm (0.0013–0.032 inches) being typical.

In Summary

There are many ways to heat treat gears. While atmosphere heat treatment (discussed above) is perhaps the most widely used technology today, other types of heat treatments, namely vacuum and induction hardening, are becoming more and more common methods. These will be discussed in Part Two.

About the Author

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.

Ask The Heat Treat Doctor®: Why and How Do We Heat Treat Gears? Part One Read More »

CO₂-Neutral Heat Generation Technology Progress

February 17, 2026 / ANNEALING TECHNICAL CONTENT, ATMOSPHERE AIR FURNACES TECHNICAL CONTENT, FOUNDRY CASTING TECHNICAL CONTENT, HARDENING TECHNICAL CONTENT, MANUFACTURING HEAT TREAT TECH

A new study from the Umweltbundesamt (the Federal Environment Agency in Germany) outlines a clear, technically grounded pathway for achieving CO₂-neutral process heat across energy-intensive industries. This Technical Tuesday installment highlights the study’s key findings, offering North American heat treaters a concise look at the technical feasibility, economic pressures, and strategic choice involved in moving beyond fossil-fuel-based thermal processing.

This informative piece was first released in Heat Treat Today’s January 2026 Annual Technologies To Watch print edition.