Metallography and nitriding, while two separate things, actually have a relationship. Do you know what it is? Would you be surprised to know that one enhances the other? Read this best of the web article to clearly see how metallography results help the nitriding of ferrous alloys be the best that it can be.
Find out how metallography is an essential tool in the nitriding process. Take a look at some basic definitions and peruse some microscope images that help explain what nitriding does to different materials. Dig even deeper with discussion of problems such as not enough stress relief and the presence of sulfides. Metallography helps explain, in this article, what nitriding can do for many different applications.
An excerpt:
Producing the best nitrided layers for the given application requires a good cooperation between designers of the product and the manufacturing companies making it . . . . Metallography of the parts, or samples which run together with them, is extremely important for verifying results of this thermochemical treatment and assessing the properties of the layer formed during, the data are also used for maintaining a good predictability of the process.
Today’s Heat Treat Radio episode illuminates how Gary Sharp, founder and CEO of Advanced Heat Treat Corp, began the company. Heat Treat Radio host and Heat Treat Todaypublisher, Doug Glenn, will hear from Gary about the technical highlights and capabilities of ion nitriding, including: common applications, real-world benefits, and true limitations.
Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.
The following transcript has been edited for your reading enjoyment.
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Advanced Heat Treat Corp.’s Beginnings with Ion Nitriding (01:20)
Doug Glenn: We’re going to be talking about nitriding, specifically, ion nitriding. Gary Sharp has a long history with the technical aspects of ion nitriding.
Tell us a little bit about yourself and Advanced Heat Treat.
Gary Sharp: I started with John Deere in the Chemistry department. I have a degree in Science Chemistry. Then I went into various other areas: production support and different management positions before I ended up ultimately leaving to start Advanced Heat Treat.
The way I became oriented and introduced to ion nitriding was a company who we had done work with wanted to sell John Deere some new technology. They came in for some meetings, and we had several meetings with top management. But in 1979/80, that was one of the first real downturns in the ag market. So, Deere wasn’t really interested in taking on a new technology at that time. While I was still interested, I talked to various management people. Ultimately, I got permission to invest and investigate, on my own, the ion nitriding process.
Doug Glenn: Even while you were still at Deere?
Doug Glenn (l) and Gary Sharp (r) Source: Heat TreatToday
Gary Sharp: While I was still at Deere, but I had to do it on my own. All of my vacations, my holidays, and things were spent doing market surveys, talking to various potential users and so forth, in the marketplace, to see if it had a “fit.” Because, obviously, when an equipment supplier comes in, everything is nice and rosy and so forth and the equipment works for everything. That’s not always the case, of course.
After discussion with the management, they gave us permission to continue our investigation. From 1979 to 1981, we did a lot of research. I took vacations and went and did market surveys with different potential customers and found out that ion nitriding still seemed to have a lot of the glitz and the shine from the company that came in to talk to us.
We went ahead and, after a period of time and evaluation, put together an investor base. We put together a building, equipment was purchased, and then we began to do ion nitriding. We started with a 25 KW lab unit and a 160 kW unit that we would transfer from development into production-sized lots.
Doug Glenn: You say “we,” so this was not Deere though — these were the people outside of Deere — yourself and some others, right?
Gary Sharp: Yes, myself and several others, at the time. Unfortunately, some things happened and partnerships are not always the easiest. That dissolved, and it was pretty much me and my wife and our employee base. That was the start anyway.
The trouble is, early on, the small lab unit worked fine, and we could do out development; but it didn’t transfer over to the larger production unit. We kept having power supply issues. This went on for months and months and months. Ultimately, I had to get legal involved. That churned around for over a year, probably. Here we’re trying to have a startup business and, at the same time, we’re fighting with everything else. The building came together, the lab we put in worked well, but we just had issues.
We weren’t aware of how many pieces of equipment had been sold in the marketplace by this company until we got into legal, and then we started having more serious discussions. They replaced the power supplies with a new source and solved the arc suppression problems and some of the things that were taking place. Basically, we would get into the range of an unstable arc discharge; and instead of heat treating we were melting the parts. Unfortunately, we couldn’t keep many customers doing that! That got resolved, we got additional equipment in the settlement, and then we took off.
Doug Glenn: When did you actually start the company?
Gary Sharp: 1981/82 timeframe.
From the market surveys, we knew there was considerable interest. Obviously, we went back to some of those folks and started doing some development work with them, particularly on applications where ion nitriding was a significant advantage over some other treatments. Where I was in Iowa, they were doing gas nitriding on cylinder liners. Again, with the market going south for a while, they weren’t interested in any new technologies; and they just continued to do the gas nitriding.
Doug Glenn: So, you ran through a little bit of legal issues. When would you say was the first time you felt you were up and running?
Gary Sharp: I think last week, maybe! It took a while.
In our investigation, we found out they had sold quite a few pieces of equipment; and they’d all been mothballed and put on the shelf. In a way, it was a bad deal, but it was good for us because we had the solutions to fix them.
I went around and purchased equipment, 10 cents on the dollar, and bought additional capacity. We would get up to around 70–80% capacity on one vessel, and then I’d go buy another. I’d get that up and established in our plant. It worked out quite well for us, being dumb and stupid, I guess.
Gary Sharp: It kind of evolved over time. We’ve got 50+ units now — ion nitriding as well as gas nitriding. The nitriding field has been our baby for many, many years. We’ve done a lot of development with other suppliers to make sure the applications they run are using the right process.
What Is Nitriding? (09:03)
Doug Glenn: Let’s talk a little bit about nitriding. Then I want to hone in a little bit more into ion nitriding. What is nitriding? How is it different from ion nitriding? What are we doing, and why do we do it?
Gary Sharp: Nitriding is a case hardening process. It is used on a variety of components to improve wear, abrasion resistance, fatigue strength, etc. It’s generally a lower temperature process (than, say, carburizing or anything like that), so you don’t have the resultant distortion and post machining requirements that you do with some of the existing treatments that are out there.
Nitriding is a case hardening process. It is used on a variety of components to improve wear, abrasion resistance, fatigue strength, etc.
As you mentioned, there is ion nitriding, there is gas nitriding, there is salt bath nitriding. All three do similar things, they just have different requirements. Obviously, there are materials and chemistry that are involved with each of those materials, but you can nitride almost anything, at least putting a compound zone or an outer layer on that’s very abrasion resistant. Where the process gets developed is when you have alloy (Chromalloy, malatium, aluminum); these are nitride formers which, combined with the nitrogen at and below the surface, give you a diffusion zone that has longevity and a very high hardness.
Doug Glenn: In the simplest terms, nitriding is in one sense hardening the surface of a metal by infusing nitrogen, basically. It’s done in a variety of ways, and that’s what I wanted to ask you a question about.
If the total universe of nitriding is 100%, what percent of that, do you think, is gas nitriding, salt nitriding and ion nitriding? Your best guess.
Gary Sharp: I’ve heard different numbers. Ion nitriding has grown significantly over the years. Up until that point, gas nitriding and salt bath nitriding were probably 70/80%, I would guess. Ion nitriding is quite visual — it has a purple glow. That’s why I’ve got this purple tie on.
Doug Glenn: I was going to ask why you have the purple tie on. I wondered why www.ahtcorp.com is purple.
The purple glow Source: Advanced Heat Treat Corp.
We won’t go into the details of gas nitriding or salt nitriding; that can be a topic for another day. Let’s talk a little bit about how ion nitriding gets the nitrogen into the surface of the metal. How does that happen? How does that differ from, if you will, gas and/or salt?
Ion Nitriding (13:40)
Gary Sharp: It’s a diffusion process. If you look at a piece of equipment, a hearth plate is a cathode in a DC circuit. The vessel wall is the anode, and the gas is your carrier.
Through the transfer of energy, you bombard the part with ions and neutral atoms. They transfer their kinetic energy, and that is what actually heats up the parts. In the early years, that was the only way you could heat the parts. Later came more developed equipment.
Now, you have auxiliary heating in the walls which adds some advantage, but it also adds a little more complexity in terms of keeping and maintaining a current density on the part adequate to diffuse into the metal itself. Sometimes you put it in a vessel, and you turn on the power supply. All the energy is coming from somewhere else, and you don’t actually diffuse or harden the part itself. It’s been solved, obviously, over the years.
Doug Glenn: Are you making a positively and negatively charged item?
Gary Sharp: The ions bombard the surface.
Doug Glenn: Right. The ions bombard it because they’re attracted magnetically?
Gary Sharp: Yep. And they transfer the kinetic energy. That’s what heated the workpiece up in the early equipment. Like I said, in later equipment, they had auxiliary heating, as well, in the chambers.
Applications of Ion Nitriding (15:17)
Doug Glenn: Typically, what are some of the more common applications? Is it mostly agriculture, like John Deere?
Gary Sharp: By no means. When I was still at Deere and left Deere, we made sure we didn’t have conflict of interest. I didn’t even solicit any Deere parts, and that went on for quite a few years. Since, we’ve done parts for them and so forth.
Anything that has high wear and abrasion. One of the advantages that we haven’t talked about is the ability to selectively harden and the ease of masking. “Ease of masking” means instead of using copper paints or stop-off materials, you can just interrupt the plasma from touching that surface. If you have some threads, you just put a nut on there. It blocks the plasma from touching the threads, and they won’t get hard. It is a physical block. And you have maybe an 8–10 thousandths/8–15 thousandths gap and you still don’t diffuse beyond the masking itself.
There are a lot of ways of masking parts with ion nitriding. Those are generally done on customers’ parts that are repeating, so you don’t have to paint them every time they come in. You let the copper paint dry and all of that. We would just use mechanical masking and just use them over and over. They basically last forever.
Doug Glenn: I’ve heard one of the other real advantages of ion nitriding is blind holes and areas like that where gas flow wouldn’t necessarily get. Even salt might have a little of bit of difficulty getting in there.
Gary Sharp: It is an advantage. There is an L/B ratio we must be aware of. You conform that plasma to go down in the hole, if it intersects itself with the other side (it’s called hollow cathoding). That is extremely hot and can melt the parts.
That’s what we learned early on, before we got some of the equipment issues resolved, is that we would get in that unstable arc discharge range. We’d basically melt the work piece. And the customers weren’t happy with that!
Doug Glenn: It’s a bad day when you open the furnace to a pool of metal. That is not a good day!
We’ve got certain benefits there. Any industry, you’re saying, can do it, anywhere where there’s high resistance. So, automotive parts, yes?
Gary Sharp: Automotive, aerospace. We did the submarine gear for the Seawolf-class submarine, 35,000 pounds, 160 inch diameter. That ran, probably, 400+ hours. Not because it was big, but because they had an extremely deep case requirement. The diffusion took longer, particularly at the lower temperatures that you run, versus other kinds of treatments.
Sea-wolf class submarine Source: Wikipedia.com/Defense.com News photo
Doug Glenn: With ion nitriding, you are typically below the temperature where distortion could occur, I believe. So, you shouldn’t have to do post hardening processes.
Gary Sharp: Yes. That is one of the big advantages, for sure. We found that one of the reasons a lot of our customers transition out of one process into ours was because we eliminated some subsequent operations which they typically had to have and reduced their cost. Even though it wasn’t a direct cost in nitriding, it definitely affected that.
Challenges with Ion Nitriding (
Doug Glenn: What are some possible challenges with ion nitriding?
Gary Sharp: Loading a chamber and the part spacing you need, depending on what level of backing they’re going to run at, will determine how wide the plasma is. That, in turn, affects then how close you can put parts together or close to each other so that you still get treatment on both products or both pieces. And it does allow you to do mixed loads of different types of things, depending on the level of vacuum and how wide the plasma that you’re conducting surrounds that part.
This is a concern, and that’s why we review all those. Generally, we even run some test samples for the customer. We let them compare our metallurgy with theirs before they commit even further production loads.
We had a steering torsion bar we probably ran 10,000 pieces in a load. We masked the bottom portion of that because it got cross-drilled in the assembly and, of course, they didn’t want to have to drill through a hardened piece of material. Consequently, it worked out really well for us; and we did that for 15/20 years.
Special Consideration: Parts Cleaning (22:17)
Doug Glenn: I have heard that when you’re ion nitriding, part cleanliness is a critical part. Can you address that?
Parts cleaning Source: Advanced Heat Treat Corp.
Gary Sharp: Yes. Of course, we clean everything before it goes in the chamber. Typically, it has been either with an alkaline wash or vapor degrease to get rid of any contaminants off the surface. The early part of the cycle, when you turn the DC power supply on, you begin to sputter. So, any oxides and things like that on the surface get sputtered away before you actually ramp up and start the diffusion phase of the cycle.
Cleaning is important. If you have plating, that often blocks out. If you have dirty parts, that will prevent nitriding. Or, an even worse case, it will sputter off and onto other parts and then you contaminate those as well. Cleaning is an important part of the equation.
Doug Glenn: Are there any other common misperceptions about ion nitriding that you would like to address?
Gary Sharp: I don’t know any more. Back then, when we first started, that’s how we learned some of the things we did, of course. The spacing is important, the gaps are important so that you don’t hollow cathode. And, as you touched on a little bit ago, the cleanliness; if it’s really dirty and contaminated, you’re going to have a void in that area and it won’t nitride. Even a fingerprint could cause an issue.
Cleaning parts it’s getting more difficult, right now, with the push to restrict the use of vapor degreasing and things like that. We have to come up with other cleaning methods that are suitable and still meet the end-product requirements.
Wear and abrasion are big benefits. Treating parts at a low enough temperature that you don’t have distortion. You don’t have to set up and post heat treat machine. Those are all key benefits from the process itself.
It’s repeatable. Over and over, we do thousands and thousands of parts and loads at our different locations. It’s been quite successful for us.
Ion Nitriding and FNC (26:07)
Doug Glenn: Can you put ion nitriding (or nitriding, generally) in perspective with things like ferritic nitrocarburizing, maybe carbonitriding? Where does it fall on the scale? What are the differences between those processes?
Gary Sharp: Ion nitriding is most effective when you have Chromalloy, malatium, aluminum, and those types of elements in your product. Of course, with carburizing, that’s not a requirement. With carbonitriding, typically, it isn’t a requirement. Both of those processes are done at considerably higher temperatures which then gets you back into the questions: Is the part going to distort, do we have to post heat machine?
Doug Glenn: I have one last question for you about people who, potentially, could use your services, but I want to dive a little bit deeper into your company before we wrap up. You’ve got three locations, now, correct?
Gary Sharp: We have four.
Doug Glenn: Four locations? Where are they?
Gary Sharp: Three of them do ion nitriding and the nitriding process. Here, in Waterloo, we have the corporate headquarters where we have the largest ion nitriders. As a matter of fact, we are installing one right now that will do parts upwards of about 30 feet. We have Waterloo, Iowa, and we have the two facilities here in essence where we started. It was risky enough, leaving John Deere, without going somewhere else.
Here, in Waterloo, we have the corporate headquarters where we have the largest ion nitriders. As a matter of fact, we are installing one right now that will do parts upwards of about 30 feet.
Then we added Michigan. Dr. Ed Rolinski was our key “go-to” guy up in Michigan. He lived with me for a year and half. Meanwhile, we were building the facility in Michigan; so he could go back to it.
Then we started a plant in Cullman, Alabama. We’ve got the central Midwest pretty well covered with all types of applications. We’re starting to add some other types of treatments: the black oxide treatment® to kind of subsidize the ion nitriding, if you will.
Doug Glenn: Let me wrap up with this question: Let’s say there is a company out there, a manufacturer, who currently is doing some sort of a case hardening process. They’re thinking, “I wonder if I should look into nitriding/ion nitriding.” What would be your guidance for them? What questions should they be asking themselves?
Gary Sharp: Companies have to go through the some of the same steps we did early on — testing, making sure the parts/the treatment they select is repeatable, and it fits their end-use.
It’s rather expensive equipment. Some equipment is in excess of $600,000–$700,000 apiece. Depending on the size, they can get even more expensive than that.
We do make some of our equipment, now. We have in the past, particularly, when there were things that weren’t available.
If you’re looking to outsource ion nitriding, you’d start first with the material chemistry and see what materials are used. It has to fit the requirements of the end application, as well. That’s probably the biggest thing.
Then, if it’s got alloy in it and you figure out your case steps and your diffusion requirements, next you would do some development testing on parts and see how it worked in the application and go from there.
Doug Glenn: And it’s probably best just to ask an expert! At least call and check it out.
Is there any part (or maybe there is more than one) that if you have this part, you shouldn’t even consider ion nitriding — it’s just not going to work?
Gary Sharp: In ion nitriding, a key thing to be cautious of — assuming the material is compatible with the nitriding reaction — is wide holes, or holes we can’t conform the plasma tight enough to reach. In those cases, you’d have hollow cathode and then you’d have a melting issue or damage to the parts.
Parts that have to be nitrided all over can also be problematic. Oftentimes, in those cases, you would nitride for half cycles and then flip them because where it’s sitting is actually getting masked, where it’s sitting on the hearth plate or on your fixture plate or something similar. So, those are the kinds of applications that you have to give more thought to.
Doug Glenn: We appreciate your time, Gary. You folks have been around a long time, and your reputation is one for doing great work. I hope people will get in touch with you.
About the expert: Gary Sharp founded Advanced Heat Treat Corp., “AHT” for short, in 1981. The company initially went to market with its UltraGlow® ion nitriding & ion nitrocarburizing services, but since then, has expanded its offerings to also include gas nitriding, gas nitrocarburizing and UltraOx® as well as more traditional heat treatments such as carburizing, induction hardening, carbonitriding, through hardening and more.
For more information: Contact with Gary or learn more about Advanced Heat Treat Corp. at www.ahtcorp.com, or call 319-232-5221.
Independence Day is right around the corner, and Heat TreatToday wanted to share some red, white, and blue processes from across the heat treating industry. We're highlighting induction hardening, gas nitriding, and hydrogen generation. Curious? Get ready for Independence Day with this red, white, and blue Technical Tuesday.
Induction hardening in action Photo Credit: Contour Hardening
Induction hardening is a bit out of place in the heat treating world. There are no huge pieces of equipment, long wait times, or mysterious happenings behind furnace walls. But using those red hot coils for hardening can be a game-changer, because induction hardening offers selective hardening, increased part strength, and uninterrupted process flow.
"The induction coil is a copper conductor that is shaped in order to harden the specified area of the part. The current that flows through the coil is what produces the magnetic field, which in turn heats the part. Coils are typically part specific, since they need to be precisely constructed to heat a particular portion of the part."
White layer from nitriding Photo Credit: SECO/VACUUM
Gas nitriding is a valuable case hardening process. In gas nitriding, a white layer made up of a nitrogen-rich compound is formed. This white layer is hard and wear-resistant, but is also very brittle.
"This compound layer depth is dependent on processing time. In the more traditional two-stage process, the case depth produces a gradient of hardness from surface to core that commonly ranges from 0.010-0.025”, with minimal white layer, typically between 0-0.0005”."
Water and electricity: that's all the materials that are needed to generate hydrogen on site. Water electrolyzers for hydrogen generation are compact, portable, and reliable, as well as being safer than storing gases. Could the future of heat treating — and perhaps the end of natural gas — be "blue"? Now, unless you live on the beach in the Bahamas, the water you're used to probably isn't blue, but you catch our drift.
"Electricity and water come into a plant in pipes and wires and are highly reliable. Additionally, there are no hydrogen storage tanks taking up a large amount of unusable space."
Find heat treating products and services when you search on Heat Treat Buyers Guide.com
There seems to be no limit to what heat treated titanium alloy products can do -- just listen to the name: "titans." However, its poor tribological performance means that a surface treatment is necessary in most applications.
This best of the web article shares a quick technical run-down of all you need to know about plasma/ion nitriding of titanium alloy products with guidelines and graphs to show you the way.
An excerpt:
At the moment, nitriding is the most common and rational diffusion treatment which can be used for all-over surface hardening of titanium products. Nitrided titanium has a gold/yellow color of the TiN nitride enhancing attractiveness of the treatment in many applications.
To create a durable and corrosion resistant barrel, guns in the 19th century were made with blackening, a process related to heat treat. This application also increased the general look, reduced light reflection, and increased wear resistance in general.
This best of the web will cover general blackening of ferrous metals and summarize key points about nitriding and nitrocarburizing with blackening.
An excerpt:
"There are three types of blackening in common use: Caustic Black Oxidizing, Room Temperature Blackening and Low-temperature Black Oxide."
Over the past year, we’ve seen numerous new technologies in the way of research, new partnerships, and conversations throughout the industry. So in honor of today being #NationalTechnologyDay, we’re sharing an original content article about just several of these new technologies that are changing the work of heat treaters across North America.
Research
Using HIP to Advance Oregon Manufacturing Innovation Center Programming– “‘Today’s globally competitive manufacturing industry demands rapid innovations in advanced manufacturing technologies to produce complex, high-performance products at low cost,’ observes Dr. Mostafa Saber, associate professor of Manufacturing & Mechanical Engineering Technology at Oregon Tech.”
College Students Implement a NEW Heat Treat Solution with Induction? – “‘We were in shock,’ Dennis admitted, ‘because we didn’t expect it to [work].’ The expectation, Dennis continued, was that something would go wrong, like the lid would not be able to clamp down, or the container would leak.”
The Age of Robotics with Penna Flame Industries – “The computerized robotic surface hardening systems have revolutionized the surface hardening industry. These advanced robots, coupled with programmable index tables, provide an automation system that helps decrease production time while maintaining the highest quality in precision surface hardening.”
Heat Treat Radio: Five experts (plus Doug Glenn) discuss hydrogen combustion in this episode. An easily digestible excerpt of the transcript circulated by Furnaces Internationalhere and is available to watch/listen/read in full for free here.
Heat Treat Radio: Get on-the-ground projections of what technologies Piotr Zawistowski believes will be bringing in the future. Watch/listen/read in full here
Heat Treat Radio: HIP. The Revolution of Manufacturing, that is, according to Cliff Orcutt. Watch/listen/read in full here
Heat Treat Radio: Will indentation plastometry find its way into North America? If you’ve been listening to James Dean, it seems like it already has. Watch/listen/read in full here
Heat Treat Radio: Fluxless inert atmosphere induction brazing. That’s a mouthful! But what is it? Watch/listen/read in full here
Welcome to another Technical Tuesday for 18 hard-hitting resources to use at your heat treat shop. These include quick tables, data sets, and videos/downloadable reports covering a range of heat treat topics from case hardening and thermocouples to HIPing and powder metallurgy.
Defining Terms: Tables and Lists
Table #3 Suggested Tests and Frequencies for a Polymer Quench Solution (in article here)
Case Hardening Process Equipment Considerations (bottom of the article here)
Two simulations of a moving billet through heating systems (in article here)
Fourier’s Law of Heat Conduction (in article here)
Webinar on Parts Washing (link to full webinar at the top of the review article here)
Materials 101 Series from Mega Mechatronics, Part 4, Heat Treatment/Hardening here
Heat Treat TV: Press-and-Sinter Powder Metallurgy here
BONUS: 39 Top Heat Treat Resources
Heat Treat Today is always on the hunt for cutting-edge heat treat technology, trends, and resources that will help our audience become better informed. To find the top resources being used in the industry, we asked your colleagues. Discover their go-to resources that help them to hone their skills in the 39 Top Heat Treat Resources on this page of the September print magazine.
What's the future of ferritic nitrocarburizing and how does it compare to other hardening processes? When it comes to metal hardening, there are many variations on central processes, including recent innovations in how to apply hardening processes.
This Technical Tuesday brings you a quick overview of how hardness technologies differ, specifically nitriding and FNC, and how certain heat treaters have developed these specific hardness technologies.
Understanding the Various Hardening Processes
If you want to know the future, the best you can do is understand the past and present. Let’s begin with looking at the most common hardening processing methods. Here are a few excerpts from “Elevate Your Knowledge: 5 Need-to-Know Case Hardening Processes” by Mike Harrison, engineering manager of Industrial Furnace Systems Division at Gasbarre Thermal Processing Systems:
Read more about these 5 processes in Mike Harrison's article. Click to read.
Carburizing: “Gas carburizing is a process where carbon is added to the material’s surface. The process is typically performed between 1550-1750°F, with carburizing times commonly between 2-8 hours [this spec is disputed, and times may run up to 24 hours]; of course, these values can vary depending on the material, process, and equipment. The most common atmosphere used for atmosphere gas carburizing is endothermic gas with additions of either natural gas or propane to increase the carbon potential of the furnace atmosphere.”
Nitriding: “Gas nitriding is a process where nitrogen is added to the material surface. The process is typically performed between 925-1050°F; cycle times can be quite long as the diffusion of the nitrogen is slow at these temperatures, with nitriding times typically ranging from 16 – 96 hours or more depending on the material and case depth required. Nitriding can be performed in either a single or two-stage process and has the potential to produce two types of case, the first being a nitrogen-rich compound layer (or “white layer”) at the surface that is extremely hard and wear-resistant but also very brittle.”
Carbonitriding: “Despite its name, carbonitriding is more closely related to carburizing than it is to nitriding. Carbonitriding is a process where both carbon and nitrogen are added to the material surface. This process is typically performed in a range of 1450-1600°F [this spec is disputed, and temperatures may go up to 1650°F] and generally produces a shallower case depth than carburizing.”
Ferritic Nitrocarburizing (FNC): “In the author’s opinion, just like with carbonitriding, ferritic nitrocarburizing (FNC) is named incorrectly as it is more closely related to nitriding than it is with carburizing. FNC is a process that is still mostly nitrogen-based but with a slight carbon addition as well. The added carbon helps promote compound layer formation, particularly in plain carbon and low alloy steels that do not contain significant nitride-forming alloys. This process is typically performed in a range of 1025-1125°F with cycle times much shorter than nitriding, typically 1-4 hours.”
Low Pressure Carburizing (LPC): “Low-pressure carburizing (LPC), or vacuum carburizing, is a variation of carburizing performed in a vacuum furnace. Instead of the atmospheres mentioned previously, a partial pressure of hydrocarbon gas (such as acetylene or propane) is used that directly dissociates at the part surface to provide carbon for diffusion. After LPC, the workload is transferred to a quench system that could use oil or high-pressure gas, typically nitrogen.”
Nitriding
Learn more about the basics of hardening at Heat Treat Radio. Click to listen,
Gas nitriding, a process over 100 years old, is a hardening process that involves diffusing nitrogen into the surface of steel to create a hard, wear-resistant case. Among many benefits, the part will have enhanced fatigue properties, anti-galling properties under load, and a resistance to softening at elevated temperature. This makes it an excellent choice for the aerospace industry.
There is some recent history regarding problems related to the “white layer”. In a typical microstructure, the “white layer” is a nitrogen-rich surface layer and the diffusion layer exists beneath it.1 It is essential that the surface layer be controlled to avoid an overly brittle part. Mark Hemsath the vice president of Sales – Americas for Nitrex Heat Treating Services, elaborated on this in a Heat TreatRadioepisode:
"Doug Glenn: I assume, with all the modern day technology and whatnot, we're able to control that white layer and/or depth of nitriding layer through your process controls and things of that sort."
"Mark Hemsath: Yes. Nitriding has been around a long time, but one of the problems that they had was controlling the white layer. Because they basically would just subject it to ammonia and you kind of got what you got. Then they learned that if you diluted it, you could control it. That's with gas nitriding. Then plasma nitriding came around and plasma nitriding is a low nitriding potential process. What that means is it does not tend to want to create white layer as much. It's much easier to control when the process itself is not prone to creating a lot of white layer, unlike gas. Now, in the last 10 – 15 years, people have gotten really good at controlling ammonia concentrations. They've really learned to understand that."
"ZeroFlow nitriding is ammonia-based gas nitriding," commented Dr.Maciej Korecki, PhD Eng., vice president of the Vacuum Furnace Segment at SECO/WARWICK Group. "It is distinguished by the fact that the nitrogen potential is controlled by introducing the right portion of ammonia at the right time and only ammonia, instead of a continuous flow of a mixture of ammonia and diluent gas."
"Consequently, the ZeroFlow method uses the minimum amount of ammonia needed to achieve the required nitrogen potential and replenish the nitrogen in the atmosphere, taking into account the situation where no ammonia is supplied to the furnace at all, no flow, hence the suggestive name of the solution," he continued. "Using ammonia alone in the nitriding process, we are dealing with a stoichiometric reaction (as opposed to some traditional methods), that is, one that is uniquely defined and predictable based on the monitoring of a single component of the atmosphere. Therefore, the ZeroFlow process controls very precisely through the analyzer only one gas, obtaining an improvement in the quality and repeatability of the results compared to various traditional methods."
According to Dr. Korecki, the process is about going back to the basics of nitriding: "The inventor of the method is Prof. Leszek Maldzinski of the Poznan University of Technology, who developed the theoretical basis and confirmed it with research. Then, more than 10 years ago, a partnership between SECO/WARWICK and the Poznan University of Technology initiated a project to develop and build the first industrial furnace designed to perform the ZeroFlow nitriding processes. The furnace was launched at SECO/WARWICK's research and development department (SECO/LAB®), where the method has been implemented and validated on dozens of industrial-scale processes."
Ferritic Nitrocarburizing
This nitrogen-based process can produce a deeper compound layer than nitriding, which is great for industrial machinery applications where this deep layer is needed for increased wear resistance and the critical strengthening of a deep case depth is not essential.
FNC has gone through a technical evolution with different heat treaters in the industry developing their own unique applications with method in mind. We'll look at two recent examples: AHT's Super Ultra Ox and Bodycote's Corr-I-Dur.
Edward Rolinski Senior Scientist Advanced Heat Treat, Corp. (Source: https://www.ahtcorp.com/)
According to experts at Advanced Heat Treat Corp. (AHT), Edward Rolinski (Dr. "Glow"), Jeff Machcinski, Vasko Popovski and Mikel Woods, "Thermochemical surface engineering of ferrous alloys has become a very important part of manufacturing. Specifically, nitriding and nitrocarburizing (FNC) processes are used since their low temperature allows for treatment of finished components. They are applied to enhance the tribological and corrosion properties of component surfaces.2 In many situations, nitriding replaces carburizing even if the nitrided layer is not as thick.3 A post-oxidizing step, applied at the end of FNC, leads to significant enhancement of corrosion properties by formation of a magnetite layer (Fe3O4).
"AHT’s newly developed process, UltraOx® Hyper, results in superior wear and corrosion resistance and allows for good control of the parts’ blackness. The latter is very important when the treatment is used for firearms. While the parts’ corrosion resistance improves with nitriding alone, the additional steps in UltraOx® Hyper significantly extend corrosion resistance. AHT is committed to achieving its customers’ desired metallurgical and cosmetic results through R&D and investing in state-of-the-art equipment. These innovations allow for flexibility in these areas."
In recent news, wave energy pioneer CorPower Ocean will be using Bodycote's thermochemical treatment, Corr-I-Dur®, for CorPower’s high-efficiency WECs. Image Source: www.waterpowermagazine.com
From Bodycote, they say that their proprietary Bodycote thermochemical treatment “Corr-I-Dur® is a combination of various low temperature thermochemical process steps, mainly gaseous nitrocarburising and oxidising.”
They explain, "In the process, a boundary layer consisting of three zones is produced. The diffusion layer forms the transition to the substrate and consists of interstitially dissolved nitrogen and nitride precipitations which increase the hardness and the fatigue strength of the component. Towards the surface it is followed by the compound layer, a carbonitride mainly of the hexagonal epsilon phase. The Fe3O4 iron oxide (magnetite) in the outer zone takes the effect of a passive layer comparable to the chromium-oxides on corrosion resistant steels.
"Due to the less metallic character of oxide and compound layer and the high hardness abrasion, adhesion and seizing wear can be distinctly reduced. Corr-I-Dur® has very little effect on distortion and dimensional changes of components compared to higher temperature case hardening processes."
How to Implement?
We’ve seen a lot of development in way of nitriding and ferritic nitrocarburizing (FNC), but for many heat treaters, you inherit specific processes and traditions of accomplishing heat treatment and do not have the chance to understand how to implement each process. Read the full 21 point comparative resource at FNC vs. Nitriding
Conclusion
The more informed you are, the better decisions you can make. For example, knowing these recent developments in metal treating and hardening is sure to help you decide whether to shift directions in how you company process parts for electric vehicles, or if you are ready to expand your offerings for your aerospace clients. It is clear that each of these processes have a future all-their-own. It’s up to you to decide whether that future should be yours, too.
For more information on the basics of hardness, listen to the what, why, and how of hardening with Mark Hemsath, an expert on metal hardness and vice president of Sales – Americas for Nitrex Heat Treating Services, on this Heat TreatRadio episode with Doug Glenn, publisher of Heat TreatToday. You can also review the resources below that were referenced in today’s article.
2 “Thermochemical Surface Engineering of Steels”, Woodhead Publishing Series in Metals and Surface Engineering: Number 62, Ed. Eric J. Mittemeijer and Marcel A. J. Somers, Elsevier, 2015, pp.1-769.
3 J. Senatorski, et. al, Tribology of Nitrided and Nitrocarburized Steels”, ASM Handbook Vol 18, Friction, Lubrication and Wear Technology, ed. G. Totten ASM International, 2017, pp. 638-652.
Senatorski, J. Tacikowski, E. Rolinski and S. Lampman, “Tribology of Nitrided and Nitrocarburized Steels”, ASM Handbook Vol 18, Friction, Lubrication and Wear Technology, ed. G. Totten ASM International, 2017.
“Thermochemical Surface Engineering of Steels”, Woodhead Publishing Series in Metals and Surface Engineering: Number 62, Ed. Eric J. Mittemeijer and Marcel A. J. Somers, Elsevier, 2015, pp.1-769.
Your parts need heat treated to herculean surface hardness but with a soft, ductile core. That is to say, you are looking at case hardening processes, most likely one of these: gas carburizing, low-pressure carburizing, carbonitriding, gas nitriding, and ferritic nitrocarburizing.
Mike Harrison at Gasbarre Thermal Processing Systems brings us a Technical Tuesday article about what case hardening is and how five of the most common processes vary by (1) comparing the specific guidelines for each temp and time, (2) identifying equipment used to perform each process, and (3) providing a chart (at the end!) to understand different process considerations.
Mike Harrison Engineering Manager of Industrial Furnace Systems Division Gasbarre Thermal Processing Systems
Case hardening falls into a class of heat treatment processes that typically involve the addition of carbon and/or nitrogen to the material through solid-gas reactions at the surface followed by diffusion. These processes are performed for any number of reasons that generally include increasing strength and wear resistance, but in all cases the end result is a harder, higher-strength surface with a softer, more ductile core.
Case hardening processes can be divided into two subsets: those that include quenching to harden, such as gas carburizing, low-pressure carburizing (LPC), and carbonitriding; and those that do not include quenching, such as gas nitriding and ferritic nitrocarburizing (FNC). This article will provide a brief look into each process, the types of equipment used, and considerations for implementation.
Diffusion + Quenching Processes
These processes involve heating the workload to austenitizing temperature, which is above the upper critical temperature for the material in question, then supplying and allowing the desired element(s) to diffuse into the part surface, followed by rapid cooling (quenching) to create a phase change to martensite that strengthens the material. Tempering is then performed to create a material that has the desired final strength and ductility properties. The result is a high concentration of added elements on the surface that continually decreases through diffusion until eventually matching the same concentration as the base material; this gradient similarly produces a hardness that is higher at the surface, gradually diminishing until reaching the core. Higher alloyed steels may also see a microstructural change in the core from quenching that produces a core with higher hardness than the previously untreated material, but lower than the surface hardness produced.
Atmosphere Gas Carburizing
Gas carburizing is a process where carbon is added to the material’s surface. The process is typically performed between 1550-1750°F, with carburizing times commonly between 2-8 hours; of course, these values can vary depending on the material, process, and equipment. The most common atmosphere used for atmosphere gas carburizing is endothermic gas with additions of either natural gas or propane to increase the carbon potential of the furnace atmosphere. Common case depths achieved are around 0.005-0.040”, with deeper cases possible through a combination of longer treatment times and/or higher temperatures.
Fig. 1 – Integral quench furnace: "The atmosphere gas carburizing process can be performed both in batch and continuous equipment."
The atmosphere gas carburizing process can be performed both in batch and continuous equipment. On the batch side, traditionally an integral quench (IQ) furnace is used (Fig. 1); it consists of a heating chamber where the workload is heated and exposed to the carburizing atmosphere, then the workload is transferred to an attached quench tank for cooling. The entire furnace system is sealed and under protective atmosphere to preserve the part surface and maintain safe control of any combustible gases. For batches of large product, a pit furnace can be used for carburizing with the workload being transferred via an overhead crane into and out of the furnace to a quench tank.
For continuous processing, a belt furnace can be used. The product is placed on a belt and then progresses through the furnace at the desired temperature and atmosphere composition; the carburizing time can be varied by adjusting the belt speed through the furnace. At the end of the furnace, the parts drop off the belt into the quench tank. Then, a conveyor pulls the parts out of the tank and drops them on another belt to be washed and tempered. For continuous processing of heavier loads pusher furnaces, rotary retort, rotary hearth, and roller hearth furnaces can be used.
Fig. 2 – Endothermic gas generator: "To achieve a carburizing atmosphere endothermic gas is typically used, which is produced by an endothermic gas generator that heats a combination of natural gas and air to create a mixture that is approximately 40% hydrogen, 40% nitrogen, and 20% carbon monoxide."
To achieve a carburizing atmosphere endothermic gas is typically used, which is produced by an endothermic gas generator (Fig. 2) that heats a combination of natural gas and air to create a mixture that is approximately 40% hydrogen, 40% nitrogen, and 20% carbon monoxide. This mixture is generally considered carbon-neutral, meaning it will neither add nor deplete carbon from the surface. To increase the carbon concentration the endothermic gas needs to be enriched with a gas (typically natural gas or propane) that will help produce additional carbon monoxide, which will “boost” the carbon potential and drive carbon diffusion into the material.
A less common carburizing atmosphere comes from a nitrogen-methanol system, where nitrogen gas and liquid methanol are combined and injected into the furnace. Upon exposure to the high furnace temperature the methanol will decompose to hydrogen and carbon monoxide. Natural gas or propane additions are still required in order to provide carbon for absorption into the surface of the steel.
Low-Pressure Carburizing
Low-pressure carburizing (LPC), or vacuum carburizing, is a variation of carburizing performed in a vacuum furnace. Instead of the atmospheres mentioned previously, a partial pressure of hydrocarbon gas (such as propane or acetylene) is used that directly dissociates at the part surface to provide carbon for diffusion. After LPC, the workload is transferred to a quench system that could use oil or high-pressure gas, typically nitrogen. LPC with gas quenching can be an attractive option for distortion prone complex geometries as the cooling rates are slower than oil quenching; however, given the slower cooling rate, it becomes very important to choose a higher alloyed steel that will achieve the desired hardness.
Fig. 3 – Vacuum furnace with oil quench
LPC typically provides faster carburizing times when compared to traditional gas carburizing. This can be attributed to a more efficient reaction of the hydrocarbon gas used and to the option of using higher carburizing temperatures, typically up to 1900°F. This is made possible by the type of internal furnace construction of vacuum furnace design, although care must be taken at higher temperatures to avoid undesirable grain growth in the material. LPC also has the benefit of eliminating the potential for intergranular oxidation, since it is running in a vacuum system.
LPC is typically performed in a single-chamber vacuum furnace, with oil quenching or high-pressure gas quenching done in a separate chamber (Fig. 3). Continuous vacuum furnaces can also be used in applications that require increased throughput (Fig. 4).
Fig. 4 – Continuous vacuum furnace
Carbonitriding
Despite its name, carbonitriding is more closely related to carburizing than it is to nitriding. Carbonitriding is a process where both carbon and nitrogen are added to the material surface. This process is typically performed in a range of 1450-1600°F and generally produces a shallower case depth than carburizing. Carbonitriding is used instead of carburizing for plain carbon steels that do not contain enough alloying content to respond well to quenching, as the added nitrogen can provide a higher hardenability in the case to allow for proper hardness development.
Atmosphere carbonitriding can be performed in the same equipment as is used for carburizing. The furnace atmosphere is still typically endothermic gas-based and includes the addition of ammonia to provide the nitrogen. Vacuum carbonitriding with both hydrocarbon and ammonia additions can also be performed in the same equipment as used for vacuum hardening and low pressure carburizing.
Diffusion Only Processes
These processes involve heating the workload to a temperature below the austenitizing temperature, allowing the desired element(s) to diffuse into the part surface, then slow cooling. The increase in hardness at the material surface comes only from the addition of the diffused element(s), and not from a phase change due to quenching. As these processes are performed below the lower critical temperature (i.e., below the austenitizing range), the desired core hardness and microstructure need to be developed through a separate heat treatment prior to case hardening. Generally, the process temperature selected should be at least 50°F below any prior treatment temperatures to avoid impact to the core properties.
Gas Nitriding
Gas nitriding is a process where nitrogen is added to the material surface. The process is typically performed between 925-1050°F; cycle times can be quite long as the diffusion of the nitrogen is slow at these temperatures, with nitriding times typically ranging from 16 - 96 hours or more depending on the material and case depth required. Nitriding can be performed in either a single or two-stage process and has the potential to produce two types of case, the first being a nitrogen-rich compound layer (or “white layer”) at the surface that is extremely hard and wear-resistant but also very brittle. This compound layer depth is dependent on processing time. In the more traditional two-stage process, the case depth produces a gradient of hardness from surface to core that commonly ranges from 0.010-0.025”, with minimal white layer, typically between 0-0.0005”. Nitriding is typically performed on higher alloyed steels or steels specifically designed for the nitriding process (e.g., Nitralloy®) as it relies on the formation of nitrides to create the increased hardness, which is achieved through the use of nitride-forming alloys such as aluminum, molybdenum and chromium. Pre and post oxidation treatments can be incorporated into the cycle to achieve certain benefits. Since the process does not require quenching to harden, it has the potential of producing a product that is more dimensionally stable and may not require any post-process finishing.
Fig. 5 – Horizontal retort nitriding furnace: "Traditionally, pit furnaces have been used for nitriding as they can accommodate larger load sizes and can be easier to seal as gravity helps keep the lid sealed; however, horizontal designs have gained in popularity in recent years."
This process is most commonly performed in batch equipment; while it is possible to use a continuous furnace, keeping the ends of furnace sealed to contain the atmosphere can be challenging. Traditionally, pit furnaces have been used for nitriding as they can accommodate larger load sizes and can be easier to seal as gravity helps keep the lid sealed; however, horizontal designs have gained in popularity in recent years (Fig. 5). In either case, the furnaces are usually a single-chamber design with the load sealed inside an Inconel or stainless steel retort.
To achieve a nitriding atmosphere, ammonia (not nitrogen) is used to supply the atomic nitrogen necessary for diffusion. At the process temperatures used, ammonia does not readily dissociate on its own; rather, it dissociates when exposed to a heated steel surface (iron acting as a catalyst) into atomic nitrogen and hydrogen. To control the amount of nitrogen available for nitriding, the dissociation rate of the ammonia can be measured with high dissociation rates (high hydrogen content) providing a lower nitriding potential and low dissociation rates (low hydrogen content) leading to more nitriding potential. The depth of the compound layer can be varied through control of the nitriding potential, with higher nitriding potentials producing a thicker compound layer.
For more precise atmosphere control, an ammonia dissociator can be used to provide gas to the furnace that has already been split to dilute the atmosphere with hydrogen to more quickly achieve a high dissociation rate in the furnace. The ammonia dissociator is a heated box with a small retort inside; the ammonia is passed through this retort that contains a catalyst to promote the dissociation of the ammonia, and the resulting gas mixture is cooled and then injected into the furnace.
Ferritic Nitrocarburizing
In the author’s opinion, just like with carbonitriding, ferritic nitrocarburizing (FNC) is named incorrectly as it is more closely related to nitriding than it is with carburizing. FNC is a process that is still mostly nitrogen-based but with a slight carbon addition as well. The added carbon helps promote compound layer formation, particularly in plain carbon and low alloy steels that do not contain significant nitride-forming alloys. This process is typically performed in a range of 1025-1125°F with cycle times much shorter than nitriding, typically 1-4 hours. The compound layer produced is usually much deeper than nitriding at 0.0005-0.0012”, with case depths reaching up to 0.025”, although in many applications a case depth may be difficult to measure. FNC is usually performed instead of nitriding in applications where the deeper compound layer is needed to increase wear resistance, but the added strength of a deep case depth is not as critical.
FNC can be performed in the same equipment used for nitriding, as long as a hydrocarbon gas is available to the furnace such as carbon dioxide or endothermic gas. FNC can also be performed in an IQ furnace using a mixture of ammonia and endothermic gas; for cooling, the parts can be oil quenched or slow cooled in a top cool chamber (if equipped).
Considerations
Case hardening processes are some of the most common heat treatments performed, but each process has its own unique needs. The table below provides a summary of the considerations that need to be made when selecting the optimum process. This list is by no means exhaustive; it is encouraged to work with a furnace manufacturer familiar with each process to help select the correct process and equipment needed.
About the Author: Mike Harrison is the engineering manager of the Industrial Furnace Systems division at Gasbarre. Mike has a materials science and engineering degree from the University of Michigan and received his M.B.A. from Walsh College. Prior to joining Gasbarre, Mike had roles in metallurgy, quality, and management at both captive and commercial heat treat facilities, gaining nearly 20 years of experience in the thermal processing industry. Gasbarre provides thermal processing equipment solutions for both atmosphere and vacuum furnace applications, as well as associated auxiliary equipment and aftermarket parts & service.
For more information: Contact Mike at mharrison@gasbarre.com
How well do you know hardness processing? Can you draw the line where nitriding and ferritic nitrocarburizing (FNC) differ? In this Technical Tuesday feature, skim this straight forward data that has been assembled from information provided by four heat treat experts: Jason Orosz and Mark Hemsath at Nitrex, Thomas Wingens at WINGENS LLC – International Industry Consultancy, and Dan Herring, The Heat Treat Doctor at The HERRING GROUP, Inc.
Let us know what you think! What is the next comparison you'd like to see? What facts were you surprised by? Email Heat TreatDaily editor Bethany Leone at bethany@heattreattoday.com.
Nitriding
Descriptor
Ferritic Nitrocarburizing
480º-590C (896º-1094ºF) typical
Temperature Range
565º-590ºC (1049ºF-1094ºF) typical
Wrought and powder metallurgy materials including alloy steels (e.g., 4140), stainless steel (e.g., 304L, 420), tool steels (e.g., H11, H13) and special nitriding steels (e.g,, Nitralloy 135M, Nitralloy EZ) are typical examples. Many other steel grades are possible.
Materials Commonly Processed
Plain and medium carbon steels (e.g., 1015, 1018, 1045), alloy steels (e.g., 4140, 4340) and tool steels (e.g., H11, H13) are typical examples. Many other steels grades are possible.
Wear (as in abrasion resistance), bending, torsional and rolling contact, fatigue resistance, lubricity, and adhesive strength improvements.
Materials Commonly Processed: Why to Process Them with These Methods
Wear resistance, lubricity, fatigue, and corrosion resistance are primary benefits with improved fatigue strength and adhesive strength possible.
3-48 hours at temperature. May be up to 72 hours.
Relative Cycle Times
2-6 hours at temperature.
Pit retort furnaces and front load retort furnaces for gas nitriding, although bell retort furnaces have also been used.
Equipment Types Used for the Process
Pit retort furnaces and front load retort furnaces for gaseous ferritic nitrocarburizing. Bell retort furnaces have also been used.
Ammonia and nitrogen or ammonia and dissociated ammonia.
Atmospheres Used/Required
Ammonia and nitrogen and carbon-bearing gas such as CO2, CO, or endothermic gas.
Dies, gears, pump bodies, springs, gun barrels, shafts and pinions, pins, brake rotors and may other types of component parts produced from bar, plate, rod, forgings and castings formed by stampings, machining, rolling, forging, casting, etc.
Typical Parts Processed
Wear plates, washers, clutch plates, gas pistons, brake pistons, brake rotors, barrels, slides, differential cases and other types of component parts produced from bar, plate, rod, etc., and formed by stampings, rolling, machining, casting, etc.
Cost is often higher for gas nitriding as opposed to other case hardening processes (including FNC) based on the type of component parts run. In many cases, cost is a function of the longer cycle time and/or more labor involved.
Relative Cost Per Unit
Cost is often lower than many other case hardening processes (including gas nitriding) based on the type(s) of component parts run. In many cases, cost is a function of a shorter cycle time and/or less labor involved.
Basic specifications are easily achieved with good equipment and/or controls; difficulty increases when attempting to produce specialized layer compositions/phases.
Ease of Use/Control
Basic specifications are easily achieved with good equipment and/or controls; difficulty increases when attempting to produce specialized layer compositions/phases. Hardware/control requirements are more complicated than for nitriding when controlling for carbon potential.
It can range from very simple to medium-high depending on application.
Relative Expertise Necessary to Perform
Medium-high depending on the application. The user will want to look for clean parts, a good loading system, and PLC controlled cycle.
Aqueous (clean chemistry) including rinse/dry, vapor degreasing (clean chemistry).
Cleaning Requirements
Aqueous (clean chemistry) including rinse/dry, vapor degreasing (clean chemistry).
Time, temperature, gas flow, nitriding potential (Kn) and/or percent dissociation, hydrogen sensors.
Controls
Time, temperature, gas flow, nitriding potential (Kn), carbon potential (Kc) and oxygen potential (Ko). Hydrogen sensor and oxygen (carburizing) sensor may be used.
electric and gas-fired equipment
Fuel Source
electric and gas-fired equipment
Hardness (surface, core), case depth determination (via microhardness – typically core hardness + 50 HV), microstructure (compound and diffusion zone depths), composition, core structure, presence of absence of nitride networking (aka nitride needles), and the presence or absence of cracking or spalling of the case.
Testing Required
Hardness (surface, core), case depth determination (via microhardness – typically core hardness + 50 HV), microstructure (compound and diffusion zone depths), composition, core structure, porosity (type and depth), and the presence or absence of cracking or spalling of the case.
Warm wall plasma nitriding, as well as advances in controls, sensors, temperature uniformity, and reduced gas volumes.
Latest Advances
Black oxide, hydrogen sensors, and fast cooling techniques as well as advances in controls, sensors, and temperature uniformity.
(1) simple equipment, (2) can offer beneficial tribological changes part/metal, (3) performed after part machining, (4) little-to-no distortion.
Pros (Strengths)
(1) fast, cheap, repeatable results, (2) excellent corrosion resistance, especially with (black) oxide, (3) performed after part machining, (4) minimal distortion/almost distortion free
(1) long cycle time, sometimes a multi-day process if deep case is required, (2) effective pre-cleaning required, (3) weldability becomes reduced, (4) ammonia is used, (5) embrittlement with too much white layer.
Cons (Weaknesses)
(1) Focused on part surface, mainly with inexpensive materials, (2) effective pre-cleaning required, (3) weldability becomes reduced, (4) ammonia is sometimes a concern.
Source: Advanced Heat Treat Corp.
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