Ferritic Nitrocarburizing

In this Heat Treat Radio episode, host Doug Glenn converses with Mike Holly on his extensive experience in ferritic nitrocarburizing (FNC). Listen as they discuss Mike’s career at General Motors, where he implemented FNC to improve brake rotor performance. This episode delves into the technical aspects of FNC, its benefits such as enhanced wear and corrosion resistance, and its application beyond automotive, including military and industrial uses.

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.

Introduction (00:36)

Doug Glenn: Welcome to another episode of Heat Treat Radio.

I have the great privilege today of talking with Mike Holly who I think you’re going to find very fascinating; I know I have in the conversations we’ve had so far. We’re primarily going to talk about ferritic nitrocarburizing (FNC) because Mike has some great experience in that area. But first I want to welcome you, Mike, and give you an opportunity to tell us a bit about you and your work history.

Mike Holly: I’m currently retired but I am working as an engineering consultant on my own, primarily in the areas of heat treatment, casting, welding, coding, and plating. I specialize in automotive and heavy truck applications. As far as my education, I’m a graduate metallurgical engineer with a bachelor’s from Wayne State University in Detroit and a master’s from Purdue. I have 43 years of experience in the auto and heavy truck industry; 32 of those years were with General Motors who I retired from. I was assigned to the materials engineering group in Warren, Michigan, and I specialized in driveline, exhaust, steering, chassis structures, and brake applications, primarily metal applications.

FNC and Brake Rotors (02:30)

Doug Glenn: The topic that we want to focus on today is FNC. Although if you think of anything else that might be of interest to our thermal processing people, feel free to deviate. How did you get introduced to ferritic nitrocarburizing or case hardening in general?

Mike Holly: I’ve always been involved with heat treatment and case hardening as a metallurgical engineer working on heavy gearing applications. I’m very familiar with FNC and way back in the mid-2000s (about 2005), we were looking at our warranty. In brakes, we saw an opportunity to improve the performance of our brake rotor by reducing brake judder, or pedal pulsation, which caused a lot of customer dissatisfaction. It caused a lot of warranties, knowing that these vehicles would be brought in to be serviced.

We were aware of FNC being done on brake rotors. It had been tried, but brake rotors are a highly dimensional, critical part, and control of distortion is paramount. With prior efforts, that distortion was completely out of control. And that’s why it never went anywhere. So, another team member and myself at GM took it offline and worked out the details so we could FNC-finish machined rotors with no subsequent grinding.

And we were able to do that, working with a company in Detroit at the time called Kolene. We were working in salt, but later on we did change the process to gas. The learnings between salt and gas pretty much transferred completely. We issued some patents, both for the FNC process itself and as it applies to brakes and some subsequent processing to improve the corrosion resistance of the rotor. My name is not on the patent as my prior employer owns the rights.

Doug Glenn: That is often the case, right? If you’re working for somebody, it’s their patent and not yours. How many patents were you involved with?

Mike Holly: I believe the number is 14 different patents. Some relate to the process directly; some relate to the interaction and the selection between the brake rotor and the friction material. There are quite a few patents that my prior employer has on this process. The first application was in 2009 in the Cadillac DTS and the Buick Lucerne. That’s where the rotors were first used.

Success with FNC (05:36)

Doug Glenn: Backing up to 2005, what do you think had made the FNC unsuccessful up to that point?

Mike Holly: Control of the output: The FNC process that was being used produced almost a solid white layer and we could not get the stopping power out of the friction material. This has to do with the application of something called a transfer layer. We discovered that you need porosity to get the transfer layer down.

Also, orientation of the brake rotor in the process is important; the patents tell you in the specs to orient the parts vertically.

Doug Glenn: Are you talking about the orientation of the rotor in the furnace?

Mike Holly: Yes. So it wasn’t anything we invented.

To try to control distortion further, we stress relieved the castings. We took all the residual stresses out from the founding, or the casting, of the part prior to machining, and then put the parts through ferritic nitrocarburizing, fully machined, no other grinding necessary; doing so, we’re able to maintain the critical dimensions.

A brake rotor is a safety critical part, so there are a lot of steps and validations to get that implemented.

Doug Glenn: It sounds like before 2005, and correct me if I’m wrong on this one, Mike, they were FNCing unfinished parts? They were FNCing the rotors before they were machined?

Mike Holly: No, they were doing finished parts and discovered that the dimensions, but the lateral runout and the thickness were so out of control that they would have to go in and subsequently grind to get it back in the dimension. But the FNC case depth is only 10 to 20 microns. You may wind up just grinding the case right off!

What Is FNC? (08:38)

Finish machining FNCed parts really can’t be done without removing the FNC, and then you lose the benefit. It’s a difficult matter to heat treat finished machined parts. It is done. But it was control of dimensions that made the difference.

Doug Glenn: Let’s take a step back then. I want to talk some very basics. You can give us a little metallurgy lesson for people who might not know what FNC is. Can you tell us about what we are doing in this process?

Mike Holly: Ferritic nitrocarburizing is a case hardening heat treatment. We are actually making a composite material. It’s within the families of nitriding, carbonitriding and carburizing. These are all done at different temperatures, and they produce different case depths. But again, you are making a composite material.

FNC is a thermal chemical treatment. We diffuse carbon and nitrogen into the surface of the iron. This strengthens the iron and provides not only a wear-resistant case but corrosion resistance. That’s a peculiar advantage to FNC.

We can specify for steels, stainless steels, gray irons, nodular irons, a whole host of ferrous materials. FNC can be performed in a gaseous atmosphere, molten salt or even a fluidized bed. You involve two gases: a source of carbon, which could be carbon dioxide or natural gas, and a source of nitrogen, which is typically ammonia.

The process is done subcritical, which means below the critical temperature of like 723°C (1333°F) — it’s well below that. It’s performed at around 560°C to say 590°C (1040°F to 1090°F). It produces a very hard wear and corrosion-resistant case from 10 to 20 microns and thickness.

Benefits of FNC (10:35)

So, what are the benefits? Why would we even do this? For one thing, it’s done at such a low temperature that it’s a low distortion heat treatment; we’re not going through the transformation temperature.

Doug Glenn: For example, just for those who don’t know, like carburizing — that means going above critical.

Mike Holly: That’s right. With FNC, we get an improved fatigue durability due to the higher surface strength. Ferritic nitrocarburized parts have a compressive residual stress on the surface, and that’s beneficial for fatigue. It’s resistant to adhesive and abrasive wear, it provides a fairly good surface finish, and, very importantly, it improves corrosion resistance compared to other processes.

And a critical environmental concern is there’s no hazardous waste treatment or landfill involved. These gases are readily available. There’s really no waste treatment that we have to concern ourselves with.

Why don’t we do every gear this way? It has to do with the case depth; these are very shallow cases. For heavily loaded parts like ring and pinion high point gearing, we need a thicker case to resist the rolling contact fatigue.

In that application we have to go to carburizing or carbonitriding. And for some shafts where we get very high bending stress, we have to use induction hardening, which is a case hardening treatment that doesn’t use diffusion. You’re just modifying the microstructure of the surface.

FNC has a unique niche: It’s subcritical, has good wear and corrosion resistance, and it improves the fatigue properties.

Doug Glenn: I want to ask you about other applications for FNC besides brake rotors.

First, let me ask you this since you’re talking about the shallow case depth. I’m thinking to myself, you’ve got the rotor and you’ve got your friction product (which we would consider to be the pads that are mounted to the caliper, let’s say on a car). Are those pads not also kind of grinding off the shallow case depth of the rotors?

Mike Holly: It could if you had an aggressive enough friction material. In one of the designs that we had to make was selection of friction materials. And at the time the non-asbestos organic friction materials worked very well with FNC.

But as we go up in aggressiveness, one of the projects I’m working on is improving the case wear resistance of the FNC brake rotors. We’re doing that by alloying gray iron with niobium. We alloy with niobium and form niobium carbides in the case. This greatly improves the wear resistance on the iron side. So that’s how we’re addressing the more aggressive friction materials that would typically be used in Europe.

Applications of FNC (14:51)

Doug Glenn: I want to come back to that niobium, too, so we’ll probably hit on that again. What other applications of FNC have you seen?

Mike Holly: It’s used where wear distortion and corrosion resistance are very important. Many lightly loaded gears will fit into this category. Struts, the devices that hold up your hoods, they’ll be FNC. Some locking mechanisms are FNC. Brake backing plates are currently done. And I think one of the biggest applications is clutch pack discs, which are small 1040, 1050 steel materials (that may not be the only alloy that’s used). They’re FNCed to improve the wear resistance in the case.

An upcoming application I’m working on is chassis cradles and frames. We stamp these pieces out of steel, and we weld them. But when we weld them, the weld heat affected zones can lose strength. What we’ve come up with is by using a niobium alloy, a high strength, low alloy steel, and FNC heat treating it, all the weld heat affected zones have good fatigue performance, along with the rest above the cradle. That’s something I worked on at GM, and there’s a patent on that. 

And brake rotors are the latest application which has benefited from FNC treatment. They provide very long-term durability, reduce brake judder, and they’re very commonly used for electric vehicles. Because of the regenerative cycle, there is not a lot of friction application. We have to be very concerned about corrosion buildup on an electric vehicle application.

Doug Glenn: When you start mentioning about car frames and things of that sort, have you gotten at all involved with this giga cast thing for Tesla? I mean is there any FNC going on there?

Mike Holly: Well, I’m not sure what Tesla is doing, but with chassis structures, you’re not only balancing strength. Strength is important; you’re also balancing stiffness. Stiffness could be related to the metal. Now steel has very high Young’s modulus value compared to aluminum. The way you have to make that up with aluminum is through section properties: Thickness and shape.

There’s always competition between steel and non-ferrous materials, whether it be cast aluminum or fabricated aluminum and steel. They each have their advantages, and there have been many vehicles made with both types of construction. Where stiffness is critical, typically steel dominates. That’s the story of chassis structures.

Doug Glenn: When we spoke before, I think you mentioned that there are some non-automotive applications for FNC like golf clubs and some other things?

Mike Holly: I have seen it performed at a company in Michigan where they’re doing, for example, very large gates that are used for hydroelectric plants. They’re FNCing the gate to improve its erosion resistance from water. It’s done in many military applications for devices that would hold onto ordinance. It can be used on stainless steels to improve their wear and strength. There are non-automotive applications for sure.

FNC at General Motors (19:52)

Doug Glenn: I want to ask you a question about the business side of FNC. A lot of times there’s a lot of inertia to keep things the way they are, right? A lot of our advertisers have trouble breaking in with new technologies. From your perspective as one of the lead guys on this for GM, what did it take to get the FNC process into your production schedule?

Mike Holly: First, we had to prove that this is something that would benefit the client. The client would benefit twofold: The vehicles would resist distortion and corrosion; that would improve the performance of the brake in terms of resisting pedal pulsation.

Also, warranties can be very costly. Adding this type of enhancement reduces warranty costs. But you do have to balance the cost reduction of warranty versus the cost of the process. Initially it was very costly, but we wanted to see how it would perform in real time. And at game speed, which means in the customer’s hands.

There was a very willing group at GM, the Cadillac people, who wanted to be first. And they were willing to do this. It turned out quite well. And since that time, it’s been adopted by many car platforms including many competitors.

Doug Glenn: Do you have any idea what it was about the guys in the Cadillac DTS division that made it more attractive, more palatable to them than others?

Mike Holly: They wanted to be first. They wanted to offer a premium vehicle with premium performance. They advertised it in their brochures.

When it was adopted by the truck platforms, which was a really big deal in terms of volume, it was actually advertised on one of the Super Bowls early on. I still have that.

Doug Glenn: That would be very interesting to see a Super Bowl ad talking about brake rotors.

Mike Holly: Brakes and FNC. You know, the customer is king, and you have to provide something that they’re willing to go along with. Ultimately, we have to make money. Those were key characteristics.

Starting Out with FNC (23:26)

Doug Glenn: At that point did you just jump in full bore — buy the equipment and do it yourself? Or did you first start by doing some outsourcing of it?

Mike Holly: It was originally done in the existing supply base. We used existing heat treaters. The furnaces were not optimized for brake rotors; parts were being shipped a lot.

Before we started purchasing equipment, we wanted to make sure this was going to operate in real time at game speed as we expected. As the platforms were added, it was very clear from the beginning (and we know this from highly machined gearing) that the best thing is to have the heat treat shop right in the manufacturing facility. That way you’re not shipping these very dimensionally critical parts all over the place. And the dunnage is expensive.

Today the FNC operations are co-located for the most part with the machining plant. And in many cases, you’ll see the foundry, the machining plant and FNC all in the same locale. This eliminates shipping and transferring costs, maintaining your highly machined parts and eliminating the handling. These are heavy parts, and the furnaces have to be designed to accept the thermodynamic load of large parts. And it’s preferred to do it by the ton — a lot of parts at once. And these are batch processes, so they’re very receptive to that.

Part Fixturing (25:23)

Doug Glenn: Earlier you mentioned the criticalness of fixturing. Is there anything more you can say about that? We don’t want to disclose any secrets.

Mike Holly: Generally, our patents will just say vertical orientation. The heat treat suppliers all have different furnaces, so that’s for them. They design their own racking, and that’s their property. They don’t have to disclose that.

The OEMs just require dimensional control. So, show us statistically that your lateral runout, your thickness and your wheel mount surface meet our specs. And, of course, the guidance that the parts should be oriented vertically and should be stress relieved before machining is out there.

As far as the intimate details of the rack and how heavily loaded the furnace is, that’s all their efficiencies, and they own that. I don’t reveal that to anybody. That’s theirs. It’s not for me to cross fertilize the industry with that.

Early Players in FNC (26:49)

Doug Glenn: For posterity’s sake, it would be nice to know who some of the early players were in this. Obviously, your DTS Cadillac division were kind of the end users. But who were the people outside of GM who helped out?

Mike Holly: I’ll give some credit here: I mentioned Kolene. I think they’re out of the salt bath business now. The original salt bath heat treater was KC Jones in Hazel Park, Michigan, and then the gas processing was basically first implemented at Woodworth in Detroit.

Doug Glenn: I’m familiar with them, and I think they’re still doing it, right? From what I understand, Woodworth’s got a huge business in that.

Mike Holly: They are still doing it. They’re a very dominant player, but other players have entered the market and been very successful. It can be done. And from the OEMs perspective, competition is great.

I was involved in developing processors not only in North America, but in Asia and South America.

Doug Glenn: Were there are a lot of hoops to jump through for the folks at Woodworth or Kolene, for example? Do you have any tips or suggestions for companies who are wanting to supply stuff like that to GM?

Mike Holly: Initially there were a lot of lessons learned. We were able to work through that — mainly to get the scrap rate down. Now it’s down to very low levels. There’s continual learnings like stress relief, for example. It’s since been discovered that not all brake rotors need to be stress relieved. Depending on the geometry of the rotor, they may not develop a lot of residual stresses in the casting operation. Or the casting operations could be different if you have, say, a vertical part line with very long shakeout, the cooling rate is rather slow. We’ll develop minimal residual stresses that you may not have to stress relief. But at the end of the day, the dimensions must be met, and 100% of these parts are typically checked for dimensions.

FNC and New Technologies (29:39)

Doug Glenn: Let’s jump back to the process a little bit. This may have to do with some technology moving forward. But is there any alternative to FNC at this point? Any competitive processes?

Mike Holly: The latest change occurring that’s driving new ideas is the Euro 7 regulation, the dust emission. And I can describe that if you’re interested in a very short description.

They’re basically new rules from the European Commission. They’re intended to provide cleaner vehicles in terms of emissions and air quality. The latest implementation date appears to be 2026. They have a rollout date of when you have to meet the requirements. And it is particularly focused on brakes and tire-related emissions.

This is according to the SAE; I’ll give them credit where credit is due. They basically tell us that with Euro 7, brake particle emissions (size in the PM10 range; inhalable particulate around ten microns and smaller like dust and pollen and 2.5 microns) must reduce by 25% to 30% to a maximum of, say, seven milligrams per kilometer. 

It’s a very complicated regulation. I think the latest data I’ve seen is 20, 35, but even if it’s 2035, we have to start working on that today.

The two technologies that I think are going to come to the forefront is going to be FNC and laser cladding, which you may have seen coming out of Europe. In laser cladding, we’re going to clad the brake rotor, the thermal spraying type of application with a very hard wear-resistant layer of titanium carbide. That will require post-grinding.

What I’m working on is FNC and enhancing the case properties by alloying the iron with niobium. Now, is this an entirely new idea? I don’t think so. Most metallurgists will tell you that even in carbides and grades we use different steels to improve either the case or core properties. Alloying additions are well-known in the heat treat industry. I’m boosting the hardness of the FNC case with niobium carbides. It also benefits the core by improving the strength of the core.

I think those are the two technologies involved.

I think niobium plus FNC is certainly the low-cost approach. Will it be compatible with all friction materials? In the most aggressive friction materials out there, you might have to go to laser cladding. But I think for the majority of friction materials, FNC on its own or FNC plus niobium will work, and they’re very low-cost type additions. Niobium alloying with cast iron is very well-known, and it’s been done in the past. It doesn’t require a lot of capital investment. If you already have FNC-heat treated rotors, you don’t have to buy furnaces. In my opinion, it is the low-cost option to accomplish the objective of meeting Euro 7.

Doug Glenn: I want to go back to that process of niobium a little bit just to be clear. The niobium is alloyed into the rotor to start with, right?

Mike Holly: That’s correct.

Doug Glenn: You’re not infusing it with….?

Mike Holly: No.

Doug Glenn: Ok, you’ve got the niobium and the carbides in the rotor to start with, and you’re just FNCing it as usual.

Mike Holly: It’s an alloy furnace addition at the foundry. It has been done in either electric or cupola melting. There is a heavy truck rotor application that was niobium alloyed for many years, and that was advertised as a 1 million-mile rotor. It had a very high niobium addition, so it affected the machinability of the part.

In the heavy truck industry, it’s all about uptime — keeping the trucks out of the shop and on the road. It accomplished the client’s objective.

Doug Glenn: You mentioned advertising again. I’ve got to go back and find this DTS advertisement on the Super Bowl.

Mike Holly: I think it was a truck application, Silverado Sierra.

Doug Glenn: I’ve got to find that.

The cladding process, if we’re talking about which one of these processes might win out if there was competition between them, is the cladding process done piece by piece? How do they clad a rotor? In FNC you’re not doing it piece by piece.

Mike Holly: One at a time.

Doug Glenn: Do you think the cost element will be the deal-breaker there, besides the fact that you’re adding cladding and post-grinding?

Mike Holly: Yes, those are very costly. But the most costly part of it is the materials. You have to put an adhesion layer down, that’s basically a 316-type stainless steel all done with laser type thermal spray application and then a second layer of the carbide.

There are a couple carbides that could be used; titanium carbide is the favorite now. Niobium carbide could be used. Tungsten carbide can be used, but that has some environmental effects; I think tungsten has fallen out of favor. 316 contains both nickel chromium and molybdenum. Nickel is traded on the London Metal Exchange. Your ability to control costs with nickel is minimal. Nickel and molybdenum, especially, is used in other applications such as high temperature alloys. So, you’re going to get competition from the turbine engine material.

In the case of FNC, ammonia, natural gas, carbon dioxide, and propane are all readily available worldwide. They are not controlled by any LME (London Metal Exchange) or anything like that.

Also, once you grind the surface, you have to deal with the grinding swarf. You cannot just put nickel to drain; that has to be treated. And, of course, you would like to recover it.

But I don’t want to throw the laser cladding people completely under the bus; it produces a very hard, wear-resistant layer.

Doug Glenn: It sounds like there may be applications where the cladding makes sense, but for your everyday truck and car you probably don’t need that high end rotor.

Mike Holly: I think we have to get back to basics. What does the brake do? It’s an energy conversion device. It’s converting mechanical energy to heat, or in the case of regenerative braking, it’s charging a battery. There’s the brake rotor, the metallic surface and the friction material. It has to be looked at as a system. What are the performance objectives that we intend to meet? And what is the desired durability and cost?

Doug Glenn: It seems like from what you’re describing FNC would have a huge cost advantage.

Mike Holly: I think so.

Current State of Brake Rotor Industry (39:05)

Doug Glenn: In your consulting work which you mentioned earlier, you’re working on improving the wear life of these rotors using FNC by incorporation of niobium?

Mike Holly: Yes. I published an SAE paper recently, and I’m going to publish another one in the upcoming North American colloquium and also in EuroBrake. My clients are sponsoring various tests and evaluations both here, in Europe and in South America. We’re getting a lot of good data, but competition makes us better. It truly does. You see it at these brake meetings. There’s always the cladding people, and there’s always the FNC people.

Doug Glenn: What is the leading brake event in the United States?

Mike Holly: In my opinion, it would be the SAE (Society of Automotive Engineers) Brake Colloquium. But there’s also the regular SAE congress. In Europe, it would be EuroBrake. And I think there’s comparable activities in Asia.

Doug Glenn: I just thought of a question I wanted to ask you before: You said Euro 7 is for brakes and tires, and they’re concerned about the particles created by both when they’re used — tire wear on the roads or brake friction?

Mike Holly: Yes. And they’re concerned about the microplastics from the tire. I think the tire people have a bigger job than the brake people do. But brakes are a fairly significant challenge.

Doug Glenn: I’m laughing because I’m thinking it depends how you drive. Some people are a little heavier on the brakes than others. 

Are you fairly confident that Euro 7 will come to the U.S. at some point?

Mike Holly: I’m not a regulations expert, but I think it likely will. It’s more of a political question. I understand from talking to some contacts in Asia that they plan on adopting it. We’ll see; it’s definitely going to add cost.

Doug Glenn: Yes, most regulations do.

Final Thoughts (42:18)

Doug Glenn: Is there anything else you would like to add before we wrap up?

Mike Holly: I not only work on brakes; I’ve also worked in suspension springs. Some of those are microalloyed to improve their properties. I can do CQI-9 audits. I’ve worked on coatings and platings (hard chrome or electroless nickel). If someone would need an extra hand, I get to help out.

Doug Glenn: You’ve got my vote. When did you retire from GM?

Mike Holly: I retired in 2021, and I currently live near Green Bay, Wisconsin.

Doug Glenn: And you’ve built your own consultancy, which is great. Thanks for taking the time to visit with us. I appreciate your expertise.

Mike Holly: Thank you.

About The Guest

Mike is currently a consultant with Mike Holly Metals LLC, specializing in heat treatment, coating, casting, metal forming and joining operations. He has 42 years of experience in industry, including 32 years at the General Motors Materials Engineering department where he was assigned to support automotive and truck chassis applications. He holds 15 patents and was key in the development of Ferritic Nitrocarburizing Brake Rotors. Mike has a Bachelor of Science in Metallurgical Engineering from Wayne State University and a Masters from Purdue University.

Contact Mike at mike.holly72@att.net.

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