Looking to a Future with FNC or Nitriding?

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:

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

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 Treat Radio episode:

"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."

Recently, SECO/WARWICK shared their nitriding technological developments in their product, ZeroFlow.

"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.

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."

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 Treat Radio episode with Doug Glenn, publisher of Heat Treat Today. You can also review the resources below that were referenced in today’s article.

References

¹ Daniel H. Herring, “The Heat Treat Doctor”, “Case Hardening of Steel, Part Three: Gas Nitriding,” PowerPoint Presentation, © 2004 – 2010 The HERRING GROUP, Inc.

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.

Resources

1. Herring, “Case Hardening of Steel, Part Three: Gas Nitriding,” PowerPoint Presentation, © 2004 – 2010 The HERRING GROUP, Inc.
2. Harrison, “Elevate Your Knowledge: 5 Need-to-Know Case Hardening Processes,” Heat Treat Today, https://www.heattreattoday.com/processes/hardening/hardening-technical-content/comparative-study-of-5-case-hardening-processes/.
3. Hemsath and D. Glenn, “Heat Treat Radio: Metal Hardening 101, Part 2 of 3,” Podcast and Transcript, Heat Treat Today, https://www.heattreattoday.com/media-category/heat-treat-radio/heat-treat-radio-metal-hardening-101-with-mark-hemsath-part-2-of-3/.
4. Orosz, T. Wingens, and D. Herring, “Nitriding vs. FNC,” Heat Treat Today, https://www.heattreattoday.com/processes/nitrocarburizing/nitrocarburizing-technical-content/integrated-nitriding-and-fnc/.
5. 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.
6. “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.