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History of Cronidur 30
It was known that nitrogen could replace carbon to form hard martensite. Both are small “interstitial” elements that create the hard structure necessary for knives. Or in this case, Cronidur 30 was developed for bearings. There have been several steels designed for bearings that also see regular use in knives, such as 52100, 154CM, and BG42. And 440C has a long history of being used in bearings. Nitrogen is also beneficial for corrosion resistance, while carbon is almost always detrimental. Nitrogen improves pitting resistance. It is also less prone to form chromium nitride than carbon is to form chromium carbide. That leaves chromium “in solution” to improve corrosion resistance. You can learn more about how nitrogen works in this article on nitrogen-alloyed knife steels.
Nitrogen contributes somewhat less to hardness than carbon. This creation comes from sources cited in this article.
One of the major reasons nitrogen isn’t used more often is because the nitrogen gas is not very soluble in liquid steel. It tends to bubble out when attempting to add it to steel. So with conventional steel production the nitrogen is often limited to the 0.08-0.12% range. Maybe enough to make a difference but not enough to replace much of the carbon in a knife steel needing to be 60+ Rc. One approach to increasing the amount of nitrogen is to produce the steel under high pressure. In the late 1980s Vereinigte Schmiedewerke of Krupp-Kloeckner-Thyssen (VSG) built a pressurized electroslag remelting furnace (PESR) which was capable of having pressure up to 40 bar with a capacity of 20 tons [1]. In the early 1990s they used this technology was used by researchers at Rurh-Universitat Bochum and fa*g Bearing Co. VSG to develop high hardness bearing stainless steels [2][3].
Images of PESR from [4]
The 40 bar pressure limited them to about 0.4% nitrogen, which is not enough to reach hardness of 58 Rc and above. So instead they experimented with different combinations of carbon and nitrogen to achieve the desired hardness levels. These were produced in a 100kg laboratory furnace which was also capable of 40 bar pressure for increased nitrogen. They used a basis of 15% Cr and 1% Mo, using 1.4116/X50Cr15Mo as their base steel to replace carbon with nitrogen.
Through these test heats they found that more carbon was better for higher hardness, with the “X30” composition reaching similar hardness levels to the X50Cr15Mo. This resulted in a similar composition to what would be named Cronidur 30. However, they also tested a composition with increased vanadium. The vanadium addition was to help with hardness when tempered in the high temperature regime (secondary hardening). This grade does not seem to have been pursued, however.
Corrosion testing found greatly improved corrosion resistance when compared with X50Cr15Mo and 440C due to the replacement of carbon with nitrogen, putting nitrogen in solution and also leaving more chromium in solution as opposed to forming chromium carbides. This is shown in a current density-potential curve in sulfuric acid below. The “X30” steel had a significant drop in passive current density, and an increase in breakthrough potential, which demonstrates an improvement in corrosion resistance.
They also found that the reduction in chromium carbide vs other martensitic stainless steels like X50Cr15Mo and 440C led to improved toughness because the carbide size was greatly reduced and the overall volume of carbide/nitride is lower than in those martensitic stainless steels.
Image from [5]
Names of the Steel and Final Composition
So the “X30” grade was released as Cronidur 30, though it is now being sold under several other names such as Zapp LC 200 N, Alpha Knife Supply Z-Finit, and Bohler N360. Knife enthusiasts now may know it better by the name LC200N as sold in knives such as by Spyderco. My supposition is that all of these different versions are ultimately being produced by VSG with the same large PESR facility that they developed. I haven’t been able to confirm this but it seems likely. Perhaps the different companies are buying the ingots and then forging and hot rolling it for sale.
Heat Treating of Cronidur 30/LC200N/Z-Finit/N360
Below is the tempering chart from the Zapp LC200N datasheet [6]:
Those hardness values are roughly equal to those that we have found in our experiments of the steel. These were heat treated either by myself (CATRA) or Warren Krywko (toughness coupons). A summary of the heat treatments we have done are shown in the table below:
It seems that the hardness of the steel tops out around 60 Rc. It may be possible to optimize the heat treatment parameters and achieve 61 Rc or so but using the relatively safe 1900°F austenitize leads to about 59-60 Rc.
Microstructure
Below are micrographs of the steel taken by me, and for comparison I have other low carbide/nitride steels AEB-L and 14C28N, and a large carbide steel 440C. The structure of LC200N is very fine as was reported in the articles about the steel. We would expect this to lead to excellent toughness of the steel.
LC200N (1905°F austenitize)
AEB-L (1975°F)
14C28N (1950°F)
440C (1900°F)
Toughness
As was previewed in the microstructure section, the toughness of Z-Finit/LC200N/Cronidur 30 is excellent, similar to AEB-L and 14C28N. The fine carbide/nitride structure and the low volume of those particles gives it very high toughness.
Edge Retention
The edge retention of LC200N/Cronidur 30/Z-Finit/N360 is pretty good given its relatively low volume of carbide/nitride. When compensating for hardness it did better than AEB-L and Nitro-V, and matches 14C28N. 14C28N can be heat treated to higher hardness, however, giving it potential for better edge retention. These steels are lower in edge retention than higher carbide steels and vanadium-containing steels like 440C, 154CM, S30V, etc. The toughness-edge retention balance of LC200N is similar to those other low carbide grades like AEB-L and 14C28N, but with very excellent corrosion resistance, as will be shown below.
Corrosion Resistance
I have tested LC200N with my standard 1% saltwater test and also the more difficult 3.5% saltwater test, and in both cases I found no rust spots on the steel. This puts it at the top of the charts for corrosion resistance of knife steels, along with other high corrosion resistance grade Vanax. This is better corrosion resistance to AEB-L and 14C28N which I have been comparing it to above. In the images below 24 hours of saltwater spray is on the left and 48 hours on the right, for the 1% saltwater test.
LC200N
14C28N
AEB-L
Conclusions and Summary
Cronidur 30, also sold as LC200N, Z-Finit, N360, and others, has an interesting history. It was developed as a bearing steel using a specialized production process (pressurized electroslag remelting), to increase the nitrogen content of the steel through high pressure. The partial replacement of carbon with nitrogen gives the steel improved corrosion resistance and toughness when compared with the carbon version, X50Cr15Mo. LC200N has a max hardness of about 60 Rc, which is probably its biggest limitation. Its edge retention and wear resistance is similar to other low carbide steels like AEB-L and 14C28N. The steel has excellent corrosion resistance and toughness as well. This gives LC200N a very good combination of properties. If higher hardness is desired at the cost of some corrosion resistance, 14C28N is a good alternative. If you want another high corrosion resistance grade but with higher edge retention, Vanax is also available.
[1] Pant, Paul, Peter Dahlmann, Wolfgang Schlump, and Gerald Stein. “A new nitrogen alloying technique‐a way to distinctly improve the properties of austenitic steel.”Steel research58, no. 1 (1987): 18-25.
[2] Chin, H. A., R. W. Bursey, D. D. Ehlert, R. Biroscak, E. Streit, and W. Trojahn. “Cronidur 30‐An Advanced Nitrogen Alloyed Stainless Steel For Advanced Corrosion Resistant Fracture Tough Cryogenic Bearings.” InAdvanced Earth-to-orbit Propulsion Technology 1994: Proceedings of a Conference Held at NASA George C. Marshall Space Flight Center, Marshall Space Flight Center, May 17-19, 1994, vol. 2, p. 321. National Aeronautics and Space Administration, Marshall Space Flight Center, 1994.
[3] Berns, Hans, and Werner Trojahn. “High-nitrogen Cr-Mo steels for corrosion resistant bearings.” InCreative Use of Bearing Steels. ASTM International, 1993.
[4] https://www.totalmateria.com/page.aspx?ID=CheckArticle&site=kts&NM=461
[5] https://www.progressivealloy.com/pdf/cronidur30.pdf
[6] https://www.zapp.com/fileadmin/_documents/Downloads/materials/powder_metallurgic_tooling_steel/special_material/en/LC200N_Datasheet.pdf
