On Monday, September 26, 2022
Alloy steels are generally more adaptable than carbon steels for general hardening by quenching and tempering or aging . Increases in hardness can also be achieved for corrosion resistant steels such as martensitic 410 SS if general heat treating is applied. After quench and temper treatment, martensitic stainless steels have not only significant corrosion resistance but also the needed strength and hardness. However, certain applications also require good tribological properties, but friction reduction at the surface cannot be achieved only by the general/conventional heat treatment. In situations like that, nitriding is the thermochemical process which is utilized very often. Stainless steels can be nitrided either by plasma/ion nitriding, salt bath or preactivated gas nitriding processes [1-3]. Ion nitriding is the preferred method, especially when treated components require only partial treatment and in situations like that, mechanical masking can easily be used, Fig. 1.
Plasma nitriding compressor cylinders. Note masking of the threaded holes with bolts. Picture courtesy of Advanced Heat Treat Corp. Monroe, Michigan.
Surface hardness of nitrided stainless steels exceeds hardness achieved by their general heat treating and it can be even more than 1000 HV (over 70 HRc equivalent). This allows for enhancing surface properties of many components, such as, for example, knife blades, Fig. 2.
Knife blade made of 410 SS after low temperature plasma nitriding. Mag. 100 X. Picture courtesy of Advanced Heat Treat Corp. Monroe, Michigan.
Nitriding of Martensitic Stainless Steels
Ion nitriding of 410 SS can be carried out at temperature as low as 400° C (752° F) and as high as 593° C (1100° F) . Kinetic at low temperature is slow, see Fig. 2 but properties of the layer produced this way are extraordinary.
Kinetics of ion nitriding for the martensitic 410 stainless steel. Based on the numerical data from Marchev .
Plasma nitriding of AISI martensitic stainless steel at 400° C (752° F) was extremally effective in improving friction and wear properties compared both to the baseline, quenched-and-tempered condition as well as to samples nitrided at higher temperatures . The effectiveness of this low- temperature nitriding treatment is in reducing the friction and wear. Such a good performance is thought to be due to the gradual nature of both the decrease of nitrogen content with depth and accordingly, the hardness increases due to this martensite formation . Friction was reduced significantly as can be seen from the graph showing change of coefficient of friction even as compared to the higher temperature treatment .
Coefficient of friction µ as a function of number of rotations of coupons of martensitic AISI 410 stainless steel plasma nitrided at 454° C (849° F) and 399° C (750° F) for 96 hours in nitrogen-rich atmosphere containing 65% N2+35%H2. Graph adopted from Marchev and others .
Regardless of nitriding temperature, hardness of the layer is very high for both martensitic and ferritic SS. Also, effect of prior heat treating on final hardness after nitriding is not very significant, see, Fig. 5. The hardness of nitrided layer stay very high, above 1000 HV.
Comparison between the microhardness profiles of plasma nitrided AISI 410 stainless steel coupons with and without preliminary heat treatment; plasma nitriding at 510° C (950° F) for 24 hours in gas mixture containing 65% N2+35% H2. Adopted from K. Marchev .
Also, preactivated gas nitriding can produce a very uniformed nitrided layers in 410 stainless steels with a high hardness as shown in Fig. 6.
Hardness profile of AISI 410 stainless steel sample gas nitrided at 538° C (1000° F) in a commercial load.
Plasma nitriding of martensitic stainless steel at temperature below 482° C (900° F) leads to formation a martensitic phase in the compound layer when higher temperatures produce a typical compound zone; ε and/or ɣ’ with precipitates of CrN and α . Properties of those layers are similar but not identical.
It should also be mentioned that austenitic stainless steels of 300 series cannot be hardened by the general/conventional techniques. Instead, nitriding is the process which is efficient here and has been used very effectively in a broad range of temperatures [2-4]. Nitrided layers formed this way on the surface are very hard and have very good tribological properties as well as excellent corrosion resistance if formed at low temperatures [3, 4].
- ASM Handbook, Vol. 4. Steel Heat Treating Fundamentals and Processes, 2013. 767 pages.
- K. Marchev, PhD Dissertation, “Processing, Characterization and Kinetics of Plasma Nitrided Stainless Steels”, Northeastern University, 1994.
- K. Marchev, C. V. Cooper, B. C. Giessen, “Observation of a compound layer with very low friction coefficient in ion-nitrided martensitic 410 stainless steel”, Surface and Coatings Technology, 99 (1998), 229-23
- E. Rolinski, “Effect of plasma nitriding temperature on surface properties of stainless steel”, Surface Engineering, 3(1987) 35-40.
Who is Doctor Glow?
Dr. Edward Rolinski, aka Doctor Glow, has been studying the plasma/ion nitriding phenomenon since the 1970s and is arguably one of the most knowledgeable people in North America when it comes to nitriding.
The doctor has written countless articles and whitepapers in industry publications and manuals. Some of his most noteworthy contributions include the chapter on "Controlling Plasma Nitriding" in ASTM International (2017) as well as "Nitriding of Titanium Alloys" in the ASM Handbook (2016).
Dr. Edward Rolinski is the Senior Scientist at Advanced Heat Treat Corp. (AHT) and has been employed at AHT since 1994.
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