Heat Treating Stainless Steels for Hardening and Corrosion Resistance

posted On Tuesday, May 10, 2022 in Blog

Many engineers ask themselves and others “How do I increase hardness of my stainless-steel component?” “What can heat treating stainless steel do for me?” Before we try to answer these questions, we should understand what stainless steel is, why is it called stainless steel?

Why Is It Called Stainless Steel?

Stainless steels (SS) are typically classified, according to their microstructure, into the following types: austenitic, ferritic, martensitic, duplex (ferritic–austenitic), and precipitation-hardenable (PH). The most common types are the austenitic, ferritic, and martensitic stainless steels and they have been known for 100 years [1, 2]. Austenitic stainless steels have also been classified as 300-series steels. Ferritic as well as martensitic SS are known as 400 series SS. These alloys contain 13-27% Cr and the purpose of the chromium is to provide a composition that will normally develop a passive surface. Many, but not all, stainless steels also contain 8-10% nickel, which is more noble than iron [3]. Precipitation hardenable stainless steels can be austenitic (A-286), martensitic (17-4PH, 15-5PH and others) and semi-austenitic (17-7PH, 15-7PH and others).

Heat Treating Stainless Steels for Hardness

General heat treating of stainless steels may involve quenching and tempering/aging applied to 400-series martensitic and age hardening SS. Treatment like that allows for hardening martensitic SS to the level of 60 HRc. The other SS cannot be hardened to such a high hardness level in conventional way.

In those situations, thermochemical surface engineering comes as a rescue way of solving such problems. In many engineering situations, SS components must have a good surface hardness to withstand tribological and bending fatigue stresses of the applications. Therefore, the primary treating method used is nitriding and in unique situations carburizing.

Heat Treating Stainless Steels with Nitriding

Nitriding is a process which can be carried out for austenitic SS in a very broad temperature range from 350 to 800°C (662-1225°F). Occasionally, even higher temperatures are used for solution nitriding. This method, in the range above 1000°C, is mainly used for hardening martensitic and ferritic stainless-steel grades. The effect of such a treatment for austenitic and duplex stainless-steel grades, however, is limited [11]. Also, like in any of the high temperature treatments, distortion of the treated components may be a problem because of the rapid temperature changes. Therefore, nitriding applied for the austenitic steels is the only effective and preferred treatment. Fig. 1 demonstrates approximate hardness profiles for 321 SS samples plasma nitrided at various temperatures.

Microhardness-depth profiles for 321 austenitic stainless steel nitrided at temperatures indicated.

Fig. 1. Microhardness-depth profiles for 321 austenitic stainless steel nitrided at temperatures indicated. Adopted from E. Rolinski [5].

It should also be noted that the non-magnetic properties of austenitic SS are altered by nitriding. Formation of the multiphase, including ferrite, structure of the nitrided layer changes ferromagnetic properties of such a steel. This problem can be minimized by nitriding at higher temperature, see Fig. 2.

Heat Treating Stainless Steel - Fig. 2 Effect of nitriding temperature on content of magnetic phases

Fig. 2 Effect of nitriding temperature on content of magnetic phases and surface hardness of austenitic stainless steel: initial state 0.0% magnetic phases, 286 HV5. Adopted from E. Rolinski [5].

Layers formed at high temperatures have very much the same tribological properties as the layers formed at lower temperatures, see Fig. 3 [5].

Heat Treating Stainless Steel with Nitriding - Fig. 3. Variation of linear wear friction time

Fig. 3. Variation of linear wear friction time with the applied pressure of 400 MPa for nitrided (at 585 and 785°C) and untreated 321 stainless steel. Adopted from E. Rolinski [5].

Nitriding Stainless Steels for Corrosion Resistance

It has been known that nitriding of austenitic stainless steels in the conventional temperature range of 450-600° C (840-1100°F) lowers their corrosion resistance, because of the multi-phase structure of the nitrided layer. In those situations, low temperature treatment, below 450°C (840°F) can be carried out for forming supersaturated solution with carbon, nitrogen or both, i.e. expanded austenite layer known also as S-phase or M-phase [7, 8]. This type of layer has very good corrosion resistance and high hardness. Low temperature surface hardening like that, either by nitriding, carburizing or nitrocarburizing, has been shown to improve the resistance of austenitic stainless steels significantly due to formation of this supersaturated solid solution, while maintaining or even improving their corrosion resistance [7-10]. Unfortunately, its thickness is limited to about 0.020 mm (0.0008”) but the layer has also a very good hardness, above 1000 HV [9, 10]. See Fig.4.

Fig. 4. Photomicrograph of 316L steel sample after ion/plasma nitrocarburizing

Fig. 4. Photomicrograph of 316L steel sample after ion/plasma nitrocarburizing at 420°C (788°F). Etched with Marbles.The white etched zone near the surface is the expanded austenite phase.

Heat Treating Duplex Steels with Nitrocarburizing

Duplex steels such as 2205 can have corrosion resistance as well as surface hardness enhanced by the same nitrocarburizing treatment. In this situation, not only austenite but also ferrite is converted into the expanded austenite layer, see Fig. 5.

Heat Treating Stainless Steel with Nitrocarburizing

Fig. 5. Photomicrograph of 2205 sample after ion/plasma nitrocarburizing at 420°C (788°F). Etched with Marbles. The white etched zone near the surface is the expanded austenite phase.

Heat Treating Precipitation Hardened Steels Such as 17-4 PH

Precipitation hardenable steels are very good candidates for nitriding and can be treated to a very high hardness exceeding 1000 HV, see Fig. 6.

Fig. 6. Hardness profile in the precipitation hardenable 17-4 PH sample after plasma nitriding at 52

Fig. 6. Hardness profile in the precipitation hardenable 17-4 PH sample after plasma nitriding at 521°C (970°F).

The temperature of nitriding for precipitation hardened steels must be lower by 25°C (45°F) than the aging temperature to avoid dimensional changes of the treated components and lowering their core hardness. The same rule applies to martensitic SS where tempering temperature is the factor determining nitriding temperature.

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"Heat Treating Stainless Steel for Hardening & Corrosion" References

1. J. Beddoes and J.G. Parr, “Introduction to Stainless Steels”, 3rd Edition, 1999. ISBN: 978-0-87170-673-7.

2. Angelo Fernando Padilha, Ronald Lesley Plaut and Paulo Rangel Rios, Chapter 12 “Stainless Steel Heat Treatment” in 2006 by Taylor & Francis Group, LLC.

3. Van Vlack, “Elements of Materials Science & Engineering”, IV Edition, 1980.

4. ASM Handbook, Volume 4A, 2013,” Steel Heat Treating Fundamentals and Processes”, Ed. Dossett and G.E. Totten, editors, pp. 619-646.

5. E. Rolinski, “Effect of plasma nitriding temperature on surface properties of stainless steel”, Surface Engineering, 3(1987) 35-40.

6. H. Berns, R. L. Juse, J. W. Bowman and B. Edenhofer, “Solution Nitriding of Stainless Steels-a New Thermochemical Heat Treatment Process” heat Treatment of Metals, 2000, 2, 39-45.

7. Zhang ZL, Bell T. Structure and corrosion resistance of plasma nitrided stainless steel. Surface Engineering. 1985;1(2):131-136.

8. K. Marchev, C. V. Cooper, J. T. Blucher, B. C. Giessen, “Condition for the formation of a martensitic single-phase compound layer in ion-nitrided 316L austenitic Stainless steel”, Surface and Coatings Technology, 1998, 225-228.

9. Sun Y. “Kinetics of low temperature plasma carburizing of austenitic stainless steels”. Journal of Materials Processing Technology, 2005;168:189-194.

10. K. V. Werner; H. L. Che, M.K. Lei, T. L. Christiansen, M. A. J. Somers, “Low Temperature Carburizing of Stainless Steels and the Development of Carbon Expanded Austenite”, HTM J. Heat Treatm. Mat. DE GRUYTER 77 (2022) 1, 3-15.

11. A. Bauer, K. Schreiner, “Dimensional Stability of Low Temperature Surface Hardened Stainless Steel Components”, HTM J. Heat Treatm. Mat. DE GRUYTER 77 (2022) 1, 16-28.