One Minute Mentor: Low-Temperature Surface Hardening of Stainless Steel
April 18, 2017
Source: ASM International
The graph shows a schematic time-temperature-transformation (TTT) diagram of austenite with a high nitrogen or carbon content. In order to prevent the development of chromium nitrides/carbides, a long-time low-temperature treatment could be applied. Alternatively, a high-temperature treatment (above 1050 °C, or 1920 °F) combined with fast cooling can be chosen.
The corrosion-resistant nature of stainless steels is due to the presence of a very stable passive layer, mainly formed by the alloying element chromium and oxygen from the environment. Unfortunately, many stainless steels are susceptible to galling, which often leads to expensive materials solutions in applications where stainless steel would otherwise have been the preferred material choice for its combination of strength and anticorrosion properties.
Surface engineering is common practice for improvement of wear performance. The improvement of hardness and wear properties relies on the development of a surface-adjacent case consisting of either a compound layer and an underlying diffusion layer, as for nitriding/nitrocarburizing; or a martensitic transformation, as for carburizing.
Stainless steel poses two challenges in this respect. First, the native passive layer is impenetrable by nitrogen and carbon atoms. Second, nitrogen and carbon react with chromium to form nitrides/carbides, thus withdrawing chromium from solid solution, with the result that corrosion protection is lost.
A remedy for dissolving nitrogen or carbon into stainless steel without the formation of chromium nitrides or carbides would be choosing a high temperature at which nitrides/carbides are no longer stable; or a low temperature at which the incubation time for nitrides/carbides is longer than the treatment time to achieve a sufficiently thick case.
Volume 4D, Heat Treating of Irons and Steels ⇒ Heat Treated High-Alloy Steels ⇒ Low-Temperature Surface Hardening of Stainless Steel
Materials Processing and Treatment | Heat Treating
Materials Processing and Treatment | Surface Engineering
Metals and Alloys | Carbon and Alloy Steels
Metals and Alloys | Stainless Steels