High-interstitial nano-crystalline martensitic stainless steels; a new synthesis route

Martensitic stainless steels containing significant fractions of both nitrogen and carbon, hence high-interstitial, are high-end material solutions for e.g. bearings. Unfortunately, the superior performance of these alloys is coupled with costly manufacturing processes that include casting under high pressure nitrogen environment conditions. We have developed a cost-effective alternative route to obtain this class of materials, not involving high-pressure casting. The solution consists of nitriding conventional carbon-rich martensitic stainless steels at high temperature in the solid state, followed by a multi-step post-nitriding heat treatment cycle. In the present contribution, the process is applied to synthesize a hard steel containing 12%Cr-0.6%C-0.3%N that has nano-sized martensitic matrix decorated with fine Cr-rich precipitates and tiny regions of retained austenite smaller than 20 nm in size. The main metallurgical processes involved in the synthesis of the material are elucidated applying advanced materials characterization techniques, including backscatter electron diffraction, X-ray diffraction and transmission electron microscopy. The data is presented in the framework of the physical metallurgy of steel.