Ultrafine-grained high-interstitial martensitic stainless steel via thermochemical processing

Martensitic stainless steels containing a significant fraction of C substituted by N present interesting properties but are hard to obtain with conventional processes. In this work, a thermochemical process involving (i) high temperature gas nitriding leading to a N- and C-rich austenite, followed by (ii) deliberate decomposition of austenite and (iii) hardening of the material by re-austenitization was applied to a conventional, high carbon, martensitic steel. The evolution of the microstructure in the involved processing stages was investigated by combining conventional metallographic techniques with X-ray diffraction, electron-backscatter diffraction and transmission electron microscopy. The results show that an ultrafine-grained martensitic alloy containing 0.65 wt% C and 0.31 wt% N can be obtained by the sequence of heat treatment steps. • Thermochemical nitrogen alloying of martensitic stainless steel can provide grain refinement. • Deliberate decomposition of high interstitial austenite precedes fine austenite/martensite. • High- interstitial austenite decomposes into ferrite/nitride and ferrite/carbide regions. • Thermo-Calc overestimates the carbon solubility in CrN and understimates it in Cr 2 (C,N).