Phase changes of the AISI 430 ferritic stainless steels after high-temperature gas nitriding and tempering heat treatment

This study examined the phase changes, nitride precipitation, heat treatment, hardness variations and tempering effect of AISI 430 ferritic stainless steel after high-temperature gas nitriding (HTGN) at 1050 °C and 1100 °C in a nitrogen gas atmosphere. The strong affinity between nitrogen and Cr enabled the permeation of nitrogen into AISI 430 ferritic stainless steel. The surface layer of the steel was changed into martensite + ferrite and Cr2N at 1050 °C, while martensite plus a rectangular type retained austenite were observed at 1100 °C after HTGN. When the nitrogen-alloyed AISI 430 steel was tempered, the precipitation of (CrFe)N nitride and a secondary hardening effect resulted in an increase in hardness to a maximum value at 450 °C. Above 450 °C, nitride precipitation decreased the hardness considerably. The HTGN treatment improved the corrosion resistance, while the precipitation of high density of Cr2N deteriorated the corrosion resistance.