The Effect of Nitrogen on Martensite Formation in a Cr-Mn-Ni Stainless Steel

The influence of nitrogen (0 to 0.27 wtYo) on martensite formation in an experimental low-nickel stainless-steel alloy (Fe-17Cr-7Mn-4Ni) has been investigated. The alloys containing 0.1 wtYo or more nitrogen are fully austenitic at room temperature; those containing less nitrogen consist of a mixture of austenite, martensite and S-ferrite. The alloys containing less than 0.2 wtYo nitrogen are metastable and undergo a transformation from austenite to martensite on deformation. Transmission electron microscopy investigations suggest that, within the nitrogen range considered in this investigation, the addttion of nitrogen causes an increase in stacking fault energy which in turn inhibits the nucleation of martensite. As the 12wnitrogen alloys (less than 0.2 wtYo nitrogen) undergo deformation, s-martensite (with the [110], and [1210]. zone axes parallel) is observed at the intersection of stacking faults. With increasing strain, the presence of a'martensite is observed in conjunction with the c-martensite, and only a'-martensite is observed at very high strains. Both the Nishiyama-Wasserman and Kurdjumov-Sachs orientation relationships are observed between austenite and a'-martensite. The transformation to martensite during deformation causes a signifcant variation in room-temperature mechanical properties, despite the overall narrow range in composition considered.