The effect of nitrogen on martensite formation in a Cr-Mn-Ni stainless steel

The influence of nitrogen (0 to 0.27 wt%) on martensite formation in an experimental low-nickel stainless-steel alloy (Fe-17Cr-7Mn-4Ni) has bee:n investigated. The alloys containing 0.1 wt% or more nitrogen are fully austenitic at room temperature; those containing less nitrogen consist of a mixture of austenite, martensite and delta-ferrite. The alloys containing less than 0.2 wt% 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 addition of nitrogen causes an increase in stacking fault energy which in turn inhibits the nucleation of martensite. As the low-nitrogen alloys (less than 0.2 wt% nitrogen) undergo deformation, epsilon-martensite (with the [(1) over bar 10](gamma) and [(1) over bar 2 (1) over bar 0](epsilon) zone axes parallel) is observed at the intersection o,f stacking faults. With increasing strain, the presence of alpha'-martensite is observed in conjunction with the epsilon-martensite, and only alpha'-martensite is observed at very high strains. Both the Nishiyama-Wasserman and Kurdjumov-Sachs orientation relationships are observed between austenite and alpha'-martensite. The transformation to martensite during deformation causes a significant variation in room-temperature mechanical prop,erties, despite the overall narrow range in composition considered.