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 a n 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 a lloys containing less than 0.2 wt% nitrogen are metastable and undergo a transformation from austenite to martensite on deformation. Transmi ssion electron microscopy investigations suggest that, within the nitr ogen 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 w t% nitrogen) undergo deformation, epsilon-martensite (with the [(1) ov er bar 10](gamma) and [(1) over bar 2 (1) over bar 0](epsilon) zone ax es parallel) is observed at the intersection o,f stacking faults. With increasing strain, the presence of alpha'-martensite is observed in c onjunction with the epsilon-martensite, and only alpha'-martensite is observed at very high strains. Both the Nishiyama-Wasserman and Kurdju mov-Sachs orientation relationships are observed between austenite and alpha'-martensite. The transformation to martensite during deformatio n causes a significant variation in room-temperature mechanical prop,e rties, despite the overall narrow range in composition considered.