Evaluation of Strain-Induced Martensite Formation and Mechanical Properties in N-Alloyed Austenitic Stainless Steels by In Situ Tensile Tests

Abstract In this study, the influence of various nitrogen contents (0.12-0.23 wt.%) on the mechanical properties, especially the strain-induced α´-martensite formation behavior, of the austenitic stainless steel X3CrMnNiMoN17-8-4 was investigated by temperature dependent in situ tensile tests. With the aid of in situ magnetic measurements during tensile test, the correlation between the strain-induced α´-martensite formation and the inflection points in the true stress-strain curve could be verified. In addition, a connection between the in situ measured α´-martensite formation rate and the strain hardening curve was established. As the temperature decreases, the formation of a large strain-induced α´-martensite fraction allows a strong increase in strength accompanied by a simultaneous decrease in elongation. The α´-martensite volume fraction increase and the triggering stress for the strain-induced martensite formation decreases with decreasing nitrogen content from to 0.23 wt.% to 0.12 wt.%. The deformation mechanisms taking place at various temperatures during tensile test were analyzed by microstructure analysis. As expected, a transition from Transformation Induced Plasticity to Twinning Induced Plasticity behavior was observed with increasing temperature. Compared to the other examined steels, the steel with 0.19 wt.% nitrogen has the highest tensile strength of 856 MPa accompanied by an excellent total elongation of 75% at RT.