Strengthening mechanism and martensite transformation behavior in grain-refined low-Ni austenitic stainless steel

Low-Ni (less than 3 wt%) austenitic stainless steels with different C contents were designed and heat treated to utilize both the grain refinement and carbide strengthening, and their effects were systematically investigated to elucidate the mechanism of tensile property improvement. Grain refinement to 2-3 mu m with homogeneously distributed Cr23C6 carbides resulted in excellent combination of yield strength (YS) and tensile elongation (YS x Tensile elongation of nearly 30,000 MPa %) without yield point elongation. The strengthening due to grain refinement was complemented by precipitation hardening. Grain refinement inhibited deformation-induced martensite transformation (DIMT) to decrease the strain hardening rate, while fine Cr23C6 carbides uniformly distributed within austenite grains promoted DIMT and increased strain hardening rate by depleting Cr and C concentrations. Two competing effects of grain refinement and Cr23C6 carbide formation on DIMT resulted in the increase of strain hardening rate as the latter was more dominant on austenite stability. Strengthening mechanisms were clearly explained by quantifying the contributions from solid solution, grain refinement and precipitation hardening. Stacking fault energy and driving force for gamma -> alpha' varied with C contents and annealing treatment, indicating that the optimum combination of C addition and thermomechanical treatment can yield the high performance low-Ni austenitic stainless steels.