A combined optimization of alloy composition and aging temperature in designing new UHS precipitation hardenable stainless steels

Alloy composition and proper heat treatment conditions are of paramount importance in maximising mechanical properties of precipitation hardenable stainless steels. Three base stainless steel alloys for ultimate strength levels employing carbides, Cu particles and/or Ni-rich precipitates are designed via a computational approach coupling a genetic algorithm with optimization criteria based on thermodynamic, kinetic and mechanical principles. The combined effects of 11 alloying elements (Al, C, Co, Cr, Cu, Mo, Nb, Ni, Si, Ti and V) are investigated on the basis of: a suitable martensite start (Ms) temperature, the suppression of undesirable phases, a minimal Cr concentration in the matrix and the potency of the precipitation strengthening contribution. The optimal aging temperature is derived from precipitation strengthening optimization and predicted values match experimental optima for existing alloy grades rather well. This thermodynamic justification of optimal precipitation temperatures in UHS steels has not been given before. For the optimized alloys considered the results of a sequential optimization of composition and precipitation temperature do not differ significantly from those of an integrated optimization.