STRENGTHENING AND TOUGHENING OF 280 kg/mm2 GRADE MARAGING STEEL THROUGH CONTROL OF MICROSTRUCTURE.

The effects of solution and aging treatments on microstructure, tensile properties, and fracture toughness were investigated on maraging steels of different composition. It was found that tensile ductility is strongly dependent upon prior gamma grain size. Unstable fracture under low stresses occurs due to the decrease in ductility above a critical grain size, which depends on the strength level and fracture toughness of the steel. On the other hand, fracture toughness is independent of prior gamma but strongly dependent on undissolved precipitates which can greatly reduce fracture toughness. Therefore, a simultaneous improvement of ductility and toughness would be obtained in the fine grained martensite structure without undissolved precipitates. Special treatments, including thermomechanical treatment, were developed to utilize these findings.