STRENGTHENING MECHANISM OF SINTERED AND HEAT-TREATED COMPACTS MADE FROM PARTIALLY PREALLOYED STEEL POWDERS

Extensive study has been executed for an optimization of chemistry of partially prealloyed steel powders with Ni and Mo for the production of high strength and high toughness sintered components via double-pressing, double-sintering and heat treatment. The austenite precipitates at Ni-rich phases in the sintered and heat-treated compacts made from partially prealloyed steel powders with 0.6% graphite. The detailed analysis of stress-strain curve of sintered and heat-treated compact indicates that deformation is localized at untransformed austenite and strain induced martensite with very high strength is formed at the initial stage. The tensile strength increases with the amount of strain induced martensite. Condensed carbon areas are observed in the specimen with higher amount of austenite. The tensile strength depends also on the carbon concentration in martensite. The maximum tensile strength increases with the decrease of Ni content. This is attributed to the decrease of sintered density and the increase of untransformed austenite for plastic deformation. The sintered and heat-treated compacts made from the 2%Ni-1%Mo steel powder contain a sufficient amount of austenite which transform almost completely to martensite during loading. Therefore, this compact shows an ultra high strength of 1920MPa and a high unnotched Charpy absorbed energy of 53J.