Abstract
The transformation behavior of retained austenite on subzero and tempering, and its effect on mechanical properties of a cold work tool steel containing 1%C, 0.8%Cr, and 1.3%W have been studied. About 15% retained austenite is present after oil-quenching, and transforms to bainite in the tempering parameter range of 6.8 to 7.8. Isothermal transformation curves of the retained austenite below 300°C coincide with those of primary austenite. The retained austenite reheated at 150°C and above on tempering is thermally stabilized, and is hard to transform to martensite by subzero cooling. The exsistence of the austenite greatly raises the twist to fracture and the energy expended in twisting a specimen to fracture in the static torsion test. It is known that the austenite is also mechanically stable. The decomposition of retained austenite on tempering lowers the twist and torsional energy to fracture, and raises the bending and torsional yield strength. Therefore, it is regarded that the retained austenite that is mechanically stable improves the torsional toughness, and renders the decrease in bending and torsional yield strength.
