Destabilization of retained austenite during multiple tempering of high-speed W-Mo-V steel

Abstract

Constant demands for quality improvement of high-speed steels have contributed to the prolonged importance of tests, such as test in structural changes and properties derived from the applied heat treatment. Generally, tools from high-speed steel are treated by multiple tempering after quenching in order to reduce amount of retained austenite and value of internal stresses to the lowest level. A review of literature shows that processes taking place during the first tempering have been studied comprehensively and in great detail while very little attention has been paid to testing structural changes in the succeeding tempering cycles. Having this in mind, the differential dilatometric method has been used to test the influences of quenching temperature (1170-1230°C), tempering temperature (540-605°C), and the number (1 to 5) and tempering time, on the process of destabilization of retained austenite at circumstances of multiple continuous tempering of quenched W-Mo-V high-speed steel. The acquired results have contributed to the conviction that the customary regime of tempering for this type of high-speed steel could be rationally modified. We emphasize that the change in tendency of the dilatometric cooling curve in the austenitic-martensitic transformation region is the result of self-tempering of secondary martensite. The explanation proposed in literature for this dilatometric effect is from our point of view totally incorrect.