Relative Dimensional Change Evaluation of Vacuum Heat-Treated JIS SKD61 Hot-Work Tool Steels

JIS SKD61 hot-work steel is usually used as precision mold material for die casting; hence, it demands a higher level of dimensional stability during the hardening process, especially for fairly large sections. This study investigates the microstructural evolution and measures the relative dimensional changes in various tempering states. The results show that the retained austenitic contents of all quenched and tempered SKD61 steel specimens were less than 2%. When the tempering temperature reached 500 °C, the retained austenitic content decreased from 1.35 to 0.45%. TEM investigations revealed that a large number of secondary carbides, molybdenum-rich M₂C and vanadium-rich MC carbides, precipitated near the dislocations when the tempering temperature reached 525 °C. A secondary hardening phenomenon and evident expansion phenomenon occurred as the tempering temperature exceeded 500 °C. These phenomena were mainly contributed by the precipitation of secondary carbides in hot-work steels. The reason is that only 0.9% of the retained austenite transformed into martensite as the tempering temperature reached 500 °C, allowing the hardness and dimensional change to be neglected.