SKD11 steel is a tool steel material. SKD11 steel must have strong and tough mechanical properties. So to improve the quality and ability of SKD11 steel to accept external loads and forces, it is necessary to heat treatment. The heat treatment carried out in this research is hardening and hardening – tempering. The hardening process is carried out by heating SKD11 steel at austenitic temperature of 950 °C and holding time of 30 minutes. Then fast cooling (quenching) with water cooling media. As

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... for the hardening – tempering process with temperature variations of 400 °C, 500 °C, 600 °C and a holding time of 60 minutes. In the as-quenching process the microstructure formed is martensite and residual austenite which cannot be transformed. Increasing the tempering temperature affects the martensite formed into tempered martensite consisting of ferrite and cementite carbide. In addition, with increasing tempering temperature the volume of carbide distribution will decrease and have a smoother shape. As the volume of carbide distribution decreases and the ferrite structure becomes more consistent, the ductility of the steel will increase and the hardness will decrease. The hardness value of SKD11 steel with hardening of 950 °C is 61.5 HRc. While the steel with hardening - tempering at a temperature of 400 °C = 57.3 HRc, 500 °C = 56.3 HRc, and 600 °C = 49,5 HRc. Increasing the tempering temperature will increase the ductility and decrease the hardenability on SKD11.