SUS 630 Steel hardness is 30.9 HRC and Chromium content is 12%, based on its use at PT. Pupuk Kalimantan Timur recommended a hardness of 55 HRC, to increase material hardness can be done by means of material engineering, namely Hardening. The aim of the study was to determine the effect of variations in holding time and cooling media on the hardness and microstructure of steel sus 630. The hardening process was carried out to reach 1020 0C then holding time with variations of 10, 20, 30, 40, and 50 minutes then dipped into water , salt water, and oil to room temperature. Microstructure was tested using an optical microscope with 400x magnification and hardness using Rockwell C hardness with a load of 150 kg. The observation of the microstructure after hardening of the structure formed is the martenstie lath, residual austenite, and metal carbide. The hardness test results were highest in water cooling media (34.2 HRC), salt water (34.2 HRC), and oil (34.1 HRC) and raw material (30.9 HRC). From this study it can be concluded that the hardening process can increase the value of violence.

Keywords : Hardening, Holding Time, Cooling, Hardness, microstructure