The presence of retained austenite in the hardening process of tool steel often causes the lower hardness compared to the hardness requirements and poor dimensional stability in the tool steel. The purpose of the present research is to determine the relationships between the tempering process with and without cryogenic treatment to the hardness and retained austenite amount of as-hardened D2 tool steel. The austenitizing temperature was 1020 °C, the tempering temperatures have variations of 180 °C, 280 °C, 380 °C, 480 °C, and 580 °C, and the subzero treatment has a temperature of −172 °C, followed by tempering at 180 °C, 380 °C, and 580 °C. This study aims to determine the appropriate treatment to obtain a minimum retained austenite percentage to prevent and mitigate the failure of AISI D2 tool steel in the industrial application process. An optical microscope with image processing software (Image-J analysis), as well as Brinell and Vickers hardness testing, is the characterization method used in this work. In general, plate martensite, bainite, retained austenite, and primary and secondary carbides are the phases contained in the microstructure. Tempering temperatures have the effect of increasing the secondary carbide precipitation and decreasing the retained austenite content (γr 3,671%-2,769%). However, the cryogenic treatment can provide a more efficient martensitic phase transformation process and minimal retained austenite content (γr 2,257%-1,199%). The increase in tempering temperature causes a decrease in hardness at a temperature of 180 °C-380 °C. On the other hand, the secondary hardening and phase transformation phenomena cause an increase in the hardness of the as-tempered sample at a temperature of 480 °C, before the sample reexperiences a significant decrease in hardness at a temperature of 580 °C due to diffusion that decreases the carbon content.