Alloy steel possesses high strength, hardenability, fatigue strength, and good impact toughness. It is widely used for making various machine parts, automobile components, shafts, gears, connecting rods, and more. Hardening and tempering develop the optimum combination of hardness, strength, and toughness in engineering steel, thereby providing components with high mechanical properties. Hardening and tempering temperatures are crucial factors that affect the mechanical and metallurgical properties of 42Cr4Mo steel. In this research work, 42Cr4Mo alloy steel samples were subjected to hardening and tempering processes. The hardening temperatures were set at 830°C, 850°C, and 870°C, while the tempering temperatures were maintained at 590°C and 650°C. The test results show that hardening at 830°C and tempering at 590°C achieve high strength, which decreases as the temperature increases. Different hardening temperatures and constant tempering temperatures will be optimized to achieve the desired hardness, ultimate tensile strength, yield strength, impact resistance, and metallurgical properties. These parameters significantly contribute to determining the appropriate.