The high temperature tempering temperature range is generally 500-650 degrees Celsius. The process method of "quenching + high temperature tempering" is usually called quenching and tempering treatment. The tissues tempered at high temperatures are tempered soxonite and ferrite. The ferrite in tempered soxonite is a polygonal ferrite in which recrystallization occurs. After high temperature tempering, the steel has good comprehensive mechanical properties of strength, plasticity and toughness, with a hardness of 25-35HRC, and is widely used in various important structural parts with complex stresses made of carbon structural steel and low-alloy structural steel, such as engine crankshafts, connecting rods, connecting rod bolts, automotive half shafts, machine tool gears and spindles. It can also be used as a preparatory heat treatment for some precision workpieces (such as measuring tools, molds, etc.).
The change of hardness with tempering temperature after tempering of steel at different temperatures is shown in Figure 3-24, and the relationship between the mechanical properties of steel and the tempering temperature is shown in Figure 3-25.