Tempering is a heat treatment process in which the quenched metal workpiece is heated to a temperature range below the critical temperature, kept warm for a certain period of time, and then cooled to room temperature. It can usually be divided into the following stages, each with its own unique mechanism:
Low-temperature tempering stage (150℃-250℃)
Organization transformation: martensite begins to decompose, and supersaturated carbon atoms precipitate in the form of extremely fine carbides to form tempered martensite.
Performance change: The hardness is slightly reduced at this stage, but it still maintains a high hardness and wear resistance. At the same time, the internal stress is partially eliminated and the toughness is improved. It is mainly used for workpieces such as tools, gauges, and molds made of high carbon steel and alloy steel that require high hardness and wear resistance.

Medium-temperature tempering stage (350℃-500℃)
Organization transformation: Retained austenite begins to decompose and transforms into lower bainite or tempered martensite. At the same time, martensite continues to decompose, carbides continue to precipitate and aggregate and grow.
Performance changes: The hardness of the workpiece decreases, the strength decreases slightly, but the toughness and plasticity increase significantly, and the internal stress is basically eliminated. This stage is suitable for processing various types of springs, which can make the springs obtain good elastic limit and yield strength, while having a certain toughness.

High temperature tempering stage (500℃-650℃)
Organization transformation: Carbides continue to aggregate and grow to form coarser granular carbides, and ferrite undergoes recrystallization, transforming from needle-shaped or lath-shaped to equiaxed. Finally, a tempered troostite structure is formed.
Performance changes: The strength and hardness of the workpiece are further reduced, and the toughness and plasticity are further improved, with good comprehensive mechanical properties. The heat treatment process of quenching and high temperature tempering is usually called quenching and tempering, which is widely used in various important mechanical parts, such as shafts, gears, connecting rods, etc.

Higher temperature tempering stage (＞650℃)
Organization transformation: Carbides further aggregate and grow, ferrite grains also grow significantly, and the organization gradually tends to a stable equilibrium state.
Performance changes: The strength and hardness of the workpiece continue to decrease, and the toughness and plasticity do not change much. At this time, the main purpose is to eliminate stress or obtain stable structure and size.