The role of tempering
① Improve the structural stability so that the workpiece will no longer undergo structural transformation during use, thereby maintaining stable geometric dimensions and performance of the workpiece.
② Eliminate internal stress to improve the performance of the workpiece and stabilize the geometric dimensions of the workpiece.
③ Adjust the mechanical properties of steel to meet usage requirements.
The reason why tempering has these effects is that when the temperature rises, the atomic mobility increases, and the atoms of iron, carbon and other alloying elements in the steel can diffuse faster, realizing the rearrangement and combination of atoms, thereby making the instability The unbalanced organization gradually transforms into a stable balanced organization. The elimination of internal stress is also related to the decrease in metal strength when the temperature increases. Generally, when steel is tempered, the hardness and strength decrease and the plasticity increases. The higher the tempering temperature, the greater the change in these mechanical properties. Some alloy steels with higher alloy element content will precipitate some fine metal compounds when tempered in a certain temperature range, which will increase the strength and hardness. This phenomenon is called secondary hardening.
Workpieces with different uses should be tempered at different temperatures to meet the requirements in use.
① Tools, bearings, carburized and quenched parts, and surface quenched parts are usually tempered at low temperatures below 250°C. After low-temperature tempering, the hardness does not change much, the internal stress is reduced, and the toughness is slightly improved.
② The spring can obtain higher elasticity and necessary toughness by tempering it at medium temperature at 350~500℃.
③ Parts made of medium carbon structural steel are usually tempered at high temperatures at 500 to 600°C to obtain a good combination of appropriate strength and toughness.
The heat treatment process of quenching and high temperature tempering is generally called quenching and tempering. When steel is tempered at around 300°C, its brittleness often increases. This phenomenon is called the first type of temper brittleness. Tempering should generally not be performed in this temperature range. Some medium carbon alloy structural steels tend to become brittle if they are slowly cooled to room temperature after being tempered at high temperatures. This phenomenon is called type II temper brittleness. Type II temper brittleness can be prevented by adding molybdenum to the steel, or by cooling it in oil or water during tempering. Steels with type II temper brittleness can be eliminated by reheating them to their original temper temperatures.