What is the difference between annealing, normalizing, quenching and tempering?
1. Annealing
The steel is heated to a certain temperature and kept at this temperature, and then slowly cooled to room temperature.
Annealing includes complete annealing (a), spheroidizing annealing (b), and stress relief annealing (c).
a, Heating the steel to a predetermined temperature, holding it for a period of time, and then cooling it slowly with the furnace is called complete annealing. The purpose is to reduce the hardness of the steel and eliminate the uneven structure and internal stress in the steel.
b. Heat the steel to 750 degrees, keep it warm for a period of time, slowly cool it to 500 degrees, and finally cool it in air, which is called spheroidizing annealing. The purpose is to reduce the hardness of steel and improve cutting performance, mainly for high carbon steel.
c. Stress-relief annealing is also called low-temperature annealing. Heat the steel to 500-600 degrees, hold it for a period of time, and slowly cool it to below 300 degrees with the furnace, and then cool it at room temperature. During the annealing process, the structure does not change, and the internal stress of the metal is mainly eliminated.
2. Normalizing
The heat treatment process of heating the steel piece to 30-50°C above the critical temperature, holding it for an appropriate time, and then cooling it in still air is called normalizing.
The main purpose of normalizing is to refine the structure, improve the performance of steel, and obtain a structure close to the equilibrium state.
Compared with the annealing process, the main difference between normalizing and annealing is that the cooling rate of normalizing is slightly faster, so the production cycle of normalizing heat treatment is short. Therefore, when annealing and normalizing can also meet the performance requirements of parts, normalizing should be selected as much as possible.
3. Quenching
Heat the steel to a certain temperature above the critical point (the quenching temperature of No. 45 steel is 840-860°C, and the quenching temperature of carbon tool steel is 760-780°C), keep it for a certain period of time, and then put it in water (oil) at an appropriate speed. ) The heat treatment process of cooling to obtain martensite or bainite structure is called quenching.
The main difference between quenching, annealing and normalizing is that the cooling rate is fast, and the purpose is to obtain martensitic structure. Martensite structure is an unbalanced structure obtained after quenching of steel. It has high hardness, but poor plasticity and toughness. The hardness of martensite increases with the carbon content of steel.
4. Tempering
After the steel is hardened, it is heated to a certain temperature below the critical temperature, held for a certain period of time, and then cooled to room temperature. The heat treatment process is called tempering.
Quenched steel parts generally cannot be used directly, and must be tempered before use. Because of the high hardness and high brittleness of quenched steel, brittle fracture often occurs when it is used directly. Tempering can eliminate or reduce internal stress, reduce brittleness, and improve toughness; on the other hand, it can adjust the mechanical properties of quenched steel to achieve the performance of steel. According to the tempering temperature, tempering can be divided into three types: low temperature tempering (a), medium temperature tempering (b) and high temperature tempering (c).
a, low temperature tempering 150~250. Reduce internal stress, brittleness, maintain high hardness and wear resistance after quenching.
b, Medium temperature tempering 350~500; improve elasticity and strength.
c, high temperature tempering 500~650°C; tempering of quenched steel parts above 500°C is called high temperature tempering. After quenched steel parts are tempered at high temperature, they have good comprehensive mechanical properties (both certain strength and hardness, and certain plasticity and toughness). Therefore, generally medium carbon steel and medium carbon alloy steel are often treated with high temperature tempering after quenching. Shaft parts are most widely used.
Quenching + high temperature tempering is called quenching and tempering treatment.
To sum up (edited by Huayang Steel Pipe alice):
1. Annealing is a heat treatment process in which the workpiece is slowly heated to a temperature above the critical point in a furnace, kept for a period of time, and then slowly cooled down with the furnace. (furnace cold)
2. Normalizing is to take the heated workpiece out of the furnace and place it in the air to cool it (air cooling)
3. Quenching is to heat the workpiece to the quenching temperature (30-50 degrees above the critical point), keep it warm for a period of time, and then put it into the quenching agent to cool
4. Tempering is the heating and cooling of parts at lower temperatures after quenching
