The annealing of straight seam steel pipe is to heat the steel pipe to a temperature and keep it warm at this temperature and then slowly cool it to room temperature. There are several types of annealing, such as annealing, spheroidizing annealing, and stress relief annealing.
1. Heating the steel pipe to a predetermined temperature, holding it for some time, and then slowly cooling it with the furnace is called annealing. The purpose is to reduce the hardness of the steel and eliminate the uneven structure and internal stress in the steel.
2. Heating the steel pipe to 750 degrees, holding it for some time, slowly cooling it to 500 degrees, and then cooling it in the air is called spheroidizing annealing. The purpose is to reduce the hardness and cutting performance of steel, mainly used for high carbon steel.
3. Stress-removing steel pipe annealing is also called low-temperature annealing. The steel is heated to 500-600 degrees, kept for some time, slowly cooled to below 300 degrees with the furnace, and then cooled at room temperature. The structure does not change during the annealing process, and the internal stress of the metal is mainly eliminated.
4. Normalizing The steel pipe is heated to 30-50 ℃ above the critical temperature, and the heat treatment process of cooling in still air after holding for an appropriate time is called normalizing. The main purpose of normalizing is to refine the structure, and the properties of the steel, and to 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 used as much as possible.
5. Quenching Heat the steel pipe to a certain temperature above the critical point (the quenching temperature of No. 45 steel is 840-860℃, and the quenching temperature of carbon tool steel is 760-780℃), keep it for a long time, and then put it in water ( The heat treatment process of cooling in oil) to obtain martensite or bainite structure is called quenching. The main difference between quenching and annealing and normalizing in the process is that the cooling speed is fast, and the purpose is to obtain the martensite structure. A martensitic structure is an unbalanced structure obtained by steel after quenching. It has high hardness but poor plasticity and toughness. The hardness of martensite increases as the carbon content of the steel increases.
6. After the tempered steel pipe is hardened, it is heated to a temperature below the critical temperature, the heat treatment process is held for some time, and then cooled to room temperature, which is called tempering. Generally, the quenched steel parts cannot be used directly, but can only be used after tempering. Because of the high hardness and brittleness of hardened steel, brittle fracture often occurs when directly used. Tempering can eliminate or reduce internal stress, reduce brittleness, and improve toughness; on the other hand, the mechanical properties of quenched steel can be adjusted to achieve the performance of steel. According to the different tempering temperatures, tempering can be divided into three types: low-temperature tempering, medium-temperature tempering, and high-temperature tempering.
1) Low temperature tempering 150-250; reduce internal stress, and brittleness, and maintain high hardness and wear resistance after quenching.
2) Tempering at medium temperature 350-500; improve elasticity and strength.
3) High temperature tempering 500-650; tempering of quenched steel parts above 500 ℃ is called high-temperature tempering. After quenching at high temperatures, the quenched steel has comprehensive mechanical properties (both strength and hardness, and plasticity and toughness). Therefore, generally medium carbon steel and medium carbon alloy steel are often treated by high-temperature tempering after quenching. There are many applications of shaft parts. Quenching + high-temperature tempering is called quenching and tempering.