Annealing steps for straight seam steel pipes
Annealing of straight seam steel pipes is to heat the steel pipe to a temperature and keep it at this temperature and then slowly cool it to room temperature. Annealing includes annealing, spheroidizing annealing, and stress relief annealing.
1. Heating the steel pipe to a predetermined temperature, holding it for a period of time, and then slowly cooling 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. Heat the steel pipe to 750 degrees, keep it warm for a period of time, slowly cool it down to 500 degrees, and then cool it in the air called spheroidizing annealing. The purpose is to reduce the hardness and cutting performance of steel, mainly used for high carbon steel.
3. Stress-relieving steel pipe annealing is also called low-temperature annealing. The steel is heated to 500-600 degrees, kept for a period of 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 heat the steel pipe to 30-50°C above the critical temperature, and after holding it for an appropriate time, the heat treatment process of cooling in still air is called normalizing. The main purpose of normalizing is to refine the structure, the performance of steel, and obtain a structure close to the equilibrium state. The main difference between the normalizing and the annealing process 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. For quenching, heat the steel pipe to a 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℃), and then keep it in water at an appropriate speed ( The heat treatment process of cooling in oil to obtain martensite or bainite structure is called quenching. The main difference in the process of quenching, annealing, and normalizing is that the cooling rate is fast, and the purpose is to obtain a martensitic structure. The martensite structure is an unbalanced structure obtained after quenching of steel. Its hardness is high, but its plasticity and toughness are poor. The hardness of martensite increases as the carbon content of steel increases.
6. After the tempered steel pipe is hardened, it is heated to a certain temperature below the critical temperature, holding time, and then cooled to room temperature. The heat treatment process is called tempering. Generally, the quenched steel parts cannot be used directly, and can only be used after tempering. Because of the high hardness and brittleness of quenched steel, brittle fracture often occurs when used directly. 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 low-temperature tempering, medium-temperature tempering, and high-temperature tempering.
1) Low temperature tempering 150～250; reduce internal stress, brittleness, maintain high hardness and wear resistance after quenching.
2) Medium temperature tempering 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, hardened steel parts have comprehensive mechanical properties (both strength and hardness, but also plasticity and toughness). Therefore, generally medium carbon steel and medium carbon alloy steel often adopt high temperature tempering treatment after quenching. There are many applications of shaft parts. Quenching + high-temperature tempering is called quenching and tempering treatment.