Differences between Annealing, Normalizing, Tempering and Quenching of Seamless Steel Pipes

Many friends do not understand the meaning of annealing, normalizing, tempering and quenching of seamless steel pipes. If these concepts are confused, it is easy to cause irreversible consequences during the production and use of materials. Therefore, it is particularly important to correctly understand the various meanings of heat treatment of seamless steel pipes. Let's take a look at the heat treatment process of seamless steel pipes

a. Heating: Heating below Ac1 is mainly to stabilize the structure of steel and eliminate internal stress. Heating above Ac3 is mainly to austenitize steel.

b. Insulation: The purpose is to uniform the heating temperature of the steel pipe to obtain a reasonable heating structure.

c. Cooling: The cooling process is the key process of heat treatment of steel pipes, which determines the metallographic structure and mechanical properties of the steel pipe after cooling.

Common cooling methods include: furnace cooling, air cooling, oil cooling, water cooling, etc.

The heat treatment process of heating the steel piece to 30~50℃ above the critical temperature, keeping it warm for an appropriate time, and cooling it in still air is called normalizing. The main purpose of normalizing is to refine the structure, improve the performance of steel parts, 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 both annealing and normalizing can meet the performance requirements of parts, normalizing should be used as much as possible.

The steel pipe is heated to a temperature above or below the critical point (Ac3 or Ac1), and after a period of heat preservation, it is slowly cooled to obtain a process of approximate equilibrium state structure, which is divided into recrystallization annealing, complete annealing, isothermal degradation, spheroidizing annealing and stress relief annealing. Generally, high C, low alloy and alloy steel pipes need to be annealed to reduce their hardness and strength, improve plasticity, eliminate internal stress and uneven structure, and refine the crystal structure to facilitate the machining of steel pipes and lay the organizational foundation for the final heat treatment of steel pipes.

The heat treatment process of heating steel to a certain temperature above the critical point (the quenching temperature of #45 steel is 840-860℃, and the quenching temperature of carbon tool steel is 760-780℃), keeping it warm for a certain period of time, and then cooling it in water (oil) at an appropriate speed to obtain martensite or bainite structure is called quenching.

The main difference between quenching and annealing and normalizing in the process is the fast cooling speed, and the purpose is to obtain martensite structure. Martensite structure is an unbalanced structure obtained after steel quenching, with high hardness but poor plasticity and toughness. The hardness of martensite increases with the increase of carbon content in steel.

The process of heating steel to a certain temperature below Ac1, keeping it warm for a period of time, and then cooling the steel pipe in a certain way to obtain a relatively stable state of organization.

Main purpose: to improve the plasticity and toughness of the steel pipe, so that the steel pipe has good comprehensive mechanical properties, reduce or eliminate the residual internal stress generated by the steel pipe during quenching, and stabilize the size of the steel pipe, so that the structure of the steel pipe does not change during use.

After tempering, the steel pipe is generally cooled by air: to prevent the steel pipe from regenerating internal stress, it should be cooled slowly. For steel pipes with high-temperature tempering brittleness, rapid cooling, such as oil cooling, should be used after tempering.

Steel pipe tempering is generally divided into: low-temperature tempering 150-250℃

Medium-temperature tempering 350-500℃

High-temperature tempering 500-650℃

Finally, solid solution treatment is performed: it is essentially the quenching process of the steel pipe, but the temperature of the solid solution treatment is higher. Solid solution treatment is mainly used for A stainless steel pipes. Its purpose is to reduce the hardness and strength of the steel pipe, improve the plasticity and toughness of the steel pipe, and improve the corrosion resistance and comprehensive performance of the finished steel pipe to meet the standard requirements or user requirements.