Heat treatment technology of seamless steel pipe

There are many friends ofseamless steel pipe  annealing, normalizing, tempering, quenching meaning do not understand, if these concepts are confused, in the process of material production and use is easy to cause not late consequences. Therefore, it is particularly important to understand the meaning of seamless steel pipe heat treatment correctly. 

① Heat treatment:

a. Heating: The heating below Ac1 is mainly to stabilize the structure of the steel and eliminate the internal stress. The heating above Ac3 is mainly for the purpose of austenizing the steel.

b. Insulation: The purpose is to uniform the heating temperature of the steel pipe, in order to get a reasonable heating structure.

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

Commonly used cooling methods are: furnace cooling, air cooling, oil cooling, water cooling and so on.

② Steel pipe normalizing: refers to the steel pipe heating to austenitic temperature above, insulation for a period of time, and then in the air gently and evenly cooling, so as to obtain a stable organization of the process

Main purpose: refine the steel grain, uniform the internal structure, change the state of residual stress, improve the comprehensive performance of steel pipe.

Main functions: reduce or eliminate the steel tube formed in the deformation process of the banded structure and mixed crystal; The network carbide in eutectoid steel is eliminated to facilitate spheroidizing annealing. It is used for pretreatment of medium C steel and alloy structural steel before quenching, so as to refine the grain and make the microstructure uniform, and reduce the steel pipe defects caused by quenching process.

For low C steel and low alloy steel instead of annealing, in order to improve the cutting performance of steel pipe can be used as the final heat treatment of common structural steel pipe.

③ Steel pipe annealing: the steel pipe is heated to a temperature higher than or lower than the critical point (Ac3 or Ac1), to be held for a period of time, and then the slow cooling, in order to obtain the approximate equilibrium structure of the process, which is divided into recrystallization annealing, complete annealing, isothermal degradation, spheroidization annealing and stress removal annealing. Generally, high C, low alloy and alloy steel tubes need to be annealed to reduce their hardness and strength, improve plasticity, eliminate internal stress and uneven microstructure, refine crystalline microstructure, in order to facilitate the machining of steel tubes and lay the microstructure foundation for the final heat treatment of steel tubes.

④ Steel pipe tempering: the steel is heated to a temperature below Ac1, held for a period of time, and then the steel pipe is cooled in a certain way, in order to obtain a more stable state organization process.

Main purpose: to improve the ductility and toughness of the steel pipe, make the steel pipe obtain good comprehensive mechanical properties, reduce or eliminate the residual internal stress of the steel pipe during quenching and stabilize the size of the steel pipe, so that the structure of the steel pipe does not change in the use process.

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

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

Medium tempering 350 ~ 500℃

High tempering 500 ~ 650℃

⑤ Steel pipe quenching: the steel pipe is heated to 30 ~ 50℃ above Ac3, and after holding for a period of time, the steel pipe is cooled quickly and the process of M and B microstructure is obtained. M generally has high hardness and brittleness, is the product of A rapid cooling. In addition to M and B, there is A small amount of A residue in the quenched steel pipe microstructure, which are all unstable microstructure. After quenching, the internal thermal stress and microstructure stress of steel pipe will be generated, which can be eliminated and improved by tempering. Tempering (Q+T) can greatly improve the comprehensive performance of steel.

Solution treatment: the essence is the quenching process of steel pipe, but the temperature of solution treatment is higher. Solution treatment is mainly used for A stainless steel pipe, 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 provisions or user requirements.