Heat treatment process of carbon steel pipe

Heat treatment process of carbon steel pipe

There are many varieties of seamless steel pipes, and the steel grades (varieties) used are also different. The chemical composition of steel pipes of the same type may also be different, but after heat treatment, the steel pipes can all meet the relevant technical standards or technical conditions of the purchaser. According to product standards, carbon steel pipe heat treatment processes mainly include the following five categories.

(1) Quenching + high temperature tempering (Q+T, also called quenching and tempering treatment)

The steel pipe is heated to the quenching temperature to transform the internal structure of the steel pipe into austenite, and then rapidly cooled at a speed greater than the critical quenching speed to transform the internal structure of the steel pipe into martensite, and then tempered at high temperature to finally transform the steel pipe structure. It is a uniformly tempered sorbite structure. Quenching + tempering can not only improve the strength and hardness of the steel pipe, but also organically combine the strength, plasticity and toughness of the steel pipe to meet the purchaser's requirements for steel pipe performance.

(2) Normalization (N, also called normalization)

After the steel pipe is heated to the normalizing temperature, the internal structure of the steel pipe is completely transformed into an austenite structure, and then the air is used as the medium for cooling. After normalizing, different metal structures can be obtained, such as pearlite, bainite, martensite or their mixed structures. Normalizing can not only refine the grains, uniformize the composition, and eliminate stress, but also increase the hardness of the steel pipe and improve its cutting performance.

(3)Normalizing + tempering (N+T)

After the steel pipe is heated to the normalizing temperature, the internal structure of the steel pipe is completely transformed into an austenite structure, and then cooled in the air, and then the tempering process is used. The structure of the steel pipe is tempered ferrite + pearlite, or ferrite + bainite, or tempered bainite, or tempered martensite, or tempered sorbite. The thermal treatment process system can stabilize the internal structure of the steel pipe and improve the plasticity and toughness of the steel pipe.

(4) Annealing

After the steel pipe is heated to the annealing temperature and kept warm for a certain period of time, it is slowly cooled to a certain temperature in the furnace, and then cooled out of the furnace. Steel pipe annealing processes can be divided into: spheroidizing annealing, complete annealing, stress relief annealing, etc. The main functions of steel pipe annealing:

① Reduce the hardness of the steel pipe and increase its plasticity to facilitate subsequent cutting or cold deformation processing;

② Refine the grains, eliminate structural defects, uniform the internal structure and composition, improve the performance of the steel pipe or prepare for subsequent processes;

③ Eliminate the internal stress of the steel pipe and prevent deformation or cracking.

(5) Solid solution treatment (mainly used in the production of 18-8 austenitic stainless steel)

The steel pipe is heated to the solid solution temperature so that carbides and various alloying elements are fully and evenly dissolved in the austenite, and then rapidly cooled so that carbon and alloying elements have no time to precipitate. Heat treatment is the process of obtaining a single austenite structure. The main functions of solid solution treatment:

① Even the internal structure of the steel pipe and the composition of the steel pipe;

② Eliminate the hardening phenomenon during processing to facilitate subsequent cold deformation processing;

③Restore the corrosion resistance of stainless steel.