What are the commonly used heat treatment processes for seamless steel pipes

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

(1) Quenching + high temperature tempering (Q+T, also known as quenching and tempering): 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 rate greater than the critical quenching rate to transform the internal structure of the steel pipe into martensite, and then combined with high temperature tempering, the steel pipe structure is finally transformed into a uniform tempered bainite 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 the performance of the steel pipe.

(2) Normalizing (N, also known as normalization): A heat treatment process in which the steel pipe is heated to the normalizing temperature to completely transform the internal structure of the steel pipe into austenite, and then cooled with air as the medium. After normalizing, different metal structures can be obtained, such as pearlite, bainite, martensite, or their mixed structures. Normalizing can not only refine the grains, make the composition uniform, and eliminate stress, but also increase the hardness of the steel pipe and improve its cutting performance.

(3) Normalizing + tempering (N+T): The steel pipe is heated to the normalizing temperature, so that the internal structure of the steel pipe is completely transformed into an austenite structure, then cooled in air, and then tempered. The steel pipe structure is tempered ferrite + pearlite, ferrite + bainite, tempered bainite, tempered martensite, or tempered troostite. The heat treatment process 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 at this temperature for a certain period, it is slowly cooled to a certain temperature in the furnace and then taken out of the furnace for cooling. The steel pipe annealing process can be divided into spheroidizing annealing, full annealing, stress relief annealing, etc. The main functions of steel pipe annealing: ① Reduce the hardness of the steel pipe and improve its plasticity to facilitate subsequent cutting or cold deformation processing; ② Refine the grains, eliminate organizational defects, make the internal organization and composition uniform, improve the performance of the steel pipe or prepare for subsequent processes; ③ Eliminate the internal stress of the steel pipe to prevent deformation or cracking.

(5) Solution treatment (mainly used in the production of 18-8 type austenitic stainless steel): Heat the steel pipe to the solution temperature so that carbides and various alloy elements are fully and evenly dissolved in the austenite, and then cool it quickly so that carbon and alloy elements have no time to precipitate, and obtain a single austenite structure. The main functions of solution treatment: ① Uniform internal organization and composition of the steel pipe; ② Eliminate hardening during processing to facilitate subsequent cold deformation processing; ③ Restore the corrosion resistance of stainless steel.