There are many varieties of steel pipes, and the steel grades (types) used are also different. The chemical composition of the same variety of steel pipes 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 standard, the heat treatment process of
seamless steel pipe mainly includes the following five categories.
(1) Quenching + high temperature tempering (Q+T, also known as 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 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, finally the steel pipe structure is transformed into a uniform structure. 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 the performance of the steel pipe.
(2) Normalizing (N, also known as normalization):
The steel pipe is heated to the normalizing temperature so that the internal structure of the steel pipe is completely transformed into austenite structure, and then the air is used as a medium for cooling. Different metal structures can be obtained after normalizing, such as pearlite, bainite, martensite or their mixed structures. Normalizing can not only refine grains, homogenize composition, and relieve stress, but also increase the hardness of steel pipes and improve their 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 austenite structure, then cooled in air, and then matched with a tempering process. The structure of the steel pipe is tempered ferrite + pearlite, or ferrite + bainite, or tempered bainite, or tempered martensite, or tempered sorbite. The fire heat treatment process system can stabilize the internal structure of the steel pipe and improve the plasticity and toughness of the steel pipe.
(4) Annealing:
A heat treatment process in which the steel pipe is heated to the annealing temperature and kept for a certain period of time, and then slowly cooled to a certain temperature with the furnace and then cooled. The steel pipe annealing process can be divided into: spheroidizing annealing, complete annealing, stress relief annealing, etc.
The main function of steel pipe annealing:
①Reduce the hardness of the steel pipe and improve its plasticity to facilitate subsequent cutting or cold deformation processing;
② Refine grains, eliminate structural defects, uniform internal structure and composition, improve the performance of steel pipes 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):
The steel pipe is heated to the solution temperature, so that the carbides and various alloying elements are fully and uniformly dissolved in the austenite, and then rapidly cooled, so that the carbon and alloying elements have no time to precipitate, and a single austenite structure is obtained.
The main function of solution treatment:
①The internal structure of the steel pipe is uniform, and the composition of the steel pipe is uniform;
② Eliminate hardening during processing to facilitate subsequent cold deformation processing;
③ Restore the corrosion resistance of stainless steel.
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