There are many types of steel pipes, and the steel grades (species) used are not the same. 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 technical conditions of the purchaser. According to product standards, seamless steel pipe heat treatment processes mainly include 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 speed greater than the critical quenching speed to transform the internal structure of the steel pipe into martensite, and then with high temperature tempering, the steel pipe structure is finally transformed into a uniform Tempered sorbite structure. Quenching + tempering can not only increase 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):
After heating the steel pipe to the normalizing temperature, the internal structure of the steel pipe is completely transformed into austenite structure, and then it is a heat treatment process that uses air as a medium for cooling. After normalizing, different metal structures can be obtained, such as pearlite, bainite, martensite or their mixed structure. Normalizing can not only refine the grain, uniform the composition, 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 austenite structure, and then it is cooled in the air and then matched with the 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.
After the steel pipe is heated to the annealing temperature and kept for a certain period of time, it is slowly cooled to a certain temperature with the furnace and then discharged from the furnace for cooling. The steel pipe annealing process can be divided into: spheroidizing annealing, complete annealing, stress relief annealing and so on. The main function of steel tube annealing:
①Reduce the hardness of the steel pipe and increase 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 austenitic stainless steel
The steel pipe is heated to the solid solution temperature, so that the carbides and various alloying elements are fully and uniformly dissolved in the austenite, and then quickly cooled, so that the carbon and alloying elements are too late to precipitate and obtain a single austenite structure heat treatment process. The main function of solution treatment:
① Uniform the internal structure of the steel pipe and the composition of the steel pipe;
② Eliminate hardening during processing to facilitate subsequent cold deformation processing;
③Restore the corrosion resistance of stainless steel.
Tips: ASTM A53 covers seamless and welded steel pipe with nominal wall thickness. The surface condition is usually black and hot-dipped galvanized. ASTM A53 is produced mainly for pressure and mechanical applications, and is also used for transport of steam, water, gas line pipes.