The performance of steel material mainly refers on mechanical properties, physical properties, and process performance. Heat treatment will bring different metallurgical structure and corresponding performance for the steel pipe, so could be better applied in different industrial or the oil gas services.
There are two methods to improve the properties of steel material. One method is to adjust the chemical composition, named alloying method. The other method is heat treatment. In the field of modern industrial technology, heat treatment improve steel pipe performance at dominate position.
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 organization. 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):
After heating the steel pipe to the normalizing temperature, the internal structure of the steel pipe is completely transformed into austenite structure, and then the heat treatment process using air as the medium for cooling. Different metal structures can be obtained after normalizing, such as pearlite, bainite, martensite or their mixed structure. Normalizing can not only refine the grains, uniform the composition, and eliminate the 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 combined with the tempering process. The steel pipe structure 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 organization 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 functions of steel pipe annealing: ①Reduce the hardness of the steel pipe and increase its plasticity to facilitate subsequent cutting or cold deformation processing; ②Refine the grains, eliminate structural defects, uniform the internal structure and composition, and improve the performance of the steel pipe or for the follow-up Preparation for the process; ③eliminate 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 the heat treatment process of obtaining a single austenite structure. The main functions of the solution treatment: ①Even the internal structure of the steel pipe and the composition of the steel pipe; ②Eliminate hardening during processing to facilitate subsequent cold deformation processing; ③Restore the corrosion resistance of stainless steel.