5 Common heat treatment processes for seamless steel pipes

There are many varieties of seamless 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.

The heat treatment process of seamless steel pipes refers to the purpose of improving the mechanical properties and organizational structure of steel by controlling the heating, insulation and cooling process of the steel pipe. According to the product standards, there are mainly 5 common heat treatment processes for seamless steel pipes (SMLS):

(1) Quenching + high temperature tempering (Q+T)

The seamless 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 troostite 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)

A heat treatment process in which the seamless steel pipe is heated to the normalizing temperature, the internal structure of the steel pipe is completely transformed into austenite, and then cooled in air. 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 seamless steel pipe is heated to the normalizing temperature, the internal structure of the steel pipe is completely transformed into austenite, and then cooled in air, and then tempered. The steel pipe structure is tempered ferrite + pearlite, or ferrite + bainite, or tempered bainite, or tempered martensite, or tempered troostite. The 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 seamless steel pipe is heated to the annealing temperature and kept at this temperature for a certain period of time, then 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, complete 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 structural defects, make the internal structure 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 seamless steel pipe to the solution temperature so that the carbides and various alloy elements are fully and evenly dissolved in the austenite, and then cool it quickly so that the carbon and alloy elements have no time to precipitate, and obtain a heat treatment process with a single austenite structure.

The main functions of solution treatment:
① Uniform internal structure and composition of steel pipe;
② Eliminate hardening during processing to facilitate subsequent cold deformation processing;
③ Restore the corrosion resistance of stainless steel.

The above are common and commonly used heat treatment processes for seamless steel pipes. According to specific needs and the properties of steel, appropriate heat treatment processes can be selected to obtain ideal mechanical properties and organizational structure.

Read more: Non-destructive testing of seamless pipes or Seamless Pipe vs ERW Pipe Cost