Steel pipe heat treatment process-Annealing

Seamless steel pipe heat treatment process-Annealing

Annealing is a metal heat treatment process that slowly heats the metal to a certain temperature, keeps it for a sufficient time, and then cools it at an appropriate speed. Annealing heat treatment is divided into complete annealing, incomplete annealing and stress relief annealing. The mechanical properties of annealed materials can be tested by tensile test or hardness test. Many steels are supplied in annealed heat treatment state. The hardness of steel can be tested by Rockwell hardness tester to test HRB hardness. For thinner steel plates, steel strips and thin-walled steel pipes, the surface Rockwell hardness tester can be used to test HRT hardness. .

The purpose of annealing is to:

① Improve or eliminate various structural defects and residual stresses caused by steel casting, forging, rolling and welding, and prevent deformation and cracking of the workpiece.

 ② Soften the workpiece for cutting.

 ③ Refine the grains and improve the structure to improve the mechanical properties of the workpiece.

 ④ Prepare the organization for the final heat treatment (quenching, tempering).

Commonly used annealing processes are:

 ① Completely annealed. It is used to refine the coarse superheated structure with poor mechanical properties after casting, forging and welding of medium and low carbon steel. Heat the workpiece to 30-50℃ above the temperature at which all ferrite is transformed into austenite, hold it for a period of time, and then slowly cool down with the furnace. During the cooling process, the austenite transforms again to make the steel structure finer. .

 ② Spheroidizing annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20-40°C above the temperature at which the steel begins to form austenite, and then slowly cooled after holding the temperature. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.

 ③ Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with higher nickel and chromium content for cutting. Generally, it is first cooled to the most unstable temperature of austenite at a relatively rapid rate, and after holding for a suitable time, the austenite is transformed into troostite or sorbite, and the hardness can be reduced.

④ Recrystallization annealing. It is used to eliminate the hardening phenomenon (increase in hardness and decrease in plasticity) of metal wire and sheet during cold drawing and cold rolling. The heating temperature is generally 50 to 150°C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal can be softened.

⑤ Graphitization annealing. It is used to make cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950°C, keep it for a certain period of time and then cool it appropriately to decompose the cementite to form flocculent graphite.

 ⑥ Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve its performance. The method is to heat the casting to the highest possible temperature under the premise of not melting, and keep it for a long time, and slowly cool down after the diffusion of various elements in the alloy tends to be evenly distributed.

 ⑦ Stress relief annealing. It is used to eliminate the internal stress of steel castings and welded parts. For steel products, the temperature at which austenite begins to form after heating is 100 to 200°C, and the internal stress can be eliminated by cooling in air after heat preservation.