Regarding commonly used heat treatment processes for seamless steel pipes in industrial projects

① Heat treatment of seamless steel pipes:
a. Heating: Heating below Ac1 mainly stabilizes the steel's microstructure and eliminates internal stress. Heating above Ac3 is mainly for austenitizing the steel.
b. Holding: The purpose is to uniformize the heating temperature of the seamless steel pipe to obtain a suitable microstructure.
c. Cooling: The cooling process is a key step in the heat treatment of seamless steel pipes, determining the metallographic structure and mechanical properties after cooling. Commonly used cooling methods for seamless steel pipes include: furnace cooling, air cooling, oil cooling, and water cooling.

② Normalizing of seamless steel pipes: This refers to heating the seamless steel pipe to a temperature above austenitizing temperature, holding it at that temperature for a period of time, and then slowly and uniformly cooling it in air to obtain a stable microstructure. The main purpose is to refine the steel grain, homogenize the internal microstructure, change the residual stress state, and improve the overall performance of the seamless steel pipe. Main functions: To reduce or eliminate banded structures and impurities formed during the deformation of seamless steel pipes; to eliminate network carbides in hypereutectoid steel to facilitate spheroidizing annealing; to be used as a pretreatment before quenching of medium-carbon steel and alloy structural steel to refine grains, make the microstructure uniform, and reduce defects in seamless steel pipes generated during the quenching process. It can also be used to replace annealing for dry-carbon steel and low-alloy steel to improve the pre-cutting properties of seamless steel pipes, and as the final heat treatment for general-purpose seamless steel pipes with less stringent requirements.

③ Seamless steel pipe annealing: This process involves heating the seamless steel pipe to a temperature above or below the critical point (Ac3 or Ac1), holding it at that temperature for a period of time, and then slowly cooling it to obtain a near-equilibrium microstructure. It includes re-annealing, full annealing, isothermal degradation, spheroidizing annealing, and stress-relief annealing. Generally, high-carbon, low-alloy, and alloy steel pipes require annealing to reduce their hardness and strength, improve plasticity, eliminate internal stress and microstructure inhomogeneity, and refine the microstructure. This facilitates machining and lays the microstructural foundation for the final heat treatment of seamless steel pipes.

④ Tempering of seamless steel pipes: This process involves heating the steel to a temperature below Ac, holding it at that temperature for a period of time, and then cooling it in a specific manner to obtain a more stable microstructure.
Main purpose: To improve the plasticity and toughness of seamless steel pipes, enabling them to achieve good comprehensive mechanical properties, reducing or eliminating residual internal stress generated during quenching, stabilizing pipe dimensions, and ensuring that the microstructure remains unchanged during use. After tempering, seamless steel pipes are generally air-cooled. To prevent the re-generation of internal stress, slow cooling is recommended. For seamless steel pipes exhibiting high-temperature tempering brittleness, rapid cooling, such as oil cooling, should be used after tempering. Tempering of seamless steel pipes is generally divided into: low-temperature tempering (150~250℃), medium-temperature tempering (350~500℃), and high-temperature tempering (500~650℃).

⑤ Quenching of seamless steel pipes: This process involves heating the seamless steel pipe to 30~50℃ above Ac3, holding it at that temperature for a period of time, and then rapidly cooling it to obtain microstructures (M and B). M generally has high hardness and brittleness and is a product of the rapid cooling of A. The microstructure of the quenched seamless steel pipe, besides M and B, also contains a small amount of A, both of which are unstable structures. After quenching, the seamless steel pipe will generate internal thermal and structural stresses, which can generally be eliminated and improved through tempering. Quenching and tempering (Q+T) can significantly improve the comprehensive properties of the steel.

⑥ Solution treatment of seamless steel pipes: This is essentially the quenching process of seamless steel pipes, only the solution treatment temperature is higher. Solution treatment is mainly used for A-grade stainless steel pipes. Its purpose is to reduce the hardness and strength of seamless steel pipes, improve their plasticity and toughness, and enhance the corrosion resistance and overall performance of finished seamless steel pipes to meet standard requirements or user requirements.