Prevention of Heat Treatment Defects of Seamless Pipes

Posted by zora li on December 8th, 2022

1. Definition: According to different uses, the metal material is heated to a suitable temperature and kept warm, and then cooled in different ways to change the metallographic structure of the metal material and obtain the required microstructural properties. This process is often referred to as heat treatment of metallic materials.

The heat treatment of metal materials is divided into:
①Overall heat treatment
②Surface heat treatment
③Chemical heat treatment
The heat treatment defects of steel pipes mainly include: unqualified structure and performance of seamless pipe, excessive size, surface cracks, scratches, severe oxidation, cooling, overheating or overburning, etc.

2. Steel pipe heat treatment and process characteristics:

①Heat treatment:

One. Heating: Heating below Ac1 is mainly to stabilize the steel structure and eliminate internal stress. Heating above Ac3 is mainly to austenitize the steel.

b. Insulation: the purpose is to make the heating temperature of the steel pipe uniform, so as to obtain a reasonable heating structure.

c. Cooling: The cooling process is the key process of steel pipe heat treatment, which determines the metallographic structure and mechanical properties of the steel pipe after cooling. Commonly used cooling methods are: furnace cooling, air cooling, oil cooling, water cooling, etc.
②Steel tube normalizing: refers to the process of heating the seamless steel tube to the temperature above austenitization, holding it for a period of time, and then cooling it slowly and uniformly in the air to obtain a stable structure

One. Main purpose: refine the grain of steel, homogenize the internal structure, change the state of residual stress, and improve the comprehensive performance of the steel pipe.

b. Main function: reduce or eliminate the banded structure and mixed crystals formed during the deformation of the steel pipe; eliminate the net carbides in the hypereutectoid steel to facilitate spheroidizing annealing. It is used as pretreatment before quenching of medium carbon steel and alloy structural steel to refine the grain, make the structure uniform, and reduce the defects produced in the quenching process of steel pipe; replace annealing for low carbon steel and low alloy steel to improve the cutting performance of steel pipe , can be used as the final heat treatment of ordinary steel pipes with low requirements.

③ Steel pipe annealing: heat the steel pipe to a certain temperature above or below the critical (Ac3 or Ac1). After holding for a period of time, the steel pipe is slowly cooled to obtain an approximately balanced microstructure process, which is divided into recrystallization annealing, complete annealing, isothermal annealing, spheroidizing annealing and stress-reducing annealing. Generally, high-C, low-alloy and alloy steel pipes need to be annealed to reduce hardness and strength, improve plasticity, eliminate internal stress and uneven structure, and refine crystal structure, so as to facilitate the mechanical processing and laying of steel pipes and lay the foundation for the final heat treatment of steel pipes. .

④ Steel pipe tempering: heat the steel to a certain temperature below Ac1, keep it warm for a period of time, and then cool the steel pipe in a certain way to obtain a more stable organizational process.

One. Main purpose: to improve the plasticity and toughness of the steel pipe, so that the steel pipe can obtain good comprehensive mechanical properties, reduce or eliminate the residual internal stress of the steel pipe during quenching, stabilize the size of the steel pipe, so that the structure of the steel pipe does not change during use.

b. Steel pipes are generally air-cooled after tempering: in order to prevent the steel pipes from generating internal stress again, they should be cooled slowly. For steel pipes that are brittle after high temperature tempering, rapid cooling, such as oil cooling, should be used after tempering.

c. Steel pipe tempering is generally divided into:
Low temperature tempering 150 ~ 250℃
Medium temperature tempering 350 ~ 500℃
High temperature tempering 500 ~ 650℃

⑤Steel tube quenching: the process of heating the steel tube to 30-50°C above Ac3, and after holding it for a period of time, the steel tube is quenched to obtain M and B structures.

One. M generally has high hardness and high brittleness, and is the product of A quenching.

b. After the steel pipe is quenched, besides M and B, there is still a small amount of A remaining, which is an unstable structure.

c. After the steel pipe is quenched, the internal thermal stress and structural stress can generally be eliminated and improved by tempering. Tempering (Q+T) can greatly improve the comprehensive performance of the steel.

⑥Solution treatment: It is essentially the quenching process of steel pipes, but the temperature of solution treatment is higher. Solution treatment