Improper heat treatment of
seamless steel pipes can easily lead to a series of production problems, resulting in greatly reduced product quality and turning into waste products. Avoiding mistakes that are prone to occur during heat treatment is cost savings. What problems should we pay attention to in the heat treatment process? Let's take a look at the common problems in the heat treatment of
seamless steel pipes:
①Unqualified structure and performance of steel pipe: three elements caused by improper heat treatment (T, t, cooling method)
Wei chemical structure: The coarse grain A formed by the steel under high-temperature heating conditions forms a flaky F and is distributed on the P when it is cooled. In particular, the room temperature strength of steel is reduced, and the brittleness is increased.
The W structure with a light degree can be eliminated by normalizing at an appropriate temperature, while the W structure with a heavier degree can be eliminated by secondary normalizing. The secondary normalizing temperature is higher, and the second normalizing temperature is lower. crystallized grains.
The F-C equilibrium diagram is an important basis for formulating the heating temperature of steel pipe heat treatment, and it is also the basis for studying the composition, metallographic structure, and properties of the F-C composite crystal in the equilibrium state. Diagram (CCT diagram), which is an important basis for formulating the cooling temperature of heat treatment
②The size of the steel pipe is unqualified: the outer diameter, ovality, and curvature are out of tolerance.
The change in the outer diameter of the steel pipe often occurs in the quenching process, and the outer diameter of the steel pipe increases due to the volume change (caused by the change of the structure). The sizing process is often added after the tempering process. Changes in the ovality of steel pipes: the ends of steel pipes are mainly large-diameter thin-walled pipes. Steel pipe bending: caused by uneven heating and cooling of steel pipes, it can be solved by straightening. For steel pipes with special requirements, a warm straightening process (about 550°C) should be used.
③Surface cracks of steel pipes: It is caused by too fast heating speed or cooling speed and excessive thermal stress. To reduce the heat treatment cracks of the steel pipe, on the one hand, the heating system and cooling system of the steel pipe should be formulated according to the steel type, and the appropriate quenching medium should be selected; on the other hand, the quenched steel pipe should be tempered or annealed as soon as possible to eliminate its stress.
④ Scratches or hard injuries on the surface of the steel pipe: the relative sliding between the steel pipe and the workpiece, tools, and roller table occurs.
⑤ The steel pipe is oxidized, de-C, overheated, or overburned. Caused by T↑ and t↑.
⑥ The surface of the steel pipe heat treated by protective gas is oxidized: the sealing of the heating furnace is not good, and the air enters the furnace. The furnace gas composition is unstable. It is necessary to strengthen the quality control of the heating of the billet (steel pipe).
