Causes of problems caused by improper heat treatment of seamless tubes

Improper heat treatment of seamless tubes (smls) is likely to cause a series of production problems, resulting in greatly reduced product quality and turned into waste products. Avoiding mistakes that are easy to make during heat treatment is saving money. What problems should we pay attention to and prevent during the heat treatment process? Let's take a look at the common problems in the heat treatment of seamless tubes:

①Unqualified structural performance of steel tube: three factors caused by improper heat treatment (T, t, cooling method).

Weihua structure: when the coarse grain A formed by the steel under high temperature heating conditions is cooled, it forms a flaky F and is distributed on the P. In particular, the normal temperature strength of steel is reduced and the brittleness is increased.

The milder W structure can be eliminated by normalizing at an appropriate temperature, while the heavier W structure can be eliminated by secondary normalizing, the first normalizing temperature is higher, and the second normalizing temperature is lower , refine the grain.

The F-C balance diagram is an important basis for formulating the heating temperature of the steel pipe heat treatment, and it is also the basis for studying the composition of the F-C alloy in equilibrium, the metallographic structure and properties, the temperature transition diagram of supercooling A (TTT diagram) and the supercooling A continuous cooling transition diagram (CCT diagram) 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 of the outer diameter of the steel tube often occurs in the quenching process, and the outer diameter of the steel tube increases due to the volume change (caused by the change of the structure). A sizing process is often added after the tempering process.
Changes in the ellipticity of steel tubes: 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, warm straightening process (about 550°C) should be adopted.

③Cracks on the surface of the steel tube: It is caused by too fast heating or cooling and excessive thermal stress.

In order 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 a suitable 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.

④Scratches or flaws on the surface of the steel pipe: the relative sliding between the steel pipe and the workpiece, tools and roller table.
⑤The steel pipe is oxidized, de-C, overheated or overburned.
⑥ Oxidation of steel pipe surface heat treated by protective gas: the heating furnace is not well sealed, air enters the furnace, and the composition of the furnace gas is unstable.

It is necessary to strengthen the quality control of each link of tube blank (steel tube) heating.