Improper heat treatment of seamless steel pipes is likely to cause a series of production problems, resulting in greatly reduced product quality and turning 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 in the heat treatment process? Let's take a look at the common problems in the heat treatment of seamless steel pipes:
① Unqualified structure and performance of steel pipe: 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 secondary normalizing temperature is higher, and the secondary normalizing temperature is lower. crystal grains. The F-C equilibrium 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 the equilibrium state, the metallographic structure and properties, the temperature transition diagram of supercooled A (TTT diagram) and the continuous cooling transformation of supercooled A The map (CCT map) is an important basis for formulating the cooling temperature of heat treatment
② Unqualified steel pipe size: outer diameter, ovality, and curvature are out of tolerance. The change of 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). A 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 adopted.
③Cracks on the surface of the steel pipe: It is caused by too fast heating or cooling 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 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 hard 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. Caused by T↑ and t↑.
⑥ Oxidation of the steel pipe surface heat-treated by protective gas: the heating furnace is not well sealed, and air enters the furnace. Furnace gas composition is unstable. It is necessary to strengthen the quality control of each link of tube blank (steel tube) heating.