Improper heat treatment of seamless steel pipes can easily lead to a series of production problems, resulting in a significant reduction in product quality and becoming waste. Avoiding mistakes that are easy to occur during heat treatment is to save costs. What problems should we pay attention to during heat treatment? Let’s take a look at the common problems in heat treatment of seamless steel pipes:
① Unqualified steel pipe structure and performance: three elements caused by improper heat treatment (T, t, cooling method)
Weihua structure: The coarse grain A formed by steel under high-temperature heating conditions forms a lamellar F and is distributed on P when cooling. It is an overheated structure, which has an adverse effect on the comprehensive performance of the steel pipe, especially reducing the room temperature strength of the steel and increasing its brittleness.
Lighter W structure: It can be eliminated by normalizing at an appropriate temperature, while heavier W structure can be eliminated by secondary normalizing. The secondary normalizing temperature is higher and the secondary normalizing temperature is lower to refine the grains.
F-C equilibrium diagram: an important basis for formulating the heating temperature of heat treatment of steel pipes, and also the basis for studying the composition, metallographic structure, and performance of F-C fusion crystals in an equilibrium state. The temperature transformation diagram of supercooled A (TTT diagram) and the continuous cooling transformation diagram of supercooled A (CCT diagram) are important bases for formulating the cooling temperature of heat treatment.
② Unqualified steel pipe size: outer diameter, ovality, and curvature are out of tolerance.
Changes in the outer diameter of steel pipes: often occur in the quenching process, and the outer diameter of the steel pipe increases due to volume changes (caused by organizational changes). 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 steel pipes.
Steel pipe bending: caused by uneven heating and cooling of steel pipes, which can be solved by straightening. For steel pipes with special requirements, a warm straightening process (about 550°C) should be used.
③ Surface cracks on steel pipes: caused by too fast heating or cooling speeds and excessive thermal stress.
In order to reduce the heat treatment cracks of steel pipes, on the one hand, the heating system and cooling system of steel pipes should be formulated according to the steel type, and the appropriate quenching medium should be selected; on the other hand, the quenched steel pipes should be tempered or annealed as soon as possible to eliminate their stress.
④ Scratches or hard scratches on the surface of steel pipes: caused by relative sliding between steel pipes and workpieces, tools, and rollers.
⑤ Oxidation, decarbonization, overheating, or overburning of steel pipes. Caused by T↑ and t↑.
⑥ Oxidation of the surface of steel pipes treated with protective gas: the heating furnace is not well sealed and air enters the furnace.
The furnace gas composition is unstable.
The quality control of each link of tube billet (steel pipe) heating must be strengthened.
Post time: Apr-18-2025