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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 rendering them scrap. Avoiding common mistakes during heat treatment is key to cost savings. What problems should we focus on preventing during heat treatment? Let's take a look at some common problems in the heat treatment of seamless steel pipes:
① Unqualified microstructure and properties of seamless steel pipes: Caused by improper heat treatment (T, t, cooling method). The three key factors are:
Wei-like microstructure: Coarse grains (A) formed in steel under high-temperature heating conditions transform into lamellar grains (F) distributed on the phosphorus (P) during cooling. This is an overheated microstructure that negatively impacts the overall performance of seamless steel pipes, particularly reducing room-temperature strength and increasing brittleness.
Mildly affected Wei-like microstructure can be eliminated by normalizing at an appropriate temperature, while more severe Wei-like microstructure can be eliminated by secondary normalizing. The secondary normalizing temperature is higher, and the second normalizing temperature is lower, refining the grains.
The F-C equilibrium diagram is a crucial basis for determining the heating temperature of seamless steel pipes during heat treatment. It is also fundamental for studying the composition, metallographic structure, and properties of F-C composite crystals in equilibrium. The temperature transition diagram (TTT diagram) and the continuous cooling transition diagram (CCT diagram) for supercooled A are important bases for determining the cooling temperature during heat treatment.
② Seamless steel pipe dimensional defects: Outer diameter, ovality, and curvature exceeding tolerances. Changes in the outer diameter of seamless steel pipes often occur during the quenching process, due to volume changes (caused by microstructural changes), increasing the outer diameter. A sizing process is often added after the tempering process. Changes in the ovality of seamless steel pipes: These occur at the ends of seamless steel pipes, mainly in large-diameter, thin-walled seamless steel pipes. Seamless steel pipe bending: Caused by uneven heating and cooling, this can be resolved by straightening. For seamless steel pipes with special requirements, a warm straightening process (around 550℃) should be used.
③ Surface cracks in seamless steel pipes: Caused by excessively rapid heating or cooling rates, resulting in excessive thermal stress. To mitigate heat treatment cracks in seamless steel pipes, on the one hand, the heating and cooling regimes should be tailored to the steel grade, and a suitable quenching medium should be selected; on the other hand, the quenched seamless steel pipes should be tempered or annealed as soon as possible to relieve stress.
④ Surface scratches or hard marks on seamless steel pipes: These occur due to relative sliding between the seamless steel pipe and the workpiece, tools, or roller conveyor.
⑤ Oxidation, decarburization, overheating, or burning of seamless steel pipes: Caused by increased temperature (T) and temperature (t).
⑥ Surface oxidation of seamless steel pipes under protective gas heat treatment: This is caused by poor furnace sealing, allowing air to enter the furnace, or unstable furnace gas composition. Quality control at each stage of billet (steel pipe) heating must be strengthened.
