Improper heat treatment of seamless steel pipes can easily trigger a series of production problems, leading to a significant reduction in product quality and rendering them scrap. Avoiding common mistakes during heat treatment is crucial for cost savings. What problems should we focus on preventing during heat treatment?
① Unqualified Steel Pipe Microstructure: Caused by improper heat treatment (T, t, cooling method) – Three key factors
- Weakened 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 the steel pipe, particularly reducing its room-temperature strength and increasing its brittleness.
- Mildly affected. Weakened microstructure can be eliminated by normalizing at an appropriate temperature. More severe weakened microstructure can be eliminated by secondary normalizing, with a higher secondary normalizing temperature followed by a lower secondary normalizing temperature to refine the grains.
- The F-C equilibrium diagram is a crucial basis for determining the heating temperature of 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) of supercooled A are important bases for determining the cooling temperature of heat treatment.
② Steel pipe dimensional defects: Outer diameter, ovality, and curvature exceeding tolerances.
- Changes in the outer diameter of steel pipes often occur during the quenching process, due to volume changes (caused by microstructural changes), resulting in an increase in the outer diameter. A sizing process is often added after the tempering process. Changes in the ovality of steel pipes occur mainly at the ends of steel pipes, especially in large-diameter thin-walled steel pipes.
- Steel pipe bending: caused by uneven heating and cooling of the steel pipe. This can be resolved by straightening. For steel pipes with special requirements, a warm straightening process (around 550℃) should be used.
③ Steel pipe surface cracks: caused by excessively rapid heating or cooling rates, resulting in excessive thermal stress. To mitigate heat treatment cracks in 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 steel pipes should be tempered or annealed as soon as possible to relieve stress.
④ Surface scratches or hard marks on the steel pipe: These occur due to relative sliding between the steel pipe and the workpiece, tools, or roller conveyor.
⑤ Oxidation, decarburization, overheating, or burning of the steel pipe: Caused by increased temperature (T) and temperature (t).
⑥ Surface oxidation of 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.
Post time: Dec-03-2025