What is the main manufacturing process for 15CrMoG steel pipes?
Production includes steelmaking (electric furnace or converter), refining outside the furnace (LF/VD treatment), continuous casting or die casting into billets, hot rolling or cold rolling, and heat treatment (normalizing + tempering). During the refining process, sulfur and phosphorus content must be strictly controlled to improve purity. After hot rolling, ultrasonic testing is performed to ensure defect-free steel. Final heat treatment is key to ensuring microstructure uniformity. Domestically, this is primarily based on the GB 5310 standard.
Why do 15CrMoG steel pipes require normalizing + tempering?
Normalizing (900-930°C) refines the grain size and eliminates banding caused by rolling. Tempering (650-680°C) reduces residual stress and improves toughness and ductility. This process produces a uniform pearlite + ferrite structure in the steel pipe. Unheat-treated steel pipes tend to be hard and prone to cracking. This process offers the best balance between strength and toughness.
What are the differences in the performance impacts of cold rolling and hot rolling?
Hot rolling offers high production efficiency but lower surface roughness and dimensional accuracy. Cold rolling improves surface quality but requires an additional annealing step. Hot-rolled pipes have better low-temperature toughness, while cold-rolled pipes have slightly higher strength. Cold rolling may lead to residual stress concentrations and requires rigorous testing. Hot rolling is generally preferred for thick-walled, high-pressure pipes. Process selection should consider both cost and application.
How to control the weld quality of 15CrMoG steel pipes?
Preheat to 150-200°C before welding to prevent cold cracking. Use low-hydrogen electrodes (such as R307) and control the interpass temperature. Post-weld dehydrogenation treatment (250°C for 2 hours) is mandatory. Use X-ray or ultrasonic testing to inspect welds for internal defects. The hardness of the weld area after heat treatment must be ≤240 HB to ensure compatibility with the parent material.
What are the non-destructive testing methods used in production? Ultrasonic testing (UT) is commonly used to inspect internal cracks and inclusions; eddy current testing (ET) is used to screen for surface defects; and radiographic testing (RT) provides three-dimensional imaging analysis of welds. Hardness testing verifies the effectiveness of heat treatment. Hydrostatic testing is essential for final verification of sealing. This combination of methods ensures 100% qualified steel pipes leave the factory.
