Q1: What are the key points in the welding process for S355J2H steel pipe?
When welding S355J2H steel pipe, low-hydrogen electrodes (such as E7018) or submerged arc welding wire (such as S3Ni1) should be used. The preheat temperature is generally 100-150°C to reduce the risk of cold cracking. The welding current and voltage should be adjusted according to the wall thickness. Thin-walled pipes (<10 mm) can be welded using TIG welding to reduce heat input. Slow cooling is recommended after welding, and stress relief annealing at 550-600°C is recommended if necessary. Furthermore, a process qualification according to EN 1011-2 is required to ensure that the mechanical properties of the weld match those of the base material.
Q2: How can hydrogen-induced cracking (HIC) be avoided when welding S355J2H steel pipe?
Prevention of hydrogen-induced cracking (HIC) requires controlling three key factors: hydrogen content, restraint stress, and sensitive microstructures. Before welding, thoroughly remove oil and moisture from the groove and use a drying flux (350°C for 1 hour). Preheating can slow the cooling rate and prevent martensite formation. For thick-walled pipes, layered welding is required, with controlled interpass temperatures (≤250°C). Immediate post-weld heating (200-250°C for 2 hours) facilitates hydrogen diffusion and escape. Rigorous nondestructive testing (such as UT) can detect microcracks at an early stage.
Q3: Can S355J2H steel pipe be cold-bent? What precautions are required?
S355J2H steel pipe can be cold-bent, but the bend radius must be ≥ 2 times the pipe diameter to avoid cracking. Before bending, inspect the material for surface defects. Local preheating is recommended for thick-walled pipes (>15 mm). Residual stresses may be generated after cold bending and require vibration aging or heat treatment. For high-precision bending (such as for automotive chassis parts), mandrel or hydroforming processes are required. Mechanical properties must be retested after processing, especially impact toughness, which may be reduced due to deformation.
Q4: What are the characteristics of machining (turning, drilling) for S355J2H steel pipe? When machining S355J2H, it's recommended to use carbide tools (such as ISO P20-P30) at a cutting speed of 80-120 m/min. Due to its moderate hardness (≤170 HBW), chip breaking performance is excellent, but work hardening tendencies should be considered. Use emulsion cooling during drilling to prevent overheating and tool wear. For high-volume machining, optimizing the feed rate (0.1-0.3 mm/r) can balance efficiency and tool life.
Q5: How is thermal cutting (plasma or flame) performed for S355J2H steel pipe?
Flame cutting of S355J2H requires propane or natural gas, preheating to 200°C to minimize hardening in the heat-affected zone (HAZ). Plasma cutting is more suitable for thin-walled pipes (<20 mm), resulting in smoother cuts. Post-cutting edges require grinding to remove slag and microcracks. For critical components, magnetic particle testing (MT) is required on the cut surfaces. CNC cutting machines can improve accuracy and reduce subsequent processing.
