Q1: What are the best process parameters for welding Q235 steel pipe?
For manual arc welding (SMAW), it is recommended to use J422 electrode (E4303), with a current of 90~120A for diameter 3.2mm and 140~180A for diameter 4.0mm. Gas shielded welding (GMAW) uses ER50-6 welding wire, CO₂ gas flow rate 15~20L/min, and voltage 22~28V. The welding speed is controlled at 15~25cm/min, and the heat input is limited to 20kJ/cm to prevent overheating. Tungsten inert gas welding (GTAW) is recommended for thin-walled pipes (δ≤3mm) to reduce deformation. All welding must ensure that the interlayer temperature is ≤150℃ to avoid grain coarsening.
Q2: How to avoid deformation of Q235 steel pipe welding?
Use a symmetrical welding sequence, such as the segmented back-welding method for long welds (each segment is 200~300mm long). For large-diameter pipes, use internal support or clamps to fix them, and control the gap to 1~2mm. Preset the reverse deformation amount (about 1°~3°) before welding to compensate for shrinkage. Pulse welding can be used to reduce heat input for thin-walled pipes (δ≤5mm). Mechanical correction (such as jack cold correction) or local heating (below 600℃) can be used after welding to eliminate deformation, but over-correction should be avoided to cause work hardening.
Q3: Key points for dissimilar welding of Q235 steel pipes and stainless steel?
309L (E309L-16) or 309MoL welding materials are preferred, and their 23%Cr-13%Ni composition can dilute carbon migration. The contact surface needs to be thoroughly cleaned before welding, and the stainless steel side groove angle is increased by 5° to balance the fusion ratio. Use low heat input process (such as TIG welding) and control the layer temperature below 100℃. PT detection (penetrant flaw detection) is required after welding to check for surface cracks. The service temperature of dissimilar steel joints should be ≤350℃ to avoid thermal fatigue caused by differences in expansion coefficients.
Q4: What are the precautions for cold bending of Q235 steel pipes?
The bending radius should not be less than 3 times the pipe diameter (R≥3D), otherwise a mandrel should be used to prevent wrinkles. Annealing treatment (650℃ insulation for 1h) is required before cold bending to improve ductility. The pipe bending machine mold should be made of Cr12MoV steel with a hardness of HRC58~62 to ensure forming accuracy. After bending, the outer thinning rate (≤15% wall thickness) and ovality (≤5%D) need to be checked. For important structural parts, it is recommended to perform stress relief annealing at 600℃ after bending.
Q5: What is the effect of thread processing on the strength of Q235 steel pipes?
Rolled threads are better than cut threads, and their cold work hardening can increase the strength by 10%~15%. The thread depth should not exceed 1/3 of the wall thickness (such as 3mm wall thickness and 1mm thread depth). Taper pipe threads (such as GB/T7306) must ensure that the effective meshing length is ≥5 teeth. After processing, burrs must be removed and checked with a go/no-go gauge to avoid stress concentration. For high-pressure applications (＞1MPa), it is recommended to add a R0.3mm fillet transition at the root of the thread.
