Q1: How to determine the preheating temperature when welding Q390 steel pipe?
The preheating temperature is determined according to the wall thickness: when the wall thickness is ≤20mm, the preheating temperature is ≥100℃; when the wall thickness is 20-40mm, the preheating temperature is 120-150℃; when the wall thickness is >40mm, the preheating temperature is ≥150℃. When the ambient temperature is lower than 5℃, the preheating temperature needs to be increased by an additional 20-30℃. The preheating range should cover an area of 3 times the plate thickness on both sides of the weld (minimum 100mm). Use electric heating plates or flames for uniform heating, and infrared thermometers to monitor the temperature. The interlayer temperature is controlled below 200-250℃ to avoid performance degradation in the overheating zone.
Q2: What welding materials are recommended for welding Q390 steel pipes?
CHE557 (-30℃ impact energy ≥47J) is the first choice for manual welding rods, and it is used after drying at 350℃×1h. For submerged arc welding, SJ101+S3 welding wire (containing Mo and Ni) is used to improve the strength of the weld. ER80S-G (80%Ar+20%CO₂) is recommended for gas shielded welding, and the heat input is controlled at 15-25kJ/cm. The water content of the flux must be ≤0.10%, and the sulfur content of the welding wire must be ≤0.008%. Important structural welding materials need to undergo process qualification tests to verify matching.
Q3: How to avoid cold cracks in Q390 steel pipe welding?
Strictly control the diffusible hydrogen content (glycerol method ≤5ml/100g) and use ultra-low hydrogen welding materials. Strictly implement preheating and interlayer temperature requirements to prevent rapid cooling from producing martensite. Post-heating (250-300℃×2h) is performed immediately after welding to promote hydrogen escape. Low-strength matching welding materials (such as E7015) are used for high-constraint joints to reduce residual stress. 100% magnetic particle testing (MT) is performed 48 hours after welding to eliminate delayed cracks.
Q4: Key points for welding Q390 with dissimilar steel (such as 304 stainless steel)?
Nickel-based welding materials (such as ENiCrMo-3) are used for the transition layer, and the thickness is ≥5mm to block carbon migration. The side groove angle of Q390 is increased to 75° to reduce the dilution rate of stainless steel. The heat input is controlled at 10-15kJ/cm to avoid embrittlement of the fusion zone. Slowly cool to room temperature after welding, and water quenching is prohibited. After RT detection, microhardness test (≤350HV10) is performed on the fusion line.
Q5: What are the special requirements for the welding process assessment of Q390 steel pipes?
The impact specimen must include three locations: weld, fusion line + 2mm heat-affected zone. The test temperature is 20℃ lower than the design temperature (if used at -20℃, -40℃ impact is performed). The bending test uses a side bending specimen (when t≤38mm), and the bending core diameter is 4t. The hardness test requires the weld to be ≤280HV10 and the heat-affected zone to be ≤350HV10. The process qualification needs to cover all welding positions (especially the 6G position).
