Why is preheating required before welding alloy steel pipes? How to determine the preheating temperature?
Preheating can slow down the cooling rate and avoid cold cracks (such as hydrogen-induced cracks). The temperature depends on the carbon equivalent (CE). When CE>0.4%, preheating of 100-250°C is required. For example, P91 steel (high Cr-Mo) requires preheating of 200-250°C. Use infrared thermometers or thermocouples to monitor, maintain the interlayer temperature during welding, and slow cooling after welding.
Which welding methods are suitable for high-alloy steel pipes (such as duplex stainless steel)?
TIG (tungsten inert gas shielded welding) or laser welding is recommended to reduce heat input and deformation. Duplex steel requires controlled heat input (10-25kJ/cm) to maintain austenite-ferrite balance. The welding wire selection needs to match the base material (such as ER2209), and 98%Ar+2%N₂ shielding gas is used to prevent nitrogen loss.
How to avoid stress corrosion cracking (SCC) after welding of alloy steel pipes?
Measures include: ① Select low-carbon welding materials (such as 316L); ② Post-weld solution treatment (for stainless steel); ③ Control the environment (such as reducing chloride ion concentration); ④ Use low-stress welding processes (such as narrow gap welding); ⑤ Shot peening or polishing to eliminate surface tensile stress. For example, chemical pipelines are prone to SCC in Cl⁻-containing media and must strictly follow NACE standards.
Why is post-weld heat treatment (PWHT) required after alloy steel pipe welding?
PWHT can eliminate residual stress, improve toughness, and reduce hardness. For example, P22 steel needs to be kept at 700°C for 2 hours after welding. For thick-walled pipes (>20mm), PWHT is mandatory to avoid delayed cracking. However, austenitic stainless steel is usually not subjected to PWHT to prevent carbide precipitation.
How to detect the quality of alloy steel pipe welds?
The inspection methods include: ① visual inspection (VT) to confirm surface defects; ② radiographic testing (RT) to detect internal pores; ③ ultrasonic testing (UT) to find lack of fusion; ④ hardness test (to ensure that the HAZ hardness is ≤350HV); ⑤ metallographic analysis to observe the microstructure. For example, the API 1104 standard requires that the welds be free of defects such as incomplete penetration and slag inclusions.
