What preparations are needed before welding P22 steel pipes?
Before welding, the groove and the oil, rust and moisture within 20mm on both sides must be thoroughly cleaned to avoid hydrogen-induced cracks. According to the wall thickness, a 30-35° V-shaped groove needs to be processed, and the blunt edge retains 1-2mm to prevent burn-through. The preheating temperature should be controlled between 200-300°C, and real-time monitoring should be performed using a temperature measuring pen or infrared instrument. Low-hydrogen welding rods such as AWS A5.28 ER90S-B3 or E9018-B3 should be used as welding materials. A welding procedure qualification (WPS) should also be formulated to determine parameters such as current, voltage and interlayer temperature.
Why should the interlayer temperature be controlled when welding P22 steel pipes?
Excessive interlayer temperature (over 300°C) will cause grain coarsening and reduce weld toughness; too low interlayer temperature may cause cold cracks. The ideal interlayer temperature should be maintained at 200-250°C, forming a gradient with the preheating temperature. After each weld is completed, the slag needs to be cleaned with a wire brush before continuing the weld. When welding thick-walled pipes, segmented skip welding can be used to reduce heat accumulation. Recording the temperature of each weld is the key evidence for quality traceability.
What is the role of post-weld heat treatment (PWHT) on P22 steel pipe welds?
PWHT can effectively eliminate welding residual stress and prevent stress corrosion cracking (SCC). Heating to 620-650°C for 1 hour/inch thickness, followed by slow cooling (≤200°C/h). This process can also reduce the hardness of the weld area to below 22 HRC, which meets ASME specifications. For sulfur-containing environments, PWHT can reduce the hydrogen content of the weld and improve corrosion resistance. Care should be taken to avoid temperatures exceeding the Ac1 line (about 750°C) to cause over-tempering of the material.
How to detect the quality of P22 steel pipe welds?
Visual inspection (VT) is the first step, and it is necessary to ensure that there are no surface defects such as pores and undercuts. Radiographic testing (RT) can detect internal incomplete fusion or slag inclusions, and is accepted according to ASME Section V. Ultrasonic testing (UT) is more sensitive to cracks, especially for thick-walled pipes. Hardness testing verifies the PWHT effect, and the weld HAZ hardness must be ≤240HB. Finally, a hydrostatic test is required, with a pressure of 1.5 times the design value and a pressure of 10 minutes.
What are the special requirements for welding P22 steel pipes to dissimilar steels (such as 304 stainless steel)?
309L or nickel-based welding materials (such as ENiCrFe-3) must be selected to balance the alloy composition differences. The preheating temperature is implemented according to the requirements of the high-alloy side (200°C for P22). Minimize the dilution rate during welding and use low-current multi-pass welding. Post-weld heat treatment should be cautious to avoid stainless steel sensitization (can be limited to below 600°C). Carbon migration needs to be monitored during high-temperature service, and an isolation layer should be added if necessary.
