Q1: What pretreatment is required before welding A335 steel pipe?
Before welding, the oil and rust within 25mm on the groove and both sides must be thoroughly cleaned to prevent hydrogen and impurities from mixing into the weld. For high alloy steels such as P91, it is necessary to preheat to 200~250°C to reduce the cooling rate and avoid martensitic hardening and cracking. The groove angle is usually designed to be 60°±5°, and the blunt edge is 1.5~2mm to ensure penetration. The welding rod needs to be dried at 350°C for 2 hours and taken out as needed. A welding procedure qualification (WPS) is also required to confirm that the parameters match the parent material grade.
Q2: Which welding rod is suitable for welding A335 P91 steel?
P91 steel recommends using E9015-B9 or E9018-B9 low-hydrogen welding rods (such as Bio-Rad FOX B9), whose composition matches the parent material (9Cr-1Mo-V-Nb). The electrode coating needs to be of basic type to control the diffusible hydrogen content to <5mL/100g. For TIG root welding, ER90S-B9 welding wire can be selected. Note that the electrode needs to be controlled in coordination with the preheating temperature (200~250°C) and the interlayer temperature (≤300°C). Post-heat insulation (300~350°C/2h) is required immediately after welding to drive out hydrogen.
Q3: Why is post-weld heat treatment (PWHT) critical to A335 steel pipe?
PWHT can eliminate welding residual stress (usually required to be reduced to ≤30% yield strength) and prevent stress corrosion cracking. For P91 steel, it is necessary to keep it at 740~760°C for 2~4 hours to fully temper the martensite and improve toughness. Insufficient temperature will cause the hardness to exceed the standard (such as >250HB), and too high temperature may weaken the high temperature strength. Hardness test (such as Brinell hardness HB≤250) and local radiographic testing (RT) are required after heat treatment. ASME B31.3 stipulates that PWHT is a mandatory requirement for A335 steel pipe welding.
Q4: How to avoid hydrogen-induced cracks in the weld zone of A335 steel pipe?
Controlling the hydrogen source is the key: use ultra-low hydrogen electrodes (H4 grade), strictly dry the welding materials, and avoid construction in a humid environment. The preheating temperature must be sufficient (such as 150~200°C for P11 steel), slow down the cooling rate, and allow hydrogen to diffuse and escape. The interlayer temperature does not exceed 300°C to prevent hydrogen accumulation. Post-heat dehydrogenation treatment at 350°C/1h is performed immediately after welding. In addition, designing low-constraint joint forms (such as U-shaped grooves) can reduce the tendency to crack.
Q5: What tests should be included in the welding procedure qualification (PQR)?
PQR must include tensile tests (fracture position should be in the parent material or weld, strength ≥ the lower limit of the parent material), bending tests (side bending 4t, no defects > 3mm), and impact tests (welds and heat-affected zones meet the standards at the lowest design temperature). Metallographic examination is also required for P91 steel (martensite conversion rate > 90%). All tests must comply with ASME IX or EN ISO 15614 standards. The evaluation scope should cover all parameters in actual production (such as current, voltage, heat input).
