12Cr1MoVG is a pearlite low-alloy heat-resistant steel. This type of steel has a high Cr content and has high thermal strength and fatigue strength at 500~550℃.

Welding method

When welding high-pressure boiler tube, the quality requirements for the base weld are high. Not only does it require the weld to be fully melted, the back is flat and defect-free, but it also requires the back of the weld to be free of slag or less slag, otherwise it will affect the safe operation of the equipment. Therefore, the welding process method of manual tungsten inert gas arc welding for base, manual arc welding for filling and covering is adopted.

Welding material selection

Manual tungsten inert gas arc welding wire: ER55-B2-MnV (TIG-R31) welding wire, wire diameter φ2.5mm.

Manual arc welding electrode: R317 (E5515-B2) electrode, diameter φ3.2mm, φ4mm.

Preheating before welding

Preheating of boiler tubes before welding can slow down the cooling rate of the weld and heat-affected zone, which is conducive to avoiding the formation of hardened structure, helping the escape of hydrogen in the welding area, and preventing welding cracks in boiler tubes. Due to the high carbon content and alloying element content of 12Cr1MoV steel, the preheating temperature is selected to be 200~300℃, and the preheating area is centered on the weld and at least 100mm on both sides. During the entire welding process of the boiler tube, the interlayer temperature should be kept not lower than the preheating temperature.

Welding operation

For the welding of 12Cr1MoVG high-pressure boiler tubes, not only the welding specification parameters must be strictly controlled, but also the welding operation has a vital impact on the quality of the joint. The welding operation steps are as follows:

a. Argon arc welding primer: fix the boiler tube horizontally, use a short arc during welding, and keep the welding gun as vertical as possible to the workpiece surface, which is conducive to the protection of the welding pool by argon gas. The welding gun and welding wire can be slightly swung horizontally to ensure that both sides of the groove are melted through. At the same time, the molten pool temperature should be controlled to prevent welding penetration and welding nodules. At the joint, use an angle grinder to grind the arc pit, remove the arc closing cracks and pores and other welding defects at the arc pit, and then continue the arc welding.

b. When filling the arc welding, short arc welding should be used, and the heat input should not be too large. If long arc welding is used, not only will there be problems such as unstable arc burning, shallow melting depth, large metal splashes and aggravated burning of alloy elements, but it is also easy to produce defects such as undercut and incomplete welding. At the same time, harmful gases such as N2 and O2 in the air are easy to enter the molten pool and form pore defects in the weld. When welding, a small crescent-shaped strip can be used. Stay on both sides of the groove for 0.5~1 second. The welding slag must be thoroughly cleaned between the welding layers. When welding defects such as pores are found, use an angle grinder to remove the welding defects. When closing the arc, the arc pit must be filled to avoid arc pit cracks. The joints between the layers should be staggered and no overlap is allowed.

c. When arc welding the cover, the welding current should be slightly smaller than the filling. The correct electrode angle should be selected and the electrode should be moved evenly to prevent undercut defects at the edge of the groove. The weld excess height should be controlled at 1~3mm to avoid stress concentration in the joint during use.

Post-weld heat treatment

After welding, the boiler tube needs to be heat treated to eliminate stress. The weld joint is heated to 720-760℃ as a whole. After 1~2h of constant temperature, the weld and the near-seam area should be immediately covered with insulation materials to allow the joint to slowly cool to room temperature. The purpose of heat treatment is to eliminate or reduce the hardened structure in the affected area of ​​the boiler tube, increase plasticity and toughness, effectively reduce welding residual stress, and at the same time facilitate the escape of diffused hydrogen, thereby reducing the tendency of cold cracks.