12Cr1MoVG High Pressure Boiler Tube Welding Process

1. Welding method
When welding high-pressure boiler alloy pipes, the quality requirements for the bottom weld are relatively high. Not only are the welds required to be fully penetrated and the back surface smooth and defect-free, but they are also required to have no slag or less slag on the back side, otherwise the safe operation of the equipment will be affected. . Therefore, the welding process of manual tungsten arc welding for the base and manual arc welding for the filling and cover is used.
2. Selection of welding materials
Manual tungsten 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.
3. Preheating before welding
Preheating can slow down the cooling rate of the weld and the heat-affected zone, which is helpful to avoid the formation of hardened structures, help the escape of hydrogen in the welding zone, and prevent welding cracks. Due to the high carbon content and alloying element content of 12Cr1MoV steel, the preheating temperature is selected to be 200~300°C. The preheating area is centered on the weld seam and is at least 100mm on both sides. During the entire welding process, the interlayer temperature should be kept not lower than the preheating temperature.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4. Welding operation
The welding of 12Cr1MoVG steel not only requires strict control of the welding specification parameters, 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 pipe fitting horizontally, use short arc when welding, and keep the welding gun as perpendicular to the surface of the workpiece as possible, which is beneficial to the protection of the welding pool by argon gas. The welding gun and welding wire can swing slightly laterally to ensure penetration on both sides of the groove. At the same time, the temperature of the molten pool must be controlled to prevent welding penetration and welding defects. At the joint area, use an angle grinder to grind the arc crater to remove welding defects such as arc closing cracks and pores at the arc crater, and then continue arc welding.

b. When filling with 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 combustion, shallow penetration, large metal spatter, and increased burning loss of alloy elements, but also defects such as undercuts and incomplete welding will easily occur. At the same time, N 2 and O 2 in the air will Harmful gases such as welding can easily 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. Welding slag must be thoroughly cleaned between 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 crater must be filled to avoid Crater cracks occur. The joints between layers should be staggered and should not overlap.

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5. Post-weld heat treatment
After welding, stress-relieving heat treatment is required, and the entire welding joint is heated to 720-760°C. After constant temperature for 1 to 2 hours, the welding seam and the near seam area should be covered with insulation material 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, increase shaping and toughness, effectively reduce welding residual stress, and at the same time facilitate the escape of diffuse hydrogen, thereby reducing the tendency of cold cracking.