Welding Process for 12Cr1MoVG High-Pressure Boiler Tubes

I. welding Method

In the welding of high-pressure boiler alloy pipes, the quality requirements for the backing weld are high. The weld must have full penetration and a smooth back side with no defects. In addition, there should be little or no slag on the back of the weld. If slag is present, the safe operation of the equipment may be affected. For this reason, the welding process uses manual tungsten inert gas (TIG) welding for the backing pass and manual arc welding for the filling and cover passes.

II. Selection of Welding Materials

• Manual TIG welding wire: ER55-B2-MnV (TIG-R31) welding wire, with a diameter of φ2.5mm.

• Manual arc welding electrode: R317 (E5515-B2) electrode, with diameters of φ3.2mm and φ4mm.

III. Preheating Before Welding

Preheating can slow down the cooling rate of the weld and the heat-affected zone. This helps prevent the formation of hardened microstructures. It also helps hydrogen escape from the welding zone and prevents welding cracks. Because 12Cr1MoV steel has relatively high carbon and alloy element content, the preheating temperature is set at 200–300℃. The preheating area is centered on the weld and extends at least 100mm to each side. During the whole welding process, the interpass temperature must not fall below the preheating temperature.

IV. Welding Operation

For welding 12Cr1MoVG steel, the welding parameters must be strictly controlled. At the same time, the welding operation itself has a critical effect on joint quality. The welding operation steps are as follows:

a. TIG backing welding: The pipe is fixed in a horizontal position. A short arc is used during welding. The torch is kept as perpendicular to the workpiece surface as possible, so that the argon gas can protect the weld pool well. The torch and filler wire can be given slight side-to-side movement to ensure full penetration on both sides of the groove. At the same time, the temperature of the weld pool must be controlled to prevent burn-through and overlap defects. At joints, an angle grinder is used to grind the crater. This removes welding defects like stop-start cracks and porosity from the crater area. After that, the arc is struck again to continue welding.

b. For arc welding filling passes, a short arc should be used, and the heat input should not be too high. If a long arc is used, the arc may burn unstably, penetration may be shallow, spatter may be heavy, and alloy elements may burn off more easily. In addition, defects like undercut and incomplete fusion can occur. Harmful gases like N₂ and O₂ from the air can also enter the weld pool and cause porosity. A small crescent-shaped weaving technique can be used. A pause of 0.5 to 1 second is made on both sides of the groove. Slag must be completely removed between layers. If welding defects like porosity are found, they are removed with an angle grinder. When stopping the arc, the crater must be completely filled to prevent crater cracks. The joints between layers should be staggered and should not overlap.

c. For the arc welding cover pass, the welding current is slightly lower than that used for filling passes. The correct electrode angle must be chosen, and the manipulation should be uniform. This prevents undercut along the groove edges. The weld reinforcement should be controlled between 1mm and 3mm. This prevents stress concentration in the joint during service.

V. Post-Weld Heat Treatment

After welding, stress-relieving heat treatment is required. The welded joint is heated evenly to 720–760℃ and held at this temperature for 1–2 hours. Then, the weld and the nearby areas are immediately covered with insulating material, so that the joint cools slowly to room temperature. The purpose of this heat treatment is to eliminate or reduce the hardened microstructures in the heat-affected zone. It also increases plasticity and toughness, reduces welding residual stress, and helps diffusible hydrogen escape. This in turn reduces the tendency for cold cracking.