Key Points of Boiler Tube Welding Process

Basic principles of welding process

The welding process of boiler tube mainly includes three types: gas welding, electric welding and submerged arc welding. Among them, gas welding is suitable for thinner boiler tube walls, electric welding is suitable for thicker boiler tube walls, and submerged arc welding is suitable for thick-walled pipes as well as pipes with high welding performance requirements, such as seamless steel pipe.

During the welding process, the selection of welding materials and electrodes also needs to be considered. The welding material should be the same or similar to the material of the boiler tube to ensure the tightness of the welded joint; the selection of electrodes also needs to be determined according to the type and requirements of the welding material. For different pipe materials, like carbon steel pipe, the selection principles of welding materials and electrodes are also different.

Steps of welding process

1. Preparation before welding

Clean the welding area to ensure that the channel is unobstructed to avoid blockage during welding; at the same time, heat the welding material to reduce welding deformation.

2. Welding operation

Perform welding operations by selecting appropriate welding processes and welding equipment. For hydraulic tubes, welding parameters and operating techniques need to be adjusted during welding.

3. Post-weld treatment

After welding, the weld needs to be polished, cleaned and inspected to ensure welding quality and sealing. For pipelines with high requirements such as precision tube, the flatness and finish of the weld need to be strictly controlled.

Welding materials

The selection of welding materials is directly related to the compliance of performance indicators such as impact resistance, heat resistance, and corrosion resistance. Depending on the material of the boiler tube, the selected welding materials are also different.

For carbon steel boiler tubes, E4315, E5015 and other welding rods are generally used, which are suitable for submerged arc welding and manual welding. If the welding quality requirements are high, low-hydrogen welding rods E5016 or E5516 can be used. For special materials such as alloy steel and stainless steel, choose the corresponding welding materials for welding.

Matching principles of welding materials

1. Boiler tube welding should give priority to welding materials with similar chemical composition and mechanical properties to the parent material. For example, P91 steel pipes need to be matched with ER90S-B9 welding wire to avoid the risk of embrittlement in the heat-affected zone.

2. When welding dissimilar steels, the martensitic transformation tendency of the organization needs to be evaluated by the Schaeffler diagram.

Pre-welding pretreatment specifications

1. Use acetone to thoroughly remove grease pollutants within 50mm on both sides of the groove.

2. Use a stainless steel wire brush to treat the oxide layer until the metallic luster is exposed.

3. For boiler tubes with a wall thickness of more than 25mm, a preheating and dehydrogenation treatment of 100-150℃ is required.

Welding process parameter optimization

1. When argon arc welding is used for base welding, the current should be controlled at 90-110A, and the argon purity should be ≥99.99%.

2. The submerged arc welding filling layer adopts DC reverse connection, and the voltage is 28-32V matching the walking speed of 500-550mm/min.

3. The interlayer temperature is strictly kept within the range of not more than 50℃ above the preheating temperature.

Quality assurance measures

1. The boiler tubes after welding should be immediately subjected to stress relief heat treatment, and the heating rate should be ≤150℃/h.

2. Delayed cooling for more than 24 hours is required before 100% RT testing.

3. The hardness test value should be controlled within 120% of the hardness of the parent material.

Safety protection requirements

1. Boiler tube welders must be equipped with automatic darkening masks, flame-retardant work clothes and protective leather skirts.

2. Forced ventilation systems and gas detectors are required for confined space operations.

3. All welders must be equipped with leakage protection devices, and the grounding resistance must be ≤4Ω.

Strict acceptance process

After welding is completed, it must pass a strict acceptance inspection. This includes a detailed analysis of the chemical composition of the material, a comprehensive test of the mechanical properties of the weld, and non-destructive testing using X-rays, hydrogen cobalt lamps, ultrasonic and other equipment. For parts such as pipe flange connected to boiler tubes, their welding quality also needs to be strictly inspected. Only after this series of strict acceptance processes can the quality of boiler tube welding be ensured.

Other additional requirements

1. The inside of the boiler tube must be kept clean, and there must be no impurities such as oil and rust on the external surface.

2. The weld surface of the boiler tube should be smooth and free of defects such as cracks, pores or inclusions.

3. The post-welding treatment process of the boiler tube should be carried out in a standardized manner to ensure the uniformity of the quality of the welding area.

4. The installation layout of the boiler tube should be reasonably planned to avoid any interference with normal operation.

Summary

Welding is an indispensable part of the boiler tube production process. Correct selection of welding materials and methods, attention to welding details, and quality inspection can effectively improve the service life and safety performance of the boiler tube.

Read more: How To Weld Boiler Tubes?