During the welding process, due to factors such as low thermal conductivity and thick walls leading to difficult heat dissipation, the welding joint stays in the sensitization temperature range (450-850 °C) for a long time, which can easily form coarse cast structures. Moreover, a large amount of chromium carbide (Cr23C6) is easily precipitated at the grain boundaries, reducing the corrosion resistance of stainless steel and causing intergranular corrosion. At the same time, the residual tensile stress of the weld seam and high heat input will increase the tendency of thermal cracks. Therefore, improper selection of welding materials and processes may cause defects such as thermal cracks, intergranular corrosion, and stress corrosion.
Argon tungsten arc welding is commonly used for stainless steel welding. The small wire energy of the tungsten arc welding method can avoid a decrease in the performance of the welding joint caused by high heat input during the welding process, and prevent the occurrence of thermal cracks. However, its production efficiency is low and is suitable for bottom welding. The efficiency of stick welding is relatively high, and the heat-affected area is small, which can help ensure the welding joint quality. According to the design requirements of high-pressure hydrogen pipeline, the welding material should be selected to be chemically similar to the base material to ensure that the mechanical properties and corrosion resistance of the welded joint meet the requirements after welding.
The key to welding thick-wall stainless steel pipes is to use small wire energy and faster cooling speeds to reduce the residence time of the heat-affected zone and the sensitization temperature range, prevent the occurrence of carbide precipitation sensitization, thermal cracks, and embrittlement welding defects. The welding current of the argon tungsten arc welding method should be controlled between 110-140A, and the same flow rate should be used for protection on the front and back. The front end of the welding wire should be placed in the protective gas, and impurities should be strictly cleaned before welding. For thick-wall stainless steel pipes, argon gas should be blown on the back of the weld to provide protection and promote formation.
The welding current of the stick welding method should be controlled between 110-130A. The welding rod should not be swung horizontally during the welding process, and the width of the weld bead should not exceed 2.5 times the diameter. Short arc welding and slow shedding should be used. Multiple layers of welding should be used, and the thickness of each layer should not exceed 3mm. The temperature between layers should be strictly controlled. After each welding pass is completed, not only should the surface slag and surrounding splashes be thoroughly removed, but also surface defects such as pores and inclusions should be eliminated. The subsequent welding pass should be carried out after the previous welding pass cools to below 60°C, to avoid the precipitation of carbides and the formation of coarse austenite structures. The welding sequence should be adjusted in a timely manner during the welding process to keep the welding deformation within an acceptable range. The arc should not be started or stopped outside the groove during pipe welding to ensure high quality at the starting and stopping points.
The welding joint of the thick-wall stainless steel pipe may have hot cracks after stabilization treatment. After passing the non-destructive testing before stabilization treatment, the surface of the pipeline heating area should be cleaned, and the electric heating method should be used for treatment, while using thermocouples to monitor the temperature to ensure that the temperature is controlled between 900±10°C. There should be no less than two temperature measurement points. This helps to ensure that the heating process temperature distribution is uniform and accurate.
With the development of large-scale petroleum and chemical equipment, thick-wall stainless steel pipes will be more widely used in high-temperature, high-pressure, and hydrogen-containing equipment. The quality of welding directly affects its safety during use. Therefore, the welding technology of thick-wall stainless steel pipes is more important. Experiments have shown that controlling the temperature between welding passes and avoiding long residence times at the sensitization temperature is an effective measure to improve the welding quality of thick-wall stainless steel pipes. However, due to the need for rapid cooling for each pass, the welding process time is longer and the production efficiency is low, which increases the production cost. Therefore, using a welding method with small wire energy to reduce heat input, controlling the precipitation of brittle phases and carbides, is the fundamental solution to the welding technology of thick-wall stainless steel pipes.