During the welding process of stainless steel pipes, due to the influence of personnel, equipment, materials, methods, environment and other factors, defects are generated in the welding seam of stainless steel pipes. The internal defects of stainless steel pipe welding mainly include cracks, pores, slag inclusion, lack of penetration, and lack of fusion.
1. Cracks.
The cracks formed by cracks in the weld or heat-affected zone are called cracks. Divided into cold money lines, hot cracks, reheat cracks and so on. Welding crack is very harmful. In addition to reducing the strength of the weld, it also causes severe stress concentration and structural fracture due to the sharp gap at the end of the crack.
1. Cold cracks: cracks produced at temperatures below 200°C during the cooling process of the weld are called cold cracks.
1.1 Reasons
During the crystallization process of the weld, the hydrogen content is too high to escape, and it gathers in the heat-affected zone near the fusion line; the base material has a high tendency to harden, and under the condition of a fast cooling rate, the heat-affected zone becomes brittle and Hard martensitic structure: During the welding process, due to the uneven partial heating of the workpiece, the welding seam will produce a large tensile stress during the cooling process, and this tensile stress increases with the decrease of the welding seam temperature. Under the combined action of hydrogen, hardened structure, and stress, cracks occur.
1.2 Preventive measures
1.2.1 Reasonable selection of welding consumables. Use low-hydrogen electrode to reduce the hydrogen content, dry strictly in accordance with the regulations before welding, thoroughly clean the edge of the weld, and reduce the source of hydrogen; select suitable welding materials to make the weld and the base metal have a good match, and increase welding The plasticity of the seam metal does not produce any undesirable structures, such as grain coarsening and hard brittle martensite, etc.
1.2.2 Choose a reasonable welding process. Such as preheating before welding, controlling the temperature between layers, slowing down the cooling rate, using small current, dispersed welding and other measures to reduce the temperature difference of the bright parts, and improve the structure of the weld and the heat-affected zone, etc.
1.2.3 Timely heat treatment after welding. Allow hydrogen to escape from the weld, reduce welding residual stress and improve the structure and performance of the joint.
1.2.4 Use reasonable welding sequence and welding direction to improve welding stress and reduce welding residual stress.
1.2.5 Formulate reasonable forming and assembly processes, minimize cooling deformation, avoid forced assembly, and prevent various scars during assembly.