Austenitic stainless steel materials are required under many harsh conditions such as corrosion resistance, high temperature resistance and low temperature resistance. However, all austenitic materials are costly, so many use carbon steel where they are not in contact. This will result in the use of exotic welding where the two are in contact. Welded welding has always been a technical difficulty in welding because there are several problems in welding:
1. The plasticity and toughness of the welded joint are reduced due to the effect of dilution. A transition layer is formed in the fusion zone on one side of the carbon steel, and the high-hardness martensite structure in the transition layer reduces the plasticity of brittleness and reduces the performance. Second, carbon steel has a higher carbon content. Due to the dilution of carbon, the chromium and nickel in the fusion zone are reduced and the brittleness is increased.
2. Softening and hardening of the fusion zone due to carbon migration. Carbon has a greater solubility in liquid iron than solid iron and is larger in austenite than in ferrite. At high temperatures, chromium acts as a strong carbide element, causing carbon to migrate into the stainless steel, resulting in a decarburization layer on the carbon steel side and a carbonization layer on the stainless steel side. The pearlite of the decarburized layer becomes ferrite and softens. The carbon of the carbon-increasing layer is hardened by the formation of chromium carbides with chromium.
3. Cracks are generated due to internal stress. The coefficient of thermal expansion of stainless steel is 40% larger than that of carbon steel, and the thermal conductivity is only 1/3 of that of carbon steel. In the post-weld cooling, heat treatment and use, internal stress is generated in the fusion zone, and cracks are formed in the fusion line.