When welding carbon steel and stainless steel, measures must be taken; otherwise, severe weld corrosion will occur

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Example: A factory uses a thin-walled tube made of 309Cb stainless steel (with 23% Cr, 13.5% Ni, a maximum carbon content of 0.08%, and a Nb content that is 10 times that of the carbon content) to transport highly corrosive solutions. Since the tube needs to be heated, a carbon steel jacket is placed around it; the stainless steel tube is then welded to this jacket using 309Cb stainless steel welding rods. Leakage occurred at the weld joint between the carbon steel jacket pipe and the stainless steel pipe after being used for a very short period of time. Commentary: During welding, stainless steel and carbon steel fuse together; the stainless steel is diluted by the carbon steel. In the stainless steel pipes at the welded areas, the contents of the alloying elements chromium and nickel decrease, resulting in a level that is insufficient to maintain the corrosion resistance required in the operating environment. This leads to an increased corrosion rate, which causes perforation and leakage. Therefore, when welding stainless steel thin-walled pipes to carbon steel pipes, a stainless steel gasket plate should be used as a transition to prevent the stainless steel pipe from being diluted. Summary: Generally speaking, even when welding stainless steels together, it is possible for the corrosion resistance of the weld to be lower than that of the base material, thereby leading to selective corrosion of the weld in the service environment. This is because welding is a process of melting and recrystallization; the weld metal has a cast structure, and it suffers from defects such as grain coarsening, compositional changes, and uneven microstructure. If the electrode material is the same as the base metal, the corrosion resistance of the weld will inevitably be lower than that of the base metal. To address this issue, when selecting electrodes, it is important to ensure that the content of alloying elements in the electrodes is higher than that in the base metal, while the impurity content should be lower than that in the base metal. Additionally, the welding process is also very important. Ensuring welding quality, preventing weld defects, and eliminating the inhomogeneity and welding residual stresses caused by the heat-affected zone are all effective measures to address welding corrosion problems.