Discussion on Welding Failure Analysis of Stainless Steel and Carbon Steel

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Let’s discuss the failure analysis of welding between stainless steel and carbon steel. In a certain installation, there was 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 carbon). This tube was used to transport highly corrosive solutions; since the tube needed to be heated, a carbon steel sleeve was placed around it, and the stainless steel tube was welded to this sleeve using 309Cb stainless steel welding rods. After short use, leakage occurred at the weld between the carbon steel jacket and the stainless steel pipe. Analysis: During welding, stainless steel and carbon steel merge together, causing the stainless steel to be 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. Measures to take: When welding stainless steel thin-walled pipes with carbon steel pipes, a stainless steel gasket plate should be used as a transition to prevent the stainless steel pipe from being diluted. Note the welding process and methods: 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, 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, composition 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 welding electrodes, care should be taken to ensure that their alloy element content is higher than that of the base material, while their impurity content should be lower than that of the base material. Additionally, the welding process is also very important. Ensuring welding quality, avoiding weld defects, and eliminating the unevenness caused by the welding heat-affected zone as well as welding residual stresses are all effective measures to address welding corrosion issues.