Microstructure and corrosion characterization of weld metal in stainless steel and low carbon steel joint under different heat input

In this study, a novel method of the arc assisted tungsten insert gas (AA-TIG) backing welding and the submerged arc welding (SAW) to fill the groove was used to join 12Cr2Mo1R low carbon steel and S30408 stainless steel. The weld metal microstructure characteristics of different heat input and its microhardness and corrosion resistance were investigated in detail. The optical microscope (OM) and scanning electron microscope (SEM) was used to analyze the microstructure and composition distribution of weld zone. The cyclic potentiodynamic polarization and electrochemical impedance spectroscopy were employed to determine corrosion resistance of weld zone under different heat input. The results show that the combination welding process of AA-TIG backing welding and SAW can effectively join thick plate with high efficiency, and the weld shape is good without incomplete fusion and penetration. A higher ratio between ferrite and austenite under higher heat input, resulting in finer microstructure with increased hardness. Further, discovered by discussing the corrosion mechanism is due to the high content of Ni and C elements in the weld are likely to form Ni-C phase, which become the inducing point of pitting corrosion. It is recommended to use a higher heat input to obtain the best corrosion resistance of the welded area.