Study on microstructure characterization and mechanical properties of AISI 444 ferritic stainless steel joint by high-frequency pulse K-TIG welding

The research employed high-frequency pulse K-TIG for the welding of a 6mm thick AISI 444 ferritic stainless steel (FSS), and the welding process window was determined. Complete penetration was achieved in the state of single-pass autogenous welding. The influence law of technological parameters and weld appearance was analyzed. The microstructure characteristics of the weld zone (WZ) and heat-affected zone (HAZ) was investigated. Subsequently, the mechanical properties of welded joints are evaluated. It is found that the thermal conductivity of FSS is large, so the welding process window is relatively narrow, and the welding heat input determines whether the weld can achieve complete penetration. Both the WZ and HAZ contained secondary phase precipitates, including sigma phase, chi phase, and Laves phases. The sigma phase and chi phase were detected at the grain boundary, while the Laves phase was located both intragranularly and at the grain boundary.In addition, NbC was detected in the HAZ. In addition, NbC was detected in the HAZ. The presence of a brittle-hard phase and coarse grain significantly affected the tensile properties and impact toughness at room temperature. The tensile strength of the weld is equivalent to 71% of the parent material, which is 561 MPa. Its average impact toughness value is 29 J, which is equivalent to 42% of the BM. Although the strength and toughness of FSS joints welded with high-frequency pulse K-TIG were somewhat inferior to the BM, the process displayed a marked improvement in welding efficiency.