Mechanical and microstructural properties of S1100 UHSS welds obtained by EBW and MAG welding

The microstructures and mechanical properties of welds consisting of 20-mm-thick thermo-mechanically rolled and directly quenched S1100MC ultra high-strength steel (UHSS) plates were investigated. The welds were produced by means of metal active gas (MAG) welding and electron beam welding (EBW). Different heat inputs of the welding processes influenced the microstructure and thus the mechanical properties including impact toughness, hardness, and tensile properties. The microstructure of the MAG weld obtained when using undermatched solid filler wire consisted mainly of acicular ferrite (AF), and it appeared more polygonal when the heat input exceeded 2 kJ/mm with spray arc in the filler pass. The coarse-grained heat-affected zone (CGHAZ) showed different microstructures depending on the thermal cycles of the respective welding processes. Fresh martensite formed in the CGHAZ of the last welding pass at both the bottom and the top surfaces, as there was no reheating from any subsequent pass. The microstructure obtained with EBW without any filler material consisted of martensite and tempered martensite in the fusion zone. Martensite with small prior austenite grain (PAG) size significantly increased the hardness of the fine-grained heat-affected zone (FGHAZ) compared to the CGHAZ and fusion zone. Uniaxial tensile testing of EBW specimens indicated higher tensile strength of the weld than of the base metal, as the specimens fractured at the base metal. In contrast, fracture of MAG specimens occurred at the weld. Hence, the tensile strength of the MAG weld consisting of undermatched filler metal was obviously lower than the tensile strength of the base metal. However, the ferritic MAG weld possessed higher impact toughness than the martensitic EBW weld.