Characterization of Friction Stir Welded Multilayered Steel

Abstract Laminated compound materials consisting of ductile austenitic steel and brittle martensitic steel are characterized by a combination of high strength and ductility which is unreachable with monolithic steels. This study investigates the mechanical properties and the microstructure of friction stir butt welded multilayered steel consisting of 15 alternating layers of SUS301 austenitic stainless steel (8 layers) and SUS420J2 martensitic stainless steel (7 layers) with a thickness of 1.2mm. With optimized welding parameters defect free welds were accomplished leading to an ultimate tensile strength (UTS) of 1240MPa and a fracture elongation of 13%. This corresponds to a joint efficiency of 90%. For optimized parameters welds fracture occurred in the heat affected zone (HAZ). The hardness profile across the weld showed a pronounced hardness drop in the martensitic layers in HAZ which was formed at temperatures around 750°C slightly below Ac1, whereas in austenitic layers no significant changes in hardness were noticed. With TEM images a large amount of grain boundary precipitates was detected in HAZ of martensitic layers leading to a considerable decrease in hardness. By applying a post weld heat treatment (PWHT) the hardness drop in the martensitic layers was removed and the tensile properties were enhanced to an UTS of 1310MPa (95% joint efficiency) and a fracture elongation of 22%. For PWHT samples fracture occurred either in the unaffected parent material or within stir zone.