Electron beam and metal active gas welding of ultra-high-strength steel S1100MC: influence of heat input

This study investigates the microstructure and the hardness of thermomechanically processed joints of ultra-high-strength steel (UHSS) S1100MC, which were butt-welded by means of metal active gas (MAG) welding and electron beam welding (EBW). In MAG welding, the microstructure of the fusion zone (FZ) consisted predominantly of fine ferrite grains. Due to the formation of martensite/austenite (M/A) constituents, the hardness of the reheated weld metal (WM) was higher for vertical-up (PF) than for horizontal flat (PA) welding position. The microstructure of the heat-affected zone (HAZ) of the last welding pass consisted mainly of hard martensite in consequence of fast cooling. However, necklace-type M/A constituents and bainite phases of lower hardness formed when the HAZ of prior welding passes were reheated by subsequent welding passes. The microstructure of the EBW fusion zone was dominated by martensite. Therefore, the fusion zone had considerable higher hardness than the base material (BM). The width of the HAZ in EBW weldments is less than in MAG weldments.