Underlying causes of strength weakening of electron beam welded joints of high-speed steels

Abstract The annular butt welding of W9Mo3Cr4V (W9) high-speed steel (HSS) and S390 HSS via electron beam welding (EBW) was carried out. The welded joint was well formed. However, the shear strength of joint was only 53 MPa. The reasons for poor shear property were investigated including microstructure and defect of joints. Acerose martensite was detected inside the austenite grains. The brittle carbides in weld zone appeared like a net-like shape along the grain boundaries, which was the main cause of poor shear property. The network carbides contained the lamellar eutectic M 2 C and the rod-shaped MC. A large number of angular polyhedral carbides were identified in the heat affected zone (HAZ) at S390 side, which were VC wrapped by Mo 2 C and W 2 C. The carbides in the HAZ at W9 side were in form of a mixture of herringbone, lamellar and thin rods. The effect of dispersion strengthening of carbides and lower brittleness were formed at grain boundaries in the HAZ at both sides. Shear fracture was characterized by a brittle intergranular mode. A quantity of carbides distributed in the fracture.