Studies of the characteristics and size effects of softened zone of laser welded 20MnTiB joint

Laser welding is considered as a promising way to suppress the softening problem of welded structures of high strength low alloy steel (HSLA) in quenched and tempered state. Mapping relation between joint strength and the minimum microhardness of the softened zone was established for fibre laser welded 6 mm-thick HSLA joints, as well as that between welding speed and the minimum microhardness of the softened zone to facilitate the optimism of fibre laser welding process of 20MnTiB HSLA. By using a welding heat input as low as about 240 kJ/m, the tensile strength of a fibre laser welded 6 mm-thick 20MnTiB joint could be almost the same as the quenched and tempered base metal due to the low aspect ratio of the cross section of the softened zone (R_SZ) formed in the heat affected zone (HAZ) of welded joint. The transient strain field and the damage evolution history were compared between softened zones with very different cross-sectional aspect ratio, aiming to get a better understanding of the effects of R_SZ on the performance of as-welded 20MnTiB joint. The numerical studies were carried out to investigate the underlying mechanism of the effect of softened zone on the overall strength of the welded joint.