Effect of laser heat treatment on bending property of laser welded joints of low-alloy ultra-high strength steel

This paper investigates the effect of laser heat treatment on the bending property of low-alloy ultrahigh strength steel laser-welded joints. A laser rectangular spot is used for local heat treatment. The results indicate that the back bending angle of joints can be improved from 30° to over 90° with a joint efficiency of 76%. With the increase of laser power and scanning times and the decrease of scanning speed, the heat treatment temperature at the back of the joint increases gradually. The peak temperature under different parameters changes from 450 to 700 °C, at which tempering takes place. This is consistent with the microstructural evolution from original lath martensite of weld metal and heat-affected zone to tempered martensite after laser heat treatment. The hardness between weld and softened zone is similar under the same parameters. Analysis of variance reveals that the hardness is mainly influenced by the peak temperature instead of heating time. The weld hardness decreases gradually with the increase of the peak temperature, and hardness less than 35.0 HRC is obtained when that temperature is over 600 °C. The weld hardness is closely related with the bending property of joints. When the hardness is less than 35.0 HRC, a bending angle over 90° can be achieved. This is because laser tempering makes the hardness of joints uniform, consequently improving the bending performance of the joints.