Effect of grain orientation on microstructure and mechanical properties of laser welded joint of additive manufactured 300M steel

The mechanical properties of 300 M steel welded joint are closed related to the morphology and orientation of the grain. Laser welding was used to join the laser additive manufactured (LAM) 300 M steel. The effect of grain orientations (0 degrees, 45 degrees, 90 degrees) on the microstructure and mechanical properties of the laser welded joint was studied. The results showed that with the increase of orientation, the columnar to equiaxed transition (CET) occurred in the center of the weld joint. Microstructures of the welding line, heat-affected zone (HAZ) and base metal presented obvious changes. The HAZ of the as-welded samples showed the highest micro-hardness, then the welding line, and the base metal was the lowest. After heat treatment, the micro-hardness of the whole joint became uniform. Grain orientation had great influence on tensile strength of the sample. With the increase of orientation, the tensile strength of the joints increased while the elongation was decreased. However, the strength of the heat-treated joints was significantly higher than that of the as-weld samples and presented little difference. Fractures mode of the as-welded and heat-treated joints showed the ductile fractures.