Microstructures and Properties of Ultra-high Strength Steel by Laser Welding

Based on the experiments of laser welding of ultra-high strength steel (UHSS), joint morphology, microstructure, hardness and tensile-shear properties are investigated. The results show that the defects such as gas holes and cracks do not appear in the weld metal if the optimized parameters are employed. Microstructures of the weld metal and HAZ are mainly martensite, the widths of martensite lath aren't sensitive to the welding speed. However, the welding speed affects the grain sizes of prior autensite of the weld metal. When changing the welding speed from 20 mm/s to 35 mm/s, the mean widths of prior austenite grains decrease from 10.67 mu m to 7.68 mu m. In the HAZ, the hardness decreases apparently for the soft ferrite appears. The test results also show that the joint tensile-shear strength can satisfy the demand of production. Consequently, the laser welding quality of UHSS is reliable for manufacturing.