Local microstructure and mechanical characteristics of HAZ and tensile behavior of laser welded QP980 joints

Abstract

Microstructural evolution of the heat-affected zone (HAZ) seriously affects the mechanical properties of the welded joints. In this paper, QP980 steels were butt-welded by fiber laser. The thermocouple and Gleeble1500 system were used to achieve the thermal simulated HAZ specimens of laser welded joints, which solved the problem that it was difficult to evaluate the mechanical properties due to the narrow HAZ of the laser welding. Large amounts of granular carbides were precipitated in the block martensite of the sub-critical HAZ (SCHAZ). Carbides were not found in lath martensite, but there were dislocation tangles in the lath martensite of the SCHAZ. In addition, the number of dislocations in the ferrite of the SCHAZ was significantly reduced. The SCHAZ had the lowest strength but the highest elongation owing to the tempering. The fine-grain strengthening resulted in the highest tensile strength in the fine-grained HAZ (FGHAZ). The tensile fracture morphologies indicated a brittle fracture mechanism in coarse-grained HAZ (CGHAZ) and FGHAZ, and exhibited a ductile fracture mechanism in the inter-critical HAZ (ICHAZ) and SCHAZ. According to the overview of the joint and the local mechanical characteristics of HAZ, the numerical model of the tensile test was performed by ABAQUS. And the simulation results showed good agreement with the experimental results, which both illuminated the constraint effect of SCHAZ and made the strain concentration transferred from SCHAZ to the BM with the increase of stress.