Thermal Simulation and Fracture Resistance Evaluation of Coarse-Grain Heat-Affected Zone in Welds of Q355D Steel

Abstract

The coarse-grain heat-affected zone (CGHAZ) is often the most fragile part of the weld region due to hard phases and coarse grains generated during the welding. Characterization of the mechanical properties and fracture resistance of the CGHAZ is a vexing problem because the CGHAZ is usually not big enough for the fabrication of specimens for traditional tests. In this study, simulated CGHAZ was produced for welds of Q355D steel by a thermal simulation technique. The peak temperature was 1,350°C for all thermal simulations and different cooling rates were designed, i.e., t8/5=30 s, 60 s, 90 s, 120 s, and 180 s, to investigate the effect of the cooling rate on the solid-state phase transformation of the steel. The similarity of the simulated CGHAZ with t8/5=180 s to the CGHAZ from the weldment was verified by metallography examination and depth sensing indentation tests. Then, the mechanical properties of the simulated CGHAZ were characterized by Charpy impact tests and tension coupon tests. The simulated CGHAZ has distinctly higher yield stress and tensile stress, but lower elongation, section reduction, and Charpy impact energy than the base metal (BM) of the Q355D steel. Parameter η, representing fracture resistance in the void growth model, was calibrated for the simulated CGHAZ through tests on smooth notched tensile specimens and complementary finite element analysis. Comparison of parameter η indicates that the fracture resistance of the simulated CGHAZ is about 5.7% lower than that of the BM of the Q355D steel.