Low temperature impact toughness of high strength structural steel

Impact toughness properties of four kinds of high strength steel (HSS) were investigated with the nominal yield strength 460, 690, 800 and 960 MPa, respectively. The specialty of impact toughness among base metal, heat affected zone (HAZ) and weld metal were also compared and discussed. The correlations between the impact toughness of HSS and their nominal yield strength, plate thicknesses, testing temperature, welding methods were further studied. Combined with the scanning electron microscope (SEM) fracture surface observation, the microstructural features and corresponding fracture mechanisms have been analyzed. The results show that the impact toughness for HSS base metal deteriorated with the increasing nominal yield strength. The impact toughness of weld metal for HSS is much lower than that of the corresponding base metal. The impact performance of HAZ is discretized due to the welding thermal cycling on the HAZ where significant changes in grain structure and properties occur. The microcosmic fracture surface of HSS is examined to explain the reduction in impact performance under low temperature.