Microstructural Evolution and Fracture Toughness of Welded Joint of 1000 MPa-Grade High-Strength Steel

The microstructure and mechanical behavior of welded joints of 1000 MPa-grade high-strength steel are studied using optical microscopy, scanning electron microscopy, electron backscatter diffraction, Charpy impact tests, and crack tip opening displacement tests. The weld metal (WM) primarily comprises interlocking lath bainite and lath martensite, with small amounts of granular bainite and retained austenite in the heat-affected zone (HAZ). The tensile strength, yield strength, and elongation of the welded joints are 1084, 1022 MPa, and 17%, respectively. Both the WM and HAZ have high impact toughness at temperatures of 20 to −80 °C. The HAZ has higher toughness than the WM because of its higher content of retained austenite and high-angle grain boundaries. These factors significantly obstruct crack growth, resulting in good toughness of the entire joint. The critical crack tip opening displacement for the HAZ (0.229 mm) is significantly higher than that of the WM (0.079 mm), indicating that the fracture toughness of the HAZ surpasses that of the WM, which is consistent with the Charpy impact results.