A Study on the Impact Toughness of the Simulated Heat-Affected Zone in Multi-Layer and Multi-Pass Welds of 1000 MPa Grade Steel for Hydroelectric Applications

The microstructure and impact toughness of a steel material subjected to multi-layer and multi-pass welding with varying secondary peak temperatures were investigated using welding thermal simulation. The detailed microstructures and fracture morphologies were examined by SEM, TEM, and EBSD. When the secondary peak temperature reaches 650 °C, the microstructure resembles that of a primary thermal cycle at 1300 °C, characterized by coarse grains and straight grain boundaries. As the temperature increases to 750 °C, chain-like structures of bulky M/A (martensite/austenite) constituents form at grain boundaries, widening them significantly. At 850 °C, grain boundaries become discontinuous, and large bulky M/A constituents disappear. At 1000 °C, smaller austenitic grains form granular bainite during cooling. However, at 1200 °C, grain coarsening occurs due to the significant increase in peak temperature, accompanied by a lath martensite structure at higher cooling rates. In terms of toughness, the steel exhibits better toughness at 850 °C and 1000 °C, with ductile fracture characteristics. Conversely, at 650 °C, 750 °C, and 1200 °C, the steel shows brittle fracture features. Microscopically, the fracture surfaces at these temperatures exhibit quasi-cleavage fracture characteristics. Notably, chain-like M/A constituents at grain boundaries significantly affect impact toughness and are the primary cause of toughness deterioration in the intercritical coarse-grained heat-affected zone.