Effect of heat input on the mechanical properties of welded Q960E ultra-high strength steel butt joints

The influence of welding heat input on the tensile properties and low-temperature impact toughness of Q960E weld metals (WMs) and butt welded joints (BWJs) was investigated. Prediction models were established to correlate the heat input range (4–17.5 kJ/cm) with the yield strength, tensile strength, and elongation of both WMs and BWJs. Impact toughness was examined at different temperatures for the WMs, heat-affected zones (HAZs), and BM. Microstructure transformations were revealed in different regions of the BWJs, and their impact on the tensile properties and toughness. The results demonstrated that the yield strength, tensile strength, and elongation of the BWJs were 0.80–0.99, 0.9–1.0, and 0.44–0.82 of the BM, respectively; this was attributed to variations in the average effective grain size and Taylor factor in different regions of the BWJs, which led to distinct strength and plastic deformation capacities. The impact toughness of both the WMs and HAZs was notably diminished when compared to the BM. For the WMs, the Charpy impact absorption energy values were only 0.24–0.34, 0.50–0.59, and 0.40–0.50 of that of the BM for heat inputs of 4, 7.4, and 11.2 kJ/cm, respectively. For the same heat inputs, in the case of the HAZs, the Charpy impact absorption energy values were only 0.71–0.90, 0.63–0.70, and 0.63–0.80 of that of the BM, respectively. The presence of martensite–austenite (M–A) islands in the HAZ contributed to the decrease in the impact toughness. Increasing the heat input resulted in larger M–A components, thus further reducing the impact toughness of the HAZ. Conversely, a higher heat input for the WMs promoted the formation of acicular ferrite, thereby enhancing its impact toughness. However, owing to the abundant lath martensite present in the WM, its impact toughness was significantly inferior to that of the BM.