Crucial Microstructural Features to Determine the Mechanical Properties of Welded Joints in a Cu-Containing Low-Carbon Low-Alloy Steel After Postweld Heat Treatment

The effect of welding and postweld heat treatment on the microstructural characteristics and mechanical properties of a Cu-containing low-carbon low-alloy steel was studied. The obtained results indicated that ferrite, bainite, and coarse Cu-rich precipitates formed in the weld metal (WM) were responsible for its low impact energy of 57 J at − 40 °C. To obtain a good balance between strength and toughness in the WM, different heat treatments were employed. The obtained results demonstrated that direct tempering had a negligible effect on strength and toughness, but the addition of intercritical annealing to the conventional quenching and tempering process resulted in a significant increase in toughness and a slight decrease in tensile strength. The impact energy was increased to beyond 144 J, and the tensile strength was maintained at a high level of 958 MPa. The optimal microstructure benefiting both toughness and strength was found to primarily comprise intercritical ferrite, tempered martensite/bainite, reversed austenite, and fine Cu-rich precipitates. The toughening mechanism can be explained by the strain-induced martensitic transformation of reversed austenite and the retarding crack propagation effect of high-angle grain boundaries with a misorientation of more than 45 deg. The strengthening mechanism can be rationalized in terms of precipitation-strengthening and the strain-induced martensitic transformation of reversed austenite.