Abstract

Third generation advanced high-strength steels, e.g., quenched and partitioned steels, are forthcoming structural materials, which consist of a martensitic matrix and a substantial proportion of stabilized residual austenite for improved deformability. A novel less energy-intensive processing route of direct-quenching and partitioning advances this concept by facilitating carbon partitioning to untransformed austenite directly from the quench-stop temperature. However, a major challenge also with these steels is how to maintain structural integrity in the welded end-products after additional heat-input reaching above a temperature where given microstructure is still stable. Heat-input limiting beam welding processes are a solution to this by minimizing degradation of the heat-affected zone (HAZ) and producing even-strength welded joints for S1100 and above. In this study, we report toughness properties of an electron-beam (EB) welded 0.2C-1.5Mn-0.5Si-0.8Al-1.1Cr-0.8Ni (wt.%) direct-quenched and partitioned steel (DQ&P) having a yield strength of ∼1100 MPa, and a direct-quenched (DQ) was used as a reference. Low-temperature post-weld heat treatment (PWHT) was considered, too. Weld seam, coarse-grained HAZ, and the base materials were tested for impact toughness. Both the DQ and DQ&P base materials have excellent impact toughness transition temperatures T_28J below -100°C. The weld seam has very good low-temperature toughness already at this stage of optimisation with T_28J of -66°C, which shows robustness of the chosen alloy. Increased residual austenite content increased upper shelf toughness but not T_28J. Furthermore, both the DQ and DQP HAZs have T_28J below -70°C, pointing to the weld seam as the weakest link. PWHT reduced low-temperature impact toughness in all the cases with T_28J being above -40°C, clearly demanding reassessment of its feasibility.