Influence of Strength Mismatch on Microstructural Evolution and Mechanical Behaviors for Q390/Q690 Ferritic Steel Dissimilar Welded Joints

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Abstract
For the structural integrity of weldments, it is necessary to ascertain acceptable strength mismatch mode when dissimilar parent materials are welded together. Herein, multilayer and multi‐pass CO2 gas shielded welding is carried out to manufacture two dissimilar welded joints of Q390 and Q690 steels using two filler materials with different strength levels. A comparative assessment of both strength mismatch modes is done based on microstructural characterization and mechanical testing. The results show that the weld metal (WM) with predominantly fine acicular ferrite (about 80 pct) and granular bainite has a more excellent combination of strength and toughness compared to the WM mainly composed of polygonal ferrite and degenerated pearlite, and the weakest link of the welded joint has transferred from the WM to the Q390‐side heat‐affected zone. The acceptable strength mismatch relationship of the dissimilar welded structures is revealed, e.g., the hardness in the welded joint should present a uniform gradient distribution. Herein, the gradient difference in the equivalent yield strength (calculated from their hardness) between adjacent areas is ≈170 MPa.

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