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.
This study attempts to propose a convenient method to adjust the welding process parameters window with the aim of high volume fraction of acicular ferrite in the weld metal (WM). The samples are directly extracted from the deposition zone of two low alloy ferritic steel dissimilar welded joints (DWJs) that manufactured using different consumables, and their microstructural evolution is controlled using thermomechanical simulation technique. The results show that it is hard for the low‐alloyed WM to obtain high amount of acicular ferrite no matter how to control the cooling process, meaning that the welding consumable with only a small amount of alloying elements is not suitable for welding this DWJ. However, for the high‐alloyed WM, the acceptable window of the cooling process is determined within 10–22 °C s−1 to achieve aimed microstructure. In situ observation shows that the preexisting bainitic laths significantly reduce the growth space of acicular ferrite plates. The average lengthening rate of acicular ferrite plates is ranged from ≈5 to ≈68 μm s−1 at a cooling rate of 3 °C s−1.
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