The matching between the wire material and the base metal has an extremely important influence on the performance of the welded joint. In this work, the effects of two welding materials THQ440-NQ-II and ER50-6 with different compositions on the microstructure, mechanical properties and corrosion resistance of bridge weathering steel Q345qDNH were investigated. Although the mechanical properties of the two welded joints are lower than those of the base metal, the yield strength, elongation after fracture, impact toughness at 20 °C and À 40 °C of THQ440-NQ-II welded joints are 6.8 %, 23.5 %, 13.7 %, 18 % higher than those of ER50-6 welds, respectively. THQ440-NQ-II welds are ductile fractures and quasicleavage fractures at 20 °C and À 40 °C, respectively; while ER50-6 welds are quasi-cleavage fracture and cleavage fracture at 20 °C and À 40 °C, respectively, the ductile-brittle transition temperature of ER50-6 weld is higher than that of THQ440-NQ-II weld. Comparing the corrosion resistance in 3.5 % sodium chloride (NaCl) solution, THQ440-NQ-II weld is better than the base metal, while ER50-6 welds are lower than the base metal. This is because the nickel, chromium, copper elements contained in the THQ440-NQ-II welding wire can inhibit the formation of Widmanstätten structure and upper bainite during the welding process, refine the structure, increase the content of acicular ferrite, and reduce the ductilebrittle transition temperature of welding joint, improves the corrosion resistance of the weld.
The periodic immersion corrosion behavior and corrosion mechanism of Q420qNH steel weld, heat-affected zone and plate sample in simulated industrial atmospheric medium were studied. The corrosion type of the three samples is the non-uniform total corrosion, and the corrosion products at different stages are different. Ferric oxide and lepidocrocite (γ-FeOOH) are generated in the immersion stage and the initial stage of infrared drying, respectively. However, lepidocrocite (γ-FeOOH), ferrosoferric oxide (Fe 3 O 4 ) and goethite (α-FeOOH) generated in the later stage of infrared drying. The infrared drying process is conducive to the formation of goethite (α-FeOOH). The corrosion rate of weld is always lower than that of plate, and the I α-FeOOH /I γ-FeOOH ratio of the weld is always higher than that of plate and heat-affected zone sample. In the later stage of corrosion, the self-corrosion current density of the three samples from small to large is weld, heat-affected zone and plate, and self-corrosion potential from large to small is weld, heat-affected zone and plate. The resistance of the rust layer of the weld and heat-affected zone sample is 1.81 times, 1.29 times of that of the plate, respectively. This is because the weld is composed of a large number of small angular grain boundary structures dominated by tiny acicular ferrite in the original austenite grain, which is conducive to the improvement of corrosion resistance. In addition, the higher chromium/carbon (Cr/C), copper and nickel elements of the weld can promote the conversion from lepidocrocite (γ-FeOOH) to goethite (α-FeOOH), and improve the density and stability of weld rust layer.
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