Microstructural characterization and mechanical properties of a Q550W weathering steel welded joint under different heat inputs

Metal active gas arc (MAG) was applied to weld a hot-rolled weathering steel, three heat inputs (5100, 6000 and 7200 J/cm) were used by changing the welding speed. The effects of heat input on the microstructure on the welded joint were investigated by optical microscope (OM), field emission scanning electron microscope (FESEM), electron-backscattering diffraction (EBSD). Tensile and hardness tests were also carried out in this paper. The results indicate that the yield strength decreases with the increase in heat input and all the specimen fractured at the mixed-grained heat affected zone (MGHAZ). The MGHAZ consist of polygonal and equiaxed ferrite. The yield strength decreases by 59 MPa when the heat input increases from 5100 to 7200 J/cm, which is attributed to the weakening of grain refinement strengthening and dislocation strengthening. The plasticity also decreases by 3.6% with the increase in heat input. The microstructure is relatively uniform when the heat input is 5100 J/cm. However, the original microstructure become coarsen with the increase in heat input, resulting the non-uniform microstructure at higher heat input. The 5100 J/cm specimen could deform more coordinately during tensile process, so the specimen has highest plasticity among three heat inputs.