Effect of microstructure on the mechanical properties and corrosion resistance of a welded joint of 620-grade marine steel

Abstract

The relationship between the microstructure and the mechanical and corrosion properties of a welded joint of 620-grade marine steel was studied using metallographic microscopy, scanning electron microscopy, an energy dispersive spectrometer, transmission electron microscopy, and microhardness and tensile tests. The results showed that the strength and hardness of the weld center area (WMmid) were higher than those of the inner and outer welding surface region (WMin and WMout) because the volume fraction of the martensite-austenite (MA) constituents (21.6%) was higher than that in WMin and WMout (18.0% and 14.3%, respectively). There were numerous MnO-Al2O3-SiO2-TiO2-type inclusions located at the bottom of dimples in the fracture surface; however, the MA constituents took precedence over this kind of inclusion in inducing pitting corrosion. In contrast, pitting corrosion can be initiated by Al2O3-MgO-CaO-CaS inclusions in the heat-affected zone (HAZ) and base metal (BM). The corrosion resistance of the welded joint was in the order of weld metal > HAZ > BM. The WMmid with smaller dendrite spacing and a larger size of MA constituents had better corrosion resistance compared with the WMin and WMout. The corrosion resistance of the HAZ decreased in the sequence of coarse grain HAZ, fine grain HAZ, and intercritical HAZ.