Local variation of impact toughness in tandem electro-gas welded joint

To identify the microstructural factors causing the local brittleness in electro-gas (EG) welded joint, this study investigated the EG welded joints produced by tandem EGW process using two different ship-grade steel plates: high-nitrogen type TiN steel and conventional EH36 steel. Experimental results revealed that, in contrast to conventional steel, the TiN steel shows a large scattering in impact values at FL attributing to the localized grain coarsening limited to a very narrow area following fusion line. In addition, at the center of EG weld metal, a bundle of parallel columnar grains are formed aligning in a vertical welding direction resulting in the formation of grain boundary ferrite veins lined up in the same direction. Brittle nature of grain boundary ferrite makes the cracks propagate easily through these veins, causing a sudden drop in impact toughness at the center of EG weld metal.