Effects of Zr-Ti on the microstructure and properties of flux aided backing submerged arc weld metals

Zr-Ti alloy powders were added into the weld groove to join D36 steel by using flux aided backing submerged arc welding (FAB-SAW). The inclusions, microstructure and mechanical properties of Zr-Ti miroalloyed weld metals were investigated. The results showed that the core of inclusions was composed of Zr, Al, Ti, Si, Mn oxides, while the surface layer of inclusions was composed of TiO (or Ti2O3), ZrO2 and MnS. The first precipitated ZrO2 can act as nucleation sites for Al, Ti, Si, Mn oxides and increases the fraction of inclusions with size of smaller than 1.0 μm. Due to the refined size and increased density of inclusions, the proportion of acicular ferrite (AF) was significantly increased while the proportion of grain boundary ferrite (GBF) and ferrite side-plate (FSP) were remarkably decreased in the microstructure. The tensile properties and impact toughness at −20 °C of weld metal were obviously promoted. The nucleation of AF was ascribed to the low lattice misfit between TiO (and ZrO2) and α-Fe as well as a high inert interfacial energy of MnS. When the content of Zr was 0.09 wt %, the percentage of AF in FAB-SAW weld metal was reached to 82%, resulting in the weld metal with elongation of 25%, ultimate strength of 615 MPa and impact energy at −20 °C of 145 J.