Modifying of microstructure and toughness in the weld metal prepared by welding wire containing nanosized titanium oxides

Highlights: • A method to prepare welding wire containing nanosized titanium oxides was introduced and such welding wire was prepared. • The coarsened titanium oxides in the weld metal modified the microstructures and improved toughness in the weld metal. • Toughening mechanism of the weld metal was revealed through instrumented impact testing and fracture observation. Titanium oxides are beneficial to the weld metal for their nucleation effect on acicular ferrite. The present study puts forward a new method to introduce fine titanium oxides into the weld metal by using welding wire containing nanosized titanium oxides. In the study, a special way was introduced and it was adopted to prepared solid welding wire containing nanosized titanium oxides. The W1 weld metal was obtained by using this welding wire after multi-layer multi-pass welding; as a contrast, the W2 weld metal was prepared by using conventional welding wire of similar composition under the same welding condition. Microstructures of the two weld metals were studied by OM and SEM; nanosized titanium oxides in the welding wire and inclusions in each weld metal were investigated by SEM and TEM. Toughness of the two weld metals was measured through instrumented impact test; fracture and crack behaviour was analyzed with SEM observation. Through microstructure characterization, it is concluded that nanosized titanium oxides in the welding wire are transitted into the W1 weld metal to form Ti₃O₅ inclusions and size of the inclusions is larger than the nanosized titanium oxides in the welding wire due to cluster of the titanium oxides in the weld metal. The inclusion size just locates in the size range that is favorable for acicular ferrite nucleation. Through comprehensive analysis of the instrumented impact test, fracture and secondary crack, it is revealed that W1 weld metal shows higher crack initiation energy, higher crack propagation energy and higher impact toughness compared with the W2 weld metal, which results from finer inclusions in the W1 weld metal prepared by the special way.