Influence of Oxides on Microstructure and Notch Toughness of Weld Metal with Flux-cored Wires for High Strength Steel

In recent years, various steel structures tend to be larger and the recent trend of applying high strength steels makes it possible to reduce the structure weight. Notch toughness as well as strength is essential for materials used in the offshore structure and bridge construction industries and achieving both of them is a big challenge. While covered electrodes have been widely used in high strength steel structures in out-of-position welding, the usage of flux-cored wire (FCW) is desired because of its advantages over covered electrode in terms of operational efficiency and less skill requirement. TiO2 type FCW is most popular for all positional FCWs for mild steel and HT590MPa class steel and its excellent welding usability can be obtained with FCW for high strength steels up to HT780MPa. However, the weld metal made by TiO2 type FCW contains higher oxygen, making it more difficult to achieve superior notch toughness. By contrast, basic type FCW capable of achieving higher notch toughness is not good in usability in out-of-position welding. In this report, the effects of various oxides on microstructure, notch toughness, welding usability and oxygen content in the weld metal of high strength steel FCW are discussed.It has been clarified that notch toughness can be improved by decreasing the TiO2 activity and the density of dispersive oxide inclusions, leading to less oxygen content in the weld metal of TiO2 type FCW. In contrast to this, the oxygen content of the weld metal made by Al2O3 type FCW is high, and thus notch toughness cannot be improved even if its microstructure is refined. Furthermore, thermo-mechanical calculation has proved that welding usability in the vertical position becomes better when the solid-liquid coexistence temperature range of slag is wider.