Takeshi Hidaka

Influence of oxide inclusion compositions on microstructure and toughness of weld metal for high-strength steel

The effect of oxide inclusion compositions on microstructure and notch toughness of weld metal for HT780MPa class high-strength steel was investigated. Three different types of weld metals were prepared by varying deoxidant elements. Intragranular bainite microstructure and dispersed oxide inclusions were observed and quantified by an optical microscope and by electron backscattered diffraction analyses to reveal the affectors of notch toughness. It has been found that fracture appearance transition temperature of the weld metal depends on the size of intragranular bainite (acicular ferrite) nucleated on oxide inclusions and that the upper shelf energy is affected by the density of dispersive oxides inclusions. It has also been clarified that all oxide inclusions, which contribute to intragranular transformations, consisted of titanium-rich crystalline phases and manganese-rich amorphous phases. By detailed finite element (FE)-TEM observation, the crystalline phases were identified as MnTi2O4 having Baker–Nutting orientation relationship with the intragranular bainite. It is considered that the reason for the bainite to have the orientation relationships of both MnTi2O4 and austenite is that the MnTi2O4 is generated at the interface between the amorphous phases and austenite previous to the intragranular transformation.