Yoshitomi Okazaki

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.

Comparison of Microstructures at As Welded Zone and Reheated Zone in 9%Ni Steel Similar Composition Weld Metal

9%Ni steel similar composition weld wires are applied for constructions of liquefied natural gas storage tanks by gas tungsten arc welding and the weld metals show high toughness and strength level. In this study, the microstructures of 9%Ni steel similar composition weld metals have been investigated. It was found that fine microstructure was formed at the reheated zone and contributed to the high toughness level. The nucleation rate of blocks and packets from prior-austenite grain boundaries was larger at the reheated zone than the as-welded zone and resulted in the fine microstructure of the reheated zone. Our results showed that the lowering of Ni concentration at and near the prior austenite grain boundaries by the reverse transformation caused the larger nucleation rate at the reheated zone.