Abstract
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 electron backscattered diffraction (EBSD) analyses to reveal affectors of notch toughness.It has been found that FATT of the weld metal depends on size of intragranular bainite (acicular ferrite) nucleated on oxide inclusions and that the upper shelf energy is affected by 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 FE-TEM observation, the crystalline phases was identified as MnTi2O4 having B-N orientation relationship with the intragranular bainite. It is considered that the reason the bainite have the orientation relationships both MnTi2O4 and austenite is that the MnTi2O4 is generated at the interface between the amorphous phases and austenite previous to the intragranular transformation.
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