Yusaku Tomio

Effect of Close-Packed Plane Boundaries in a Bain Zone on the Crack Path in Simulated Coarse-Grained HAZ of Bainitic Steel

Global effect of Bain-zone boundaries and the local effect of close-packed plane (CP) boundaries on the path of secondary cleavage cracks (observed on a fractured V-notch Charpy specimen) were visualized and discussed in simulated coarse-grained HAZ of bainitic steel. Microstructural unit map (Bain-zone map and CP map) was obtained by electron backscatter diffraction method for a prior austenite grain of a few hundred micrometers. Furthermore, a correlation between CP boundaries and sites that favored formation of the blocky martensite–austenite (M–A) constituent was confirmed. It was clarified that two crack deviation/local changing factors (CP boundaries and blocky M–A) are paired.

Visualization and Analysis of Variant Grouping in Continuously Cooled Low-Carbon Steel Welds

The variant grouping behavior of a simulated heat-affected zone in low-carbon steel was tracked in situ and analyzed by laser scanning confocal microscopy and electron backscatter diffraction. By visualizing the microstructural changes and crystallographic properties under rapid cooling conditions, the transition from Bain zone grouping to close-packed plane grouping was directly studied. The results were compared with those obtained under slow cooling and the link between the variant grouping behavior and the nested microstructure of lower bainite and martensite is clarified.

Visualization of Microstructural Factor Resisting the Cleavage-Crack Propagation in the Simulated Heat-Affected Zone of Bainitic Steel

Cleavage-crack propagation behavior was investigated in the simulated coarse-grained heat-affected zone (CGHAZ) of bainitic steel using electron backscattering diffraction (EBSD) pattern analysis when a low heat input welding was simulated. From viewpoint of crystallographic analysis, it was the condition in which the Bain zone was smaller than the close-packed plane (CP) group. It was clarified that the Bain zone and CP group boundaries provided crack-propagation resistance. The results revealed that when the Bain zone was smaller than the CP group, crack length was about one quarter the size of that measured when the CP group was smaller than the Bain zone because of the increasing Bain-zone boundaries. Furthermore, it was clarified that the plastic work associated with crack opening and resistance at the Bain and CP boundaries could be visualized by the kernel average misorientation maps.

Correlation Among the Variant Group, Effective Grain Size, and Elastic Strain Energy During the Phase Transformation in 9Ni Steels

The effect of carbon content on the density of variant-pair boundaries was investigated in 9Ni steel using an electron backscatter diffraction patterns method. The changes in the density of variant-pair boundaries were correlated with the nondestructive measured values of shear modulus of the austenite phase at the phase transformation point. Furthermore, the effective grain size was correlated with the shear modulus and the density of variant-pair boundaries. These relations are discussed from the viewpoint of self-accommodation of elastic strain energy and the nucleation event in the bainite and martensitic transformations.