Bainite in steel is an industrially useful structure. However, the controlling factor of its transformation start point is not known clearly. In this study, to clarify the effect of carbon content on the bainite transformation start temperature (B s ), we evaluated the dilatation curve and the microstructure in low carbon Fe-9Ni alloys. As a result, B s decreased with increasing of carbon content. Furthermore, the driving force of partitionless transformation from fcc to bcc at B s , which was calculated considering nickel segregation, was approximately constant at 400 J/mol in all alloys. This value is consistent with the driving force required for partitionless growth of ferrite, as reported in a previous study. This consistency suggests that B s depends on the martensitic growth behavior of lath-shaped ferrite, which is determined by the supercooling starting from the T 0 line.
The crystal orientation of nucleating austenite during reverse phase transformation of a C-Mn steel has been investigated by in situ EBSD technique using a heating stage of FE-SEM at temperatures ranging from RT to 800 C. It has been found that the multiple interfaces between a nucleating austenite grain and the surrounding parent ferrite grains are preferentially selected as the Kurdjumov-Sachs (K-S) relationship. More than 50% of austenite grains are surrounded with two or more parent ferrite grains satisfying the K-S relationship with a deviation within 7 . Based on these ndings, a nucleation model at triple junction is proposed, which supposes that the orientation of nucleating austenite is selected to satisfy the K-S relationship or the orientation relationship close to K-S relationship at two of the interfaces to ferrite grains, and at the other interfaces to minimize the misorientation from the K-S relationship.
We have investigated the crystallographic orientation relationship between ferrite and austenite and the interfacial planes between them of a low-carbon steel formed by the transformation from ferrite to austenite upon heating. Three-dimensional investigation using EBSD measurement with FIB serial sectioning technique is carried out on samples quenched from an early stage of the reverse transformation. The prior austenite orientation of martensite in the quenched microstructure is determined based on an analysis of the variants in the Kurdjumov-Sachs (K-S) relationship and the three-dimensional ferrite-austenite microstructure is successfully reconstructed. The crystallographic analysis on the three-dimensional microstructure has revealed that the nucleus of reverse-transformed austenite maintains the K-S relationship or close to it with two or three adjacent ferrite grains. The significance for reverse-transformed austenite to select these specific orientation relationships was estimated through a statistical assessment. In addition, the influence of crystallographic relationship between ferrite and austenite to nucleation and growth of reverse transformation is discussed.
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