MORPHOLOGY AND PHASE TRANSFORMATION OF GRANULAR BAINITE AND GRANULAR STRUCTURE

Study was made of the microstructural morphology and phase transformation mechanism of the "ferrite+islands" formed either as granular bainite in upper region of temperature range of bainitic transformation or as so called "granular structure" in temperature range of pro-eutectoid transformation in the low carbon alloy steels. The granular bainite stands out in surface relief during its transformation Its ferrite grows up longitudinally and form parallel laths The K-S orientation relationship is kept between ferrite and austenite The morphology of its islands appears as the shape of short lath and with a tendency of being parallelly distributed in the bainite-ferrite matrix. The granular structure does not stand out in surface relief in the formation of equiaxial ferrite. It's ferrite can grow across the grain boundary of parent phase, and has no strict orientation relationship with parent phase The shape of the islands is irregular, and its arrangement on the matrix is also irregular. The grain boundary of austenite couldn't be revealed clearly Except the difference of the transformation temperature, the granular structure possessed Widmanstatten ferrite is similar to the granular bainite in characteristics. Finally, the models of the growth of granular bainite and granular structure are suggested.