The .GAMMA..RAR..ALPHA. Transformation in Low Carbon Irons

The influence of cooling rate on transformation temperatures in low carbon irons is reviewed. Plateaux are obtained in plots of transformation temperature vs.cooling rate. Comparison with later published TTT diagrams show that these plateaux correspond with the nose of 'c' curves on TTT diagrams. This leads to a proposed TTT diagram for iron. Transformations in Fe-Ni, Fe-Cr and Fe-C alloys, and low carbon irons are reviewed. The morphology and kinetics of the individual transformations, namely equi-axed ferrite, massive ferrite, bainitic ferrite, lath (massive) martensite and twinned martensite are then discussed. A recent theoretical treatment of the equi-axed ferrite transformation is summarised. This theory indicates that the kinetics of the equi-axed ferrite transformation is controlled by growth. Correlation is made between microstructures obtained in binary alloys and those obtained in Fe-Nb-C alloys and commercial HSLA steels.