Investigation of bainitic isothermal transformation products in unalloyed ductile cast iron

The properties of the products of isothermal transformation of undercooled austenite into bainite in unalloyed ductile cast iron were investigated using X-ray diffraction. The following parameters were investigated: the fraction of austenite in the cast iron matrix, the crystal lattice parameter, and the width of the diffraction lines of the alpha and gamma phases. The structures were studied using a TEM. It was observed that the temperature T-A and time tau(A) of the isothermal transformation significantly influence the nature of the alpha and gamma phases. The transformations are determined by the diffusion of carbon, and the maximum carbon content is approximately twice the equilibrium carbon content at the austenitising temperature. The lattice parameter of the a phase in the range of T-A studied decreases with increasing cooling time but increases in the upper bainite range. The increase in this lattice parameter results from the typical process of bainitic transformation during the retained austenite eutectoidal reaction (stage III). The crystal structure of the gamma phase in the upper bainitic region is more perfect than in the lower range. Within the investigated temperature range of T-A, bainitic ferrite continually improves its crystal structure. (C) 1996 The Institute of Materials.