Pearlite Growth Rate in Fe-C Binary and Fe-X-C Ternary Steels

Abstract

The diffusion path during the pearlite transformation has been studied from the point of view of mixed diffusion-controlled growth. It is reason- able that the solutes do not choose only one path to move along when they have multiple available paths. This research has provided better fit for reported growth rate measurements of pearlite than volume and boundary diffusion models in isolation. Local equilibrium is assumed to be maintained at ferrite/austenite and cementite/austenite interfaces. For the Fe-C binary system, the tie-lines which describe the interfacial compositions that maintain equilibrium at the interfaces are unique at a given temperature. Unlike the binary system, the local equilibrium condition in ternary system that contains an interstitial and substitutional solute needs to take account of the kinetic effect of solute because the complexity of the system is increased. Pearlite transformation involves the cooperative growth of ferrite and cementite, thus, this also needs to be considered to find the appropriate local equilibrium condition. Two principles are required: one is mass conservation and the other is solute symmetry: the growth rate should have the same result when it is calculated by diffusion of any solute. The analysis is supported with transmission electron microscopy results and gives good agreement with Mn, Ni, and Co. The local equilibrium condition for the pearlite transformation has been analysed for various diffusivity ratios of the two solutes and two different compositions. Two criteria can be distinguished when diffusivity ratio goes to infinity by observing the partitioning of substitutional solute between ferrite and cementite. The boundary between PLE and NPLE for pearlite is expected from the solution of the mixed diffusion equation.