Phase transformation theory applicable to reaustenitisation that takes place upon heating low-alloy steels is described. The assumption of local equilibrium is made. Diffusivity of interstitial solute in austenite is treated as concentration dependent and no diffusion is allowed in cementite and ferrite. Under these assumptions, the process is controlled by the diffusion of the interstitial solute in the growing phase. A variational dual extremum principle is used to calculate the interstitial solute concentration profile for specified interface motions. These results are compared with those obtained for a constant diffusivity equal to a weighted average of the concentration-dependent diffusivity and to a variety of simple approximations to the full problem.
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