Effect of alloying elements on the γ to α transformation in steel. I

Abstract

A newly constructed computer program was used to simulate partitionless growth of α in γ of Fe-Ni-C alloys, taking into account so-called solute drag by evaluating the dissipation of Gibbs energy due to diffusion inside the interface and in the nickel spike being pushed in front of it. It could be shown how the conditions at the α/γ interface vary with the velocity. A continuous change from paraequilibrium to quasi-paraconditions could be illustrated in the phase diagram. By combination with an approximate analytical growth equation, it was possible to derive the thickness of α as function of time. The growth velocity was assumed to start at very high values but decreased due to the pile-up of carbon. For alloy compositions outside the limit for quasi-paraconditions and just inside it the growth was predicted to stop suddenly when critical conditions are approached during the reaction. For alloy compositions further inside the limit, there was no such stop, except by the action of impingement of diffusion fields for carbon. Somewhere between the lines for paraequilibrium and quasi-paraconditions there is a rather rapid drop of the final thickness of α. In order to explain experimental information on this drop one must accept an appreciable tendency of segregation of nickel to the α/γ interface.