Diffusion interaction coefficients βCi and thermodynamic interaction coefficients ϵCi of carbon in alloyed austenitic steels

Abstract

Diffusion experiments on carbon redistribution in welded FeNiCrSiMnC steels have been carried out between 750 and 1150°C. Experimental samples had the form of sandwich-like couples. It is stated that the carbon diffusivity depends on the chemical composition of the individual steels. This dependence can be expressed by an exponential function of the products β C i N i, where β C i is the diffusion interaction coefficient and N i the concentration of the alloying element i. The activity of carbon in the welded steels is discussed on the basis of Wagner's model containing the activity interaction coefficients ϵ C i. Special attention is paid to the investigation of silicon influence on carbon diffusivity and activity. It is confirmed that silicon decreases the carbon diffusivity ( β C Si < 0) and increases its activity ( ϵ C Si > 0). This effect is explained by a repulsive interaction of silicon on carbon atoms which are associated with neighbouring iron atoms, and by an attactive SiC interaction on next-nearest interstitial sites.