Carburisation of Fe-X (X=Si, Mo, V) diffusion couples

Experimental studies of carburisation of diffusion couples Fe/Fe-4 at.% Si, Fe/Fe-7.5 at.% Mo and Fe/Fe-20 at.% V are reported. The diffusion couples were carburised at 700 ∘C for 10 h in a strongly carburising CO-H2-H2O-H2S gas mixture imposing a carbon activity aC=100. The growth of cementite at the surface of the couples was found to depend on the concentration of the alloying element X in the diffusion couple, starting from the pure Fe-part up to a limiting alloy composition. The alloy content of cementite was determined by EPMA measurements showing that cementite formed according to a regime of local equilibrium without partitioning (LENP). The observed upper composition limit for cementite formation is related to an upper limit of carbon dissolution in ferrite. It is shown that cementite dissolves a small amount of Si, in contrast to the general assumption and thermodynamic description in thermodynamic databases. The observed upper limit of LENP reaction was at about 0.0005 at.% Si. This suggests that the upper solubility of Si in cementite in equilibrium with ferrite may be of the order of 1 at.% Si.