Co-deposit of iron and chromium on a high carbon content steel by pack cementation

Pack cementation chromizing is a process of chromium deposit by activated cementation. When the substrate is a high carbon content steel, a layer of carbides M(7)C(3) and M(23)C(6)(M=Cr,Fe) is formed. Consequently the substrate surface composition evolves significantly during the treatment. The thermodynamic modelling carried out aimed at studying the influence of this evolution on the deposit mechanisms when using ferrochromium as source of chromium. Both the condensed solutions properties of the C-Cr-Fe system (with the aid of the sub-lattices model) and the stoichiometrics substances (condensed and gazeous) properties of the C-Cr-Fe-N-H-Cl system (with the aid of COACH databank) have been modelled. The two equilibriums which take place have also been modelled (with the aid of the GEMINI2 computation code, by minimizing the Gibbs energy). Firstly the equilibrium between the ferrochromium and the halide compound which are present in the cement. Secondly, the equilibrium between the gazeous phase of this first equilibrium and the substrate surface. For this last equilibrium we have carried out some computations which describe the evolution of the substrate surface composition. Our computations show that in our experimentals conditions (source of chromium and halide compound: ferrochromium with 30% of iron and ammonium chloride; substrate:XC65 steel; temperature:980 degrees C): In the beginning of the treatment there is only a chromium deposit according to the reactions of exchange ((CrCl2)+<Fe>(substrat)--><Cr>(depose)+(FeCl2) and reduction ((CrCl2)+(H-2)--><Cr>(depose)+2(HCl)) which occur in similar proportions then, when the M(23)C(6) carbide is present in the substrate surface, the reaction of reduction becomes strongly predominant until it occurs a co-deposit of chromium and iron, according to the reaction of reduction of those two elements. Comparison of chromium and iron activities in the ferrochromium and M(23)C(6), carbide explains this evolution : the activities of chromium and iron in the ferrochromium can be simultaneously higher than the activities of those two elements in the M(23)C(6) carbide, that is to say in the substrate surface. Quantitative analysis carried out by electron microprobe has shown that our thermodynamic modelling was realistic.