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 activa ted 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. Consequen tly 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 f errochromium as source of chromium. Both the condensed solutions prope rties of the C-Cr-Fe system (with the aid of the sub-lattices model) a nd the stoichiometrics substances (condensed and gazeous) properties o f 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 modell ed (with the aid of the GEMINI2 computation code, by minimizing the Gi bbs energy). Firstly the equilibrium between the ferrochromium and the halide compound which are present in the cement. Secondly, the equili brium between the gazeous phase of this first equilibrium and the subs trate surface. For this last equilibrium we have carried out some comp utations which describe the evolution of the substrate surface composi tion. Our computations show that in our experimentals conditions (sour ce 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 acc ording to the reactions of exchange ((CrCl2)+<Fe>(substrat)--><Cr>(dep ose)+(FeCl2) and reduction ((CrCl2)+(H-2)--><Cr>(depose)+2(HCl)) which occur in similar proportions then, when the M(23)C(6) carbide is pres ent in the substrate surface, the reaction of reduction becomes strong ly predominant until it occurs a co-deposit of chromium and iron, acco rding to the reaction of reduction of those two elements. Comparison o f chromium and iron activities in the ferrochromium and M(23)C(6), car bide 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 sub strate surface. Quantitative analysis carried out by electron micropro be has shown that our thermodynamic modelling was realistic.