Thermodynamic aspects of gas-metal heat treating reactions

In the heat treatment of many alloyed steels, the relationship between the composition of the atmosphere and the carbon content of the steel in the austenite region is often re-quired in order to establish the proper heat treating atmosphere. Information regarding the atmospheric conditions under which the oxides and carbides of the various alloying elements are formed may also be necessary for the development of a suitable heat treat-ment environment. Both of these quantities can be estimated from available thermody-namic data. For the control of surface carbon, a generalized equation is given that can be used to approximate the relationship between the P2co/-PCO2 ratio in the furnace atmosphere and the carbon content. The effect of alloying elements such as Cr, Ni, Mo, Si, Mn, V, W, Co and Cu can be taken into account in the relationship based on existing data. Standard free energies of formation are used to estimate the stability region of the oxides and carbides of the common alloying elements. These data are presented in both modified Richardson diagrams and thermochemical phase diagrams.