Carburization Mechanisms of High Chromium Alloys

Carburization is a major mode of corrosive degradation of high chromium alloy materials in many high temperature processes. The process generally involves the transfer of carbon from the gas phase onto the alloy surface followed by diffusion of carbon to the alloy interior with concurrent precipitation of chromium-rich carbide phases. The kinetics of this overall process has been determined by thermogravimetry in the temperature range 1173-1373 K using Fe-20 Cr and Ni-30 Cr model alloy systems and H~-CH4 gas mixtures to provide the carburizing medium. Mathematical expressions for the overall kinetics have been developed in terms of surface reaction and diffusion steps. Both processes contribute to carburization, the influence of the surface reaction step increasing with increase in temperature. Additions of sulfur as H2S to the gas phase slows down the surface reaction step sufficiently so that the whole carburization process can become surface reaction controlled.