Sulfur effects on the internal carburization of Fe-Ni-Cr alloys

Several commercial and laboratory-cast model austenitic alloys have been exposed in both sulfur-free carburizing environments and also in carburizing atmospheres to which additions of H₂S have been made. These studies were concentrated over the temperature range 1223–1323 K at a fixed carbon activity (a_c=0.8) with sulfur activities ranging from 2.2×10^–12 bar to 1.4×10^–9 bar. Under conditions of sulfur adsorption, e.g., 5.5 × 10^–11 bar at 1273 K, the blocking of adsorption sites for methane resulted in a transition from the parabolic kinetics observed during sulfur-free carburization to surface controlled linear kinetics. Higher levels of H₂S promoted the formation of a surface layer of chromium sulfide which reduced internal carburization but became a problem itself. The role of minor alloying elements has been established and the use of thermodynamic phase stability diagrams in defining the optimum conditions for sulfur inhibition of carburization evaluated.