INFLUENCE OF H2S ON METAL DUSTING

Presence of H2S in a carburizing, atmosphere causes S-adsorption which retards carbon transfer and deposition and can suppress metal dusting of iron and steels. In the latter process cementite Fe3C is an interm ediate, graphite deposition would initiate its decomposition but graph ite nucleation is prevented by adsorbed sulfur. Thus continued Fe3C gr owth can be observed in the presence of H2S. Thermogravimetric studies in CO-H-2-H2O-H2S mixtures have been con ducted at 500 degrees C at v arious carbon activities a(C) and H2S/H-2-ratios. With increasing a(C) higher H2S/H-2-ratios are needed to suppress metal dusting, with incr easing H2S/H-2-ratio the kinetics of Fe3C growth change from diffusion controlled parabolic kinetics to linear carbon transfer controlled ki netics. At very high a(C) greater than or equal to 1000 besides Fe3C a lso the Hagg carbide Fe5C2 was observed as an outer layer on the cemen tite.