Localized Coking on Austenitic Heat-resisting Steels in CO-H2 Mixtures at 923 K.

Abstract

The coking behavior on the austenitic heat-resistant steels (SUS309S and SUS310S) and iron in CO-H2 mixtures at 923 K was studied. Two types of carbon, laminar carbon and filamentous carbon, deposited on iron in large amount, while only filamentous carbon slightly deposited on the high temperature steels because of the formation of noncatalytic chromium-rich carbides on the surface instead of laminar carbon. On the steels, filamentous carbon deposited at some defects such as pittings and cracks occurred on the carbide scale. Thus, the increase in such surface defects by a severe surface finishing caused the acceleration of filamentous carbon deposition. The deposition of filamentous carbon on the surface defects or the localized coking is probably due to the depletion of chromium or the enrichment of iron at the exposed subsurface and thereby to the formation of an more unstable carbide. The behavior of filamentous carbon deposition on the heat-resistant steels seems to be essentially the same as that on iron. That is, the formation and decomposition of unstable carbides such as Fe3C and (Fe, Cr)3C may be involved in the process of filamentous carbon deposition on both iron and the heat-resistant steels.