A KINETIC AND MORPHOLOGICAL-STUDY OF THE COKING OF SOME HEAT-RESISTANT STEELS

The coking behaviour of a range of austenitic, heat-resistant steels h as been examined in the temperature range 700-1000-degrees-C. At and b elow 800-degrees-C, catalytic coke in the form of bundles of filaments formed at localized defect sites in the carbide scales. A wide range in weight-gain kinetics resulted from the differing efficacy of the no n-catalytic carbide scales in excluding carbon from the catalytically active substrate. At and above 900-degrees-C, catalytic coke formation gave way to pyrolytic coke formation and internal carburization becam e significant. Parabolic kinetics resulted from the fact that internal carburization was rate-determining. Carburizing alloys gained weight an order of magnitude faster than did alloys protected by oxide films. This was a consequence of dissolution of carbon into the alloy direct ly from the gas stream being much faster than the rate of coke formati on on the alloy surface. Oxide-protected alloys all gained weight at a similar rate, the rate being that of coke deposition on coke. Oxide f ilms containing aluminium were more effective in excluding carbon from the alloy than chromium-containing oxides. However, under reducing co nditions, preformed oxide films were not beneficial in limiting carbur ization in the longer term, because they were prone to spalling, crack ing and conversion to non-protective carbide.