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

The coking behaviour of a range of austenitic, heat-resistant steels has been examined in the temperature range 700-1000-degrees-C. At and below 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 non-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 became 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 directly from the gas stream being much faster than the rate of coke formation on the alloy surface. Oxide-protected alloys all gained weight at a similar rate, the rate being that of coke deposition on coke. Oxide films containing aluminium were more effective in excluding carbon from the alloy than chromium-containing oxides. However, under reducing conditions, preformed oxide films were not beneficial in limiting carburization in the longer term, because they were prone to spalling, cracking and conversion to non-protective carbide.