Effects of Silicon Content and Oxidation Potential on the Carburization of Centrifugally Cast HK-40

The influence of oxidation potential on the gas carburization resistance of cast HK-40 tube sections was evaluated in equilibrated hydrogen-ethane-steam mixtures or equivalents at 1366 K. Below an oxidation potential roughly corresponding to the stability limit of SiO2 as a surface species, carburization was pronounced in all alloys, with only a slight reduction in the extent of carbon pickup with high alloy silicon contents. At higher oxygen contents in the gas, resistance was proportional to both alloy silicon content and to oxidation potential. The results are interpreted in terms of silica stability and the kinetics of silica formation. It is suggested that more realistic assessments of materials performance in carburizing environments will require the oxidation potential to be taken into account.