Effect of pre-oxidization on the cyclic coking and carburizing resistance of HP40 alloy: With and without yttrium modification

The pre-oxidation results demonstrate that rare earth elements play an important role on the scale adherence of alloy after the oxidation at elevated temperature. The better scale adherence of the yttrium modified alloy permits a superior cyclic coking and carburizing resistance. This is because the integrity chromia layer, which prevents fast ion diffusion, can retard the outer catalytic coking and the inner carburizing of pre-oxidized alloy. As for the alloy under the cast condition, severe coking and carburization has been detected, which was also observed on the pre-oxidized samples where the scale suffered from severe spalling.