Thermal degradation of alloy CCA617 in pure steam at 750 and 850°C

Abstract

Oxidation and oxidation-induced microstructure evolution of CCA617 alloy was studied in pure steam at 750 and 850°C. Results showed that the alloy was oxidised approximately following a parabolic law. Temperature increase greatly promoted the external and internal oxidation. A single (Cr, Mn)2O3 layer with distribution of minor TiO2 and MnCr2O4 formed at 750°C whereas a duplex oxide scale formed at 850°C consisting of a thin MnTiO3 outer layer and a thicker (Cr, Mn)2O3 inner layer. TiO2 and Al2O3 were formed internally especially along grain boundaries. Intragranular Mo-rich M23C6 type carbides were also observed. The carbides dissolved into the matrix as oxidation progressed. Based on detailed compositional and microstructural characterisation of the oxidised alloy, the mechanisms of external and internal oxidation as well as dissolution of the intragranular carbides are presented.