Acoustic emission studies during the high temperature oxidation of 2.25Cr–1Mo steel at 1173 K

Abstract

Breakaway oxidation behaviour of 2.25Cr–1Mo steel has been studied at 1173 K, using the acoustic emission (AE) technique. AE parameters, i.e. events, ring down counts (RDC), rise time (RT) and event duration (ED) show negligible increase during isothermal heating at 1173 K, up until a point where a sudden increase in AE activity was observed. This point, after 120 minutes, corresponds to the onset of breakaway oxidation as confirmed by thermogravimetry and microscopy. It is, therefore, confirmed that breakaway oxidation in 2.25Cr–1Mo steel results from mechanical failure of the scale. This is one of the means for release of growth stresses in the oxide, generated as a result of conversion of metal into its various oxides. The results do not support the postulate of a chemical theory for the occurrence of breakaway oxidation in 2.25Cr–1Mo steel, as no depletion of chromium was observed in the underlying alloy as well as no trace of Cr2O3 was observed in the underlying matrix. The AE technique also detects the breakaway point earlier than the thermogravimetric method because microcracking in the oxide scale preceded the onset of mass gain by some considerable time. The AE results have been correlated with those of scanning electron microscopy (SEM) and electron probe micro analysis (EPMA).