Oxidation behaviour of 2.25Cr-1Mo steel and its characterisation using acoustic emission

Abstract

2.25Cr-1Mo steel is widely used for high temperature applications as heat exchangers and pipings in power plants. The microstructural instability and discontinuity of oxide scale are problems associated with their long exposure at high temperature. This effect limits the life cycle of the component. In the present investigation, in situ oxidation behaviour of this steel has been studied using acoustic emission (AE) technique. The potential of this technique is described in detecting online occurrence of breakaway oxidation. The breakaway oxidation in 2.25Cr-1Mo steel is shown to be due to mechanical failure of the scale which is one of the means of releasing stresses generated during oxidation. The results do not support the chemical theory of breakaway oxidation as reported by other investigators. The analysis does not reveal depletion of chromium either in underlying scale or as traces of Cr2O3 in the matrix. The beneficial effects of AE technique in detecting breakaway point earlier than the thermogravimetric method are discussed. The microcracking in the oxide scale is observed to proceed after a considerable duration of onset of weight gain. Frequency analysis of the AE signal obtained during various stages of oxidation, namely prebreakaway, post-breakaway and internal cracking of the oxide layers resulted in differentiation of these processes on the basis of their predominant frequency contents present in the corresponding frequency spectra of AE signals.