Detection of breakaway oxidation and spalling in the oxide scales of 2¼Cr-1Mo steel using acoustic emission technique

Abstract

The onset of breakaway oxidation and in-situ cracking and spalling of the oxide layer formed on 2¼Cr-1Mo steel have been detected by the use of the acoustic emission (AE) technique. Various AE parameters, viz., acoustic emission event counts vs time, acoustic emission counts vs time, and voltage level vs time have been used to detect these phenomena. It has been found that variations in AE parameters are marginal during heating at 600, 700, and 800°C, and a sudden rise in these parameters occurs during cooling. Increase in AE activity during cooling has been related to spalling of the oxide layers. At 900 and 950°C, a considerable increase in AE parameters (except voltage level) has been detected after certain times at the respective temperatures. By comparing with corresponding thermogravimetric curves, this increase in AE parameters has been attributed to the onset of breakaway oxidation. A very large increase in AE parameters during cooling from these temperatures has been related to in-situ cracking.