Detection of Crack and Spallation of Oxide Scales using Acoustic Emission Technique

Acoustic Emission has been finding increasing application for monitoring of aging infrastructures. The materials undergo degradation mechanism during long service exposure at high temperature. Experiments have been carried out on recently developed steel grades P91, P92 and E911 used in thermal power plant materials using acoustic emission (AE) technique under isothermal oxidation in air at 950°C and 1000°C upto 5h. AE parameters count and voltage level shows negligible during isothermal heating for the alloys at these temperatures until the test duration, however a sudden increase is AE activity is found while cooling. An enormous increase in AE activity occurs after the start of cooling has been related to spallation of oxide layers. A very large increase in AE parameters for P92 and E911 alloy at 1000°C during cooling from this temperature has been related to cracking of oxide scale. The oxidation specimen was analyzed using various surface analytical techniques such as scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX).