Study of the oxidation behaviour of 9 Cr-1 Mo steel using the acoustic emission technique

The onset of breakaway oxidation andin situ cracking of the oxide formed on 9 Cr-1 Mo steel in air at 900 and 950°C has been detected by acoustic emission (AE) technique. AE parameters, i.e. AE counts and AE event counts, show negligible increase during isothermal heating at these temperatures, until at a point, where a sudden increase in AE activity is found. This point corresponds to the onset of breakaway oxidation. An enormous increase in AE activity after the start of cooling has been attributed to the separation of scale from matrix as a result of thermal stresses arising during cooling. Peak amplitude distribution is measured andb values are calculated which help in distinguishing the phenomenon at 900 and 950°C in terms of the amplitudes of the AE emission released