Effect of variation of external stress on the oxidation behaviour of 9Cr-1Mo steel at 973 K

Specimens of 9Cr-1Mo steels were subjected to external stresses in the range of 20 to 40 MPa, The behaviour of scale on oxidation was examined at a temperature of 973 K. The integrity of the scale was recorded in-situ with the aid of an acoustic emission unit which responded to scale cracking. Investigations revealed that the integrity of the scale was nearly intact during oxidation at the isothermal condition. However, it was affected during the cooling of the specimen due to differential in the thermal expansion coefficients of the oxide and the alloy. Specimens were also subjected to oxidation without the application of external stress in order to delineate the effect of applying external stress. The durations of experiment under each applied stress conditions were in the range of 25 to 140 hours. Post-oxidation examination of the scale was carried out using scanning electron microscope. In the case of the specimens without external stress, spalling was preceded by buckling. Nearly the same behaviour was noticed when external stresses of 20 and 28 MPa were applied. However, at a stress of 40 MPa the scale showed wedge-type cracking prior to spalling. Analysis by energy dispersive X-Ray spectrometry revealed the segregation of silicon and chromium at the oxide ridges. Such segregations were more predominant in the case of the specimen subjected to external stress.