Behavior of oxide scale on 9Cr-1Mo steel under varying external stresses

The effects of different external stresses on the oxidation behaviour of 9C r-1Mo steel were investigated Tensile specimens were subjected to stresses of 11, 20, 28, and 40 MPa and the oxidation behavior was studied at a tempe rature of 973 K. The elongation of the specimen was determined by an extens ometer. An acoustic-emission unit was employed to monitor the in the oxide scale. The oxide scale was found to undergo buckling before spalling, in th e case of unstressed specimens, and in the case of those specimens subjecte d to a stress up to 28 MPa. The specimen will 40 MPa str ess showed the dev elopment of cracks. The application of external stress up to 28 MPa (averag e strain rate for 28 MPa stress was 1.2 x 10(-7)s(-1)) had a beneficial eff ect with respect to the adherence of the oxide scale, The specimen with 40 MPa of stress (average strain rate of 5.2 x 10(-7)s(-1)) showed substantial weight gain on oxidizing up to 140 hr. The unstressed specimen revealed en hanced spallation when compared with the stressed ones. Investigation by SE M revealed the spalling of the oxide scale when the duration of oxidation,w as higher than 70 hr at all stress levels employed in the present investiga tion. Formation of cracks at 40 MPa exposed fresh areas to oxygen and cause d accelerated oxidation. Analysis by energy-dispersive X-ray spectrometry ( EDS) revealed the segregation of chromium at the oxide ridges. The segregat ion of silicon was also Significant for the specimen subjected to a stress of 40 MPa. Analysis by XRD clearly revealed the presence of oxides of chrom ium and iron (Cr2O3 and Fe2O3) and spinel-type oxide (FeCr2O4).