Effect of External Stress on the Behavior of Oxide Scales on 9Cr-1Mo Steel

Author: Pillai S.R.   Barasi N.S.   Khatak H.S.   Gnanamoorthy J.B.  

Publisher: Springer Publishing Company

ISSN: 0030-770X

Source: Oxidation of Metals, Vol.49, Iss.5-6, 1998-06, pp. : 509-530

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Abstract

Tensile specimens of 9Cr-1Mo steel were subjected to oxidation in air at a temperature of 973 K for periods of 25, 47, 70, 97, 120, and 140 hr. The integrity of the oxide scale was examined by an in-situ technique which involved the recording of the acoustic-emission activity associated with the breaking of the scale. Specimens were also subjected to oxidation for the same times after subjecting them to an external stress of 40 MPa. The application of the external stress was found to alter the integrity of the scale. Spalling of the scale occurred to a lesser extent when external stress was applied due to partial release of growth stresses because of the elongation of the specimen. The scale buckled before spalling when no external stress was applied. On the other hand, the scale developed wedge-type cracks before spalling when subjected to external stress during oxidation. Postoxidation examination of the stressed specimen revealed substantial segregation of silicon and chromium at the grain boundaries (oxide ridges). The specimens, which were oxidized without external stress, revealed a reduction in the concentration of chromium on the surface. This behavior was attributed to enhanced spalling in the case of the latter specimen compared to the former.