Influence of prior-austenite grain size on the oxidation behavior of 9 wt.% cr-1 wt.% Mo steel

The effect of grain size on the oxidation of 9 wt.% Cr-1 wt.% Mo steel (with prior-austenite grain sizes of 90, 210, and 360 μm) has been studied at 973 K in air. After the initial stages of oxidation (up to 60 min), the fine-grain specimen (90 μm) suffered heavy spallation. A similar spallation took place after 120 min in the case of the specimen with intermediate grain size (210 μm), and after 360 min in a coarse-grain (360 μm) specimen. This increase in the duration for the onset of pronounced spallation with grain size has been attributed to the smaller area of the grain boundaries which are the locations of higher growth stresses leading to spallation. This dependence of spallation characteristics on the alloy grain size has been confirmed by the incidence of higher acoustic-emission activity during AE monitoring. Scanning electron microscopy (SEM) has confirmed the occurrence of pronounced spallation of the oxides formed in the areas adjoining the grain boundaries.