Acoustic emission monitoring of the cracking/spallation in the oxide scales developed over microstructurally different regions of a chrome-moly ferritic steel

Acoustic emission (AE) monitoring has been used to confirm the difference in initiation and intensity of the cracking and spallation of the scales over the regions with different microstructures in 9Cr-1Mo steel. As indicated by in-air thermogravimetric (TG) tests at 973K, the oxide scale on 9Cr-1Mo steel with 0.26%Si and fine grain size (90μm) suffered a pronounced weight-loss after about 60min., where as the scale on the coarse grain (360μm) steel suffered weight-loss only after 240min. AE monitoring under identical conditions suggest sudden and pronounced increase in AE activity after the respective durations in the two cases. Hence, the weight losses in both cases have been ascribed to spallation of the oxide scale. As suggested both by TG and AE tests, onset of pronounced scale spallation is considerably delayed in the case of the steel with a higher (0.75%) Si-content. Hence it is concluded that increase in Si-content and grain size of 9Cr-1Mo steel improves spallation resistance at high temperatures. (author)