Materials Science and Engineering, A132 ( 1991 ) 89-95 89 Characterization of small crack growth in 12% CrMoV steel under high temperature, low cycle fatigue conditions J. C. Earthman Materials Section, Department of Mechanical Engineering, University of California, Irvine, CA 92717 (U.&A.) (Received January 31, 1990; in revised form August 27, 1990) Abstract Cylindrical specimens of 12% CrMoV steel were tested under high temperature, low cycle fatigue (HTLCF) conditions to investigate the growth of microcracks which nucleate at the surface. Results are presented which indicate that the microcracks maintain an equilibrium shape under HTLCF conditions, making it possible to estimate crack depth from a measure of the crack length along the surface. The individual growth of these microstructurally small cracks was monitored using a surface replication technique during periodic interruptions of the experiments. Crack growth rate data for different microcracks are compared on the basis of the cyclic J-integral AJ to examine the use of fracture mechanics for predicting HTLCF microcrack growth. A determination of AJ is shown to be successful in characterizing the growth of fatigue microcracks in 12% Cr steel at elevated temperatures despite differences in overall orientation at the surface. Good agreement with fatigue crack growth data for through-cracks in CrMoV steels at high temperatures further demonstrates the usefulness of AJ for predicting HTLCF microcrack growth. The results also suggest that large extrapolations of the observed correlation between AJ and crack growth rate could result in unacceptable errors owing to the relative contribution of oxidation damage at the crack tip. 1. Introduction Ferritic stainless steels used in critical com- ponents of power-generating facilities are often subjected to high strain amplitude fatigue condi- tions at temperatures up to 600 °C. In order to accurately predict failure early in the life of these components, it is important to have a good understanding of the growth of cracks while they are still much smaller than the dimensions of the component. Although high temperature fatigue crack growth has been studied with CrMoV steel specimens containing single "through'cracks" that span the entire thickness of the specimen [1-3], little experimental work has been reported on the growth of microstructurally small cracks in these materials at elevated temperatures. Differences in the fatigue crack growth be- havior of microcracks and through-cracks have been reported for high cycle fatigue conditions at ambient temperatures [4]. For example, micro- crack growth at ambient temperatures is often observed below the AK threshold established with through-crack specimens. This implies that microcrack growth below a certain AK level cannot be predicted from the data obtained with through-crack specimens. Since critical com- ponents are often subjected to a combination of elevated temperatures and stresses that exceed the elastic limit, it follows that fatigue micro- cracks should also be examined in specimens under high strain amplitudes at elevated tempera- tures. This paper presents results from a study of small crack growth in 12Cr-IMo-V steel under high temperature, low cycle fatigue (HTLCF) conditions. In addition to observations of micro- cracks at the surface, a study of internal micro- crack shape is presented. Finally, a comparison of the growth of microcracks and through-cracks under HTLCF conditions is discussed. 2. Procedures Cylindrical specimens with a gauge diameter of 10 mm and a gauge length of 15 mm were 0921-5093/91/$3.50 © Elsevier Sequoia/Printed in The Netherlands