Cyclic crack growth properties of service-exposed ferritic steels for use in thermal fatigue assessments

• Published in: Materials at high temperatures Vol. 21; no. 3; pp. 129 - 146
• Main Author: Skelton, R P
• Format: Journal Article
• Language: English
• Published: Leeds Taylor & Francis Ltd 01.08.2004
• Subjects: Assessments; Crack propagation; Cracks; Damage; Dwell; Ferritic stainless steels; Short cracks; Upper bounds
• ISSN: 0960-3409; 1878-6413
• DOI: 10.3184/096034004782749908

This paper is concerned with the laboratory isothermal (low cycle) fatigue crack growth properties (generally at 550°C) of eleven ferritic steel samples taken from power station components which have seen between 63,000 h and 175,000 h service exposure to high temperatures. Full chemical analyses and abbreviated service histories of the materials are presented. Both the short crack (LCF) and deep crack (LEFM) regimes are examined. In some cases it was possible to continue the growth of actual service cracks under laboratory conditions, by suitable machining of samples into specimens; in other cases it was necessary to start the crack artificially. It is intended that the results can be used in direct application to remanent life assessments. In many tests the creep-fatigue interaction was simulated by the addition of a 0.5 h tensile dwell, it being assumed that major damage was already present in the extracted samples. The short crack growth tests at 550°C with dwell have demonstrated that even where significant prior grain boundary damage is present, the resulting growth rates generally do not violate the upper bound relation at present used in the R5 assessment procedures. The deep crack growth rate tests at 550°C with dwell on the other hand have shown a sensitivity to microstructure such that heavily cavitated large-grained material can give growth rates in excess of the appropriate upper bound relation.