Volume 8, Issue 8 (September 2011)
The Occurrence Mechanism of Periodicity of Creep Crack Path for P92
The morphology of creep crack growth in metals is usually dominated by inter-granular cracking. However, for developed heat-resistant steels with a martensitic lath strengthening structure such as ASME Grade P92, inter-granular cracking, which is a typical fracture mechanism of creep crack growth, was restrained, and the creep crack growth path was found to be composed of a fracture unit area (FA) beyond the scale of the grain size. In spite of the fact that FA is closely related to resistance against crack growth, the occurrence mechanism of FA has not yet been clarified. In this study, by conducting creep crack growth tests for P92, two-dimensional finite element analyses, and metallurgical characterization of the microstructure, the occurrence mechanism of FA for P92 was clarified. As a result, the creep crack growth path for P92 was found to be composed of an FA beyond the scale of the grain size due to the sub-critical crack growth, and the periodic sub-critical crack growth was found to be dominated by the preceding initiation of voids along a slip line. On the basis of the proposed finite element method analysis, this crack growth mechanism was found to cause dispersion of creep damage, which causes unstable crack growth and results in periodic convexo-concave crack growth, that is, FA-cracking.