Abstract
The 22 wt.% Cr, fully ferritic stainless steel Crofer®22 H has higher thermomechanical fatigue (TMF)- lifetime compared to advanced ferritic-martensitic P91, which is assumed to be caused by different damage tolerance, leading to differences in crack propagation and failure mechanisms. To analyze this, instrumented cyclic indentation tests (CITs) were used because the material’s cyclic hardening potential—which strongly correlates with damage tolerance, can be determined by analyzing the deformation behavior in CITs. In the presented work, CITs were performed for both materials at specimens loaded for different numbers of TMF-cycles. These investigations show higher damage tolerance for Crofer®22 H and demonstrate changes in damage tolerance during TMF-loading for both materials, which correlates with the cyclic deformation behavior observed in TMF-tests. Furthermore, the results obtained at Crofer®22 H indicate an increase of damage tolerance in the second half of TMF-lifetime, which cannot be observed for P91. Moreover, CITs were performed at Crofer®22 H in the vicinity of a fatigue crack, enabling to locally analyze the damage tolerance. These CITs show differences between crack edges and the crack tip. Conclusively, the presented results demonstrate that CITs can be utilized to analyze TMF-induced changes in damage tolerance.
Highlights
Because of the current dramatic changes in electricity generation and supply, conventional thermal power plants are operated in a more fluctuating way, resulting in a combination of creep and cyclic loading of the hot section components
This was based on instrumented cyclic indentation tests (CITs), which were performed at specimens and loaded for the different numbers of oop thermomechanical fatigue (TMF)-cycles, as well as in the vicinity of a fatigue crack in a Crofer® 22 H specimen
The presented work focuses on the evolution of damage tolerance during thermomechanical fatigue (TMF-)loading, as well as the local changes in defect tolerance in the vicinity of a fatigue crack
Summary
Because of the current dramatic changes in electricity generation and supply, conventional thermal power plants are operated in a more fluctuating way, resulting in a combination of creep and cyclic loading of the hot section components. As shown by Holdsworth et al [1], this leads to accelerated crack propagation, and reduces lifetime compared to pure creep loadings. Preliminary work at materials for thermal power plant applications [2] has shown, that 22 wt.% Cr, fully ferritic stainless steel Crofer® 22 H exhibits a higher lifetime compared to advanced ferritic-martensitic (AFM) steel P91.
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