Active Crack Obstruction Mechanisms in Crofer® 22H at 650 °C

• Published in: Materials Vol. 15; no. 18; p. 6280
• Main Authors: Fischer, Torsten, Kuhn, Bernd
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 09.09.2022; MDPI
• Subjects: active crack obstruction mechanisms; Chromium; Cooling; Crack initiation; Crack propagation; Crack tips; Creep strength; Cyclic loads; Dual phase steels; Dynamic stability; Electric power production; Energy conversion; Experiments; fatigue; Fatigue failure; Fatigue strength; Ferritic stainless steels; ferritic steels; Force and energy; Fracture mechanics; frequency; Grain boundaries; Grain refinement; Grain structure; Heat; Heat storage; High temperature; Laves phase; Martensitic stainless steels; Metal fatigue; Oxidation; Oxidation resistance; Polygonization; Power plants; Precipitation hardening; Solar energy; Solid solutions; Steel; Strengthening; Structural stability; Thermal energy; United Kingdom; Germany
• ISSN: 1996-1944; 1996-1944
• DOI: 10.3390/ma15186280

Increased cyclic loading of components and materials in future thermal energy conversion systems necessitates novel materials of increased fatigue resistance. The widely used 9–12% Cr steels were developed for high creep strength and thus base load application at temperatures below 620 °C. At higher temperature, these materials present unstable grain structure, prone to polygonization under thermomechanical fatigue loading and limited resistance to steam oxidation. This seminal study compares thermomechanical fatigue resistance and long crack propagation of the advanced ferritic-martensitic steel grade 92 and Crofer® 22H, a fully ferritic, high chromium (22 wt. %) stainless steel, strengthened by Laves phase precipitation. Crofer® 22H features increased resistance to fatigue and steam oxidation resistance up to 650 °C. Both thermomechanical fatigue (crack initiation) and residual (crack propagation) lifetime of Crofer® 22H exceeded that of grade 92. The main mechanisms for improved performance of Crofer® 22H were increased stability of grain structure and “dynamic precipitation strengthening” (DPS). DPS, i.e., thermomechanically triggered precipitation of Laves phase particles and crack deflection at Laves phase-covered sub-grain boundaries, formed in front of crack tips, actively obstructed crack propagation in Crofer® 22H. In addition, it is hypothesized that local strengthening may occur near the crack tip because of grain refinement, which in turn may be impacted by testing frequency.