Active Crack Obstruction in High Temperature Ferritic Steels

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At temperatures up to 650 °C HiperFer (High performance Ferrite) steels exhibit higher creep as well as thermomechanical fatigue (TMF) performance combines with strongly reduced crack propagation rates in comparison to ferritic-martensitic 9-12 wt.% Cr steels. Strengthening of these ferritic, high chromium, stainless steel grades is achieved by a combination of solid solution and intermetallic (Fe,Cr,Si)2(Nb,W) Laves phase particle precipitation. The microstructural mechanisms, which govern the improved fatigue strength and decreased crack propagation rates, are far from being understood. These mechanisms shall be identified and analysed in detail within the framework of the proposed project. Crofer 22 H, a predecessor of HiperFer steels, demonstrated that increased fatigue life is a result of strong cyclic hardening, which is partially caused by different morphologies of Laves phase particles. Furthermore, thermomechanically induced particle generation may occur in case of sufficiently high stress or plastic deformation at temperatures between 600 °C and 650 °C. This additionally contributes to an active crack obstruction, which is supposed to be verified within the framework of the presented project.Under cyclic loading, the formation of sub-grain boundaries in front of the crack tip was observed in Crofer 22H. Such grain refinement may cause further increase in cyclic hardening potential, which in turn may lead strongly reduced crack propagation rates. Moreover, the newly formed sub-grain boundaries in the crack tip region can act as potential nucleation sites and favour the precipitation of even more Laves phase particles. Higher particle density, observed at crack tips at 650 °C under thermomechanical loading, accompanied by sub-grain formation, may be interpreted as indication of this. The proposed project focuses on the clarification of the described phenomena at the microstructural level using SEM, EDX, EBSD and TEM analysis. For this purpose, isothermal LCF (low cycle fatigue) and HCF (high cycle fatigue) fatigue as well as crack propagation tests in air at temperatures between 600 °C and 650 °C will be performed. At this temperatures the formation of the potentially embrittling (Fe,Cr)--Phase can be excluded. Additionally, test frequencies of 0.005 Hz up to 20 Hz will be used, because the HiperFer steels revealed a pronounced dependency of crack propagation on the strain rate. Furthermore, this interrelation is influenced by the test temperature, which also will be investigated in the proposed research project.Moreover, the cyclic hardening behaviour will be analysed by using instrumented cyclic indentation tests (PhyBaLCHT), enabling the determination of the evolution of the material’s cyclic properties in the plastically deformed zone in front of the crack tip. Combined with microstructural analysis, this enhances the understanding of the underlying phenomena.