Active Crack Obstruction in High Temperature Ferritic Steels

在高达650°C的温度下，HiperFer(高性能铁素体)钢表现出更高的蠕变和热机械疲劳(TMF)性能，与9-12wt.%铬的铁素体-马氏体钢相比，裂纹扩展速率显著降低。这些铁素体、高铬、不锈钢牌号的强化是通过固溶体和金属间化合物(Fe，Cr，Si)2(Nb，W)Laves相粒子析出相结合实现的。控制疲劳强度提高和裂纹扩展速率降低的微观结构机制还远未被了解。应在拟议项目的框架内确定和详细分析这些机制。HiperFer钢的前身Crofer22H证明，疲劳寿命的提高是强循环硬化的结果，这部分是由不同形态的Laves相粒子引起的。此外，在600°C到650°C的温度下，在足够高的应力或塑性变形的情况下，可能会发生热机械诱导的颗粒产生，这还有助于活动的裂纹阻碍，这应该在本项目的框架内得到验证。在循环加载下，在Crofer 22H中观察到裂纹尖端前方亚晶界的形成。这样的晶粒细化可能导致循环硬化势进一步增加，进而可能导致裂纹扩展速率显著降低。此外，裂纹尖端新形成的亚晶界可以作为潜在的形核点，有利于更多的Laves相粒子的析出。在650°C的热机械载荷作用下，在裂纹尖端观察到较高的颗粒密度，并伴随着亚晶的形成，可以解释为这一迹象。拟议的项目侧重于利用扫描电子显微镜、能谱分析、EBSD和透射电子显微镜分析在微观结构层面上澄清所描述的现象。为此，将在600°C至650°C的空气中进行等温LCF(低周疲劳)和HCF(高周疲劳)疲劳以及裂纹扩展试验。在此温度下，可以排除潜在脆化的(Fe，Cr)-相的形成。此外，将使用0.005赫兹至20赫兹的测试频率，因为HiperFer钢显示出裂纹扩展对应变率的显著依赖性。此外，这种相互关系受试验温度的影响，这也将在拟议的研究项目中进行研究。此外，将使用仪器循环压痕试验(PhyBaLCHT)来分析循环硬化行为，从而能够确定材料在裂纹尖端前面的塑性变形区的循环性能的演变。结合微观结构分析，这增强了对潜在现象的理解。

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.