Active Crack Obstruction in High Temperature Ferritic Steels

高温铁素体钢中的活性裂纹阻碍

• 批准号: 450763904
• 负责人: Professor Dr.-Ing. Tilmann Beck
• 金额: --
• 依托单位: Lehrstuhl für Werkstoffkunde (WKK)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2020
• 资助国家: 德国
• 起止时间: 2019-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/450763904?language=en
• 关键词: Active; Crack; Obstruction; Temperature; Ferritic

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.

与铁素体-马氏体 9-12 wt.% Cr 钢相比，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，同时裂纹扩展速率大大降低。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 相颗粒沉淀的结合来实现的。控制疲劳强度提高和裂纹扩展速率降低的微观结构机制尚不清楚。应在拟议项目的框架内详细确定和分析这些机制。 Crofer 22 H（HiperFer 钢的前身）证明，疲劳寿命的延长是强循环硬化的结果，而循环硬化的部分原因是 Laves 相颗粒的不同形态。此外，在 600 °C 至 650 °C 之间的温度下，如果应力或塑性变形足够高，则可能会发生热机械诱导的颗粒生成。这还有助于主动裂纹阻塞，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面形成亚晶界。这种晶粒细化可能导致循环硬化潜力进一步增加，进而可能导致裂纹扩展速率大大降低。此外，裂纹尖端区域新形成的亚晶界可以作为潜在的成核位点，有利于更多拉夫斯相颗粒的析出。在 650 °C 的热机械载荷下在裂纹尖端观察到更高的颗粒密度，并伴随着亚晶粒的形成，可以解释为这一点的迹象。拟议项目的重点是使用 SEM、EDX、EBSD 和 TEM 分析在微观结构水平上澄清所描述的现象。为此，将进行等温 LCF（低周疲劳）和 HCF（高周疲劳）疲劳以及空气中 600 °C 至 650 °C 温度下的裂纹扩展测试。在此温度下，可以排除潜在脆化 (Fe,Cr)-α-相的形成。此外，将使用 0.005 Hz 至 20 Hz 的测试频率，因为 HiperFer 钢揭示了裂纹扩展对应变率的明显依赖性。此外，这种相互关系受到测试温度的影响，这也将在拟议的研究项目中进行研究。此外，将使用仪器化循环压痕测试（PhyBaLCHT）来分析循环硬化行为，从而能够确定裂纹尖端前面的塑性变形区域中材料循环性能的演变。与微观结构分析相结合，这增强了对潜在现象的理解。