Enhancement of the fatigue limit at HCF- and VHCF-loadings with the help of thermomechanical treatment at the temperature of maximum dynamic strain ageing

借助最大动态应变时效温度下的热机械处理，提高 HCF 和 VHCF 负载下的疲劳极限

• 批准号: 408139037
• 负责人: Dr.-Ing. Stefan Guth, since 1/2021
• 金额: --
• 依托单位: Institut für Angewandte Materialien - Werkstoffkunde (IAM-WK)
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2022-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/408139037?language=en
• 关键词: Enhancement; fatigue; limit; HCF; VHCF

In the past fatigue experiments were carried out only until an ultimate number of cycles of 106 or 107 because people thought that there is no further decrease of the cyclic strength at longer lifetimes. Nowadays, we know that a lot of especially high-strength materials can fail even after more than 107 loading cycles. There are several ideas why a failure might happen even after very high cycle fatigue (VHCF). The proposer’s opinion is that the stress concentration at inclusions and the corresponding localized plasticity are the reason for a local grain refinement, which leads to a so called fine grained area (FGA), and the subsequent crack initiation during VHCF. Approaches to improve the VHCF-behaviour or to avoid the said failure mechanisms are not available in literature. So, this is the main question of the current proposal.To increase the VHCF-fatigue limit a stabilized dislocation structure with increased dislocation density has to be realized by a thermomechanical treatment (TMT) at the temperature of maximum dynamic strain ageing. Former investigations of the proposers showed that such a TMT can improve the HCF-fatigue limit of the high-strength bearing steel 100Cr6 but it was never investigated whether this TMT is also useful for other steels and for avoiding VHCF-failure.The goal of the project consequently is the increase of the loadability of high-strength steels at VHCF-loadings. Thereby the responsible underlying mechanisms have to be understood from a materials science and technology point of view.Therefore, the experiments have to show whether the TMT increases the VHCF-fatigue limit and whether or not it modifies and stabilizes dislocation structure. To that end, before testing the possible increase by HCF and VHCF loadings, it is necessary to clarify the boundary conditions of the TMT for the steels 42CrMo4 and 100Cr6. Whether or not TMT causes a change of the failure mechanism will be determined with the means of detailed fractographic investigations of the fracture surfaces. Thereby, the focus will be on the crack initiation region at non-metallic inclusions in order to detect the typical structure FGA on fracture surfaces after VHCF loading. If no such FGA on the fracture surfaces are identified after TMT this would strongly point to the view that the dislocation structure is stabilized, which means that the fatigue limit has increased by microstructural reasons.

在过去，疲劳试验只进行到极限循环次数为106或107次，因为人们认为在更长的寿命下循环强度不会进一步降低。如今，我们知道许多特别高强度的材料即使在超过107次加载循环后也会失效。有几种观点认为，即使在非常高的循环疲劳(VHCF)之后，也会发生故障。作者认为，夹杂物上的应力集中和相应的局域塑性是导致局部细化的原因，这导致了所谓的细晶区(FGA)，以及随后的VHCF裂纹萌生。改善VHCF行为或避免上述故障机制的方法在文献中不可用。因此，这是当前提议的主要问题。为了提高VHCF疲劳极限，必须在最大动态应变时效温度下通过热机械处理(TMT)来实现具有更高位错密度的稳定位错结构。前人的研究表明，这种TMT可以提高高强度轴承钢100Cr6的高强疲劳极限，但这种TMT是否也适用于其他钢，避免VHCF失效，还没有人研究过。因此，本项目的目标是提高高强度钢在VHCF载荷下的承载能力。因此，必须从材料科学和技术的角度来理解相关的潜在机制。因此，实验必须证明TMT是否提高了VHCF疲劳极限，以及它是否改变和稳定了位错结构。为此，在测试HCF和VHCF载荷可能增加的可能性之前，有必要澄清42CrMo4和100Cr6钢TMT的边界条件。TMT是否会导致失效机制的改变，将通过对断口的详细断口观察来确定。因此，将重点放在非金属夹杂物的裂纹起始区，以检测加载VHCF后断口上的典型结构FGA。如果在TMT后没有在断口上发现这样的FGA，这将强烈地表明位错结构稳定的观点，这意味着疲劳极限由于组织原因而增加。