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

在过去，疲劳试验只进行到最终循环次数106或107次，因为人们认为循环强度在较长的寿命内不会进一步下降。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，为什么即使在非常高的循环疲劳（VHCF）之后也可能发生故障。作者认为，包裹体处的应力集中和相应的局部塑性是导致局部晶粒细化的原因，从而导致所谓的细晶区（FGA），并随后在VHCF过程中产生裂纹。文献中没有改善vhcf行为或避免上述失效机制的方法。所以，这是当前提案的主要问题。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）实现位错密度增加的稳定位错结构。先前的研究表明，这种TMT可以提高高强度轴承钢100Cr6的hcf -疲劳极限，但从未研究过这种TMT是否也适用于其他钢和避免vhcf -失效。因此，该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负责任的潜在机制。因此，实验必须证明TMT是否提高了vhcf -疲劳极限，是否改变和稳定了位错结构。为此，在测试HCF和VHCF载荷可能的增加之前，有必要澄清42CrMo4和100Cr6钢的TMT边界条件。TMT是否会引起破坏机制的改变，将通过对断口表面进行详细的断口学研究来确定。因此，将重点放在非金属夹杂处的裂纹起裂区域，以检测VHCF加载后断口表面的典型结构FGA。如果在TMT后断口表面没有发现这样的FGA，这将有力地表明位错组织已经稳定，这意味着由于微观组织的原因疲劳极限已经提高。

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.