Very high cycle fatigue behaviour of nanostructured bainitic steels

该研究项目涉及纳米贝氏体钢在超高周疲劳（VHCF）下的失效。纳米贝氏体钢是一种新型钢，即使在很长的寿命下也具有很高的疲劳强度。通常，可以观察到高强度钢在很长的疲劳载荷下，即使在低于经典疲劳强度的载荷下也会失效，该疲劳强度是在1000万次循环的极限次数下确定的。这种后期失效通常伴随着疲劳断裂表面形态的改变：在裂纹萌生区域中，在高强度钢的情况下，裂纹萌生区域通常位于非金属夹杂物处，与寿命较短的情况相比，出现更粗糙的表面。这种粗糙的表面伴随着局部细化的晶粒结构，即所谓的细颗粒区域（FGA），其晶粒尺寸仅低于100 nm，因此比原始微观结构细得多。这种晶粒细化伴随着裂纹扩展的应力强度因子的阈值的降低，根据申请人的假设，这导致疲劳强度的降低。具有分级显微组织的纳米贝氏体钢显示交替的铁素体和奥氏体层片作为最小的显微组织元素，其具有在几个10纳米范围内的薄片厚度，并导致该钢状态的非常高的静强度。到目前为止，尚未研究这种精细微观结构在VHCF载荷下是否稳定，从而不允许形成FGA，这可以证明这种高强度钢类别在降低的应力水平下对VHCF失效不敏感。因此，两种化学性质不同的纳米贝氏体钢将在该项目中研究其在两种热处理条件下的VHCF行为，每种热处理条件下具有不同的细片层厚度。除了疲劳测试，这里的重点是关于可能的FGA形成的断裂表面的表征。根据第一个资助期计划的这项工作的结果，在可能的第二个资助期，将有可能更详细地调查哪些微观结构的结构成分特别有利于提高对VHCF载荷的稳定性，从而可以为部件生产更有弹性，从而更可持续的钢状态。除了关于钢的进一步发展的发现外，申请人还希望获得对FGA形成条件的新见解，这仍然是有争议的讨论主题。

The research project deals with the failure of nanobainitic steels under very high cycle fatigue (VHCF). Nanobainitic steels are a new class of steels with the potential for high fatigue strength even at very long lifetimes.In general, it can be observed for high-strength steels at very long fatigue loads that the steels fail even at loads below the classical fatigue strength, which was determined for an ultimate number of cycles of 10 million. This late failure is very often accompanied by a changed morphology of the fatigue fracture surface: in the crack initiation area, which in the case of high-strength steels is very often located at non-metallic inclusions, rougher surfaces occur than in the case of shorter lifetimes. This rougher surface is accompanied by a locally refined grain structure, the so-called fine granular area (FGA), which only has grain sizes below 100 nm and is therefore much finer than the original microstructure. This grain refinement is accompanied by a reduction in the threshold value of the stress intensity factor for crack propagation, which, according to the applicant's hypothesis, causes the reduction in fatigue strength.Nanobainitic steels with their hierarchical microstructure show alternating ferritic and austenitic lamellae as smallest microstructural elements, which have a lamella thickness in the range of a few 10 nm and cause a very high static strength of this steel state. So far it has not been investigated whether this fine microstructure is stable under VHCF loadings and thus does not allow FGA formation, which could justify the insensitivity of this high-strength steel class to VHCF failure at reduced stress levels. Therefore, two chemically different nanobainitic steels will be investigated in the project with regard to their VHCF behaviour in two heat treatment conditions each with different fine lamella thicknesses. Besides fatigue testing, the focus here is on the characterisation of the fracture surfaces with regard to possible FGA formation. Depending on the results of this work planned for a first funding period, it will then be possible in a possible second funding period to investigate in more detail which microstructural structural constituents are particularly favourable to increased stability against VHCF loading, whereby more resilient and thus more sustainable steel states could be produced for components. In addition to findings on the further development of steel, the applicant also hopes to gain new insights into the conditions under which FGA are formed, which are still the subject of controversial discussion.