Very high cycle fatigue behaviour of nanostructured bainitic steels

本课题研究纳米贝氏体钢在甚高周疲劳作用下的失效问题。纳米贝氏体钢是一种新型钢，具有很高的疲劳强度，甚至具有很长的使用寿命。一般来说，可以观察到高强度钢在很长时间疲劳载荷下，即使在低于经典疲劳强度的载荷下，钢也会失效，这是在最终循环次数为1000万的情况下确定的。这种后期失效通常伴随着疲劳断口表面形态的变化：在裂纹起裂区，高强度钢通常位于非金属夹杂物处，表面比寿命较短的钢更粗糙。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（FGA），其晶粒尺寸仅在100 nm以下，因此比原始微观结构精细得多。这种晶粒细化伴随着裂纹扩展的应力强度因子阈值的降低，根据申请人的假设，这导致疲劳强度的降低。纳米贝氏体钢具有分层组织，其最小的微观组织元素为铁素体和奥氏体相间的片层，片层厚度在10 nm左右，具有很高的静强度。到目前为止，还没有研究这种精细的微观结构在VHCF载荷下是否稳定，从而不允许FGA的形成，这可以证明这种高强度钢在降低应力水平下对VHCF失效不敏感。因此，该项目将研究两种化学性质不同的纳米贝氏体钢在两种不同细片层厚度的热处理条件下的VHCF行为。除了疲劳测试，这里的重点是关于可能的FGA形成的断裂表面的特征。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，将有可能更详细地研究哪些微观结构成分特别有利于增加抗VHCF负载的稳定性，从而可以为部件生产更有弹性和更可持续的钢状态。除了对钢铁进一步发展的发现外，申请人还希望对FGA形成的条件有新的见解，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.