VHCF of high strength steels
Increasing the HCF- and VHCF- Strength by Thermomechanical Treatment in the Temperature Range of the Maximum Dynamic Strain Aging
In the past, fatigue tests were often only carried out up to 106 or 107 load cycles, because it was thought that increased fatigue lifes would no longer be expected to decrease the fatigue strength. It is now known that many, in particular high-strength, materials can fail after significantly more than 107 load cycles. There are already a number of hypotheses as to why such failure after very high cyclic fatigue (VHCF) can occur. In our opinion it emerges that the local stress increase at the inclusion and the resulting highly localized plasticity are the cause of local grain refinement (the resulting area is often referred to as the Fine Grained Area, FGA) and could lead to the subsequent crack initiation in the VHCF regime. Measures to improve VHCF behavior or to avoid this possible damage mechanism have not yet been discussed in the literature. This is the aim of this project: With dynamic strain aging, a stabilized dislocation structure with increased dislocation density should be achieved, which would then lead to increased fatigue strength. As part of our own research, a thermomechanical treatment (TMT) based on dynamic strain aging to improve the HCF fatigue strength of steel 100Cr6 has already been investigated. It was shown that the HCF fatigue strength can be increased again in the high-strength steel. So far, however, this has only been investigated for 100Cr6 in the HCF range. The effects on other steels and on VHCF failure remain still unknown. The aim of the project is therefore to increase the strength of high-strength steels under VHCF stress and to understand the underlying mechanisms in regards to materials science. The TMT mentioned should be tested for its effectiveness in increasing the VHCF resistance and the modified and stabilized dislocation structure should be demonstrated. For this purpose, the exact boundary conditions for the TMT on the two steels 42CrMo4 and 100Cr6 are first determined. Then the increase in potential is checked by the TMT for HCF and VHCF stress. A detailed fractographic examination of the broken samples is then used to check whether the TMT has altered the failure mechanism. For this purpose, the crack initiation area around the non-metallic inclusions is analyzed by scanning electron microscopy in order to identify the fracture surface structures with FGA, which are otherwise typical for VHCF failure. If, according to the TMT, there are no FGAs on the fractured surfaces of the fatigued samples, this would be a clear indication of the microstructural cause (stabilized dislocation structure) for the increase in fatigue strength.
Contact: M. Sc. Jan Sippel