Influence of process-related microstructural and geometrical inhomogeneities on the fatigue behavior of two steel types considering the influence of chemical composition

Laser assisted cutting as well as all kinds of forming processes are nowadays standard manufacturing strategies applied in nearly every industrial sector. While in a cyclically deformed structure the material properties can be adjusted according to the local stress distribution by taking advantage of the so called transformation induced plasticity during shaping, laser cutting edges in most cases result in a detrimental reduction in fatigue strength. That is one of the major reasons why laser cutting is up to now barely used for cyclically loaded structures. The key features of a laser cutting edge are burrs, a pronounced surface relief and the heat affected zone, all of which strongly depend on the process parameters, but are also influenced by the sheet thickness and the chemical composition of the material to be cut. The kinematic hardening of a metastable material as a consequence of a deformation induced martensite formation also depends on the chemical composition. Both the thermal input as well as the deformation process will have an influence on the microstructural evolution during the processing, which in turn will have a significant influence on the complexity of the fatigue behavior and therefore makes a reliable prediction of the cyclic strength all the more difficult. For two steels – 1.0388 and 1.4301 – the changes in microstructure as a result of the process steps applied will be investigated considering the influence of the chemical composition and the process parameters. The aim of the investigation is to identify process parameters and strategies that aim at adjusting the microstructure in favor of the cyclic strength in the transition range between HCF and VHCF. The results will be used as basis for a statistically reliable fatigue life prediction, where the material physics based interdependencies between deformation induced hardening and/or transformation on the one hand and the formation and severeness of notches resulting from the laser cutting process on the other hand will be taken into account to get a better understanding of the fatigue behavior of any such processed structures.