Zuverlässigkeit von ausscheidungshärtbaren und martensitischen Stählen im additiv gefertigten Zustand unter hochzyklischer mechanischer Beanspruchung

This dissertation deals with the reliability of precipitation-hardened martensitic steels under cyclic mechanical loading up to > 109 cycles. The experimental work explores the two typical representatives of the material group X3NiCoMoTi18 9 5 (1.2709) and X5CrNiCuNb17 4 4 (1.4548) manufactured by laser beam powder bed fusion (PBF LB) and in heat treated condition. In addition to the microstructural analysis and quasi-static investigations, the focus is on fatigue tests on typical laboratory fatigue specimens using ultrasonic fatigue testing technology. Thereby, specimens from a conventionally rolled process and a PBF-LB are compared. PBF-LB process parameters with different building directions and surface conditions as well as process-inherent irregularities are taken into account. The transferability of the determined fatigue strengths is evaluated using component-like cube-shaped scaffold structures with an edge length of 80 mm. The irregularities present in the material are quantified by metallographic cross-sections and fractographic analyses of failed samples. These methods are used to determine √area values according to the concept proposed by Murakami. With the support of the Kitagawa-Takahashi-diagram, √area values and experimental strength values are linked and can be compared. The experimental findings show that both test materials exhibit similar behavior: Heat treatment in the form of solution annealing and aging results in a comparable microstructure for all material states considered. Irregularities present in the material are not only inhomogeneous in space but also logarithmically normally distributed in size. The irregularities cause a significantly reduced formability under quasi-static loading. Under cyclic loading, the irregularities determine the failure and reduce the strength. Corresponding to the irregularity distribution, the fatigue life or the endurable stress amplitude is also subject to a broad distribution. Thus typical laboratory fatigue test specimens are not suitable for characterizing the fatigue strength of materials using classic fatigue test methods. Large √area values of the samples with machined surfaces reach the dimension around 200 µm. The crack-initiating irregularities from the samples with an as built surface are also concentrated in this order of size. The Kitagawa-Takahashi-diagram shows that it is possible to determine a reliable long-term fatigue strength using the El Haddad-model. Thereby, the main challenge is to determine the largest irregularity present in the material. As a simple approximation, the size distribution of the irregularities in the material can be represented by using samples with process-typical surfaces. For the investigated materials, the largest √area value determined 215 µm results in long-term fatigue strengths of 146 MPa for the X3NiCoMoTi18-9-5 and 199 MPa for the X5CrNiCuNb17-4-4, independently of the sample surface and building direction. Compared to the rolled material, surface structure and irregularities cause a significant reduction of the long-term fatigue strength of approx. 450 respectively 300 MPa in PBF LB manufactured specimens.