Helge Knobbe

Microstructural aspects of duplex steel during high cycle and very high cycle fatigue

Fatigue experiments were conducted up to N=108 cycles on a duplex stainless steel. The investigations revealed important information: (i) most of the slip markings on {111}-slip planes form during the early stage of fatigue (~104), although many grains stay active in developing more and/or longer slip bands (investigations with SEM/EBSD). It was found that the damage in these grains often has its origin at grain or phase boundaries, growing into the interior of the grains; (ii) in most of the recrystallisation twins slip markings are present in all parts of the grain leading to the assumption that just tilted grain boundaries are not very effective as microstructural barriers. In cases where the twin grain boundary is favorably oriented, it even can promote microcrack initiation; (iii) phase boundaries are most effective in restricting dislocation movement to the softer austenite, since no evidence of growing fatigue damage was found in ferritic grains. These findings lead to the conclusion that for the material studied an endurance limit exists despite occurring plastic deformation. This behaviour is a consequence of the barrier effect induced by phase boundaries against short crack propagation and holds true as long as no inclusions are present.

Bewertung mikrostruktureller Barrieren während der Ermüdungsschädigung von Duplexstählen bei sehr hohen Zyklenzahlen

Kurzfassung Wechselverformungsexperimente unter Anwendung verschiedener Prüftechniken, wie Servohydraulik, Resonanzprüfmaschinen, Umlaufbiegung und Ultraschall, sowie eine systematische mikrostrukturelle Auswertung der Oberfläche nach vorgegebenen Lastwechselintervallen mithilfe der analytischen Rasterelektronenmikroskopie weisen darauf hin, dass Korn- und Phasengrenzen maßgeblich das Ermüdungsverhalten von Duplexstählen beeinflussen. Je nach räumlicher Orientierung der Gleitsysteme benachbarter Körner und Phasenbestandteile kann die lokale Bildung von Scherbändern erklärt werden, die auch über Grenzflächen hinaus wachsen und schließlich zu Rissinitiierung, Ermüdungsrissausbreitung und Bruch führen.

Propagation Behaviour of Microstructurally Short Fatigue Cracks in the High-Cycle- and Very-High-Cycle Fatigue Regimes

In the present paper examples for propagating and non-propagating conditions of slip bands and short fatigue cracks in a ferritic-austenitic duplex steel are given, which were quantified by means of SEM in combination with automated EBSD. To classify the results within the scope of predicting the service life under HCF- and VHCF-loading conditions a numerical model based on the boundary-element method has been developed, where crack propagation is described by means of partially irreversible dislocation glide on crystallographic slip planes in a polycrystalline model microstructure (Voronoi cells). This concept is capable to account for the strong scattering in fatigue life for very small strain amplitudes and to contribute to the concept of tailored microstructures for improved cyclic-loading behaviour.