Book/Dissertation / PhD Thesis

FZJ-2013-06282

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Lebensdauer und Schädigungsentwicklung martensitischer Stähle für Niederdruck-Dampfturbinenschaufeln bei Ermüdungsbeanspruchung im VHCF-Bereich

Kovacs, S. (Corresponding author)FZJ*IEK-2*

2014
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-89336-959-1

Please use a persistent id in citations: http://hdl.handle.net/2128/7967

Abstract: Low-pressure steam turbine blades are usually made of martensitic steels with Cr contents between 9 and 12%, which combine good corrosion resistance, high mechanical strength and sufficient ductility. The inhomogeneous flow field behind the vanes generates high-frequency oscillations above 1 kHz. In addition, the blades with lengths up to 1.5 m are operated at rotational speeds up to 3000 rpm, resulting in large centrifugal forces leading to the superposition of extremely high mean stresses. Also resonance oscillations during start-up and shutdown cannot be completely excluded. Currently, the components are designed using high safety factors against S-N curves with an assumed asymptotic fatigue limit above 107 load cycles. Nevertheless, fatigue cracks are observed even at high number of cycles, starting fromthe blade root without pre-damage by erosion or steam droplet impingement. While fatigue failure usually occurs at the surface, fatigue cracks at very high number of cycles (> 108) initiate at oxides or intermetallic inclusions below the surface. This transition between both failure mechanisms in the Very High-Cycle Fatigue (VHCF) regime is in the focus of numerous current research activities, because numbers of cycles above 108 can be attained in a viable period of time using the recently developed high-frequency testing techniques operated at 20 kHz. Also for wind turbines, gas turbines, bearings, springs, etc. VHCF issues become increasingly important.Within this work, the fatigue life and damage behavior of a martensitic Cr-steel during fatigue loading with and without high mean stresses at number of cycles to failure above 108 was analyzed. On the one hand, the studies gave insights into the relation between fatigue life and fatigue damage evolution of the investigated group of high-strength steels in the very high cycle fatigue regime (up to 2∙109). In particular, the influence of high mean stresses on the VHCF behavior (fracture origin, crack growth, fatigue life) which was not investigated in detail before is studied and the crack initiation and propagation mechanisms are analyzed by electron microscopy (SEM, TEM / FIB). With this, the work contributes to the reliable design of future low-pressure steam turbines. The results show that in particular non-metallic inclusionsin the steel cause fracture by fatigue cracks initiated in the volume under very high cycle fatigue conditions. This fatigue behavior can be described very well by means of fracture mechanics approaches over a wide range of load ratios.

Keyword(s): Dissertation

Note: Dissertation, RWTH Aachen, 2010

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Contributing Institute(s):

1. Werkstoffstruktur und -eigenschaften (IEK-2)

Research Program(s):

1. 122 - Power Plants (POF2-122) (POF2-122)

Appears in the scientific report 2013

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Database coverage:

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