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Fatigue behaviour of a steel for low-pressure steam turbine blades at Very High Cycle Fatigue with superimposed high mean stresses

Exploration of failure mechanisms in the Very High Cycle Fatigue (VHCF).

Schematic presentation of an SN curve for metallic materials with sufficiently large inclusions (? 10 ?m)Schematic presentation of an SN curve for metallic materials with sufficiently large inclusions (? 10 ?m)

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 long blades are operated at rotational speeds up to 3000 revolutions per minute, resulting in large centrifugal forces leading to the superposition of extremely high mean stresses. Also resonance oscillations during start-up and shut-down cannot be excluded completely. Currently, the components are dimensioned with high safety factors against SN curves with an assumed asymptotic fatigue limit above 107 load cycles. Nevertheless, fatigue cracks are observed even at high number of cycles, starting from the 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 in the 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 becomes increasingly important.

In this context, in 2010 the priority program 1466 of the Deutsche Forschungsgemeinschaft (DFG) launched under the name "Life ∞" with 16 participating project partners. The aim of this priority program is to identify and to conceive the occurring damage processes in VHCF, their modeling based on the mechanisms and the development of reliable and more accurate life prediction concepts for components that are subjected to extremely high numbers of cycles. Thus, a valuable contribution to the conservation of resources and the increase of the system reliability will be made.
Within the project of Research Center Jülich, the damage evolution and failure mechanisms of a ferritic turbine steel at extremely high numbers of cycles (up to 2x109) is examined. In particular, the still influence of mean stresses on the VHCF behavior (crack origin, crack growth, lifetime), which has not been investigated in depth up to now is analyzed, and the crack initiation and propagation mechanisms are elucidated by using electron microscopy (SEM, TEM / FIB

SPP 1466