VHCF Behavior and Word Hardening of a Ferritic-Martensitic Steel at High Mean Stresses

Abstract:

Low-pressure steam turbine blades undergo VHCF-loadings induced by inhomogenous flow behind the vanes resulting in excitation frequencies of ≈ 2 kHz for rotational speeds of 50 Hz and a typical number of stator vanes of ≈ 60. The VHCF loading is superimposed by considerable mean stresses caused by centrifugal forces. In the present study, the VHCF-behavior of the ferritic-martensitic turbine blade steel X10CrNiMoV12-2-2 is investigated using an ultrasonic fatigue testing system up to cycle numbers of 5∙10⁹ at stress ratios from R = -1 up to 0.7, i.e. up to very high mean stresses. Generally, crack initiation changes from the surface to internal inclusions at fatigue lives around 4∙10⁷. The transition between fatigue failure and run-outs is shifted to higher lifetime with increasing R, and fine grained areas (FGAs) at the crack initiation sites only occur at R < -0.1. However, the fracture mechanics approach proposed by Murakami consistently describes the lifetime behavior for all load ratios over 4 decades of lifetime. At R up from 0.5 considerable cyclic creep occurs, even for lifetimes above 10⁸ cycles, resulting in cyclic hardening which was proved by microhardness measurements at longitudinal sections. This effect at least partially explains the high maximum stresses close to the tensile strength of the material occurring in the VHCF regime at load ratios ≥ 0.5.

• Periodical: Key Engineering Materials (Volume 664)
• Pages: 246-254
• DOI: https://doi.org/10.4028/www.scientific.net/KEM.664.246
• Online since: September 2015
• Authors: Tilmann Beck*, Stephan A. Kovacs, Fabian Ritz
• Keywords: Cyclic Creep, Cyclic Hardening, Ferritic-Martensitic Steel, Mean Stress, ODA Formation, VHCF

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