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.

FGA formation mechanism for X10CrNiMoV12‐2‐2 and 34CrNiMo6 for constant and variable amplitude tests under the influence of applied mean loads

“…The main focus of the current work is the verification of FGA formation models proposed in literature for two steels investigated for a wide range of R values and constant amplitude as well as variable amplitude tests. For detailed information on the fatigue behaviour, the authors refer to earlier publications …”

“…For detailed information on the fatigue behaviour, the authors refer to earlier publications. [4][5][6]20,24,25,33 For X10CrNiMoV12-2-2, fracture surfaces and microstructure within the vicinity of crack initiating oxide inclusion have been investigated using SEM-and FIB technique. In earlier investigations on smooth specimen, cracks initiated within the volume at oxide inclusions with increased contents of Al and Ca.…”

Sonotrodes for Ultrasonic Welding of Titanium/CFRP-Joints—Materials Selection and Design

“…Besides Young's modulus, mass density and Poisson's ratio, the fatigue strength was considered. Therefore, the fatigue strength of the selected materials was characterised by ultrasonic fatigue testing system (20 kHz) close to ambient temperature up to an ultimate number of cycles of 2.5 × 10 9 to estimate the very high cycle fatigue (VHCF) behaviour (see [36,37]). The samples were tested in pulse/pause-sequences of 2500 ms/500 ms, leading to an effective testing frequency of 16.6 kHz.…”