13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- the mean stress influence up to a load ratio of 0.5 on the HCF and VHCF behavior of a high-strength bearing steel (tensile strength: 2316 MPa, HV749) with high compressive residual surface stresses (-373 MPa). The VHCF tests were conducted up to an ultimate numbers of cycles of 2·108 and 109, respectively. Fatigue fracture is generally initiated at inclusions with an average size of 17 µm. With increasing R the slope of the S-N curves becomes extremely flat. For R = 0 and 0.5 the measured fatigue limits lie distinctly below the Goodman approximation. To extend the relatively good knowledge of VHCF behavior of high strength fully martensitic steels to ferritic steels with intermediate strength and relatively high ductility typically used for steam turbine blades, the present study deals with the VHCF behavior of a typical representative of this type of materials up to 2·109 cycles. The investigated material is a martensitic 12% Cr steel frequently used for turbine blades in power plant applications. For this class of steels, the influence of mean stresses on the fatigue behavior is a main issue. Especially the last stages of low pressure steam turbine blades have to sustain fatigue loads with extremely high superimposed mean stresses due to a rotational speed of 50 Hz in combination with blade lengths of up to 1.5 m. High-frequency loading occurs due to inhomogeneous flow fields behind the vanes. With load frequencies above 2000 Hz and a component life up to 30 years, the number of cycles clearly reaches the very high cycle fatigue (VHCF) region above 109 cycles. Hence, VHCF loading must be considered in the design of low-pressure turbine blades. Currently, such turbine blades are designed with high safety coefficients based on S-N curves assumed to approach an asymptotic fatigue limit at N > 107 load cycles. Nevertheless, blade failures at high number of cycles still occur at corrosion pits or even at blade roots without environmental influence [12-16]. 2. Materials and methods The material tested in this study is a martensitic 12% Cr steel used for low pressure steam turbine blades in power generation. The chemical composition is shown in Table 1. Table 1 Chemical composition of X10CrNiMoV12-2-2 element C Cr Ni Mo Mn V Si weight-% 0.117 11.4 2.70 1.64 0.70 0.31 0.23 To obtain a martensitic microstructure, the blades were hardened by annealing at 1040 °C for 60 min and subsequent compressed air cooling. For stress relief purposes, the blades were subsequently tempered at 660 °C for 3 h followed by annealing at 640 °C for 4 h and very slow furnace cooling. The resulting highly tempered martensitic microstructure has finely distributed Cr-carbides with diameters of around 100 nm along the martensite laths and the former austenite grain boundaries. Precipitation-hardening results in high ultimate tensile strength combined with a sufficient elongation at fracture (Table 2). Due to its heat treatment, the microstructure is stable up to temperatures as high as 600°C and ensures safe application of this steel for power plant applications. Table 2 Mechanical properties of X10CrNiMoV12-2-2 at room temperature Vickers hardness 334 HV Yield strength 843 MPa Ultimate tensile strength 1001 MPa Elongation at fracture 17.7 % Contraction at fracture 58 % Young´s Modulus 213 GPa
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