Martensitic 12 %-Cr-steels are -among other applications- used for the last stages in low-pressure steam turbines in power plant applications. During operation these materials undergo cyclic loadings induced by inhomogenities of the flow field superimposed by high mean stresses due to centrifugal forces. Typical component lifetime of 30 years or more and frequencies > 1 kHz for the cyclic load clarify the relevance of fatigue investigations at very high number of cycles.
In the present work the fatigue behavior of X10CrNiMoV12-2-2 is investigated up to 2x10^9 load cycles for several load ratios and different stress concentration factors (1.09 < αk < 2.42) at room temperature. To perform studies under realistic conditions within acceptable time an ultrasonic testing system combined with a servohydraulic load frame to apply mean loads was developed at the authors’ institute. To limit the self-heating of the samples the tests were realized in pulse-pause mode (500 ms pulse / 3500 ms pause). During fatigue tests data from the ultrasonic generator, IR-temperature sensor, displacement, cyclic creep and the mean stress is collected and processed. Additionally, the electrical resistance was measured in certain intervals during extended cooling breaks.
Comparable to earlier studies at smooth samples (αk = 1.00), crack initiation for samples with low notch factor (αk = 1.09) changes from the surface to internal oxide inclusions at numbers of cycles to failure around 2x10^7. Above this cycle number the S-N curves for αk = 1.00 and 1.09 are identical while at lower cycle numbers to failure (surface crack initiation) the stress concentration of the notched samples results in reduced lifetime compared to the cylindrical specimens. For the higher notch factor (αk = 2.42) only crack initiation at small surface defects resulting from the machining process is observed. The maximum number of cycles where failure occurs is about 1x10^6, which means that no VHCF-failures for αk = 2.42 was observed. S-N-curves for higher load ratios (αk = 1.09 and αk = 1.00) show, similar as for R = -1, a flat slope with no significant decrease of fatigue strength vs. lifetime in the VHCF regime. Murakami’s widely accepted √area-concept describes the fracture mechanism for a broad range of R-values over four decades of lifetime for αk = 1.00 and fits well for notched samples with low stress concentration factor (αk = 1.09) and a load ratio of R = -1. Ongoing research focuses (i) on the influence of load ratios of the fatigue behavior of notched samples with different stress concentration and (ii) on determining the maximum stress concentration factor which still allows crack initiation at internal inclusions.