The Fatigue Behavior of NiCr22Co12Mo9 Under Low-Frequency Thermal-Mechanical Loading and Superimposed Higher-Frequency Mechanical Loading

Abstract

The repeated start-ups and shut-downs experienced by gas turbines cause complex thermal-mechanical loading, which may result in the low-cycle fatigue failure of components such as the walls of the combustion chambers. In addition, a higher frequency high cycle fatigue loading (HCF) is often superimposed due to mechanical vibrations and unsteady combustion processes. It is known that such a superimposed HCF loading may not only change the cyclic deformation behavior, but also reduce the lifetime considerably. In the present study, the cyclic response of a combustion chamber material, NiCr22Co12Mo9 (Inconel 617), was investigated under combined TMF/HCF loading in total strain controlled out-of-phase (OP) and in-phase (IP) TMF tests with and without superimposed HCF loading. The minimum temperature of the TMF tests was 200 °C, and the maximum temperature was varied from 750 to 1200 °C. For each maximum temperature condition, the superimposed HCF amplitude was varied between 0 and 0.2%. Cyclic hardening was observed, but it became less pronounced the higher the maximum temperature, and at the highest Tmax the material was cyclically neutral. With increasing superimposed HCF amplitude the cyclic deformation behavior was more and more determined by the HCF. Further, as a result of the superimposed HCF loading the TMF lifetimes decreased significantly. In general, the lifetimes decreased with increase in HCF amplitude and approached only 10% of the fatigue lifetime obtained in the pure TMF experiments. The relation between the applied total strain amplitude, εa,tme,and the number of cycles to failure, Nf, could be expressed as εa,tme = A ∙ Nf-b for each Tmax and type of TMF-test. With the use of this empirically determined relationship it was possible to estimate the lifetime reduction caused by a superposition of a higher frequency HCF loading on the OP- as well as IP-TMF loading.