Abstract
Materials made for modern steam power plants are required to withstand high temperatures and flexible operational schedule. Mainly to achieve high efficiency and longer components life. Nevertheless, materials under such conditions experience crack initiations and propagations. Thus, life prediction must be made using accurate fatigue models to allow flexible operation. In this study, fully reversed isothermal low cycle fatigue tests were performed on a turbine rotor steel called FB2. The tests were done under strain control with different total strain ranges and temperatures (20 °C to 625 °C). Some tests included dwell time to calibrate the short-time creep behaviour of the material. Different fatigue life models were evaluated based on total life approach. The stress-based fatigue life model was found unusable at 600 °C, while the strain-based models in terms of total strain or inelastic strain amplitudes displayed inconsistent behaviour at 500 °C. To construct better life prediction, the inelastic strain amplitudes were separated into plastic and creep components by modelling the deformation behaviour of the material, including creep. Based on strain range partitioning approach, the fatigue life depends on different damage mechanisms at different strain ranges at 500 °C. This allows for the formulation of life curves based on either plasticity-dominated damage or creep-dominated damage. At 600 °C, creep dominated while at 500 °C creep only dominates for higher strain ranges. The deformation mechanisms at different temperatures and total strain ranges were characterised by scanning electron microscopy and by quantifying the amount of low angle grain boundaries. The quantification of low angle grain boundaries was done by electron backscatter diffraction. Microscopy revealed that specimens subjected to 600 °C showed signs of creep damage in the form of voids close to the fracture surface. In addition, the amount of low angle grain boundaries seems to decrease with the increase in temperature even though the inelastic strain amplitude was increased. The study indicates that a significant amount of the inelastic strain comes from creep strain as opposed of being all plastic strain, which need to be taken into consideration when constructing a life prediction model.
Highlights
Increase of thermal efficiency in steam turbines is desired, but restrictions from the material properties limit this advancement
The steam turbine rotor steel FB2 was tested in low cycle fatigue (LCF), both with and without dwell time
The material behaviour at mid-life was modelled and the LCF tests with dwell time were used to extract creep properties
Summary
Increase of thermal efficiency in steam turbines is desired, but restrictions from the material properties limit this advancement. Yimin and Jinrui [3] investigated the low cycle fatigue (LCF) behaviour of 30Cr2MoV rotor steel at high temperatures and stressed the importance of designing rotors based on cyclic conditions while taking the creep properties of the material into account. Cyclic loading of materials above the yield limit produces plastic straining, but at high temperatures creep contribution becomes significant and has to be taken into account. [10], which can subsequently be used to evaluate the fatigue life It was shown by Mishnev et al [11] that lower strain rates produce larger inelasticity at high temperature for creep-resistant martensitic steels. This could be attributed to creep, as the material spends longer time at high stresses. The study investigates the mechanisms behind the effect of temperature on fatigue life by carrying out a microstructural analysis to inspect the contribution of inelasticity and the effect of temperature on life under LCF loading
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