Impact of frequency, hold time and atmosphere on creep-fatigue of a 9-12% Cr steel from 300 °C-600 °C

In this study the impact of frequency or rather hold time and atmosphere on the transition from fatigue dominated crack growth to creep fatigue of the ferritic/martensitic steel X20CrMoV12-1 was investigated from 300 degrees C - 600 degrees C. This temperature range is most important for power plants operated in a flexible manner. Due to the increased share of renewable sources of energy, modern thermal power plants must be operated in a flexible manner to compensate fluctuating power supply from renewables. More damaging loading scenarios, including frequent start-up and shut-down cycles as well as load fluctuations, occur much more frequently than in the past as a consequence. Fatigue damage becomes more and more important, while creep damage reduces due to shorter full power operation durations. Furthermore, loss of passivation by fatigue induced cracking and delamination of protective oxide layers may constitute strong interdependencies of increased cyclic operation and steam oxidation resistance. This results in the necessity for a innovative surveillance concepts including operation mode dependent inspection intervals of thermal power plants. Application of a damage tolerance concept, based on fracture mechanics, can help to improve remaining life assessment of existing and fatigue tolerant design of future power plants. In order to develop codes for flexibly operated components, based on damage tolerance analysis by linear elastic fracture mechanics, its validity range and especially its limitation have to be evaluated first. The study has shown that the transition from pure fatigue to creep fatigue interaction begins at 500 degrees C and 3.33 x 10(-3) Hz (300 s hold time). Furthermore this transition was found to be independent from steam atmosphere. The corresponding fracture mechanisms were investigated in detail.