Ratcheting Fatigue Behaviour of Advanced Structural Materials

The advanced power generating industries requires high operating steam temperatures and pressures, to attain higher efficiency. This led to development of alloys with superior properties at elevated temperatures. The two alloys under investigation are Modified 9Cr-1Mo steel and Inconel 617 alloy. From the application point of view, the two alloys are used as piping and tubing materials in various components such as steam generator, super heater, re heater and heat exchanger etc. The present investigation deals with the comparative study of the two alloys under asymmetrical stress controlled cyclic loading, with the help of normalized mean stress and stress amplitude with constant stress rate, at ambient and homologous temperatures. Fractographic studies of the failed specimens were also observed, at room and homologous temperatures with the aid of scanning electron microscope. The difference in fractographic features and deformation mechanism of the alloys has been elaborated. The key results of this investigation points that there is accumulation of plastic strain under asymmetrical cyclic loading. Modified 9 Cr-1Mo steel exhibited unique fracture behaviour under ratcheting which was not observed in case of IN-617 superalloy. On comparing the ratcheting of the alloys at ambient temperature and homologous temperatures it was concluded that uniform strain is a crucial parameter to consider when selecting materials for piping and tubing components that are subjected to ratcheting. Inconel 617 alloy showed remarkable strength and sustained very high strain values compared to modified steel.