Alloy 617 is a Ni-base superalloy and is a candidate material for high temperature steam piping systems and casings for advanced ultra-supercritical power generation technology because of its excellent high temperature creep strength. It is expected in the near future that the installation of variable renewable energies with large output fluctuations will increase. Therefore, adjusting the supply and demand of electricity is requested for thermal power generation. This leads to the issue of fatigue damage caused by repeated thermal stress due to the startup and shutdown procedures.

It was reported that the fatigue crack growth of alloy 617 changed from transgranular to intergranular at 750°C, with a decreasing loading frequency in a steam environment. It was also indicated that the increase in the crack growth rate in lower loading frequency conditions; however, the specific acceleration mechanism governing the crack growth has not been elucidated.

In this study, morphology of oxide and precipitates at fatigue crack tip region were analyzed by transmission electron microscope. All of examined crack tip were filled with oxides. Especially, the double layered oxide film was formed in the crack tip which propagated under the lower loading frequencies. In addition, unoxidized precipitates mainly consisting of Mo and Co were observed in the intergranular cracks. It was suggested that the changes in the oxidation morphologies depending on the loading frequency influenced the crack growth rate.

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