Creep-fatigue properties of HR6W, which is Fe-Ni base 0.08C-23Cr-45Ni-7W alloy for piping in 700°C USC power plants, are investigated at 700°C and compared with Ni base superalloy Alloy 617. The fatigue lives of HR6W and Alloy 617 are almost the same under PP (fast-fast) type strain waveform, but the life of HR6W is longer than that of Alloy 617 under CP (slow-fast) and CC (slow-slow) type strain waveforms. In the PP test of both HR6W and Alloy 617 the transgranular fracture is dominant. On the other hand, the intergranular fracture mainly occurred in the CP test. In particular, the intergranular cracking is observed in almost all areas of the fracture surface of Alloy 617, although the transgranular cracking is partly observed in the facture surface of HR6W. It is predicted by three-dimensional FE-analyses that the crack initiation life of HR6W at the ligament parts of superheater outlet headers in the boiler plant operating in the steam condition of 700°C-34.3MPa is longer than that of Alloy 617, because HR6W is superior to Alloy 617 in creep ductility and creep-fatigue properties.
