Creep-Fatigue Properties of Fe-Ni Base 0.08C-23Cr-45Ni-7W Alloy for Piping in 700°C A-USC Power Plants

Creep-fatigue properties of Fe-Ni base 0.08C-23Cr-45Ni-7W alloy (HR6W) are investigated at 700°C and compared with Ni base superalloy Alloy617 in order to examine applicability of these materials to piping in 700°C A-USC power plants. It is clarified that the fatigue lives of HR6W and Alloy617 are almost the same under PP (fast-fast) type strain waveform, but the life of HR6W is longer than that of Alloy617 under CP (slow-fast) type strain waveform. The crack initiation lives at the ligament parts of superheater outlet headers, which are made of HR6W and Alloy617, are predicted from the results of three-dimensional FE-analyses. The transient heat transfer analysis and the elastic-plastic-creep thermal stress analysis are conducted on several kinds of typical operating conditions to evaluate the inelastic strain caused by the thermal cycles during start-up and shut-down of power plants. The maximum steam temperature and pressure are selected as 700°C and 34.3MPa, respectively, and the maximum cycle time of operation is 1 year. On the basis of the analytical results, creep-fatigue and creep rupture properties of the materials, the crack initiation lives of the headers are predicted with the strain range partitioning method. Inelastic strain in the header of HR6W is larger than that of Alloy617, because HR6W has lower creep deformation resistance and a slightly higher thermal expansion coefficient. However the predicted life of the header of HR6W is longer than that of Alloy617, because HR6W is much superior to Alloy617 in creep ductility and creep-fatigue properties.