Abstract
Large scale components of the conventional 600°C class steam turbine were made of the ferritic steel, but the steam turbine plants with main steam temperatures of 700°C or above (A-USC) using the Ni-base superalloys are now being developed in order to further improve the thermal efficiency. The weight of the turbine rotor for the A-USC exceeds 10ton. A lot of high strength superalloys for aircraft engines or industrial gas turbines have been developed up to now. But it is difficult to manufacture the large-scale parts for the steam turbine plants using these conventional high strength superalloys because of their poor manufacturability. To improve high temperature strength without losing manufacturability of the large scale components for the A-USC steam turbine plants, we developed Ni-base superalloy USC800(Ni-23Co-18Cr-8W-4Al-0.1C [mass %]) which has temperature capability of 800°C with high manufacturability achieved by controlling microstructure stability and segregation property. The 700°C class A-USC materials are the mainstream of current development, and trial production of 10 ton-class forged parts has been reported. However, there have been no reports on the development and trial manufacturing of the A-USC materials with temperature capability of 800°C. In this report, results of trial manufacturing and its microstructure of the developed superalloy which has both temperature capability 800°C and good manufacturability are presented. The trial manufacturing of the large forging, boiler tubes and turbine blades using developed material were successfully achieved. According to short term creep tests of the large forging and the tube approximate 100,000h creep strength of developed material was estimated to be 270MPa at 700 °C and 100MPa at 800°C.
