The present invention relates to a novel nickel-base superalloy and, more particularly, to a gas turbine and a gas turbine nozzle which are formed of such a superalloy and, further, a combined power generating system employing such a gas turbine and a steam turbine.
Cobalt-base alloys have been widely used as materials of industrial gas turbine nozzles (stationary blades) because of their favorable corrosion-resistance and weldability. However, because the combustion temperature (the temperature at the inlet of a gas turbine) has recently been increased in order to improve thermal efficiency, there is an increasing need for an alloy having great high-temperature strength and thermal-fatigue resistance to replace cobalt-base alloys. Nickel-base superalloys, which are used for blades (buckets), have greater high-temperature strength and thermal-fatigue resistance than cobalt-base alloys. However, because conventional nickel-base superalloys have poor weldability, they are not very suitable materials for gas turbine nozzles, which require welding, for example, repair welding.
Nickel-base superalloys for gas turbine nozzles are disclosed in JP-A-60-10064 and U.S. Pat. No. 4,039,330, and an improved nickel-base superalloy for gas turbine nozzles in U.S. Pat. No. 4,810,467.
Although the above conventional art improves weldability of nickel-base superalloys, it provides no improvement in high-temperature strength, particularly, high-temperature strength over a long term, which is essential to increase the tolerable temperature and durability of the gas turbine nozzles.