Development of Ni‐Based DS Superalloy with Excellent Oxidation Resistance and LCF Properties for Power‐Generation Gas Turbines

Abstract

In advanced industrial gas-turbine systems, there is a great demand for a new, excellent superalloy which can be used for large gas-turbine blades. Because it is very difficult to produce a large-scale Ni-based Single Crystal (SC) blade, we have developed new Directionally Solidified (DS) superalloys with an excellent combination of high-temperature creep strength, lowcycle-fatigue strength, good oxidation resistance, castability and hot-corrosion resistance. In this study, three kinds of Ni-based DS superalloys were designed with the aid of the d-electrons concept. The chemical compositions of these alloys were in the range of 1.4%Ti, 8.2%Cr, 11%Co, (0-1.0)%Ru, 3.6%Ta, 9.2%W, 1.0%Re, 5.2%Al, 1.40%Hf and balanced Ni in weight percent (wt%). Creep rupture tests, hot-corrosion burner-rig tests, low-cyclefatigue tests and oxidation tests were conducted with heat-treated DS specimens of these alloys. Two of the designed alloys showed the same level of creep rupture life as those of the 2nd generation DS superalloys currently in wide use. Oxidation tests at 1313K proved that the oxidation resistance of all the designed alloys was superior to that of all the reference alloys. The results of the hotcorrosion burner-rig tests showed that all the designed alloys had better corrosion resistance than the 2nd generation DS superalloys. As for Low-Cycle-Fatigue (LCF) strength, it was found that even 0.6%Ru addition greatly improved LCF strength, and the 0.6%Ru containing Alloy-B showed better LCF strength than any commercially used reference DS superalloys. A casting test was accomplished in a model bucket of a first stage blade of a 25MWclass gas turbine.Alloy-B is potentially a suitable alloy for making gas turbine blades with prolonged lifetimes and low maintenance costs in recent 1500 °C-class or even higher class gas turbines for power generation.