Abstract
Recently, a γ’ precipitation strengthened Ni-base superalloy, USC141, was developed for 700°C class A-USC boiler tubes as well as turbine blades. In boiler tube application, the creep rupture strength of USC141 was much higher than that of Alloy617, and the 105 hours’ creep rupture strength of USC141 was estimated to be about 180MPa at 700°C. This is because fine γ’ particles precipitate in austenite grains and some kinds of intermetallic compounds and carbides precipitate along austenite grain boundaries during creep tests. Good coal ash corrosion resistance is also required for tubes at around 700°C. It is known that coal ash corrosion resistance depends on the contents of Cr and Mo in Ni-base superalloys. Therefore the effect of Cr and Mo contents in USC141 on coal ash corrosion resistance, tensile properties, and creep rupture strengths were investigated. As a result, the modified USC141 containing not less than 23% Cr and not more than 7% Mo showed better hot corrosion resistance than the original USC141. This modified alloy also showed almost the same mechanical properties as the original one. Furthermore the trial production of the modified USC141 tubes is now in progress.
