抄録
Advanced 700 ℃ class steam turbines require using Ni-based superalloys instead of conventional ferritic 12Cr steel which is insufficient in creep strength and oxidation resistance above 650 ℃. The superalloys, however, possess quite higher coefficient of thermal expansion (CTE) than the 12Cr steel. So far authors examined the influence of alloying elements on CTE and the high temperature strength of the Ni-based superalloys. Consequently “LTES700R” was developed for steam turbine, which has low CTE and sufficient creep strength. LTES700R is corrected Mo amount to inhibit the Laves phase and added W to reduce CTE instead of Mo within the range that alfa-tungsten does not precipitate. In addition, Al and Ti, which form gamma-prime, were increased to make up for deteriorating of the strength by Laves phase free. The creep rapture strength of LTES700R manufactured by laboratory forging is higher than that of advanced 12Cr steel owing to be strengthened by gamma-prime [Ni3(Al, Ti)] phase precipitates. The CTE of LTES700R is lower than that of Refractaloy 26®, and slightly higher than that of 12Cr steel. The phase of LTES700R is stable until 5000 hr heating from 550 ℃ to 750 ℃.
