Microstructure stability: Optimisation of 263 Ni-based superalloy

To reduce CO2 emissions on coal-fired power plant, A-ultra supercritical (A-USC) power plant whose steam conditions exceed 700 degrees C are being developed. At these elevated temperatures, the use of Ni-base superalloys becomes necessary. In this context and within the European project NextGenPower, focus is made on commercial Nimonic C-263 as a candidate material for turbine rotors. Nimonic C-263 is known to have low sensitivity to segregation, high workability and high weldability which are major properties for the manufacture of large shafts. Long-term creep strength is also required for this application and unfortunately Nimonic C-263 shows eta-phase precipitation after long-time exposure between 700 degrees C-900 degrees C which is detrimental for long-term creep properties. The composition of Nimonic C-263 was thus optimised to overcome the formation of eta-phase. Trial tests were made in order to study the effect of hardening contribution elements on microstructural and mechanical properties. Then, a 500 mm diameter forged rotor was made from optimised 263 alloy and shows promising properties.