According to the requirements of properties and hot working for the large forgings in 700°C ultra-supercritical (USC) coal-fired power plants, the optimization of alloy compositions, the design of hot working and heat treatment processes were conducted on the basis of Fe-Ni based alloy GH984G used for tubes in 700°C USC power plants. The γ matrix, γ' phase, the interface between γ and γ', the characteristics of grain boundary etc. were optimized by adjusting Nb/(Ti+Al) ratio and the content of Fe, Cr, C, B and P. The modified alloy has excellent strength, impact toughness, structural stability and workability. It is suitable for the manufacture of GH984G large forgings with low cost. The hot working of large forgings is complicated and the microstructures varies under different hot working processes. The hot deformation map of materials under different hot working processes indicated that the hot deformation behaviors were related to the hot working history. The hot deformation process suitable for large forgings has been developed on the basis of the results. The influence of heat treatment on the microstructure and mechanical properties was investigated and the optimized heat treatment for large forgings was given. The large forgings with 6.9 tons and 693 mm outer diameter have been produced. The tensile and stress rupture properties of the large forging are higher than the requirements of the project.
