Heat Treatment And High Temperature Performance Of A New Fe-Ni-Co Low Expansion Superalloy

Low expansion superalloys possess low expansion coefficient, excellent high temperature mechanical properties and oxidation resistance, so it was widely used in aerospace, ordnance industry, instrumentation, and other fields. With the rapid development of the aerospace industry, for low expansion superalloys overall performance put forward higher requirements, so our group members developed a new type of Fe-Ni-Co based low expansion superalloys.We selected a representative composition to prepare the alloys, and optimized organizational control and performance for alloys.By thermodynamic calculation software Jmat Pro, calculate alloy’s phase transition behavior to obtain the alloy’s equilibrium phase diagram, CCT curve, TTT curve, to provide guidance for the heat treatment process.Effect of solution treatment on the alloy structure was studied, the results show that by heat 6h at 1170 ℃, air-cooled, then heat 2h at 1220 ℃, air cooled solution can get better results.Research on the aging process on Microstructure and hardness, the results show that aging temperature have greater impact on γ ’ phase size than the aging time, with the increase of an aging temperature, the alloy γ’ phase increased in size and the hardness reduced; With a higher temperature in the primary aging process, the alloy in the secondary aging process there will be a small secondary γ ’phase precipitation. Alloy has a higher hardness when primary γ ’phase in the size range 30 nm ~ 40 nm, and uniform distribution. When primary aging temperature is at 650 ℃, γ ’phase cube is the best with the size range 30 nm ~ 40 nm, aging within 24 h average hardness HV570 ~ 580, microstructure and hardness values are stable. The room temperature tensile of the sample after 650 ℃ × 12 h / AC + 620 ℃ × 8h / AC aging treatment was 985 MPa, 650 ℃ high temperature tensile strength was 718 MPa, with good intensity.Thermal expansion properties of the alloys were studied, the experimental alloy system have more excellent low expansion properties, with 2 # alloy’s Curie temperature 448 ℃ and the average linear expansion coefficient of 25 to 400 ℃ is 9.1 × 10-6 /℃. Adding Aluminum will increase the thermal expansion coefficient of the alloy, reduce the Curie temperature of the alloy. Adding the Tantalum will reduce the thermal expansion coefficient of the alloy, but will also reduce the Curie temperature of the alloy. Appropriate adjustments of Co /(Ni + Co) value will increase the Curie temperature, lower coefficient of linear expansion of alloy in high temperature.Alloy oxidation resistance at 550 ℃ ~ 700 ℃ was studied, the alloy oxidation kinetics in this temperature range follows a parabolic law. At 550 ℃, 600 ℃, 650 ℃ is wholly antioxidant levels, at 700 ℃ is antioxidant level; Alloy isothermal oxidation diffusion activation energy was 166179 J, isothermal oxidation rate constant expression is: KT = 8.34 × 107e-166179 /(RT). The oxidation products of the alloy at high temperature oxidation after 100 h were mainly Co Fe 2O4, and forming a dense oxide film that will protect the alloy substrate.