Inconel 718 (UNS N07718): Aerospace Superalloy — Creep Strength to 700°C

Per AMS 5662: Solution anneal at 954-982°C (1750-1800°F) for 1 hour per 25 mm thickness, air cool or faster. Then age-harden: 718°C (1325°F) for 8 hours, furnace cool at 55°C/hr (100°F/hr) to 621°C (1150°F), hold at 621°C for 8 hours (total aging time approximately 18 hours), air cool. The delta phase (orthorhombic Ni₃Nb) is controlled during solution treatment to pin grain boundaries without consuming too much niobium needed for γ″ precipitation. The two-step aging was designed to precipitate both γ″ (at 718°C) and γ′ (at 621°C) for optimal high-temperature properties.

Three reasons: (1) Weldability — γ″ precipitation is slow enough that 718 can be welded in the solution-annealed condition without heat-affected-zone cracking. Waspaloy, René 41, and other γ′-hardened alloys crack during welding or post-weld strain-age cracking — a fundamental limitation. (2) Manufacturability — 718 can be hot-worked, machined, and formed with conventional tooling in the solution-annealed condition, then precipitation-hardened to full strength. (3) Cost relative to capability — while expensive per kg, 718 is significantly cheaper than powder-metallurgy superalloys (René 88DT, RR1000) used in the hottest turbine disk stages, and it provides adequate properties for compressor and fan stages where temperatures do not exceed 650°C.