Inconel 718 is a precipitation-hardenable nickel-chromium-molybdenum alloy containing significant amounts of iron, niobium, and aluminum. It is widely used for high-strength, high-temperature applications up to about 700°C (1,300°F). The alloy can be age-hardened to achieve exceptional tensile, fatigue, and creep strengths while maintaining good ductility and weldability.

The nominal chemical composition is: nickel 50.0 to 55.0 percent, chromium 17.0 to 21.0 percent, iron balance (approximately 18 percent), molybdenum 2.8 to 3.3 percent, niobium (plus tantalum) 4.8 to 5.5 percent, titanium 0.65 to 1.15 percent, aluminum 0.20 to 0.80 percent, and cobalt up to 1.0 percent. The precipitation hardening phases are gamma prime (Ni3(Al,Ti)) and gamma double prime (Ni3Nb).

Key mechanical properties (typical, aged condition):

● Tensile strength: 1,200 to 1,450 MPa

● Yield strength (0.2% offset): 1,000 to 1,200 MPa

● Elongation: 12 to 25 percent

● Hardness: 35 to 45 HRC

● Density: 8.19 g/cm³

Inconel 718 offers excellent oxidation and corrosion resistance up to high temperatures, good resistance to post-weld cracking, and outstanding stress-rupture properties. It retains useful strength and ductility from cryogenic temperatures to about 650°C (1,200°F) for long-term service, and can be used up to 700°C for shorter durations.

Common standards: AMS 5662/5663/5664 (bar, forgings), AMS 5596/5597 (sheet, plate), ASTM B637 (bar), ASTM B670 (plate, sheet).

Typical applications include:

● Aerospace: gas turbine engine discs, blades, casings, shafts, and fasteners.

● Rocket engines: turbopump components and thrust chambers.

● Nuclear reactors: fuel element spacers and other high-temperature components.

● Oil and gas: downhole tools, wellhead equipment, and high-strength fasteners in sour service.

● Automotive and industrial: turbocharger rotors, high-temperature bolts, and dies.

Inconel 718 is often compared to Waspaloy (higher temperature capability but harder to weld) and Inconel 625 (lower strength, no precipitation hardening). For applications requiring the highest strength at moderate temperatures (around 650°C), Inconel 718 is the most common choice. For service above 700°C, alloys like René 41 or Inconel 939 may be preferred. For lower strength but better corrosion resistance, Inconel 625 is an alternative.