UNS N07718, better known as Alloy 718, is a precipitation-hardening nickel superalloy used from cryogenic temperatures to about 700 degrees C. This reference summarises its composition, heat treatment, product forms and standards.
Alloy Overview and Composition
UNS N07718 contains about 50 to 55 percent nickel, 17 to 21 percent chromium, 4.75 to 5.5 percent niobium plus tantalum, 2.8 to 3.3 percent molybdenum, and controlled titanium and aluminium, with the balance iron.
The alloy is strengthened by gamma double prime and gamma prime precipitates formed during aging, giving high strength together with good fatigue, creep and corrosion resistance.
Strengthening and Heat Treatment
UNS N07718 is normally supplied solution treated and then aged in one or two steps to develop its properties. The aging treatment precipitates the strengthening phases that give the alloy its high yield strength and fatigue life.
Heat treatment must be controlled because the properties of the finished part depend on the exact cycle, and heat treatment charts are normally required as part of the documentation.
Product Forms and Specifications
UNS N07718 is available as bar, rod and wire, plate, sheet and strip, seamless pipe and tube, and forgings. In the ASTM system, bar and forgings are covered by a precipitation-hardening bar and forging standard, plate and sheet by flat-rolled standards, and seamless pipe and tube by a dedicated high-strength pipe standard.
In aerospace supply chains the same product forms are frequently specified to AMS specifications such as AMS 5662 for bar and AMS 5596 for sheet, which control chemistry, condition and mechanical property levels.
Mechanical Properties and Applications
In the aged condition the alloy reaches high tensile and yield strength with good ductility, excellent fatigue resistance and useful creep strength to about 700 degrees C. It also remains tough at cryogenic temperature.
Typical applications include gas turbine and rocket engine components, aerospace fasteners and hydraulic lines, oil and gas wellhead and downhole equipment, cryogenic vessels and springs.
Ordering and Documentation
When ordering UNS N07718, state the product form, the applicable standard and edition, the heat treatment condition, dimensions and tolerances, quantity and certification level.
Request a material test certificate with heat number, chemistry, tensile and hardness results, the heat treatment chart and, where required, ultrasonic or positive material identification reports.
Frequently Asked Questions
Q: What is UNS N07718?
A precipitation-hardening nickel-chromium superalloy, commonly called Alloy 718 or Inconel 718, used from cryogenic temperatures to about 700 degrees C.
Q: How is UNS N07718 strengthened?
By a controlled solution treatment and aging cycle that forms gamma double prime and gamma prime precipitates.
Q: What product forms are available?
Bar, rod and wire, plate, sheet and strip, seamless pipe and tube, and forgings, each covered by its own standard.
Q: What temperature range does Alloy 718 serve?
It performs well from cryogenic temperatures up to roughly 700 degrees C.
Q: What documents are normally supplied?
A material test certificate with chemistry, tensile and hardness results, the heat treatment chart and inspection reports where required.
