Chemical composition

Phosphorus and sulphur 0.015% maximum each. Limits as given in ASTM B637 and AMS 5662.

Niobium is the key strengthening element: during ageing it forms very fine gamma double-prime particles that block dislocation movement. Because these particles form slowly, 718 can be welded and cooled without cracking in the weld zone – a major advantage over other age-hardening nickel alloys.

Mechanical properties

Values for bars and forgings in the solution-treated and aged condition; requirements vary with size and with AMS specifications.

In the solution-treated condition, 718 is relatively soft and is machined, formed and welded; the final strength is developed by a two-step ageing treatment at around 720 °C and 620 °C. API 6A 718 for oil and gas uses a different heat treatment to give a lower maximum hardness for sour service.

Equivalent designations

See the full grade equivalent chart.

Inconel 718 products we supply

• Inconel round bars – ASTM B637, AMS 5662/5663, API 6A 718
• Inconel fasteners – high-strength studs, bolts and nuts
• Inconel flanges – forged flanges and rings on request
• Inconel pipes and tubes – seamless tube on enquiry
• Inconel sheets and plates – AMS 5596 on enquiry
• Inconel pipe fittings – machined fittings on request

All Inconel alloys are on the Inconel page.

Where Inconel 718 is used

Aerospace and gas turbines: turbine discs, shafts, casings, rings, fasteners and structural parts in jet engines and land-based gas turbines, where high strength and fatigue resistance up to about 650 °C are needed. Oil and gas: wellhead and christmas-tree components, hangers, valve stems, drilling tools and subsea connectors, often to API 6A 718 with hardness limits for sour service. Power and industry: high-temperature bolting, springs, pump shafts, cryogenic tanks and tooling. Nuclear and space: springs, fasteners and structural parts.

Inconel 718 compared with other alloys

Inconel 625 is not age-hardenable and has much lower strength but better corrosion resistance in seawater and acids, and is easier to fabricate into piping. Inconel 600 is used for oxidation and caustic service rather than strength. Inconel X-750 and Waspaloy are other age-hardening alloys; X-750 is often used for springs, and Waspaloy for higher temperatures than 718 can tolerate. For room-temperature high-strength corrosion-resistant parts, Monel K-500 is a lower-cost alternative in seawater.

Heat treatment and machining

Heat treatment is specified by the purchaser: solution treatment at about 940–1010 °C followed by double ageing gives the highest strength for aerospace use, while API 6A practice uses a solution and single age to meet hardness limits for sour service. Machining is usually done in the solution-treated condition with rigid set-ups, sharp carbide or ceramic tools, low speeds and plenty of coolant; final finishing may be done after ageing. Inconel 718 is welded with matching ERNiFeCr-2 filler, followed by ageing of the weldment.

Sizes and supply

Inconel 718 is supplied in the product forms listed above, in metric and imperial sizes. Because nickel alloys are produced by a limited number of mills and in smaller quantities than stainless steel, availability of each size, the delivery of special sizes and the minimum mill quantities are confirmed with each quotation. Cut lengths, cut plates and machined blanks reduce waste of an expensive alloy and can be supplied on request. Please send the full requirement – grade, specification, product, size, quantity and testing – so that the complete package can be quoted together.

Documents and inspection

Material is supplied with mill test certificates (MTC) showing heat number, chemical composition, mechanical properties and heat treatment. Third-party inspection is available on request, and positive material identification can be carried out before dispatch. Because nickel alloys look alike, every piece is marked with grade and heat number for traceability through fabrication.

For aerospace and oil and gas orders, the specification, heat-treatment condition and any grain-size, ultrasonic or hardness requirements should be stated with the enquiry.