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Nickel Inconel 718

Nickel Inconel 718

The Alloy Nickel Inconel 718 is a nickel-chromium alloy with high strength and corrosion resistance. It is used at temperatures ranging from -423° to 1300°F (-253° to 705°C). As an age-hardenable alloy, the alloy Nickel Inconel 718 can be readily fabricated in very complex parts, and its welding characteristics are also outstanding. It also has good tensile, fatigue, creep, and rupture strength, which combined with easy fabrication and corrosion resistance.

Due to its combination of properties, the Nickel Inconel 718 is used for a wide range of applications, such as liquid-fueled rockets, rings, and casings, various formed sheet metal parts for aircraft and land-based gas turbine engines, cryogenic tankage, and for fasteners and instrumentation parts.

Tech Steel & Materials offers the alloy Nickel Inconel 718 in 9 sub-type specifications, each in multiple and customized shapes/forms:

We offer a fully customized line of tube drawing and seamless tubing for Nickel 718. Learn more about our Custom Tubing Services.

The Composition of Nickel Inconel 718

The composition of the particular alloy is the following:

Element Limiting Chemical Composition (%)
Nickel (plus Cobalt) 50%-55%
Chromium 17%-21%
Niobium (plus Tantalum) 4.75%-5.50%
Molybdenum 2.80%-3.30%
Titanium 0.65%-1.15%
Aluminum 0.20%-0.80%
Cobalt 1% max
Carbon 0.08% max
Manganese 0.35% max
Silicon 0.35% max
Phosphorus 0.015% max
Sulfur 0.015% max
Boron 0.006% max
Copper 0.30% max
Iron Balance*

*Not exclusively to the element mentioned, but that one predominates other elements that are used only in minimal quantities.

Fabrication and Working Instructions

It is highly recommended to follow the next instructions when Nickel Inconel 718 is heated, in order to minimize oxide layers forming.

  • Fuels used for the heating must be extremely low in sulfur;
  • Both the furnace and material should be heated at the correct temperatures;
  • Before putting it into the furnace, completelyclean the alloy from oil, paint, grease, and shop soil;
  • For forging or open annealing, the furnace atmosphere should be slightly reducing, with at least 2% carbon monoxide;
  • Keep the pressure in the furnace slightly positive so that there is no air infiltration;

Please note that in reducing atmospheres, a green-black film of oxide forms (easy to remove). In oxidizing atmospheres, on the other hand, a black scale forms that is very difficult to remove.

Nickel Inconel 718 joint efficiency when welding is about 100% for parts that have been annealed, welded and aged. Due to the slow response, it can be age-hardened directly, without any sort of stress relief. Welding should be done with the gas tungsten-arc process, using the proprietary Nickel Filler Metal 718.

If the Alloy Nickel Inconel 718 is cold-worked or precipitation hardened, use of silver brazing compounds is not recommended, as they can cause stress and crackling to the alloy. Brazing compounds with cadmium should also be avoided because that element can aggravate the cracking that started from silver or other elements.

Physical Constants

Density Annealed: 0.296 lb/in3
Annealed and aged: 0.297 lb/in3
Melting Range 2300° to 2437°F  (1260° to 1336°C)
Specific Heat at 21°C (70°F) 0.104 Btu/lb-°F
Permeability at 200 oersted and 21°C (70°F) Annealed material: 1.0013
Annealed and aged material: 1.0011
Curie Temperature °C (°F) Annealed material: <-320°F (<-196°C)
annealed and aged material: -170°F (-112°C)

Heat Treating and Mechanical Properties of Nickel Inconel 718

The Alloy Nickel Inconel 718 is specified as solution annealed and precipitation hardened by the precipitation of the secondary phases into the metal matrix. The precipitation takes place at 1100° to 1500°F (593° to 815°C). In order for the metallurgical reaction to properly take place, the aging constituents, such as aluminum, titanium, and niobium, must be dissolved in the matrix. Otherwise, the alloy may not reach full strength. For that to happen, solution annealing is utilized.

Overall, three heat treatments are utilized for the Nickel Inconel 718:

  • Solution anneal at 1700 to 1850°F (925° to 1010°C) followed by rapid cooling, usually in water. Then, precipitation hardening is done at 1325°F (718°C) for 8 hours and furnace cool to 1150°F (620°C). Hold at 1150°F (620°C) for a total aging time of 18 hours, followed by air cooling.
    This treatment is optimal when there is a concern of rupture life, notch rupture life, and rupture ductility. It is also associated with high fatigue strength and room-temperature tensile and yield strengths.
  • Solution anneal at 1900° to 1950°F (1037° to 1065°C) followed by rapid cooling, usually in water. Then, precipitation hardening is done at 1400°F (760°C) for 10 hours and furnace cool to 1200°F (650°C). Hold at 1200°F for a total aging time of 20 hours, followed by air cooling.
    This treatment is preferred for tensile-limited applications. It produces the best transverse ductility in heavy sections, better impact strength, and low-temperature notch strength. Compared to the other treatment though, it has a tendency to produce notch brittleness in stress rupture.
  • Solution anneal at 1850° to 1900°F (1010° to 1037°C), followed by aging at 1450°F (790°C) for six to eight hours and then air cooled. Produced under the NACE specification MR0175, this treatment is used for oil field applications.

The material can be purchased in its mill annealed or stress relieved condition if it’s to be welded, machined or formed.