Application and development trend of 718 nickel-based high-temperature alloy

Alloy 718 is a nickel-based high-temperature alloy selected where heat resistance, corrosion resistance and fatigue resistance must hold together: hot-section parts of aerospace engines, drilling equipment, chemical reactors and high-temperature furnaces. This article covers its composition, melt-and-cast preparation, solution and aging heat treatment, mechanical properties and application fields, then looks at development trends. In piping systems the same alloy is best known as bolting material that keeps flange gaskets sealed through repeated thermal cycles.

Composition and Preparation of Alloy 718

Nickel is the base element, generally above 50% of the composition. Iron balances the alloy, and chromium, cobalt, molybdenum and aluminium provide the strengthening response that heat treatment later unlocks. Small additions of titanium, zirconium and carbon complete the chemistry.

• Base metal: nickel above 50%, iron as the balance.
• Major additions: chromium, cobalt, molybdenum and aluminium.
• Minor additions: titanium, zirconium and carbon.
• Crystal structure: face-centred cubic, dense and mechanically strong.

Production follows the usual superalloy route. The elements are melted by vacuum induction melting or electroslag remelting to obtain a uniform melt, the melt is poured into a pre-designed mould and solidifies into shape, and the resulting material is heat treated to reach its final properties. Melting practice matters: a uniform melt is what makes the response to solution and aging treatment repeatable from heat to heat.

Heat Treatment: Solution and Aging

Alloy 718 is supplied in the heat-treated condition. Two stages do the work: solution treatment first, then aging.

Solution treatment heats the material to 1020-1100 °C and holds it for about 1 hour so that the alloying elements dissolve uniformly, then cools it at a controlled rate. The stage removes residual stress and over-hard phases and improves plasticity and hot workability.

Typical heat-treatment parameters reported for alloy 718. Exact cycles depend on section size and on the material specification that applies.

Aging follows, normally in two steps: low-temperature aging first, then high-temperature aging. Three points decide whether the cycle works in production.

• Temperature control - hold the set point in a narrow band, otherwise the properties of the material deteriorate.
• Time control - overlong low-temperature aging reduces creep resistance, while overlong high-temperature aging grows the grains and lowers strength and hardness.
• Composition control - silicon, aluminium, titanium and carbon all change the response to solution and aging treatment.

Mechanical Properties and Yield Strength

Alloy 718 combines high strength, high toughness and fatigue resistance. Tensile strength can exceed 1400 MPa and yield strength passes 1200 MPa, while hardness and wear resistance allow the material to be used for parts that move at high speed.

Yield strength is the stress at which plastic deformation begins, and it is the number that sets service limits. Strength is controlled by the face-centred cubic crystal structure and by composition: nickel provides solid-solution strengthening, and elements such as chromium and iron add solid-solution strengthening and precipitation hardening.

• Temperature - yield strength falls as service temperature rises.
• Stress level and strain rate - both shift the measured value.
• Composition and alloying content - adjusting the ratio raises yield strength.
• Heat treatment condition - solution and aging cycles change the grain structure.
• Cold deformation - raises yield strength noticeably but reduces plasticity and toughness.

In practice this is why allowable stresses for bolting on pipe flanges and other loaded parts are taken from the yield strength at service temperature rather than from room-temperature data, and why the alloy is specified only where that margin is really needed.

Applications and Development Trends

Two fields absorb most of the material, and a third group of parts takes the rest.

• Aerospace - turbine blades, combustion chamber components and bearings in the hot section.
• Oil, gas and chemical - chemical reactors, catalyst carriers, high-temperature furnaces and drilling equipment.
• High-speed machinery - parts that need hardness and wear resistance alongside strength.

Demand is expected to keep growing in both main fields. Research is aimed at higher high-temperature strength and better corrosion resistance, at lower preparation cost and at better sustainability. As materials science and melting and casting practice advance, the range of parts that can justify alloy 718 should widen rather than narrow.

Frequently Asked Questions

What are the main elements in alloy 718?

Nickel above 50% with iron as the balance, plus chromium, cobalt, molybdenum and aluminium, and small additions of titanium, zirconium and carbon.

Which heat treatment temperatures are used for alloy 718?

Solution treatment at 1020-1100 °C for about 1 hour, then aging at about 720 °C for about 8 hours and about 980 °C for about 8 hours.

Why does yield strength drop as temperature rises?

Yield strength decreases with temperature, so the service temperature limit is derived from the yield strength at that temperature and not from room-temperature values. Stress level and strain rate also affect the result.

Does cold deformation improve alloy 718?

It raises yield strength noticeably, but it also reduces plasticity and toughness, so the amount of cold work is balanced against the ductility the part needs.