Precipitation Hardening Nickel-Chromium Alloy

Alloy 718 is a precipitation hardening nickel-chromium alloy with a great combination of corrosion resistance and high strength together with high weldability and ease of fabrication. The improved properties of the alloy are obtained by adding molybdenum, niobium, titanium, aluminum and iron. The distinct blend of properties helps its usage for components in the marine, aerospace, chemical, nuclear, offshore and petrochemical industries.

Alloy 718 is stocked by Columbia Metals in the solution treated condition as per AMS 5662 which is highly suitable for heavy machining, forming or welding as it is in its most malleable state. It is also stocked in the API 6A specification, which is widely used in oil & gas industry applications.

It can be precipitation hardened after fabrication to achieve the right strength levels. It can be solution treated and age hardened at several temperatures to fulfill the requirements of individual specifications. Each heat treatment can offer the user with a different set of mechanical and physical properties suitable for different applications.

According to the AMS 5662 specification, the material should be supplied in the solution treated condition (i.e. heat treated within the range 941- 1010°C). The subsequent age hardening process offers Alloy 718 the best combination of rupture life, notch rupture and rupture ductility. The superior properties are achieved due to the development of a fine grain structure that also provides its highest tensile and yield strengths through a wide range of temperatures from -253°C to 705°C.

Alloy 718 also offers excellent corrosion resistance in a range of media. It excels in a range of environments including organic acids, inorganic acids (not strongly oxidising), hydrogen sulphide and sour gasses. It is also particularly resistant to chloride stress-corrosion cracking. The chromium content present in the material results in an excellent resistance to oxidation and sulphur compounds up to temperatures of around 980°C. The molybdenum content aids in the overall pitting resistance of the material.

Nominal Composition (%)

The composition of alloy 718 is given the table below:

Ni Cr Fe Nb Mo Ti Al
S3 19 18 S 3 0.9 0.5

Properties of Alloy 718

The key properties of Alloy 718 are listed below:

  • Excellent strength levels
  • Outstanding corrosion resistance
  • High resistance to stress rupture
  • Resistance to stress corrosion
  • Excellent pitting resistance
  • Superb high temperature strength
  • Oxidation resistance up to 982°c
  • Good forming / fabrication
  • Very high resistance to creep
  • Easily joined / welded

Mechanical Properties

Mechanical properties of Alloy 718 are provided in the table below:

  As Delivered Solution Treated
Hardness (HB) 277
  Age Hardened Room Temperature (Longitudinal) <127mm dia
Ultimate Tensile Strength (N/mm2) 1276
0.2% Yield Strength (N/mm2) 1034
Elongation (%) 12
Reduction of Area (%) 15

Typical Physical Properties

  Age Hardened Alloy 718
Density (g/cm3) 8.22
Melting Range (°C) 1260 - 1335
Young's Modulus (GPa) 204
Thermal conductivity (0-200°C - W/m°K) 14.2
Coeff. of Thermal Expansion (m/m°K x 10-6) 13.5
Magnetic Permeability 1.0011

Machining Properties of Alloy 718

Alloy 718 can be machined readily in the solution treated condition but the material's high strength and work-hardening characteristics should be taken into account.

It is possible to hot work the material between 985-1150°C or cold worked whilst in the solution treated condition due to its high elongation. However, inter stage annealing (between 925 to 1010°C) may be required for more complicated cold working should be followed by annealing and quenching to restore the best balance of properties. This material can also be readily welded by the gas tungsten arc (TIG) process and it is especially resistant to post weld cracking.

Applications of Alloy 718

This integration of properties results in a material that can be called a "Superalloy". Specific applications include valves, fasteners, choke stems, down-hole tooling, cryogenic storage tanks, gas turbines, jet engines, pump bodies and nuclear reactor parts. It is also suitable for shear blades and extrusion dies where conventional tool steels do not offer the required high temperature strength.

This information has been sourced, reviewed and adapted from materials provided by Columbia Metals.

For more information on this source, please visit Columbia Metals.

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