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Alloy 718

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Alloy 718

W-Nr: 2.4668
ASTM: B 637
SAE AMS: 5662, 5663

Alloy 718 was initially developed for the aerospace industry but its excellent strength and corrosion resistance
were recognised by the oil industry and it is now widely used in this field also.
Alloy 718 is a nickel – chromium alloy which can be heat treated to give high strength, good corrosion
resistance, ease of formability and which can be welded with good resistance to strain age cracking. The alloy
can be used at temperatures up to 700ÂșC.

The majority of alloy 718 is vacuum induction melted (VIM) followed by vacuum arc remelting (VAR) or
electro-slag remelting (ESR). However some material is electric arc melted, argon oxygen decarburised
(AOD) followed by double VAR. The major alloying elements are molybdenum, niobium + tantalum, aluminium
and titanium. These give the alloy a combination of high strength and good corrosion resistance when the
alloy is precipitation hardened. The heat treatment of alloy 718 differs depending on its application. This is
because the aerospace material requires maximum strength for operation at high temperature whereas the
oilpatch material is limited to a maximum hardness in order to ensure it is resistant to stress corrosion
cracking.

Alloy 718 for the oil industry is heat treated such that the hardness does not exceed 40HRC which is the
maximum allowed by NACE MR-01-75 to prevent stress corrosion cracking. The major applications in this
field are gate valves, choke stems, fasteners, tubing hangers and fire safe valves.
Alloy 718 for aerospace and power generation is heat treated to give maximum strength and high creep
resistance with typical hardness values exceeding 42HRC. The major applications are components for gas
turbines, aircraft engines, fasteners and other high strength applications.