Inconel 718 Alloy
General Characteristics of Inconel 718 Alloy
Inconel 718 is a nickel-based precipitation-hardening alloy designed for very high yield strength, tensile strength and creep rupture properties at temperatures up to 1300ºF (705ºC.) The alloy has excellent weldability.
| Chemical Analysis | |
|---|---|
| C | Carbon 0.08 max |
| Mn | Manganese 0.35 max |
| P and S | Phosphorus and Sulfur 0.015 maxx |
| Si | Silicon 0.35 max |
| Ni(+Co) | Nickel Colbalt 50.0/55.0 |
| Cr | Chromium 17/21 |
| Co | Cobalt 1.00 max |
| Cu | Copper 0.15 max/td> |
| Al | Aluminium 0.35/0.80 |
| Mo | Molybdenum 2.80/3.30 |
| Ti | Titanium 0.65/1.15 |
| Cb + Ta | Columbium + Tantalum 4.75/5.50 |
| B | Boron 0.001/0.006 |
Applications
The alloy is used in jet engine and high-speed airframe parts such as wheels, buckets and spacers, and high-temperature bolts and fasteners. Inconel 718 is also used in the oil and gas drilling and production industries due to its high strength and resistance to chlorides, stress corrosion and sulfide stress cracking. Within these industries the alloy has been used for valves, pump shafts and wellhead components.
Forging
Alloy 718 should be forged from a maximum furnace temperature of 2050ºF (1120ºC) and should not be soaked for an overly long time at this temperature. Uniform reductions during forging will prevent the formation of a duplex grain structure. Working in the range 1700/1850ºF (925/1010ºC) will improve the forging’s strength if its subsequent service temperature is below 1100ºF (595ºC.)
Heat treatment
Heat treatments may be adjusted to give required properties. For the best combination of tensile and stress rupture properties, the following quite complex cycle is recommended :
1 hr at 1750/1800ºF (955/980ºC) and air cool, then 8 hrs at 1325ºF (720ºC) and cool at 100ºF/hr (56ºC/hr) to 1150ºF (620ºC), hold for 8 hrs and air cool.
For the best room temperature and cryogenic tensile properties, the cycle is :
1-2hrs at 1950ºF (1065ºC) and air cool, then 8 hrs at 1325ºF (720ºC) and cool at 100ºF/hr (56ºC/hr) to 1150ºF (620ºC), hold for 8 hrs and air cool.
Machinability
Inconel 718 may be readily machined in either the annealed or age-hardened conditions. The age-hardened condition gives a better surface finish, the annealed condition a longer tool life.
Weldability
The alloy may be welded in either the aged or annealed conditions. Welding an age-hardened material will result in the formation of a softened heat-affected zone
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