Forged rods of alloy 718, designated UNS N07718, are one of the most widely specified precipitation-hardening nickel-base superalloys in high-temperature manufacturing. Modern industry keeps pushing service temperatures and pressures upward, and components must hold their shape, strength and corrosion resistance under those conditions. Alloy 718 answers that demand with a combination of high yield strength, creep and fatigue resistance, oxidation resistance and good fabrication behaviour, which is why it appears in aerospace, power generation, nuclear, chemical and oilfield equipment.
Composition and Strengthening Mechanism
The alloy is built on a nickel-chromium-iron matrix and hardened by the precipitation of gamma double prime phase, based on nickel and niobium, with additional gamma prime phase formed from aluminum and titanium. That dual precipitation reaction is what allows the material to reach very high strength while remaining weldable and resistant to strain-age cracking.
| Element, percent | Limit |
|---|---|
| Nickel | 50.0 to 55.0 |
| Chromium | 17.0 to 21.0 |
| Iron | balance |
| Niobium plus tantalum | 4.75 to 5.50 |
| Molybdenum | 2.80 to 3.30 |
| Titanium | 0.65 to 1.15 |
| Aluminum | 0.20 to 0.80 |
| Cobalt | 1.0 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 |
Standards and Specifications for Forged Rods
ASTM B637 / ASME SB-637 for precipitation-hardening nickel alloy bar, forgings and forging stock
Aerospace material specifications covering solution treated and aged bar and forgings, including AMS 5662, AMS 5663 and AMS 5664 grades
Sour service qualification to NACE MR0175 / ISO 15156 where hydrogen sulfide is present
Supplementary requirements for ultrasonic inspection, grain size and stress rupture testing when a critical rotating part is ordered
Because alloy 718 is used for highly stressed components, the purchase specification should state the required condition, the heat treatment cycle, the acceptance level for ultrasonic inspection and any grain size requirement in addition to the chemistry.
Forging Practice and Heat Treatment
Forging is carried out in a narrow temperature window. A practical working range is about 1010 to 1120 C, and the final reduction should be completed above roughly 950 C so that the part retains a uniform, fine grain structure. Overheating must be avoided because it coarsens the grains and reduces fatigue life and ductility.
Solution treatment: hold at about 950 to 980 C, then cool rapidly in air or water
Aging: hold at about 720 C for 8 hours, then furnace cool at a controlled rate of about 50 C per hour to about 620 C
Second aging step: hold at about 620 C for a further 8 hours, then air cool
Forging stock is usually supplied in the annealed or solution treated condition, and the aging treatment applied after final machining where distortion control matters
Mechanical Properties and Temperature Capability
| Property, aged condition | Typical value |
|---|---|
| Tensile strength | 1275 MPa min |
| Yield strength, 0.2 percent offset | 1035 MPa min |
| Elongation in 50 mm | 12 percent min |
| Reduction of area | 15 percent min |
| Hardness | 331 to 388 HB |
| Continuous service temperature | up to about 650 C |
| Short-term exposure | up to about 980 C |
The alloy also retains toughness at cryogenic temperature, which makes it suitable for equipment handling liquefied gases at the same time as it serves high-temperature duty in gas turbines.
Applications in High-Temperature Alloy Manufacturing
Aerospace: turbine discs and blades, combustion chamber components, shafts, rings and high-load fasteners
Power generation: gas turbine discs, blades, combustion liners and transition pieces
Nuclear engineering: structural components and spring elements that must tolerate high temperature, pressure and radiation
Chemical and petrochemical: reactors, steam generators, high-pressure vessels and process piping
Oil and gas: wellhead parts, valves, drill collars and downhole tools, particularly where sour service qualification is required
Cryogenic equipment: components for liquefied gas service, where toughness must be maintained at very low temperature
The common thread in these duties is that the material must keep a stable microstructure under load, because strength loss and dimensional change, not simple corrosion, are the usual causes of failure in high-temperature equipment.
Frequently Asked Questions
Q: What is the UNS number of alloy 718?
UNS N07718. The alloy may also be quoted with a Werkstoff number and with aerospace material specification numbers, but the UNS designation plus the product specification is the clearest way to order it.
Q: How is the strength developed?
By precipitation hardening during the two-step aging treatment. Gamma double prime phase, formed with niobium, provides most of the strength, supported by gamma prime phase from aluminum and titanium.
Q: What is the maximum service temperature?
Continuous service is normally limited to about 650 C because the strengthening phases start to coarsen above that level, although the material can survive short excursions up to about 980 C.
Q: Can forged rods be machined before aging?
Yes, and for close-tolerance parts it is the preferred sequence, because aging and any solution treatment cause dimensional movement. Machining allowances should account for the expected shrinkage and distortion.
Q: Is the alloy suitable for sour service?
The alloy is widely used in oilfield equipment and can be qualified to NACE MR0175 / ISO 15156, but the acceptable hardness, stress level and temperature limits depend on the specific application standard.
Q: Why is forging rather than casting used for these rods?
Forging breaks up the cast structure, closes internal porosity and refines grain size, which raises fatigue life and toughness. Castings can reach large sizes but cannot match the fatigue performance of a properly forged and heat-treated rod.
