Alloy 718 is the nickel alloy that holds high strength at temperature. It gets that strength from a heat treatment rather than from the press alone: a solution treatment followed by a controlled ageing cycle precipitates fine particles through the metal, and those particles block the movement that would otherwise let the part deform. In the aged condition it reaches roughly 830 to 1000 N/mm² proof stress and 32 to 40 HRC, with corrosion resistance that stands up in seawater.
Longrui Forging forges, solution treats and ages Alloy 718 in house, in Yunhe County, Zhejiang, China: valve bodies and covers, flanges, hubs and custom parts, from 5 kg to 520 kg per piece, on the 6,000 T multi-directional press. Forging to ASTM B564.
What Alloy 718 Is
The grade is written as Alloy 718, UNS N07718 (Werkstoff 2.4668). Inconel 718 is Special Metals’ trademark for it, so drawings, heat records and certificates here carry the alloy and UNS names rather than a producer’s brand.
| Element | Typical composition (%) |
|---|---|
| Nickel | 50.0 – 55.0 |
| Chromium | 17.0 – 21.0 |
| Niobium + tantalum | 4.70 – 5.50 |
| Molybdenum | 2.80 – 3.30 |
| Titanium | 0.65 – 1.15 |
| Aluminium | 0.20 – 0.80 |
| Iron | Balance |
Some background that explains why the grade keeps coming up: 718 was developed for steam power plant, then taken up by aerospace for its oxidation resistance at temperature, and it is now the most widely specified grade in the nickel family, including in oil and gas. It is effectively non-magnetic, and its melting range is about 1260 to 1340 °C.
How 718 Gets Its Strength
Three alloying additions do the work: niobium, titanium and aluminium. During the solution treatment they dissolve into the nickel base, and during the ageing cycle that follows they come back out as very fine precipitates, dispersed through the metal. The precipitates pin the microstructural features that would otherwise slide under load, which is what turns a moderately strong nickel alloy into a high-strength one.
Two consequences for a buyer:
- The heat treatment is part of the specification. 718 is quoted in several strength levels depending on how far the ageing is taken, so the drawing or the purchase specification has to name the condition. We work to the level the specification calls for and record the cycle.
- Strength has a ceiling, and it is not the oxidation limit. Most of the mechanical strength is retained up to about 650 °C; above that the grade is chosen for oxidation resistance rather than for strength, and creep becomes the design case. Data sheets quote higher figures for short-term exposure, so it is worth naming the service temperature and the design code rather than picking the highest number on a table.
What 718 Demands in the Forge
Like the other nickel alloys, 718 keeps much of its strength while hot, so the press works harder and the usable temperature window is narrower than for carbon or stainless steel. On top of that, this grade has a further step after forging: solution treatment, then ageing.
On a 718 job we hold the billet inside the window the grade allows, with the temperature monitored rather than estimated, then solution treat and age the part to the strength level the specification names, recording the cycle so the certificate can show it. Testing covers chemistry, tensile properties, impact at low temperature, hardness and ultrasonic inspection, all carried out on site; what each stage leaves behind is set out on the quality control page.
Machining and Welding Inconel 718
Machining is where customers most often ask for advice, because 718 is a difficult material to cut:
- It work-hardens as it is cut, so cutting forces climb and the tool has to be fed rather than rubbed.
- Its thermal conductivity is low, which traps heat at the tool tip instead of carrying it into the chip.
- The precipitates that give the alloy its strength are abrasive, so tool wear is fast and speeds and feeds are set conservatively.
Two practical points we pass on: 718 machines more easily in the solution-treated condition, and gives a better surface finish in the aged condition, which is why complex parts are often roughed before ageing and finished after. Welding is straightforward compared with the machining, normally with a matched filler metal.
Alloy 718 in Oil and Gas
The combination that puts 718 on wellhead and downhole drawings is high strength plus resistance to stress corrosion cracking in sour and chloride service. The specifications that come with it:
- API 6A and API 6A CRA for wellhead and tree equipment
- NACE MR0175 / ISO 15156-3, which sets the material and hardness requirements for hydrogen sulphide service
- AMS 5662 and AMS 5663 where the part is for aerospace work: 5662 covers the solution-treated condition, 5663 the same part after ageing
Where a hardness limit is attached to the condition, we work to the limit and confirm it on the test certificate.
What We Forge in 718
| Part | Range | Supplied as |
|---|---|---|
| Ball valve bodies and covers | 1/2″ through 12″, Class 150 to 2500 | Forged, solution treated and aged |
| Flanges and ring parts | To drawing | As above |
| Hubs, sleeves, custom shapes | To drawing, to 520 kg | As-forged, rough or finish machined |
Machining envelope is Ø600 × 1000 mm. Weight range on the press runs from 5 kg up to 520 kg per piece.
If you are choosing between the two high-nickel grades, the difference is short: 625 has the higher alloy content and the better corrosion resistance, 718 has the higher strength and the lower cost. The comparison is set out on the Alloy 625 page. Where the service is a wet acid rather than hot chlorides, Alloy 825 covers the duty at lower cost.
Standards, Documents and Testing
718 parts are forged to ASTM B564, the nickel alloy forging specification, and leave with a certificate carrying the heat number. Where a project requires witnessed testing, a 3.2 with an outside countersignature can be arranged, quoted as a separate item rather than built into the part price. Aerospace and oil and gas orders often name AMS or API specifications on top, and the raw stock arrives with its mill certificate. Each finished piece is checked for identity with a handheld XRF spectrometer before packing.
Buying Inconel 718 Forgings: What to Check
An Inconel 718 forging supplier is being asked to hold two things at once, and both are recorded rather than claimed: the alloy, and the heat treatment that turns it into a high-strength part. Four checks settle most of it.
- Heat traceability end to end. The heat number on the mill certificate should follow the part through cutting, forging, heat treatment and machining to dispatch, so that the chemistry on the certificate belongs to the piece in the crate.
- The ageing cycle, named on the certificate. Strength in 718 comes out of the solution treatment and the ageing that follows, not out of the press alone. If the condition is not named and recorded, the strength level of the part is not known.
- The forging standard and the test scope. ASTM B564 is the forging specification. Ultrasonic inspection, tensile and hardness tests on the part itself, and impact tests where the service calls for them, are what a buying specification usually names.
- An identity check before dispatch. A handheld XRF reading taken on the finished part, matched against the heat number, closes the loop between what was ordered and what is shipped.
Where a Forging Pays for Itself
This is the grade where a forging earns the most. 718 costs more per kilo than the steels around it, and machinists rate it among the worst to cut, so arriving near-net, and roughed before ageing, takes cost out of the most expensive operation in the shop. Because the forging, the solution treatment, the ageing and the finishing are all done here, the heat record, the hardness result and the machined dimensions belong to one order and one heat number instead of being passed between suppliers. Presses run from 2,000 to 6,000 tonnes, with parts from 5 kg to 520 kg.
Related Reading
- Duplex, Stainless & Special Steel Forgings
- Grade 91 Forgings (ASTM A182 F91)
- Material Test Certificates
Frequently Asked Questions
Can you forge Alloy 718 valve bodies?
Yes. Body and cover forgings in 718 are routine work here, and because the ageing cycle is recorded, the hardness and mechanical results travel with the parts.
What strength does Alloy 718 reach?
In the aged condition, typically 830 to 1000 N/mm² proof stress with a tensile strength above 1000 N/mm² and hardness of 32 to 40 HRC, depending on the ageing cycle and the section size. The condition has to be named on the drawing or the purchase specification.
Is Alloy 718 suitable for sour service?
It is one of the grades used for hydrogen sulphide service, and NACE MR0175 / ISO 15156-3 is the specification that governs it. Where the standard sets a hardness limit for the condition, we work to it and report the result.
What is the difference between Alloy 718 and Alloy 625?
718 is age hardened, so it reaches higher strength, and it costs less. 625 is a solid-solution alloy with a higher molybdenum content, so it resists corrosion better in most environments. The full comparison is on the Alloy 625 page.
Do you issue EN 10204 documents with 718 parts?
Yes. A 3.1 certificate comes as standard, traceable to the heat. A 3.2, countersigned by an inspector outside our organisation, is provided when the specification requires witnessed testing, and it is quoted separately.
What is the composition of Inconel 718?
Nickel 50.0 to 55.0%, chromium 17.0 to 21.0%, niobium plus tantalum 4.70 to 5.50%, molybdenum 2.80 to 3.30%, titanium 0.65 to 1.15%, aluminium 0.20 to 0.80%, iron balance, as set out in the table above. The grade is also written UNS N07718 or 2.4668. A delivery here comes with the heat analysis rather than typical figures, so the values can be checked against the standard.
Send the drawing, the specification and the service conditions; the quotation will follow.