Alloy 718 in Short
Alloy 718 (UNS N07718) is a precipitation-hardenable nickel-chromium-iron alloy that combines high yield strength with good resistance to oxidation and to a broad range of process media. Unlike solid-solution grades such as alloy 625, its strength is developed by an aging treatment that precipitates gamma-prime and gamma-double-prime phases, driven mainly by the niobium and titanium content. Designers select it when a part has to carry load at 600-700 C, or when it must survive repeated thermal cycling without losing dimensional stability.
The same hardening mechanism that produces that strength is also the source of nearly every limitation of the grade. The disadvantages of alloy 718 are rarely about corrosion; they are about cost, metal removal rate, joining discipline and the temperature ceiling of the strengthening phases.
Material Cost and Availability
Alloy 718 contains 50.0-55.0% nickel, 17.0-21.0% chromium, 4.75-5.50% niobium plus tantalum and 2.80-3.30% molybdenum, so its price tracks nickel and molybdenum markets rather than the iron market. Melt practice adds to that base: rotating and fracture-critical parts are usually produced by vacuum induction melting followed by vacuum arc remelting, which is a more expensive and lower-yield route than air melting. Plate, heavy bar and large forgings have the longest lead times in the supply chain, while thin sheet and small-diameter bar are normally available from stock.
Machining Behaviour
Alloy 718 is difficult to machine for three reasons that act together: it work-hardens rapidly, it has low thermal conductivity, and it tends to stick to cutting tools. Heat generated at the cutting edge does not escape into the chip or the workpiece, so edge temperatures rise, and the deformed layer left by a previous pass is harder than the base metal.
Practical consequences on the shop floor:
- Rubbing is the main cause of premature insert failure. Feed rates should be kept high enough to cut below the work-hardened layer rather than skating across it.
- Carbide grades with a tough substrate and a wear-resistant coating are the normal starting point; ceramic and whisker-reinforced grades are used for finishing where the machine tool is rigid.
- Copious high-pressure coolant directed at the cutting zone, and climb milling on profile operations, extend tool life noticeably.
- Interrupted cuts and thin-walled sections demand light radial engagement with full depth of cut to avoid chatter and deflection.
Welding and Post-Weld Cracking
The niobium-rich chemistry that makes the alloy weldable also makes it sensitive to strain-age cracking in the heat-affected zone when the welded assembly is aged. Thick sections are the worst case, because the residual stress field is large and the solution-annealing treatment that would relieve it is not practical on a finished fabrication.
Welding measures that are normally specified:
- Low heat input, stringer beads and a controlled interpass temperature not exceeding the range qualified by the procedure test.
- Matching filler metal of the ERNiFeCr-2 type, kept clean and dry.
- Stress relief or full solution treatment plus aging before any machining that demands close tolerance, so that distortion is not released after final sizing.
- For heavy-wall fabrications, designers often accept the annealed condition and lower allowable stress instead of relying on post-weld aging.
Temperature Ceiling and Oxidation Resistance
The gamma-double-prime phase that provides the exceptional yield strength of alloy 718 is metastable. Above roughly 650 C it transforms progressively to the stable delta phase, and long-term load-carrying capability drops. Oxidation resistance also falls behind that of higher-chromium or alumina-forming grades, so scaling becomes a design consideration in continuous high-temperature service. For components that operate continuously above about 700 C, a solid-solution strengthened or oxide-dispersion strengthened grade is usually the better choice even though its room-temperature strength is lower.
Composition and Specification Data
| Element | Requirement, % |
|---|---|
| Nickel | 50.0-55.0 |
| Chromium | 17.0-21.0 |
| Iron | Balance |
| Niobium + Tantalum | 4.75-5.50 |
| Molybdenum | 2.80-3.30 |
| Titanium | 0.65-1.15 |
| Aluminium | 0.20-0.80 |
| Cobalt | 1.00 max |
| Carbon | 0.08 max |
Bar, forgings and forging stock are covered by ASTM B637; plate, sheet and strip by ASTM B670. For sour service in oil and gas, the alloy is qualified under the requirements of NACE MR0175 / ISO 15156, provided the specified hardness and heat treatment condition are met. In the precipitation-hardened condition the grade is commonly specified with a minimum 0.2% offset yield strength of 1034 MPa (150 ksi) and a minimum elongation of 12%, values that vary with product form and section size.
Where the Trade-Off Is Worth It
Alloy 718 remains the default choice for high-pressure, high-temperature components where a small weight saving or a long fatigue life justifies a hard-to-machine material: turbine and compressor hardware, high-strength fasteners, downhole tools, cryogenic ducting and spring components. If the duty cycle stays below about 600 C, a grade such as alloy 625, an austenitic stainless steel or a lower-alloy precipitation-hardening steel will usually deliver adequate performance at a lower total cost. The correct question is not whether alloy 718 is expensive, but whether the alternative will survive the thermal and mechanical duty without a redesign.
Frequently Asked Questions
Q: Is alloy 718 harder to machine than alloy 625?
A: Yes, in most operations. The precipitation-hardening additions raise the shear strength and the work-hardening rate, so cutting speeds are lower and tool wear is faster than for solid-solution alloy 625 in the annealed condition.
Q: Can welded alloy 718 be re-aged after fabrication?
A: It can, but only where the assembly fits in a vacuum or inert-atmosphere furnace and can be quenched rapidly enough. For large field-erected structures, the practical route is to design around the annealed condition rather than to specify post-weld aging.
Q: What is the maximum continuous service temperature for alloy 718?
A: Roughly 650-700 C for load-bearing duty. Above that range the strengthening precipitates coarsen and transform, and a solid-solution strengthened or oxide-dispersion strengthened alloy performs better.
Q: Which product form has the longest lead time?
A: Large-diameter bar, heavy plate and closed-die forgings, because they require vacuum melting, extended soaking and, in some cases, ring rolling or press forging before final heat treatment.
Q: Does alloy 718 resist chloride stress corrosion cracking?
A: Its high nickel content gives it good resistance in many chloride environments, but it is not immune. Where chlorides, oxygen and elevated temperature combine, the stress level and the specific chemistry of the environment must both be evaluated.
