Inconel 718 vs Inconel 625

Updated October 13, 2026

The difference is how each alloy gets its strength

Alloy 718 is precipitation hardened: it is solution treated, then aged so that niobium-rich particles precipitate and block dislocation movement. Alloy 625 is solid-solution strengthened: Special Metals' bulletin says its strength comes from the stiffening effect of molybdenum and niobium on the nickel-chromium matrix, so precipitation-hardening treatments are not required. Almost every practical difference between the two follows from that.

• Strength: aged 718 has roughly two to two and a half times the yield strength of annealed 625 at room temperature and keeps a large margin up to 1300 F.
• Heat treatment: 718 needs a defined anneal and age, with pyrometry and a heat treat cert. 625 is used annealed, solution treated or cold worked.
• Welding: both weld well, but a 718 weldment must be aged, or re-annealed and aged, to get its strength back. 625 weld metal needs no postweld heat treatment.
• Corrosion: 625's 8 to 10 percent molybdenum gives it the edge in pitting and crevice corrosion.

The nickel superalloys page covers the wider family, the AMS 5662 and 5663 minimums and oxygen combustion rankings. This page focuses on the choice between these two.

Side-by-side properties, with the basis of every number

At room temperature, aged 718 has a 150 ksi minimum yield in AMS bar and sheet; annealed 625 plate typically yields near 60 ksi. Rows are labeled minimum (specification value as published by the producer) or typical (producer test data, not for design).

Both bulletins state that their tabulated data are typical and not suitable for specification purposes; the Haynes 625 data sheet carries a similar caution. Design allowables come from your program's approved source, not from this table.

Strength against temperature: where the gap closes

The gap is widest at room temperature and stays large through 1300 F. At 1200 F, typical aged 718 bar still yields about 145 ksi; annealed 625 plate yields about 44 ksi in the Haynes data. Special Metals quotes 718 for service from minus 423 F to 1300 F.

625 behaves differently. Its yield barely changes from 1000 to 1400 F in the Haynes plate data while ultimate falls off above 1200 F, and the alloy remains useful for oxidation resistance far above 718's range. The catch is thermal stability. Special Metals reports that long exposure around 1200 F hardens 625 through sluggish precipitation of a nickel-niobium phase that later transforms to Ni3Nb: annealed bar exposed 2000 hours at 1200 F rose from 69.5 to 126.5 ksi room temperature yield while elongation fell from 54 to 30 percent. Haynes attributes significant embrittlement above 1100 F to the delta phase and recommends 1100 F as the long-term limit. For long-life hot hardware, 625 is strongest as an oxidation and corrosion alloy, not as a load-carrying one.

At the cold end, 718 gains strength. The producer's short transverse forging data at minus 423 F show 237.2 ksi ultimate with a notch tensile ratio of 1.30, consistent with the producer listing cryogenic tankage among its uses.

Weldability and strain-age cracking

Both alloys are considered readily weldable, but for different reasons. Strain-age cracking happens when a precipitation-hardened weldment ages during postweld heat treatment while weld residual stresses are still relaxing, and the hardening outruns the metal's ability to accommodate strain. Alloys hardened by fast-aging aluminum-titanium precipitates are prone to it.

718 avoids most of that because its niobium-based precipitate forms slowly. Special Metals states that the slow response of 718 to age hardening lets parts be welded and directly age-hardened without an intermediate stress relief, and that better properties come from re-annealing after welding before aging. The bulletin calls its resistance to postweld cracking outstanding. The matching filler is AWS A5.14 ERNiFeCr-2.

625 does not age-harden in the normal sense, so there is no postweld aging step to crack. Special Metals says weld metals from Filler Metal 625 and Welding Electrode 112 have high strength and toughness from cryogenic temperatures to 1800 F and need no postweld heat treatment to keep strength and ductility. Niobium also stabilizes 625 against sensitization during welding. That is why bellows, flex hose, ducting and welded manifolds so often end up in 625.

Two shop cautions from the 718 bulletin: silver brazing compounds can cause stress cracking in cold-worked or aged 718, and cadmium-bearing brazing alloys are best avoided because cadmium can aggravate cracking from other sources. For weld process and inspection, see sheet metal and aerospace welding.

Oxygen service and corrosion

For corrosion, 625 is the stronger choice; for oxygen-wetted parts, neither alloy is chosen on a ranking alone. Special Metals credits 625's high molybdenum with resistance to pitting and crevice corrosion and lists resistance to chloride-ion stress corrosion cracking among the properties behind its use in seawater hardware. NASA MSFC-STD-3029 lists 718 in Table I, high resistance to stress corrosion cracking, in all conditions including weldments, and 625 in the annealed condition, so both are good choices for salt-air launch site hardware.

In oxygen, published promoted combustion work places 718 in a middle tier with 316 stainless and 17-4PH, behind Monel 400 and Inconel 600; the two studies cited on the nickel superalloys page do not rank 625. Whichever alloy you use in oxygen service, NASA-STD-6001B expects an oxygen compatibility assessment that accounts for pressure, thickness, flow velocity, particle impact and cleanliness, and the parts need cleaning to a specified level; see precision and oxygen cleaning.

Machinability and processing

Both alloys work harden and keep their strength at the cutting edge, so both are slow to machine. The difference is the heat treat sequence. 718 offers a choice: the producer notes annealed material machines more easily with longer tool life, while aged material gives a slightly better finish and better chip control. Many shops rough annealed, age, then finish to remove aging distortion. That route adds a setup and a heat treat cert but protects tool life and tolerance.

625 has no aging step, so parts are usually machined in their delivered condition. Special Metals gives starting turning parameters of 13 to 35 surface ft/min with high speed steel and 45 to 110 surface ft/min with coated carbide, at 0.005 to 0.020 in per revolution feed. No such table appears in the 718 bulletin, so do not read that as a direct comparison.

Quote implications: on 718, the heat treat sequence and the number of setups drive cost as much as the material. On 625, expect cost to follow cycle time and the welding or forming content. Heat treatment for 718 parts is commonly called out to AMS 2774 with pyrometry to AMS 2750; see heat treatment.

Procurement and additive manufacturing specifications

Call the AMS specification by product form. The producers' own specification lists include the ones below; confirm the current revision on ASSIST or SAE before ordering.

Note that AMS 7000 and 7001 are 625 documents; their titles give a nominal 62Ni, 21.5Cr, 9.0Mo, 3.65Nb chemistry. Do not cite AMS 7000 on a 718 build. For printed 718, ASTM F3055 plus your program's requirements, such as NASA-STD-6030 for spaceflight, is the usual starting point; see metal additive manufacturing.

Where each is used: engines, rocket hardware and GSE

The producers' own application lists show the split clearly: 718 where load and temperature combine, 625 where fabrication and environment dominate.

Ground support equipment follows the same logic. High-strength pins, fasteners and fittings that see cryogenic propellant or high load suit 718. Welded fluid lines, flex hose, bellows and parts exposed to salt air, cleaning chemicals or exhaust suit 625. Where a 718 part is welded into GSE, plan the aging step and the heat treater before the weld procedure is written.

Choose 718 when, choose 625 when

• Choose 718 when strength drives the section: shafts, fittings, fasteners, rings and brackets loaded at room, cryogenic or elevated temperature up to about 1300 F.
• Choose 718 when the part sees liquid hydrogen or oxygen temperatures and needs high strength and notch toughness there.
• Choose 718 when you can specify and verify the heat treatment, including the anneal range.
• Choose 625 when the part is welded, brazed or formed and an aging cycle would be impractical or distort the assembly.
• Choose 625 when pitting, crevice or chloride corrosion is the limiting failure mode.
• Choose 625 when the requirement is oxidation resistance at temperature and the stress is low, keeping in mind the 1100 F long-term guidance for thermal stability.

Cost and availability drivers

Neither alloy is cheap, and the drivers differ. We do not publish prices; these are the qualitative factors.

• Alloy content. 625 carries at least 58 percent nickel and 8 to 10 percent molybdenum; 718 runs 50 to 55 percent nickel plus cobalt with iron as the balance. Raw material cost tends to follow nickel and molybdenum markets.
• Heat treatment. 718 adds an aging cycle, and often a second machining setup, plus a processor cert and pyrometry flow-down.
• Form and specification. Both are widely produced in bar, sheet, plate, tube and wire, but a specific AMS specification in a specific size can be scarce. Confirm form, size and spec at quote.
• Testing. Stress rupture and elevated temperature tensile requirements in some AMS 718 documents add test time.
• Specialty metals. Both are nickel alloys with more than 10 percent other alloying metals, so DFARS 252.225-7009 applies on defense work; see DFARS specialty metals.

Callout examples for the drawing and PO

Give the alloy, UNS number, specification by form, delivered condition and the heat treatment that happens after machining or welding. Adapt to your drawing standard.

• 718 machined part: MATERIAL: NICKEL ALLOY 718 (UNS N07718), BAR PER AMS 5662. ROUGH MACHINE, PRECIPITATION HEAT TREAT PER THE AMS 5662 AGING TREATMENT, HEAT TREAT PER AMS 2774, FINISH MACHINE.
• 718 weldment: WELD PER APPROVED PROCEDURE USING AWS A5.14 ERNiFeCr-2 FILLER. AGE AFTER WELDING PER THE BASE METAL SPECIFICATION.
• 625 sheet assembly: MATERIAL: NICKEL ALLOY 625 (UNS N06625), SHEET PER AMS 5599, ANNEALED. WELD WITH ALLOY 625 FILLER PER APPROVED PROCEDURE. NO POSTWELD HEAT TREATMENT UNLESS SPECIFIED.
• 625 printed part: MATERIAL: ALLOY 625 BY LASER POWDER BED FUSION PER AMS 7000, POWDER PER AMS 7001.

On the PO, require mill certs with heat number, chemistry and tensile results in the specified condition, the heat treater's cert and pyrometry compliance for any aging after receipt, weld procedure and welder qualification records where welding is in scope, oxygen cleaning certs if applicable, and the melt source for DFARS. For cert review, see material certs and traceability.

Get 718 or 625 material and parts quoted

The common mistakes are ordering 718 in the wrong condition for the machining plan and treating 625 as if it needed aging. Send your requirement through a certified material quote request and we will source the alloy, form and AMS specification from mill-certified stock, check the cert before anything ships, and flow down heat treat, welding and cleaning requirements when the material becomes a part. See also the materials hub, certified aerospace material and Ti-6Al-4V vs ELI.

Questions

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• Nickel Superalloys and Nickel-Copper Alloys
• Metal Additive Manufacturing for Aerospace Hardware
• Aerospace Heat Treatment: Pyrometry, Process Specs and Callouts
• Ti-6Al-4V vs Ti-6Al-4V ELI (Grade 5 vs Grade 23)

Sources

• Special Metals, INCONEL alloy 718 technical bulletin
• Special Metals, INCONEL alloy 625 technical bulletin (2013)
• Haynes International, HAYNES 625 alloy data sheet (tensile vs temperature, thermal stability)
• ASTM F3055, Additive Manufacturing Nickel Alloy (UNS N07718) with Powder Bed Fusion (BSI listing)
• ASTM F3056, Additive Manufacturing Nickel Alloy (UNS N06625) with Powder Bed Fusion
• SAE AMS 7000A, L-PBF nickel alloy 62Ni 21.5Cr 9.0Mo 3.65Nb parts, SR, HIP and solution annealed
• Metal AM, SAE International issues new technical standards for aerospace additive manufacturing (AMS 7000 series)
• NASA MSFC-STD-3029A, stress corrosion cracking resistance tables
• NASA-STD-6001B, Flammability, Offgassing, and Compatibility Requirements
• DFARS 252.225-7009 specialty metals