Guides / Materials

Inconel 625 vs 718: Which Alloy Fits Your Application

September 15, 2026

Inconel 625 and Inconel 718 are the two nickel superalloys buyers specify most often, and they get confused for each other more than any other pair in the family. Both resist corrosion and hold strength well past where steel gives up, but they get there through different metallurgy, and that difference changes how each machines, where each gets specified and what a shop needs to run it well.

Composition and how each is strengthened

Inconel 625 is solid-solution strengthened. Chromium, molybdenum and niobium sit dissolved in the nickel matrix and stiffen it directly, with no heat treatment step required to reach final strength. The alloy comes off the mill close to its working properties and stays that way through welding and forming.

Inconel 718 is age-hardenable. It gets its strength from a precipitation hardening heat treatment that forms fine niobium-rich particles inside the nickel-chromium matrix after the part is solution annealed and aged. Before that heat treatment, 718 is comparatively soft and machines easier. After it, the part reaches the high strength that makes it useful in a hot rotating turbine section, and cutting it gets noticeably harder.

Where each grade is chosen

625 shows up wherever weldability and corrosion resistance matter more than peak strength at high temperature: marine hardware, chemical processing equipment, exhaust systems and welded assemblies where a heat-affected zone cannot be allowed to crack or lose properties. It welds without the post-weld heat treatment complications that age-hardenable alloys can bring.

718 is the default in aerospace and gas turbine work: engine fasteners, turbine discs, brackets and hot-section hardware that need the highest strength-to-weight ratio in the nickel family at continuous operating temperatures up to roughly 700°C. It is typically the most requested grade among the shops we match, ahead of every other nickel alloy combined.

Machining differences

Both alloys work-harden under the tool, so a light finishing pass can cut harder than the rough pass that came before it. That part is common ground. The real difference is where the hardness comes from and when it shows up.

625 holds a fairly consistent hardness through the whole machining sequence, since there is no heat treatment step changing the material partway through the job. A shop can often rough and finish in one setup sequence without planning around a hardness jump.

718 is a different sequencing problem. Machined before aging, it cuts closer to a standard stainless steel. Many shops rough it in that condition, then age it, then come back for finishing cuts and any features that need to hold dimension at full hardness. That means a second setup, a heat treatment step in the middle of the job, and more coordination between the shop and its heat treat supplier. It is one reason 718 quotes usually run higher than 625 quotes for a comparable part.

Corrosion resistance and weldability

625 is generally the stronger performer in aggressive wet corrosion: seawater, acids and chloride-bearing environments where pitting and crevice corrosion are the concern. Its weldability is also better across a wider range of joint types, since there is no aging response to disturb in the heat-affected zone.

718 holds up well in oxidizing, high-temperature service and has good general corrosion resistance, but it is chosen for strength at temperature first and corrosion resistance second. It can be welded, but post-weld heat treatment needs to be planned around the aging response, and it is less forgiving than 625 in a welded joint that will see repeated thermal cycling.

Tolerance, lead time and how to decide

Both alloys typically hold tolerances in a similar range, around ±0.02 to ±0.05 mm depending on feature and part size, though 718 finishing cuts after aging are the ones most likely to push a shop toward the loose end of that range on a tight budget. Lead time runs close between the two for prototype quantities, mostly driven by raw material availability rather than the alloy itself, though a 718 job that requires an aging cycle between rough and finish machining adds days to the schedule that a 625 job does not need.

If the part sees a welded assembly, marine or chemical exposure, or does not need peak strength at high temperature, 625 is usually the more direct answer and often the faster one to get quoted. If the part goes into a hot rotating section, needs the highest strength-to-weight ratio in the nickel family, or the drawing already calls out 718 by spec, that decision is largely made already. When it is not obvious, describe the service temperature, the corrosion environment and whether the part will be welded, and a shop that runs both grades can confirm which one actually fits. For the other grades and the five service facts that decide between them, see our guide to nickel alloy selection. For creep and long-term limits of 625 and 718 above 600°C, see our guide to high temperature nickel alloys.

See also: the Inconel 625 reference page for composition, ASTM B446 grades and the RFQ checklist.

Frequently asked questions

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