Inconel 625 or 718: which one your part actually needs
Both are nickel-chromium superalloys. Both are expensive. Both are miserable to machine. But they solve different problems, and picking the wrong one shows up either as a part that fails early or as an invoice that did not need to be that large.
The short version
Inconel 625 is a solid-solution alloy. Its strength comes from what is dissolved in the nickel matrix, so it arrives strong and stays that way — no heat treatment required, no aging cycle, excellent weldability. It is the corrosion alloy: seawater, acids, sour gas, flue gas.
Inconel 718 is precipitation-hardened. Gamma double-prime precipitates form during an aging cycle and roughly double the yield strength. It is the strength alloy: turbine discs, fasteners, high-load structure up to about 650 °C.
If your part must survive chemistry, look at 625. If it must survive load, look at 718.
Properties side by side
| Property | Inconel 625 | Inconel 718 |
|---|---|---|
| Condition supplied | Annealed | Solution treated, or solution + aged |
| Yield strength, room temp | ~415 MPa | ~1,035 MPa (aged) |
| Tensile strength | ~830 MPa | ~1,275 MPa (aged) |
| Useful temperature ceiling | ~980 °C (oxidation), ~650 °C (load) | ~650 °C under load |
| Hardness | ~180–240 HB | ~331–400 HB (aged) |
| Strengthening | Solid solution (Mo, Nb) | Precipitation (gamma double-prime) |
| Weldability | Excellent | Good, with post-weld aging |
| Relative bar cost | Baseline | 10–25 % higher, varies with form |
The yield strength gap is the headline. Aged 718 is roughly two and a half times stronger than annealed 625 at room temperature. If the part is load-bearing, that difference usually decides it on its own.
What each alloy does to machining cost
Both alloys work-harden aggressively and both destroy tooling, but they do it at different price points. The mechanism behind that is worth understanding before you finalise the geometry — work hardening in superalloys.
625 in the annealed condition cuts at around 20–30 m/min with coated carbide. It is gummy, it builds up on the edge, and it demands rigid workholding. Ceramic inserts push roughing speeds far higher — 150 m/min and beyond — but only on a machine with the horsepower and rigidity to use them.
718 in the solution-treated condition machines roughly like 625. 718 in the aged condition is a different job: 380 HB material that wants ceramic or CBN tooling and eats carbide.
That gives you a real decision on the drawing: machine soft then age, or buy aged bar and machine hard.
- Machine, then age. Cheaper cutting, longer tool life. But the aging cycle at 720 °C and 620 °C moves the part — typically 0.1–0.3 % dimensional change — so critical features need grind stock left on, and a second operation after heat treat.
- Buy aged, machine hard. No distortion, one operation, dimensionally predictable. But slower cycles, expensive tooling and a higher hourly rate.
Full parameters for both conditions are in Inconel 718 machining.
For tight-tolerance parts with few critical features, machining aged material usually wins on total cost even though the cutting is slower. For large parts with many features, soft machining plus a finishing operation usually wins. A shop that machines nickel alloys weekly can tell you which side your part falls on in about ten minutes with the drawing in front of them.
Where each one actually gets used
625 turns up in: subsea and offshore hardware, chemical process valves and fittings, heat exchanger components, flue-gas desulphurisation, marine exhaust, sour-service oilfield parts, bellows and expansion joints.
718 turns up in: gas turbine discs, blades and casings, rocket engine components, downhole tools, high-temperature fasteners and springs, cryogenic structure, high-pressure valve bodies where load rather than chemistry governs.
There is overlap. A high-pressure valve body in a corrosive line might justify either, and the answer depends on whether the wall thickness is driven by pressure or by corrosion allowance.
Questions worth answering before you send the RFQ
- Is the governing load mechanical or chemical? This is the whole decision in one question.
- What is the actual service temperature, and for how long? Both alloys behave differently over 10,000 hours than they do in a short test.
- Does the part get welded? 625 welds without a post-weld aging cycle. 718 needs one, and the cycle brings distortion with it.
- What condition should material arrive in? Say “solution treated and aged per AMS 5663” rather than just “Inconel 718”. The condition changes both cost and process route.
- Who supplies the material? Mill certification, heat-lot traceability and in some industries approved-source lists are normal for nickel alloys. Confirm requirements before quoting, not after.
A note on substitution
Buyers sometimes ask whether a cheaper alloy will do — 316L, a duplex stainless, or Hastelloy C-276 in the other direction. Sometimes yes. 316L handles plenty of mildly corrosive service at a fraction of the price, and super duplex beats 625 on strength-per-dollar in seawater below about 300 °C.
That question is worth asking early, when it can change the drawing, and worth dropping once the design is frozen. Substituting a nickel alloy late in a programme is rarely the saving it looks like.
If you are still choosing between alloys, send us the drawing and the service conditions. We will tell you what the shops quoting it would want to know — and if a cheaper material genuinely does the job, we will say so.