Inconel 718 vs. 625 in Aerospace: Strength, Temperature and the 650 °C Wall
Between them, Inconel 718 and 625 account for an enormous share of aerospace nickel alloy consumption. They are not competitors so much as neighbors with a hard border: 650 °C. Understanding why that border exists tells you which alloy your part needs.
718: the strength king (below 650 °C)
Inconel 718 (UNS N07718) is age-hardenable: niobium and titanium form γ″ and γ′ precipitates during heat treatment that pin dislocations and drive yield strength past 1,100 MPa. No other widely available superalloy combines that strength with such good weldability — which is why roughly half the weight of a modern jet engine is 718: disks, shafts, casings, fasteners, rocket motor cases.
The same precipitates are its ceiling. Above ~650 °C the γ″ phase over-ages and dissolves, and strength falls off a cliff. 718's kingdom ends there.
625: the endurance champion
Inconel 625 (UNS N06625) takes the opposite route: solid-solution strengthening from molybdenum and niobium, no heat treatment needed, no precipitates to dissolve. Yield is "only" 414 MPa, but it keeps useful strength to 800 °C+ and survives oxidation to 982 °C. Add near-immunity to corrosion and you get the alloy of exhaust systems, ducting, bellows, seals and marine/aero fasteners in corrosive zones.
The practical selection table
- Rotating parts, high stress, <650 °C (disks, shafts, bolts) → 718, aged to AMS 5662/5663.
- Hot structures, corrosive zones, welded assemblies (exhaust, ducting, bellows) → 625.
- Parts that are both highly stressed AND hot? → This is where buyers get into trouble. Neither alloy fits; you're in Waspaloy / Rene territory. Tell us early.
- Repair welding on aged 718? Possible but requires re-solution and re-aging — plan the heat treatment with the repair, not after it.
Buyer's note on certification
Aerospace 718 is bought to a specification, not a grade name: AMS 5662/5663 (bars), AMS 5596 (sheet), with tensile, stress-rupture and microstructure data per heat. Every 718 bar and forging we ship carries full MTC EN 10204 3.1 traceability with the spec revision stated on the certificate — because in aerospace, the paperwork is the product.