Inconel Heat Treating: 718, 625, Hastelloy, Monel

Karl Gillihan

🔥 INCONEL / NICKEL SUPERALLOYS

Heat Treating Inconel and Nickel Superalloys: Solution Anneal and Age Hardening 718 and 625

Nickel superalloys split sharply into two camps for heat treatment: the precipitation-hardening grades like Inconel 718 that gain enormous strength from controlled aging, and the solid-solution-strengthened grades like Inconel 625 and most Hastelloys that are only annealed, never age-hardened. Knowing which camp your alloy falls into is the difference between specifying a meaningful treatment and asking for one that does nothing.

AS9100NADCAPISO 9001

Two Strengthening Mechanisms, Two Completely Different Heat Treat Routes

Inconel 718 is precipitation hardenable, it derives its remarkable high-temperature strength from gamma double-prime (Ni3Nb) and gamma prime precipitates that form during a controlled aging cycle. Inconel 625, by contrast, is solid-solution strengthened, its strength comes from molybdenum and niobium dissolved in the nickel matrix, and there is no precipitation aging that will harden it. The same is true of most Hastelloy grades (C-276, C-22) and of Monel 400, these are annealed, never aged. This is the single most important distinction for a buyer. Specifying an aging cycle for 625 or Hastelloy accomplishes nothing useful and can even harm corrosion resistance by precipitating embrittling intermetallics. Conversely, leaving 718 in the solution-annealed condition leaves most of its strength unrealized. The corrosion-resistant solid-solution alloys are heat treated only to solution anneal: heating to dissolve carbides and homogenize the structure, then rapid quenching to keep everything in solution and preserve the corrosion resistance that is the whole point of these alloys.

Inconel 718 Aging: The AMS 5662 Cycle and Why It Takes 18+ Hours

The standard 718 precipitation cycle is long. After solution annealing (typically 1700 to 1850F per the spec), the part is aged in a two-step cycle: roughly 1325F for 8 hours, furnace cooled to about 1150F, then held for a total aging time around 18 hours. This drawn-out double-aging is what nucleates and grows the gamma double-prime precipitates that take 718 from around 60 ksi yield in the annealed state to 150+ ksi yield aged, with excellent strength retention up to 1200F. The specific cycle matters and is tied to the application. AMS 5662/5663 specify a higher solution temperature for better tensile strength, while AMS 5664 uses a lower solution temperature favored for oil and gas service where stress-corrosion and hydrogen-embrittlement resistance matter more than peak strength. There is no improvising here, aerospace and energy buyers call out the exact AMS spec and condition. Because aging is a low-temperature, slow cycle, distortion is modest, so 718 is frequently rough machined, solution treated, then finish machined and aged, or machined fully in the annealed condition and aged last where geometry permits. The long cycle drives cost and scheduling more than any distortion concern.

Solution Annealing 625, Hastelloy, and Monel for Corrosion Service

For the solid-solution alloys, heat treatment is about corrosion performance, not strength. Inconel 625 is solution annealed around 1900 to 2100F depending on whether you want the softer Grade 1 (annealed) or the higher-strength Grade 2 (solution annealed at higher temperature for elevated-temperature creep service). Hastelloy C-276 and C-22 are solution annealed near 2050F and rapid quenched specifically to dissolve the chromium and molybdenum carbides that would otherwise sensitize the alloy and ruin its resistance to pitting and crevice corrosion in aggressive chemical and sour-gas environments. The rapid quench is non-negotiable for these alloys. Slow cooling through the carbide-precipitation range, or excessive heat input during welding, precipitates intermetallics and carbides at grain boundaries that destroy corrosion resistance, the same sensitization mechanism that plagues austenitic stainless but with even more catastrophic consequences in the severe environments these alloys are chosen for. Monel 400 is annealed mainly to soften after cold work and relieve stress, since it too is non-age-hardenable (Monel K-500 is the age-hardenable exception, strengthened by aluminum and titanium additions). For all of these, the buyer specifies an anneal to restore the as-supplied corrosion-resistant condition, never an aging treatment.

Distortion, Atmosphere, and the Cost of Getting Superalloy Heat Treat Right

Nickel superalloys are typically vacuum or hydrogen-atmosphere heat treated to prevent oxidation and intergranular attack from furnace gases, and the high solution temperatures (up to 2100F) demand serious furnace capability. Bright annealing in vacuum or pure hydrogen gives clean parts without a scale that would otherwise need pickling, important on corrosion-critical surfaces. These alloys work harden aggressively during machining, so stress relief between roughing and finishing is common to stabilize parts before the final aging or anneal. Distortion during the high-temperature solution treatment can be significant on thin or long parts, fixturing and slow heat-up help, and aerospace parts are often machined with allowance and finished after the controlling thermal cycle. The cost reality is that superalloy heat treating is among the most expensive in the industry: long cycles, high temperatures, vacuum furnaces, mandatory certified pyrometry, and full traceability stack up. Buyers should budget accordingly and not be surprised that aging Inconel 718 to an AMS spec is a multi-day, premium-priced operation.

Frequently Asked Questions

No, Inconel 625 cannot be age hardened the way 718 can, because the two alloys strengthen by fundamentally different mechanisms. Inconel 625 is solid-solution strengthened, its strength comes from molybdenum and niobium atoms dissolved in the nickel-chromium matrix, and there is no precipitation aging cycle that increases its room-temperature strength. Inconel 718 is precipitation hardenable, gaining its high strength from gamma double-prime and gamma prime precipitates that form during a controlled aging cycle, which is why 718 nearly triples its yield strength after aging while 625 does not respond. For 625, the only meaningful heat treatment is solution annealing around 1900 to 2100F followed by a rapid quench, done to dissolve carbides, homogenize the structure, and preserve the alloy's outstanding corrosion and oxidation resistance, not to add hardness. In fact, holding 625 in the wrong temperature range can precipitate embrittling intermetallic phases that harm both ductility and corrosion resistance. If you need a high-strength, age-hardenable nickel alloy, you specify 718; if you need maximum corrosion resistance in a tough, weldable alloy, you choose 625 and anneal it.

The long aging time for Inconel 718 is dictated by the physics of forming its strengthening precipitates. After solution annealing, 718 is age hardened in a deliberate two-step cycle: roughly 1325F for about 8 hours, then a slow furnace cool to around 1150F, then a further hold for a total time near 18 to 20 hours. This extended double-aging is what nucleates and grows the gamma double-prime (Ni3Nb) precipitates, and to a lesser extent gamma prime, that give 718 its strength. A short cycle would leave the precipitates too few and too fine to fully strengthen the alloy, while the controlled two-temperature profile ensures a uniform, stable precipitate distribution that performs reliably at service temperatures up to 1200F. The exact cycle is governed by aerospace and energy specifications like AMS 5662, 5663, and 5664, and buyers call out the specific spec because it ties the heat treatment to the required mechanical and environmental performance. The long cycle is the main reason 718 aging is expensive and schedule-driving, the furnace is tied up for the better part of a day per load.

Solution annealing restores the corrosion resistance that makes Hastelloy worth using. Grades like C-276 and C-22 are highly alloyed with chromium and molybdenum for resistance to pitting, crevice corrosion, and aggressive acids and sour-gas environments. During welding or slow cooling, chromium- and molybdenum-rich carbides and intermetallic phases precipitate at grain boundaries, pulling those protective elements out of the surrounding matrix and creating sensitized, corrosion-vulnerable zones, the same mechanism that plagues austenitic stainless but with far more severe consequences given the environments Hastelloy serves. Solution annealing heats the alloy to around 2050F to dissolve those carbides and put everything back into solid solution. The quench is critical because the part must cool rapidly through the carbide-precipitation temperature range, if it cools slowly the carbides simply re-form and you have accomplished nothing. That is why these alloys are solution annealed and water or rapid-gas quenched, and why post-weld heat input must be controlled. There is no aging step, Hastelloy is not age hardenable, the entire purpose of the heat treatment is corrosion-resistance preservation, not strengthening.

Nickel superalloy heat treating is among the most expensive in the industry. Solution annealing 625, Hastelloy, or Monel runs roughly $4 to $10 per pound at production volume because of the high temperatures (up to 2100F), vacuum or hydrogen-atmosphere furnaces, and rapid-quench requirements, with lot minimums of $400 to $900 at certified shops. Age hardening Inconel 718 to an AMS spec costs more, often $6 to $15 per pound, because it combines a solution treatment with the long 18-plus-hour two-step aging cycle, certified pyrometry, and full lot traceability. Lead times typically run 10 to 20 business days for aerospace and energy work under AS9100 or NADCAP, driven by furnace scheduling, the long aging cycle for 718, witnessed pyrometry, and per-lot mechanical testing. The 18-hour 718 cycle alone ties up a furnace load for nearly a day, so batching dominates scheduling. Expedited service is limited and carries steep premiums because dedicated high-temperature vacuum furnace time is scarce. Budget extra if parts need stress relief between machining steps or post-treatment straightening.

Last updated: July 2026

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