Exotic Alloys · Nickel Superalloys · Inconel
Inconel 625 vs 718 —
Properties, AMS Specs, and Machining Strategy
Link types
Exotic alloy / article
Spec / standard
Process / method
Cost / caution
Inconel is a trademarked family of nickel-chromium superalloys produced by Special Metals Corporation. Of the many grades in the family, 625 and 718 are by far the most commonly machined in precision shops like ours here at C&W Manufacturing. They share the same nickel-chromium base but differ fundamentally in how they derive their strength. This guide covers both grades, all governing
AMS specifications, and practical cost guidance for engineers and buyers specifying these materials.
The Core Distinction
Inconel 625 is a solid-solution strengthened alloy. Its strength comes from the interactions between atoms in the crystal lattice, not from a subsequent heat treatment. It is used in the annealed condition and cannot be significantly strengthened by aging. Its great virtue is exceptional corrosion resistance combined with good weldability and toughness from cryogenic temperatures to over 982°C (1800°F).[1]
Inconel 718 is a precipitation-hardening alloy. It can be age hardened after solution annealing to achieve tensile strengths more than double its annealed condition. This makes it the preferred grade when high strength is the primary requirement. It is the most widely used superalloy in the world, accounting for roughly one-third of all superalloy production by weight.[1][2]
At a Glance
625 shown in annealed condition. 718 shown in both annealed and aged (AMS 5664) to illustrate the dramatic strength change after precipitation hardening.[1][3]
Inconel 625
"The corrosion grade" — solid solution, used annealed
Max service temp.~1800°F / 982°C
Heat treatable?No — annealed only
Inconel 718
"The high-strength superalloy" — precipitation hardened
Tensile (aged)185 ksi
1276 MPa · 150 ksi annealed
Yield (aged)150 ksi
1034 MPa · 75 ksi annealed
Elongation12% aged / 30% annealed
Hardness≤444 HB (~45 HRC) aged
Max service temp.~1300°F / 704°C
WeldabilityGood (anneal before/after)
Heat treatable?Yes — double aging cycle
MachinabilityDifficult (catastrophic when aged)
Strength Comparison
Tensile strength at room temperature
6061 Aluminum (ref)45 ksi / 310 MPa
304 Stainless (ref)84 ksi / 579 MPa
17-4 PH H900 (ref)190 ksi / 1310 MPa
Inconel 625 (annealed)120 ksi / 827 MPa
Inconel 718 (annealed)150 ksi / 1034 MPa
Inconel 718 (aged, AMS 5664)185 ksi / 1276 MPa
Why Exotic Alloys Cost More
The cost premium for nickel alloys / high-temp alloys (HTA) isn't arbitrary. It's the result of multiple compounding factors.[2]
Material Cost
10–30×
Inconel 718 bar can run $40–80/lb vs $3–5/lb for 6061 aluminum. Nickel, chromium, and molybdenum are expensive raw materials.
Machining Cost
5–15×
Exotic alloys destroy tooling rapidly. Cycle times are much longer, requiring slower speeds and careful chip management.
Lead Time
2–6×
Less commonly stocked. Specialty bar sizes may be mill-order only. Heat treatment adds processing time for 718.
In plain terms: when to justify the cost
If your part operates above 600°C, lives in a corrosive chemical environment that would destroy
316 stainless, or must maintain high strength under cyclic load at elevated temperature, then Inconel isn't a luxury. If your part operates at room temperature in a benign environment and someone specified Inconel because it "sounds strong," that's an opportunity to re-evaluate. Consider
17-4 PH stainless first — if the application doesn't require service above approximately 300°C, 17-4 PH at H1025 may provide adequate strength at 2 to 3 times the cost of 304, compared to 8 to 12 times for Inconel.
AMS Specifications — Inconel 625
625 is a solid-solution alloy. It cannot be precipitation hardened. The AMS specifications cover different product forms, all in annealed condition.[3]
AMS 5596Inconel 625 — Sheet, Strip, Plate
Condition
Annealed — 1093°C (2000°F) min, rapid cool. No aging.
UTS minimum
120 ksi (827 MPa)
Yield minimum
60 ksi (414 MPa)
Applications
Chemical processing, heat exchangers, marine components, exhaust systems, welded fabrications
AMS 5666Inconel 625 — Bar, Rod, Wire, Forgings
Condition
Annealed. Same thermal requirements as
AMS 5596.
UTS minimum
120 ksi (827 MPa)
Applications
Precision machined components, shafts, fittings, fasteners, offshore structural members
625 drawing callout
For machined bar: Nickel Alloy Bar, Inconel 625 (N06625), Annealed, per AMS 5666. For plate: Nickel Alloy Plate, Inconel 625, Annealed, per AMS 5596. Since 625 cannot be aged, there is no condition suffix to specify.
AMS Specifications — Inconel 718
718 has multiple governing specifications based on product form and heat treatment condition. The most important distinction: AMS 5662 (annealed, for machining before aging) vs AMS 5664 (solution treated and double aged, final strength).[4][5]
AMS 5662Inconel 718 — Bar, Annealed (Pre-Machining Condition)
Condition
Solution annealed only. NOT aged. Softest machineable condition for 718.
UTS minimum
150 ksi (1034 MPa)
Yield minimum
75 ksi (517 MPa)
Hardness max
363 HB (~38 HRC)
Availability
Widely stocked — most common 718 bar from distributors
AMS 5664Inconel 718 — Solution Treated and Double Aged (Peak Condition)
Condition
Solution annealed 980°C (1800°F) + double aged: 718°C (1325°F) / 8 hr → furnace cool to 621°C (1150°F) / 8 hr + air cool
UTS minimum
185 ksi (1276 MPa)
Yield minimum
150 ksi (1034 MPa)
Applications
Jet engine discs, structural aerospace fasteners, turbine shafts, downhole tooling
AMS 5663Inconel 718 — Direct Aged (DA) — Maximum Strength UTS minimum
200 ksi (1379 MPa) — highest achievable for 718
Yield minimum
170 ksi (1172 MPa)
Applications
Specialty aerospace/defense only. Rarely specified. Confirm availability and lead time.
Critical: always specify the condition
A drawing that calls out "Inconel 718" without specifying the
AMS specification and condition is ambiguous. The correct callouts: for bar to be machined then aged,
Nickel Alloy Bar, IN718, Annealed, AMS 5662. For bar already at peak strength,
Nickel Alloy Bar, IN718, Solution Treated and Aged, AMS 5664. If the part will be rough machined from
AMS 5662 stock, then aged per
AMS 2774, then finish machined, the material note on the drawing should reference AMS 5662 and the required final condition should be stated as a post-process requirement.
Full Property Comparison
All values for standard conditions. 625 annealed per AMS 5666. 718 shown in both annealed (AMS 5662) and double aged (AMS 5664). Reference materials included for context.[1][6]
Property Comparison Chart
Toggle between properties to compare 625, 718 (both conditions), and reference materials visually.[1][6]
Material property comparison
625 annealed, 718 annealed, 718 aged (AMS 5664), with 17-4 PH H900 and 316 SS as reference.
Tensile strength
Yield strength
Elongation %
Max service temp (°F)
Typical Components by Grade
Inconel 625 — Corrosion and environment-driven applications
625 is specified when the operating environment is the primary design challenge, not the mechanical load. Its combination of exceptional corrosion resistance, excellent weldability, and stability across a wide temperature range (cryogenic to 982°C) makes it the default choice for chemical processing, marine, and exhaust system components. Typical machined components include valve bodies and seats for chemical service, heat exchanger tube sheets and baffles, marine hardware and fasteners, exhaust manifolds and ducting, bellows and expansion joints, and flare tips for oil and gas.[1]
Inconel 718 — Strength and temperature-driven applications
718 is specified when the part must carry significant mechanical load at elevated temperature. The ability to reach 185 ksi tensile while maintaining reasonable ductility and corrosion resistance makes it the aerospace industry's workhorse superalloy. Typical machined components include turbine disc forgings and rings, compressor blades and vanes, high-strength aerospace fasteners and studs, rocket engine components, downhole tooling for oil and gas, cryogenic tankage fittings, and nuclear reactor internals.[1][2]
When to Choose 625 vs 718
The decision usually comes down to one question: is the primary design driver corrosion resistance or mechanical strength?
Choose 625 when
The part operates in a corrosive environment (marine, chemical, acidic, or high-chloride) and the mechanical loads can be met by 120 ksi tensile / 60 ksi yield. The part needs to operate above 704°C where 718 loses its age-hardened strength. Weldability is critical and post-weld heat treatment isn't practical. The part operates at cryogenic temperatures where 625's low-temperature toughness is advantageous.
Choose 718 when
The part must carry loads that exceed 625's 120 ksi tensile or 60 ksi yield. The application requires high strength at temperatures up to 704°C (1300°F), such as turbine components and engine hardware. The part is a structural fastener, shaft, or fitting where high strength-to-weight ratio at temperature is a hard requirement. Fatigue life at elevated temperature is a governing design criterion.
Cost Guidance for Engineers
Material selection has a direct and significant impact on part cost when specifying nickel alloys / high-temp alloys (HTA). Two decisions at the drawing stage can reduce cost dramatically without compromising performance.
⚠ Before specifying Inconel: consider 17-4 PH stainless
If the application doesn't require service above approximately 300°C and the corrosion environment is moderate,
17-4 PH stainless at H1025 may provide adequate strength (155 ksi tensile) at a fraction of the cost. The machining cost difference is even more dramatic. We machine both Inconel and 17-4 PH regularly at C&W and can advise on which material best fits your application.
If annealed strength is sufficient, keep it annealed
Inconel 718 in the annealed condition (
AMS 5662) delivers 150 ksi tensile strength, which exceeds
17-4 PH H900 yield and far exceeds any stainless or alloy steel. If your structural analysis shows the design works at 150 ksi, eliminating the
double aging cycle removes heat treatment cost ($500–2,000+ per lot depending on size), eliminates 1 to 3 weeks of heat treat lead time, avoids aging distortion that requires additional finish machining, and reduces overall machining cost significantly because annealed material is easier to cut. For many applications, annealed 718 at 150 ksi is more than adequate. Ask the question before defaulting to
AMS 5664.
Material Cost
10–30×
Inconel 718 bar runs $40–80/lb vs $3–5/lb for 6061 aluminum. Budget accordingly during design trade studies.
Machining Cost
5–15×
Longer cycle times and rapid tool wear. Aged material costs significantly more to machine than annealed.
Lead Time Impact
2–6×
Less commonly stocked. Add 1 to 3 weeks for heat treatment if aging is required.
Design tips that reduce cost
Use the minimum strength condition that satisfies the design. Don't default to
AMS 5664 when
AMS 5662 works.
Minimize tight tolerances on features that must be machined after aging. Aging causes slight distortion, and finish machining in aged Inconel is the most expensive operation.
Avoid deep, narrow pockets and small-diameter holes where possible. These features are disproportionately expensive in nickel alloys.
Engage C&W early. A review of the drawing before release can identify significant cost-saving opportunities in operation sequence and tolerance placement. Our engineering team is available during the design phase, not just at quoting.
Which Grade Is Right for Your Part?
Inconel 625
Best for: Corrosion-first applications
When the environment is the challenge, not the load. Excellent weldability. Cannot be aged. Higher service temperature than 718 (982°C vs 704°C).
Chemical processingMarine hardwareHeat exchangersExhaust systemsCryogenicOil & gas
Inconel 718
Best for: High-strength at elevated temperature
When you need strength above 120 ksi at temperatures up to 704°C. Always specify the AMS condition. Consider annealed (150 ksi) before defaulting to aged (185 ksi).
Turbine discsCompressor bladesAerospace fastenersRocket componentsDownhole tooling
Need help selecting the right Inconel grade or condition?
Our engineering team at C&W reviews material and heat treatment callouts as part of every quote. We can advise on whether annealed strength is sufficient for your application, coordinate with certified heat treat vendors, and verify certs on receipt.
