Inconel 718
Also: Alloy 718, Inconel 718, IN718, UNS N07718, Nickel alloy 718, W.Nr. 2.4668, NiCr19Fe19Nb5Mo3, Haynes 718, Allvac 718, AMS 5662, AMS 5663
Inconel 718 is the workhorse superalloy: 1240 MPa (180 ksi) tensile to 650 °C (1200 °F), weldable, printable, slow to machine. Values solution treated and aged.
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At a glance
| Density | 8.19 g/cm³ (0.296 lb/in³) |
|---|---|
| Tensile strength | 1240 MPa (180 ksi) min; typically 1375 MPa (200 ksi) |
| Yield strength | 1035 MPa (150 ksi) min; typically 1100-1170 MPa (160-170 ksi) |
| Hardness | 36-44 HRC (331 HB min) |
| Elongation | 12% min; typically 15-20% |
| Modulus of elasticity | 200 GPa (29 Msi) |
| Thermal | Melts at 1260-1336 °C (2300-2437 °F); thermal conductivity ~11 W/m·K; retains strength to about 650 °C (1200 °F), oxidation resistant to about 980 °C (1800 °F) |
| Composition | Ni 50-55%, Cr 17-21%, Fe balance (about 18%), Nb+Ta 4.75-5.5%, Mo 2.8-3.3%, Ti 0.65-1.15%, Al 0.2-0.8% |
Typical values, Solution treated and aged per AMS 5662 / AMS 5663 (solution 940-1010 °C, age 720 °C 8 h, furnace cool to 620 °C, hold to 18 h total). Solution treated only (as shipped under AMS 5662): roughly 400-550 MPa (60-80 ksi) yield and 40-45% elongation. Confirm against the mill certificate and the purchase spec.
- MachinabilityPoor
- About 10-15% of the free-machining steel benchmark: it work-hardens under a rubbing tool, holds heat at the edge and abrades carbide. Rigid machines, positive-rake coated carbide or ceramic, low speed, constant feed above the work-hardened layer, and high-pressure coolant. Plan 8-15x the cycle time of 6061.
- WeldabilityGood
- The most weldable precipitation-hardening superalloy: TIG, plasma, laser and EB with ERNiFeCr-2 (718) filler. Weld in the solution-treated condition, then solution treat and age to restore strength; low risk of strain-age cracking compared with gamma-prime alloys.
- FormabilityFair
- Sheet forms in the solution-treated condition with high forces and springback; anneal between severe operations. Hot working is done at 950-1100 °C (1750-2000 °F). Formed parts are heat treated after forming.
- Corrosion resistanceExcellent
- Resists oxidation to about 980 °C (1800 °F), sour gas (NACE MR0175 / ISO 15156 listed), seawater, and chloride stress-corrosion cracking. Not the choice for hot reducing acids or wet chlorine; Hastelloy C-276 covers those.
Inconel 718 is a nickel-chromium-iron superalloy strengthened by precipitation of a niobium-rich gamma double-prime phase. It combines very high strength from cryogenic temperatures to about 650 °C (1200 °F) with oxidation resistance to about 980 °C (1800 °F), corrosion resistance that covers sour gas, seawater and most process chemicals, and, unusually for a precipitation-hardened superalloy, good weldability, because the strengthening phase forms slowly enough that welds do not crack on aging. It is the most widely used superalloy: roughly a third of the weight of a modern jet engine is 718.
It is bought in two conditions. AMS 5662 material is solution treated and left soft for the customer to machine and then age; AMS 5663 is solution treated and aged at the mill, harder (36-44 HRC) but dimensionally stable. Machining in the solution-treated state is faster and then aging adds a small, predictable growth; machining after aging is slower but holds the tightest tolerances. AMS 5664 uses a higher solution temperature for better notch rupture life in engine parts.
Machining is the cost. 718 work-hardens as it is cut, holds heat at the tool, and abrades carbide, so it runs at about 10-15% of free-machining steel speeds and eats inserts; wire EDM and creep-feed grinding do much of the detail work in engine parts. It is also the most printed superalloy, and additive 718 with HIP and a full heat treatment reaches near-wrought properties.
Heat treatment
Precipitation hardening per AMS 2774. Standard cycle (AMS 5662 / 5663): solution treat at 940-1010 °C (1725-1850 °F) for about 1 hour, air cool; age at 720 °C (1325 °F) for 8 hours, furnace cool at 55 °C (100 °F) per hour to 620 °C (1150 °F), hold until 18 hours total, air cool. AMS 5664 solution treats at 1040-1065 °C (1900-1950 °F) and ages at 760 °C (1400 °F) then 650 °C (1200 °F) for better notch rupture life. Aging adds about 0.05-0.1% growth; machine in the solution-treated condition and allow for it, or finish-machine after aging. Vacuum or protective-atmosphere furnaces are standard.
Tempers and conditions
- Solution treated (AMS 5662, to be aged by the purchaser)
- Solution treated and aged (AMS 5663)
- High-temperature solution treated and aged (AMS 5664)
- Annealed sheet and plate (AMS 5596)
- Direct aged (rings and forgings)
Stock forms
- Round bar and billet
- Plate
- Sheet and strip
- Forgings and seamless rolled rings
- Wire and fastener stock
- Seamless tube (limited)
- Powder for additive manufacturing
Relative cost
Tier 5 of 5. Roughly 8-15x the price of 304 stainless per pound, with machining time 8-15x that of aluminum. A machined 718 part commonly costs 10-20x the same part in 6061; printing and near-net forging exist to reduce that.
Typical applications
- Jet engine disks, shafts, casings, blades and fasteners
- Rocket engine turbopumps and combustion components
- Industrial gas turbine parts
- Oil and gas downhole tools, wellhead and subsea components
- Cryogenic tanks, valves and fittings
- High-temperature fasteners, springs and bellows
- Nuclear reactor internals and fuel hardware
- Extrusion dies and hot-work tooling
Process compatibility
How Inconel 718 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| CNC Machining | PoorPoor | Done every day, expensively: low speed, constant feed, ceramic or coated carbide, high-pressure coolant. Machine in the solution-treated condition when tolerances allow and age afterward. Can Inconel 718 be machined? |
| CNC Turning | PoorPoor | Turned shafts and rings are standard engine work; ceramic inserts at high speed for roughing, carbide for finishing. Insert life is measured in minutes. |
| Wire EDM | ExcellentExcellent | Hardness is irrelevant to EDM, so slots, fir-tree profiles and blanks are cut by wire; remove the recast layer on fatigue-critical surfaces. |
| Sinker EDM | GoodGood | Cooling holes, cavities and shaped features in aged 718 that a cutter cannot reach economically. |
| CNC Grinding | GoodGood | Creep-feed grinding with open, soft wheels is the production route for slots and blade roots; watch for burn and work-hardening. |
| Welding | GoodGood | TIG, plasma, laser and EB with 718 filler; weld solution-treated material and heat treat afterward. Grind out any crater cracks; clean all sulfur and lead contamination before heating. Can Inconel 718 be welded? |
| TIG Welding | GoodGood | ERNiFeCr-2 filler, argon shielding, low heat input and stringer beads; the usual repair and fabrication process. |
| Forging | ExcellentExcellent | The primary route for disks, rings and shafts; closed-die and ring-rolled forgings in solution-treated or direct-aged condition, then finish machined. |
| Investment Casting | GoodGood | Cast 718 is used for casings and structural engine parts; HIP and a solution-and-age cycle follow casting. Vacuum foundries only. |
| Metal 3D Printing | ExcellentExcellent | The most printed superalloy: laser powder bed and directed energy deposition, then stress relief, HIP and a full solution-and-age cycle to reach near-wrought properties. |
| Vacuum Heat Treating | ExcellentExcellent | Solution and age cycles are run in vacuum or protective atmosphere per AMS 2774 to keep the surface clean; the heat treater needs the spec and the required hardness. |
| Heat Treating | ExcellentExcellent | Precipitation hardening is the source of its strength; specify AMS 5662 plus purchaser aging, or buy AMS 5663 pre-aged. |
| Laser Cutting | FairFair | Sheet and thin plate cut with nitrogen assist; the edge is hard and slightly heat-affected. Waterjet for thicker plate. |
| Carburizing | Not recommendedNot recommended | A nickel alloy with no martensitic transformation; it cannot be case hardened. Its hardness comes from aging, and any wear surface uses a coating or hard facing. |
Compare Inconel 718
Design and sourcing tips
- Specify the condition and the spec: "Inconel 718 per AMS 5662, age per AMS 2774 after machining" or "AMS 5663 (solution treated and aged)" when the shop should not heat treat.
- Decide who ages the part. Machining before aging is faster and cheaper; machining after aging holds the tightest tolerance. Allow about 0.05-0.1% growth if aging follows machining.
- Keep machined geometry simple: every deep pocket, small radius and thin wall costs far more in 718 than in steel. Move detail to wire EDM or grinding where possible.
- Design welds in solution-treated material and heat treat the assembly afterward; do not weld aged parts unless the drawing accepts the local loss of strength.
- Do not specify 718 above about 650 °C (1200 °F) for load-bearing service; gamma double-prime overages and strength drops. Alloy 625, Waspaloy or a cast superalloy covers higher temperatures.
- Require certs to the AMS spec, the heat treatment record and hardness on the report; 718 is commonly counterfeited or substituted with 625 in low-grade supply chains.
Frequently asked questions
- What is the difference between Inconel 718 and Inconel 625?
- 718 is precipitation hardened: about 1240 MPa (180 ksi) tensile after aging, for structural parts to about 650 °C (1200 °F). 625 is solid-solution strengthened: about 830-930 MPa (120-135 ksi) tensile annealed, with better corrosion resistance in acids and chlorides, easier welding and forming, and useful strength to higher temperatures. Choose 718 for strength, 625 for corrosion and fabrication.
- Is Inconel 718 hard to machine?
- Yes, it is among the hardest common engineering metals to machine. It work-hardens under the tool, conducts heat poorly and abrades carbide, so speeds are about 10-15% of free-machining steel and insert life is short. Shops that run superalloys use rigid machines, ceramic or coated carbide, high-pressure coolant, and wire EDM or grinding for detail. Expect 8-15x the cycle time of aluminum.
- Can Inconel 718 be welded?
- Yes, and it is the most weldable of the age-hardening superalloys. TIG, plasma, laser and electron beam welds with 718 filler (ERNiFeCr-2) are routine. The alloy should be welded in the solution-treated condition and then solution treated and aged to restore full strength; welding aged material leaves a soft zone and raises the cracking risk.
- What temperature can Inconel 718 withstand?
- It keeps most of its strength to about 650 °C (1200 °F) and is used for load-bearing parts up to that temperature; above it the strengthening phase overages. It resists oxidation to about 980 °C (1800 °F) for unloaded parts, and it stays tough to cryogenic temperatures, which is why it is used in liquid hydrogen and oxygen turbopumps.
- Is Inconel 718 magnetic?
- No. Inconel 718 is an austenitic nickel alloy and is non-magnetic at all temperatures, in both the solution-treated and aged conditions. A magnet is a quick check against substitution with a magnetic alloy, though not against 625 or stainless, which are also non-magnetic.
Processes that commonly use it
Related materials
Where to verify these values
- AMS 5662, AMS 5663 and AMS 5664 (bar and forging minimums by condition)
- AMS 5596 / 5597 and ASTM B637 / B670
- AMS 2774 (heat treatment of wrought nickel alloy parts)
- Special Metals Inconel alloy 718 data sheet and ASM Handbook, Volume 2
