Inconel 718 Properties, Uses & Machining Limits
Inconel 718 is a nickel-based, precipitation-hardenable alloy used where strength must survive demanding temperatures and loads. Its practical limits depend on the product form, heat treatment, service environment and manufacturing route.
For a designer or machine shop, “718” alone is not enough information. Start with the required final condition, then choose stock, machining allowances and a qualified joining or heat-treatment sequence that can deliver it.
What is Inconel 718?
Alloy 718 is identified as UNS N07718 and commonly as W.Nr. 2.4668. INCONEL is a Special Metals trademark. The material combines a nickel-rich matrix with chromium, iron, niobium, molybdenum and smaller additions that support its heat-treatment response.
Precipitation hardening means that a controlled aging treatment develops fine strengthening precipitates inside the metal. Consequently, solution-treated stock and fully aged finished parts can share the same chemistry while having very different strength and machining behavior.
Composition identifies the alloy, not its final properties
The producer lists nickel plus cobalt at 50–55%, chromium at 17–21%, niobium plus tantalum at 4.75–5.50%, and molybdenum at 2.80–3.30% by mass, with iron as the balance. Titanium and aluminum are also controlled. These headline ranges are an identification aid; the complete chemistry and product specification belong on the material certificate.
Source: Special Metals alloy 718 bulletin, composition and heat-treatment sections. The ranges shown are not a complete purchasing specification.
Which properties can you use for an initial estimate?
Use physical properties for preliminary calculations, then obtain condition-specific design data. Density, conductivity and melting range do not establish the allowable stress, fatigue life or creep life of a finished part.
On a narrow screen, swipe the table horizontally. Keyboard users can focus the table region and use the arrow keys.
| Property | Reported or converted value | Condition and practical meaning |
|---|---|---|
| Density | About 8.19 g/cm³ annealed; 8.22 g/cm³ annealed and aged | Converted from 0.296 and 0.297 lb/in³. Useful for mass estimates; allow for actual part volume and stock tolerances. |
| Thermal conductivity | About 11.1 W/(m·K) annealed; 11.4 W/(m·K) annealed and aged | At 70°F (about 21°C), from the producer’s Table 5. Conductivity varies with temperature; use a temperature-dependent dataset for thermal models. |
| Specific heat capacity | 435 J/(kg·K) | At 70°F (about 21°C). This describes heat storage per unit mass, not the rate at which heat flows through a part. |
| Melting range | 1260–1336°C (2300–2437°F) | A melting interval, not a service-temperature rating or a laser welding setpoint. |
Source: Special Metals, Tables 2 and 5 and physical constants. Typical data, not specification limits. Table 5 conductivity values are calculated from electrical resistivity; 77 and 79 Btu·in/(ft²·h·°F) convert to approximately 11.1 and 11.4 W/(m·K).
Why there is no single tensile-strength value
Strength must be tied to product form, section size, heat treatment, test direction and temperature. For example, the producer’s Table 14 reports a room-temperature transverse test on a pancake forging after solution treatment at 1800°F for 1 hour and water quenching, followed by 1325°F for 8 hours, furnace cooling to 1150°F and holding to complete 18 hours of total aging: 204.5 ksi (about 1410 MPa) ultimate tensile strength, 176 ksi (about 1213 MPa) 0.2% offset yield strength and 16% elongation. The reported forging was 5.5 inches in diameter and 1 inch thick, with tests taken transverse from center to midsection. Those are results for that specimen and treatment, not guaranteed properties for any bar or printed part labeled 718.
How does heat treatment change the decision?
Solution treatment establishes the starting condition for later aging; aging develops strength. Choosing the cycle is part of selecting the material condition, so it should happen before a shop commits to the manufacturing sequence.
The producer describes the following two broad treatment families. They explain why different 718 conditions exist; they are not complete furnace instructions.
Lower solution-temperature family
Solution range: 1700–1850°F (927–1010°C).
Aging outline: 1325°F (718°C) for 8 hours, furnace cool to 1150°F (621°C), then hold until total aging time reaches 18 hours; air cool.
The bulletin associates this family with a useful balance of rupture behavior, notch-rupture ductility and fatigue strength.
Higher solution-temperature family
Solution range: 1900–1950°F (1038–1066°C).
Aging outline: 1400°F (760°C) for 10 hours, furnace cool to 1200°F (649°C), then hold until total aging time reaches 20 hours; air cool.
The bulletin describes benefits for transverse ductility and impact properties in heavy sections, together with a stress-rupture notch-brittleness tradeoff.
Source: Special Metals, heat treatment and mechanical properties. Total aging time includes the first hold, furnace cooling and final hold; it is not an additional 18 or 20 hours at the second temperature. The applicable specification must define solution soak, cooling method, furnace controls and acceptance tests.
Is about 700°C a safe operating limit?
No single temperature establishes safe service. The producer describes uses extending to 1300°F (about 704°C), but that statement does not specify stress, exposure time or required life. A brief thermal excursion, a continuously loaded fastener and a rotating fatigue-critical part require different evidence. Use the governing design allowables and the qualified material condition; do not substitute the melting range or a short tensile test for long-term creep and rupture data.
Why is Inconel 718 difficult to machine?
High strength and work hardening make tool selection and engagement important. The producer reports easier machining and longer tool life in annealed material, while aged material can give better finish and chipbreaker action.
If the approved route allows it, rough-machine before final aging and leave enough stock for later finishing. Agree the allowance from the part geometry, heat-treatment distortion experience and final tolerances; there is no universal allowance for all 718 parts.
Swipe horizontally to compare the symptom, checks and next action.
| What you observe | What to check first | Useful next action |
|---|---|---|
| Rapid wear or unpredictable insert life | Actual stock condition and hardness; cutting speed; tool grade; whether wear is gradual or the edge is chipping. | Start from the tool maker’s data for that operation and material condition. Compare controlled trials by wear pattern and acceptable parts per edge. |
| Chatter or a changing surface pattern | Tool overhang, workholding, wall thickness, runout and changing engagement around the feature. | Resolve instability before raising removal rate. Change one engagement or setup variable at a time and inspect the resulting surface. |
| Poor finish during light finishing passes | Edge wear, whether the cut is rubbing, and actual chip thickness at the chosen cutter engagement. | Check minimum effective chip thickness with the tooling supplier. Simply reducing feed can make a rubbing problem worse. |
| Unexpected failures after changing tool material | Whether coolant practice was carried over from carbide to ceramic tooling. | Follow the exact tool family’s instructions. Sandvik’s ceramic milling guidance uses dry cutting to avoid thermal shock; its carbide milling example uses coolant. |
Basis: Special Metals, machining section and Sandvik Coromant, ceramic milling in superalloys. The checks above are a diagnostic sequence, not proven diagnoses for an unexamined part. Ceramic milling guidance must not be generalized to all ceramic turning or other operations.
Can Inconel 718 be welded, including with a laser?
718 is weldable, but a joint still needs a process qualified for its actual condition and service duty. The producer highlights its resistance to postweld cracking relative to other precipitation-hardening nickel alloys; this does not guarantee a defect-free weld or base-metal properties throughout the joint.
For laser welding, separate equipment capability from joint qualification. Rated power alone does not establish penetration, acceptable defects or the properties after the final thermal cycle.
What a meaningful trial should establish
- Match the production joint. Use the actual alloy condition, thickness, joint geometry, fit-up, filler strategy and access constraints.
- Record the process. Include beam mode, delivered power, focus or spot definition, travel speed, shielding and fixture arrangement.
- Test the required outcome. Apply the agreed cross-section, defect inspection and mechanical or service-related acceptance tests after the required postweld treatment.
A visually smooth bead is only one observation. The qualification plan should establish which measurements are required and how the production joint will be accepted.
Material weldability background: Special Metals, fabrication and welding. The trial sequence is a planning guide; it does not claim an Oceanplayer Laser 718 qualification or replace the applicable welding procedure.
Printed 718 needs its own material route
A powder chemistry match does not make a printed part equivalent to wrought stock. Build parameters, defects, orientation and subsequent thermal processing all belong in the qualification record.
A NIST-published study comparing three laser powder-bed fusion conditions found that defect structure dominated the strength differences in its tested specimens. That is a reason to verify the complete process, not to copy an energy-density number onto another machine.
Connect the build record to the finished part
ASTM F3055 covers full-melt powder-bed fusion of UNS N07718 using laser or electron-beam melting. Its scope includes feedstock, manufacturing plans, thermal processing, microstructure, mechanical properties and inspection.
Define the required post-processing route, including whether hot isostatic pressing is required, and test the final condition. A stress-relief-only part and a solution-treated and aged part should not share a generic property claim.
Sources: ASTM F3055-14a(2021); NIST publication on LPBF 718 defects, microstructure and tensile properties.
Inconel 718 or 625: what changes?
Start with the service requirement, then compare qualified conditions. 718 is precipitation hardenable. Alloy 625, UNS N06625, derives much of its strength from solid-solution strengthening and does not need precipitation hardening to obtain that strengthening mechanism.
When the main requirement is high strength in an aged condition, 718 is a natural candidate to evaluate. When corrosion performance and fabrication requirements lead the decision, review 625’s data for the actual environment. Neither alloy is a universal corrosion winner, and neither can replace the other on a drawing solely because both are nickel alloys.
The comparison should include required stress and life, temperature, process fluids, joining sequence and delivered product form. For a closer look at the alternative, see Inconel 625 properties, uses and machining considerations.
Strengthening and alloy identity: Special Metals alloy 625 bulletin and the alloy 718 bulletin.
Where 718 is used—and what each use demands
Turbine disks and engine hardware need relevant fatigue, temperature and inspection evidence. High-strength fasteners need the specified strength and dimensional condition. Oilfield pressure equipment needs the correct material qualification for its pressure duty and environment. “Used in aerospace” or “used in oil and gas” describes an application family; it does not certify a particular heat, component or manufacturing route.
Match the specification to the stock and final condition
A specification number helps only when its scope matches what you are buying. The examples below show the distinction between product form, thermal condition and application. Verify the contract’s required revision and all applicable size limits.
Swipe horizontally to read the purchasing checks in the final column.
| Specification example | What the published scope addresses | Purchasing check |
|---|---|---|
| SAE AMS5662P | Bars, forgings and flash-welded rings in the solution-treated condition, plus stock for further conversion; precipitation hardenable. | Confirm product dimensions and scope. “Precipitation hardenable” does not mean already aged to the finished component’s properties. |
| SAE AMS5596N | Sheet, strip, foil and plate, 1 inch (25.4 mm) and under, in the specified solution-treated condition. | A flat-product requirement is not automatically interchangeable with a bar or forging requirement. |
| ASTM F3055 | Additively manufactured UNS N07718 using full-melt powder-bed fusion. | Specify the build and final processing condition, qualification evidence and inspection requirements. |
| API 6ACRA | Age-hardened nickel alloys for pressure-containing and pressure-controlling equipment, supplementing API 6A. | Confirm the required edition and purchaser requirements. A general aerospace material certificate does not itself establish acceptance for this duty. |
Scope references: SAE AMS5662P; SAE AMS5596N; ASTM F3055; API’s 6ACRA introduction. API’s 2015 notice states that 6ACRA replaced 6A718; it does not establish the latest edition. If a legacy drawing specifies 6A718, resolve the requirement with the purchaser before substituting documents.
What to put in an RFQ
State the alloy designation; product form and dimensions; specification and revision; delivery condition and required final condition; operating temperature and environment; required mechanical tests, inspection and traceability. For sour service, supply the actual service conditions and applicable purchaser/material qualification requirements rather than relying on a generic “sour-service suitable” label.
For a machining or welding quotation, also identify which party owns heat treatment, finishing and final acceptance. This prevents a quote for machinable solution-treated stock being compared with one for fully processed, tested parts.
Discuss a 718 laser welding application
Send Oceanplayer Laser the material specification and condition, thickness, joint drawing, access constraints and required inspection results. These details make it possible to discuss a relevant equipment and sample-evaluation plan.