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Certifications: EN 10204 3.1 / 3.2 material test certificates, NACE MR0175 compliance documentation, PMI verification, and complete export documentation packages.
Inconel
718
(UNS N07718)
A world-class technical reference for EPC contractors, aerospace and oil & gas engineers, procurement heads, and TPI inspection agencies specifying Inconel 718 nickel-chromium-iron-niobium superalloy — covering the gamma-double-prime precipitation hardening mechanism, the critical NACE MR0175 hardness constraint, notorious machining difficulty, strain-age cracking weldability concerns, and the QC and documentation discipline required for critical aerospace and high-pressure/high- temperature oil & gas equipment supply.
Precipitation Hardening
& Selection Logic
Inconel 718 (UNS N07718) is a precipitation-hardenable nickel- chromium-iron superalloy — the single most widely used nickel- based superalloy in the world by volume, prized for its exceptional combination of very high strength, good corrosion and oxidation resistance, and useful mechanical properties across an unusually broad temperature range from cryogenic to approximately 650–700°C.
1.1 — What Inconel 718 (UNS N07718) Is
Inconel 718 is a nickel-chromium-iron-based superalloy with a nominal composition of approximately 50–55% nickel, 17–21% chromium, 4.75–5.5% niobium (plus tantalum), 2.8–3.3% molybdenum, and a controlled titanium and aluminium addition, with the balance comprising iron and minor elements. The alloy’s defining metallurgical feature — and the reason it superseded many earlier superalloys for a huge range of applications — is its precipitation-hardening mechanism based on the niobium addition, which forms a metastable gamma-double-prime (γ″, Ni₃Nb) strengthening phase during ageing heat treatment. This gamma-double-prime phase responds much more slowly to precipitation kinetics than the gamma-prime (Ni₃(Al,Ti)) phase that strengthens alloys like Monel K500 (discussed in RR Hydraulic’s dedicated reference) or other aluminium/titanium-hardened superalloys — this slower response gives Inconel 718 excellent weldability relative to most other high- strength age-hardenable superalloys, since the alloy does not rapidly harden and crack during the heat-affected zone cooling cycle of welding the way faster-responding gamma-prime alloys often do (discussed further in Section 3.3).
1.2 — Key Engineering Properties
Exceptional Strength Across a Broad Temperature Range
In the fully age-hardened (solution treated and aged, STA) condition, Inconel 718 achieves yield strength typically in the range 1030–1170 MPa — among the highest of any commercially available nickel alloy — and retains useful strength from cryogenic temperatures up to approximately 650–700°C, a temperature range broader than most competing superalloys, making it the standard high-strength material for demanding structural and fastener applications across aerospace, oil & gas, and power generation.
Good Corrosion and Oxidation Resistance
The chromium content provides good general corrosion and oxidation resistance, though Inconel 718’s primary selection driver is mechanical strength rather than the specific aqueous corrosion resistance profile of alloys like Incoloy 825 or Hastelloy C-276 (discussed in RR Hydraulic’s dedicated references) — for the most aggressive aqueous chemical process corrosion environments, a corrosion-optimised alloy rather than 718 is typically the better choice, reserving 718 for applications where high strength is the dominant requirement.
Excellent Fatigue and Fracture Toughness
Inconel 718 offers a well-documented, favourable combination of high strength with good fatigue resistance and fracture toughness — a combination that is difficult to achieve simultaneously in many high-strength alloy systems, making 718 particularly well suited to rotating and cyclically loaded aerospace gas turbine components as well as high-pressure, cyclically loaded oil & gas completion equipment.
Relatively Good Weldability for a High-Strength Superalloy
As discussed in Section 1.1, the gamma-double-prime precipitation mechanism’s slower kinetics give 718 meaningfully better weldability than most comparably high-strength age-hardenable superalloys — 718 can be welded in the solution-annealed condition and subsequently aged, or welded and re-aged, with proper procedure control, whereas many aluminium/titanium-hardened superalloys are essentially unweldable in the age-hardened condition without severe strain-age cracking risk.
1.3 — Comparison to Inconel 625
| Property | Inconel 718 (N07718) | Inconel 625 (N06625) |
|---|---|---|
| Strengthening mechanism | Age-hardenable (γ″ Ni₃Nb precipitation) | Solid-solution strengthened (Mo/Nb), not age-hardenable to the same degree |
| Typical yield strength (STA vs. annealed) | 1030–1170 MPa (STA) | 415–480 MPa (annealed) |
| Primary selection driver | Maximum strength at elevated temperature | Combined high-temperature strength + excellent aqueous/sour corrosion resistance |
| Typical application | Aerospace turbine disks/fasteners, high-strength oil & gas completion equipment | Sour service piping, marine, chemical process piping requiring both strength and corrosion resistance |
Heat Treatment Conditions
& Mechanical Reference
Inconel 718 is manufactured primarily as bar, rod, and forging stock for high-strength fastener and component manufacture, governed by specific ASTM and aerospace material specifications. Full detail on related nickel alloys is available across our standards reference library.
Submit form, condition, size, and quantity to sales@rrhydraulics.com for a certified offer.
2.1 — Governing Standards
ASTM B637 — Precipitation-Hardening Nickel Alloy Bar
The primary general industrial standard governing precipitation-hardening nickel alloy bar and rod including Inconel 718 — defines chemical composition and mechanical properties by heat treatment condition (annealed, solution treated, or fully age-hardened).
AMS 5662 / AMS 5663 — Aerospace Bar and Forging Stock
The primary aerospace material specifications for Inconel 718 bar (AMS 5662) and forgings (AMS 5663), imposing considerably more rigorous chemistry, grain size, mechanical testing, and process control requirements than the general industrial ASTM B637 baseline — the standard specifications for aerospace-critical 718 fastener and component supply.
AMS 2774 — Heat Treatment Specification
Governs the qualified solution treatment and precipitation (ageing) heat treatment process parameters for Inconel 718, ensuring the correct, documented heat treatment cycle is applied to achieve the specified mechanical property condition.
NACE MR0175 / ISO 15156
Provides the mandatory material qualification framework and, critically, maximum hardness limits for Inconel 718 used in sour (H₂S-containing) oil and gas service — one of the most specification-critical constraints for 718 in oil & gas high-pressure/high-temperature completion equipment, discussed further in Section 3.1.
2.2 — Mechanical Properties by Heat Treatment Condition
| Condition | Description | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) |
|---|---|---|---|---|
| Solution Annealed | Solution treated only, no ageing | 860–970 | 380–480 | 35–50 |
| Solution Treated and Aged (STA) | Standard age-hardened condition | 1240–1420 | 1030–1170 | 12–20 |
| Direct Aged (DA, no solution treatment) | Used for specific hot-worked/forged conditions | 1310–1450 | 1100–1240 | 10–16 |
Values are indicative — always verify the specific supplier’s heat treatment process qualification data and the current ASTM B637/AMS 5662/5663 revision for the exact mechanical property guarantee applicable to a specific ordered condition.
2.3 — Solution Treatment and Two-Step Ageing Heat Treatment
Inconel 718 achieves its full mechanical property range through solution treatment (typically 925–1010°C, temperature selected based on the desired grain size and subsequent mechanical property target) followed by a precisely controlled two-step ageing treatment — commonly 720°C for approximately 8 hours, furnace-cooled to 620°C and held for a further period, then air cooled. This specific two-step ageing profile is required to develop the optimum combination of gamma-double-prime (Ni₃Nb) and gamma-prime (Ni₃(Al,Ti)) precipitate distribution that gives 718 its full strength — a single-step ageing treatment does not achieve equivalent mechanical properties, making correct, documented two-step ageing process control an essential verification point for any critical 718 component supply.
Machining Difficulty
& Strain-Age Cracking
Inconel 718’s exceptional strength brings three specific engineering constraints that must be actively managed: a NACE MR0175 hardness ceiling for sour service, notoriously difficult machining behaviour, and strain-age cracking susceptibility during welding of age-hardened material.
3.1 — NACE MR0175 Hardness Constraint for Sour Service
3.2 — Machining Difficulty
Notoriously Difficult to Machine
Inconel 718, particularly in the age-hardened condition, is widely regarded among the most difficult common engineering alloys to machine — the combination of very high strength, significant work-hardening tendency, low thermal conductivity (concentrating cutting heat at the tool edge, similar in principle to the titanium machining challenges discussed in RR Hydraulic’s Titanium Grade 5 reference), and the abrasive nature of the hard carbide/intermetallic precipitates all combine to cause rapid tool wear and demand very conservative cutting parameters.
Specialized Tooling Requirements
Machining 718, especially in the fully aged condition, typically requires ceramic or advanced-coated carbide tooling, rigid machine tool setups to minimise vibration/chatter, generous and effective coolant application, and significantly reduced cutting speeds and feeds compared to machining carbon or standard stainless steel — production planning for 718 components must budget substantially higher machining time and tooling cost than for less demanding alloys.
Machining Before Full Age-Hardening
As with Monel K500 (discussed in RR Hydraulic’s dedicated reference), many manufacturers machine Inconel 718 components in the solution-annealed or partially-aged condition and perform the final full ageing heat treatment after machining — substantially reducing machining difficulty and tool wear compared to machining the fully hardened condition, though this requires accounting for any dimensional change during the final ageing step, which is typically minor but should be verified for precision-toleranced components.
3.3 — Strain-Age Cracking Risk in Welding
While Inconel 718’s slower gamma-double-prime precipitation kinetics give it better weldability than many comparably high-strength superalloys (Section 1.1), the alloy remains susceptible to strain-age cracking — a specific failure mode where residual welding stress combines with the onset of precipitation hardening during post-weld heat treatment or in-service thermal exposure, causing intergranular cracking in the heat-affected zone before the material can relieve the stress through ductile deformation. This risk is managed through correct welding procedure qualification (controlled heat input, appropriate preheat/interpass temperature control), welding in the solution-annealed (rather than pre-aged) condition where practical, and carefully controlled post-weld heat treatment cycles — welding procedures for 718 should be qualified specifically for this alloy and its intended heat treatment condition rather than assumed from general nickel alloy welding practice.
Industry Applications
& Documentation
RR Hydraulic maintains full traceability from certified nickel superalloy heat to finished, tested, and packed Inconel 718 component shipment. Chemical composition, mechanical, hardness, and grain size verification are standard on all project-grade supply.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 / Documentation Requirements
| Certificate | Content | EPC Requirement | When Mandatory |
|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Not acceptable for critical aerospace/HPHT supply | Never for aerospace or critical oil & gas HPHT equipment supply |
| 3.1 (EN 10204) | Heat-traceable chemical + mechanical test report | Mandatory — all EPC supply | All industrial and general oil & gas component supply |
| NACE MR0175 hardness compliance certificate | Production lot hardness test result vs. applicable limit | Mandatory — sour service HPHT supply | All confirmed or potential sour-service 718 supply |
| 3.2 (EN 10204) | 3.1 + TPI countersign | Critical / owner-specified critical items | Aerospace-critical and safety-critical HPHT completion equipment |
4.3 — Applications by Industry
Aerospace Gas Turbine Components
Inconel 718 (per AMS 5662/5663) for turbine disks, shafts, casings, and high-strength fasteners in aircraft and industrial gas turbine engines — the single largest-volume application category for 718 worldwide, leveraging the alloy’s exceptional strength retention at elevated temperature combined with good fatigue resistance for these highly cyclically loaded, safety-critical components.
Oil & Gas HPHT Completion Equipment
Inconel 718 components for high-pressure, high-temperature (HPHT) well completion equipment — packers, safety valves, and high-strength connectors — where the alloy’s exceptional strength at elevated temperature is essential, with the NACE MR0175 hardness constraint (Section 3.1) requiring careful heat treatment condition management wherever sour service is confirmed or possible.
High-Strength Fasteners and Load-Bearing Components
Inconel 718 studs, bolts, and precision mechanical components for critical aerospace, power generation, and industrial applications requiring the highest achievable strength combined with good corrosion resistance and elevated-temperature capability beyond what steel or titanium fasteners can provide.
4.4 — Export Packaging Specification
- Bar and billet stock protected to prevent contamination and mechanical damage during transit, particularly important given the alloy’s high cost and typical use in critical aerospace and HPHT applications
- Heat/lot number stamped or tagged on each item, cross-referenced to the accompanying material test certificate including the specific heat treatment condition, hardness test result, and grain size report
- Components segregated from carbon steel and other dissimilar materials during packing to avoid surface contamination affecting the alloy’s corrosion performance
- Documentation in a waterproof pocket: EN 10204 3.1/3.2 MTC, chemical composition report, mechanical properties report, hardness test report (NACE compliance where applicable), grain size report, PMI report, NDT reports, and packing list with form/condition/size breakdown per item
- ISPM-15 timber or export cartons for international shipment, with country of origin and HS tariff code documentation matched to the nickel superalloy product category
Submit your form, condition, size, and quantity to RR Hydraulic for a complete, certified commercial offer.