Inconel 718 API High-Strength Nickel-Chromium Alloy (UNS N07718)

Inconel 718 (UNS N07718) is a precipitation-hardening nickel-chromium-iron superalloy that combines very high yield and tensile strength with excellent corrosion resistance and weldability; when supplied in API/NACE/API-6A CRA heat-treatment conditions it meets the hardness and strength limits required for sour-service oil & gas equipment and is one of the primary corrosion-resistant alloys used for high-duty downhole and surface components.

what makes Inconel 718 different

Alloy 718’s performance arises from a balanced nickel-base matrix strengthened by two key precipitates (γ″ and γ′) produced during controlled aging. The alloy’s unique point is that heat-treatable microstructure permits large increases in yield and tensile strength while preserving good ductility and weldability. This makes it possible to supply components that meet NACE MR0175/ISO 15156 and API 6A CRA limits for hardness and yield — critical for sour-service environments.

Chemical composition and metallurgical features

Typical composition ranges (wt%) for Inconel 718 are approximately:

(Precise contractual chemical ranges should follow the specification cited on purchase order — API, AMS or ASTM variants list slightly different tolerances.)

Metallurgical notes

• Precipitation phases: γ″ (Ni₃Nb) is the principal hardening phase in 718; γ′ (Ni₃(Al,Ti)) also contributes. Control of Nb/Ti/Al and the thermal history dictates precipitate size and volume fraction.

• Carbides & delta (δ) phase: Excessive δ (Ni₃Nb, orthorhombic) or coarse carbides can degrade toughness and ductility; solution anneal and aging schedules are used to minimize harmful phases.

• Solidification practice: Multiple melting routes (VIM/VAR or ESR) are used for high-integrity aerospace or oil & gas forgings.

Mechanical properties — typical and high-strength API variants

Inconel 718 is sold in a range of conditions from annealed (lower strength) to peak-aged (highest). API/NACE oilfield grades are age-hardened and often produced to meet specified minimum yield strengths or hardness limits.

Representative mechanical table (typical aged / peak condition)

API 6A CRA / ‘API-718’ classes: API/supplier practice commonly divides 718 API into classes by minimum yield: ~120 ksi (≈827 MPa) and ~140 ksi (≈965 MPa) and some high-strength product variants claim 150 ksi in specific product forms. These variants are engineered with precise aging to achieve the targeted yield while controlling hardness for sour service.

Heat treatment, age-hardening and microstructure control

The typical sequence to obtain the high strength is:

1. Solution anneal (solution treatment): high temperature (≈980–1035 °C / ≈1800–1900 °F) for homogenization and to dissolve Nb and other phases, followed by cooling (air cool).

2. Aging (two-step commonly used): e.g., 718 standard aging cycle: 720 °C (≈1320–1350 °F) for 8 hours, cool to 620 °C (≈1150 °F) and hold for 8 hours, then air cool (numerous proprietary variants exist). This produces a fine γ″/γ′ precipitate distribution that gives the high yield and tensile strength.

Practical tips

• Control of aging: Small deviations in aging temperature/time alter strength and hardness. For NACE/MR0175 compliance, producers use tailored sequences to stay below the maximum hardness limit (commonly 40 HRC) while achieving minimum yield.

• Section size effects: Larger cross sections age and cool more slowly; mechanical properties can vary with section thickness. Specifications sometimes require property tests on representative section sizes.

International standards and oil & gas API / NACE requirements

Important documents and specifications commonly cited for 718 in oil & gas:

• API 6A / API 6A CRA (supplement): defines requirements for age-hardenable nickel alloys used in wellhead, tree and downhole equipment. It sets processing and testing expectations for materials used in pressure equipment.

• NACE MR0175 / ISO 15156 (part 3): provides requirements for cracking-resistant materials used in H₂S-containing environments; Alloy 718 can be listed with temperature/pressure limits and hardness constraints. Compliance requires specific heat treatment and hardness limits (often Rockwell C ≤ 40 in the aged condition).

• AMS / ASTM / ASTM B637 / AMS5662 / AMS5663: aerospace and industrial procurement specs that describe solution annealed, aged or other conditions with associated acceptance tests.

Key contractual points for purchase orders

• Specify UNS number (UNS N07718).

• Reference the exact standard (API 6A CRA rev X, NACE MR0175 ed Y).

• State the required minimum yield and maximum hardness explicitly.

• Include required testing (mechanical, PMI/Chemistry, NDT, microstructure) and traceability (heat numbers, mill certificates).

Corrosion behaviour and environmental limits

Alloy 718 offers robust resistance to general corrosion, pitting and crevice corrosion in many chloride environments and has reasonable resistance to H₂S-induced cracking provided the material meets the NACE/ISO hardness and heat-treatment controls.

H₂S (sour) environments

• NACE MR0175/ISO 15156 classifies CRAs and sets temperature/partial pressure limits. For UNS N07718, Table A.32 places constraints on maximum temperature and pressure for sour service and emphasizes the need for controlled heat treatment and hardness monitoring. Failure to meet the hardness limit increases susceptibility to sulfide stress cracking and hydrogen embrittlement.

Other environments

• Seawater and chloride brines: alloy 718 resists pitting better than many stainless steels but can still suffer localized attack under stagnant or highly aggressive chloride conditions.

• Oxidation resistance: service temperatures up to ~650–700 °C (≈1200–1300 °F) are typical for strength retention; for continuous oxidation resistance at higher temperatures, other nickel alloys may be preferred.

Fabrication, welding and joining

One reason 718 is widely used is its weldability compared with many high-strength alloys.

• Weldability: 718 welds well using matching filler metals; problems arise if post-weld heat treatment (PWHT) is neglected. Solution annealing plus aging on welded assemblies is common for critical parts.

• Weld filler selection: use matched or approved fillers that will allow post-weld aging to produce compatible mechanical and corrosion behaviour.

• Distortion and residual stress: due to high strength, correct fixturing and controlled weld sequences are required for tight tolerance parts.

• Machining: In the aged condition 718 is tough to machine due to high strength—many fabricators machine in the soft (solution annealed) condition and then apply the aging cycle.

Applications in oil & gas and aerospace

Oil & Gas (primary focus for API 6A CRA 718):

• Packers, hangers, couplings, downhole connectors, safety/relief valves, seals, and high-strength fasteners. 718’s combination of strength and sour-service capability makes it a mainstay for completion and drilling equipment.

Aerospace & gas turbines:

• Turbine discs, shafts and fasteners historically used older variants of 718 (AMS conditions). Aerospace demand drove early development of the alloy due to its creep-resistance and fatigue properties.

Other sectors:

• Nuclear components, high-pressure pump shafts, and certain chemical processing equipment where high strength and corrosion resistance are needed.

Inspection, testing and qualification for sour service

Common test matrix for API 718 supply includes:

• Positive Material Identification (PMI): confirm composition.

• Mechanical tests: tensile, yield, elongation on representative samples.

• Hardness testing: Rockwell C and/or Brinell across the lot — mandatory for NACE MR0175 compliance.

• Microstructural examination: for precipitate distribution, grain size, and absence of detrimental phases.

• Non-destructive testing (NDT): magnetic particle (for ferromagnetic inclusions), liquid penetrant, ultrasonic as specified.

• Hydrogen embrittlement screening / SSC testing: required in certain sour applications; NACE guidance indicates when SCC testing is compulsory.

Procurement forms, marking and traceability

Suppliers typically provide 718 in bars, forgings, plates, rings, seamless rod and machined components. For API/NACE use orders commonly require:

• Mill test certificates (EN 10204 type 3.1 or equivalent).

• Heat number traceability from melt to finished piece.

• Documentation of heat treatment cycles carried out (temperature/time profiles).

• Hardness records and mechanical test reports for each lot/batch.

Comparative selection (718 vs other common alloys)

Selection logic: where very high yield strength combined with sour-service resistance is required, 718 API variants are preferred.

Design considerations — fatigue, creep and temperature

• Fatigue: 718 has strong fatigue performance in the aged condition; however, surface finish, residual stresses, and heat treatment variations are critical. Shot-peening and surface treatments can raise fatigue life.

• Creep: 718 retains useful creep resistance up to ≈650–700 °C. For continuous exposure above those ranges other nickel superalloys (e.g., alloys 713, 625 variants) may be selected.

• Temperature limits for sour service: NACE/ISO tables give bounds; engineers must cross-reference the actual temperature and H₂S partial pressure to determine if 718 is permitted for the specific service.

Representative case notes and failure modes to avoid

Common root causes when 718 components fail in the field:

• Improper heat treatment: insufficient aging or incorrect solution anneal leading to either low strength or localized brittleness.

• Exceeding NACE hardness limits: components hardened above allowable HRC or with high residual stresses can suffer sulfide stress cracking.

• Incorrect section-to-heat treatment matching: large forgings need adjusted aging or post-machining controls because properties vary with section thickness.

• Mixed metallurgy without appropriate galvanic isolation: connectors combining dissimilar metals in seawater can encourage localized attack on the less noble component.

Quick-reference tables

Table A — Typical chemical composition (normalized)

(Specification ranges vary slightly per AMS, ASTM or API variants — always specify which document you require.)

Table B — Example heat treatment cycles (illustrative)

(Actual cycles may vary; suppliers usually document heat-treatment profiles.)

FAQs

1) Is Inconel 718 suitable for sour (H₂S) service?
Yes — provided the material is produced and heat-treated to meet NACE MR0175 / ISO 15156-3 and API 6A CRA requirements (including maximum hardness limits and documented heat treatment). Verify supplier test reports.

2) What’s the difference between AMS and API 6A CRA 718?
AMS specs (e.g., AMS5662/5663) commonly define aerospace heat-treatment conditions and acceptance tests; API 6A CRA adds oil & gas-specific processing, testing and hardness limits tailored for sour service. Choose the spec that matches the application.

3) What hardness is acceptable for NACE compliance?
Typically Rockwell C ≤ 40 in the solution-annealed + aged condition, but you must confirm the edition and specific table for the application (NACE/ISO has temperature-dependent constraints).

4) Can 718 be welded and still be used in sour service?
Yes — but welded assemblies must receive correct solution anneal and aging cycles (or specified post-weld heat treatment) and be verified by hardness and microstructural inspection to ensure NACE compliance.

5) What are typical minimum yield values for API 718 classes?
Common industry practice supplies classes around 120 ksi and 140 ksi minimum yield strength; some suppliers offer a special 150 ksi high-strength product in restricted forms. Always specify the required minimum in the PO.

6) Are there limits on temperature for 718 in sour service?
Yes. NACE MR0175 / ISO 15156 tables set temperature/pressure guidance; engineers must validate the specific combination of temperature and H₂S partial pressure against those tables.

7) How does section thickness affect mechanical properties?
Large sections cool and age differently; yield and tensile properties and hardness can vary with section size. Suppliers often test representative sections or provide derating guidance.

8) Is 718 magnetic?
In most conditions 718 is weakly magnetic or nearly non-magnetic due to its austenitic nickel matrix; however, cold working or certain precipitate distributions can make it slightly magnetic.

9) What NDT and qualification testing are typically required for critical parts?
PMI, tensile testing, hardness testing, microstructural examination, and appropriate NDT (UT/MT/PT) plus any hydrogen embrittlement or SSC testing mandated by the purchase spec.

10) Alternatives if the application demands higher general corrosion resistance?
If pitting or chloride resistance is paramount, consider alloys like 625, 725 or more noble CRAs; if higher temperature creep resistance is required, evaluate other nickel-base superalloys tailored for creep.