API 6ACRA / UNS N07718 / Alloy 718 – Age-Hardened Nickel Alloy for Sour Oil & Gas Service

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API 6ACRA is a specification developed by the American Petroleum Institute (API) outlining requirements for age-hardened nickel-based alloys used in oil and gas drilling and production equipment. 

This standard ensures integrity and reliability of components operating in harsh environments containing hydrogen sulfide (H₂S) and other aggressive chemicals. 

UNS N07718 (Alloy 718) is the most widely specified alloy under this standard, offering an exceptional combination of high strength, excellent corrosion resistance, and outstanding weldability for critical downhole and surface applications. .


We offer custom grades, sizes, and non-standard materials. Contact us with your requirements.

1. Product Specifications & Physical Data

Alloy 718 is a precipitation-hardenable nickel-chromium-iron-molybdenum alloy strengthened by niobium, titanium, and aluminum additions. It is typically double vacuum melted (VIM + VAR) to ensure high purity and cleanliness .

  • Available Forms: Bar, billet, wire, bored bar with bespoke internal diameters .

  • Specifications: API 6ACRA, NACE MR0175/ISO 15156, ASTM B637, AMS 5662/5663 .

Chemical Composition (Weight %)API 6ACRA / UNS N07718
Nickel (Ni)50.0 - 55.0
Chromium (Cr)17.0 - 21.0
Niobium (Nb)4.75 - 5.50
Molybdenum (Mo)2.80 - 3.30
Titanium (Ti)0.65 - 1.15
Aluminum (Al)0.20 - 0.80
Carbon (C)0.08 max.
Manganese (Mn)0.35 max.
Silicon (Si)0.35 max.
Iron (Fe)Balance


Physical PropertiesMetricImperial
Density8.19 g/cm³0.296 lb/in³
Melting Range1260 - 1336°C2300 - 2437°F
Modulus of Elasticity205 GPa29,700 ksi
Coefficient of Expansion (20-100°C)13.0 µm/m·°C7.2 µin/in·°F


Mechanical Properties (API 6ACRA)120 ksi Grade140 ksi Grade150 ksi Grade
Tensile Strength (min.)150 ksi (1034 MPa)150 ksi (1034 MPa)150 ksi (1034 MPa)
Yield Strength (0.2% offset, min.)120 ksi (827 MPa)140 ksi (965 MPa)150 ksi (1034 MPa)
Elongation (min.)20%18%15%
Reduction of Area (min.)35%30%25%
Hardness (HRC)32-4035-4238-45
Charpy Impact @ -75°F (-60°C) (min.)35 ft·lbs35 ft·lbs35 ft·lbs

We offer custom grades, sizes, and non-standard materials. Contact us with your requirements.

2. Operating Conditions & Selection Criteria

Alloy 718 is specified for oil and gas applications requiring high strength, excellent sour service resistance, and reliable performance in H₂S and CO₂ environments.

  • Service Temperature Range: Suitable for high-temperature applications with de-rating factors; maintains strength at elevated temperatures up to 700°C (1290°F) .

  • Sour Service Compliance: NACE MR0175/ISO 15156 approved for use in H₂S environments. The alloy meets SSC resistance requirements at specified hardness levels .

  • Corrosion Resistance: Excellent resistance to H₂S, CO₂, and chloride-induced stress corrosion cracking in aggressive oil and gas environments .

  • Strengthening Mechanism: Age-hardened through precipitation of gamma-prime and gamma-double-prime phases (Ni₃(Nb,Ti,Al)) during heat treatment .

  • Toughness: Excellent sub-zero impact properties even at higher strength levels; Charpy impact values meet API 6ACRA requirements at -75°F (-60°C) .

  • Weldability: Outstanding weldability with resistance to post-weld cracking—significantly better than other precipitation-hardened nickel alloys .

3. Fabrication & Material Comparison

  • Melting: Double vacuum melting (VIM + VAR) ensures high purity and absence of non-metallic inclusions critical for sour service .

  • Heat Treatment:

    • Solution Anneal: 980-1010°C (1800-1850°F) × 1-2 hours, air cool

    • Age Hardening: 720°C (1330°F) × 8 hours, furnace cool to 620°C (1150°F) × 8-18 hours, air cool .

  • Machining: Good machinability in solution-annealed condition; carbide tooling recommended .

  • Welding: Excellent weldability using GTAW and GMAW processes; matching filler metal recommended; no preheating required .

Material ComparisonAlloy 718 (120 ksi)Alloy 925Alloy 72517-4PH
Yield Strength (min.)120-150 ksi110-120 ksi120-130 ksi90-110 ksi
H₂S ResistanceExcellent (NACE)Excellent (NACE)Excellent (NACE)Limited
Maximum Service Temp700°C550°C650°C315°C
WeldabilityExcellentGoodGoodModerate
Primary UseDownhole, wellhead, subseaDownhole, valvesSubsea, connectorsSurface equipment

Applications

  • Downhole Tools: Packers, safety valves, pumps, hangers, connectors, couplings

  • Wellhead Equipment: Gate valves, choke stems, tubing hangers, fire-safe valves

  • Subsea Systems: Subsea connectors, manifolds, pressure-containing equipment

  • Completion Hardware: Polish bore receptacles (PBRs), completion components

  • Fasteners: High-strength bolts, studs, nuts for sour service

Contact ZYTC Alloy for certified API 6ACRA UNS N07718 Alloy 718 with complete traceability, EN 10204 3.1/3.2 certificates, NACE MR0175 compliance, and competitive pricing. Available in bar, bored bar, and custom forms for oil and gas sour service applications.

📧 Email: daisy@zytcsteel.com

📲 WhatsApp/WeChat: +86-17335753350

🌐 Website: https://www.zytcalloys.com/product-API-6ACRA-UNS-N07718-Nickel-Alloy-for-Sour-Oil-Gas-Service.html

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