Inconel 718 (UNS N07718) – Precipitation-Hardening Nickel-Chromium Alloy for High-Strength Applications

Short Description:

Inconel 718 (UNS N07718 / W.Nr. 2.4668 / GH4169) is a precipitation-hardenable nickel-chromium-iron alloy with exceptional high-temperature strength and corrosion resistance. Womic Steel supplies Inconel 718 in multiple product forms including seamless pipe, welded pipe, steel bar, steel plate, steel coil, strip, flanges, fittings, forgings, wire, fasteners, and valves for aerospace, gas turbine, marine, nuclear, and oil and gas applications .

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Product Overview

Inconel 718 is the most widely used nickel-based superalloy in the world, often described as the "workhorse" of high-temperature alloys. Developed by the International Nickel Company in the 1950s, the alloy achieves its exceptional strength through the precipitation of γ″ (Ni₃Nb) and γ′ (Ni₃(Al,Ti)) phases during controlled double-aging heat treatment. With tensile strengths reaching 1,275-1,380 MPa, Inconel 718 offers one of the highest strength-to-weight ratios among nickel alloys while maintaining excellent fabricability and weldability.

Key characteristics include:

Exceptional high-temperature strength up to 700°C (1300°F)
Precipitation hardening through γ″ and γ′ phase formation during aging
Excellent creep-rupture resistance at 650-700°C
Good oxidation and corrosion resistance in diverse environments
Outstanding fatigue resistance and fracture toughness
Excellent weldability – one of the most weldable nickel-based superalloys
Wide temperature capability from cryogenic (-253°C) to 700°C

Chemical Composition

Element	Content (%)	Role
Nickel (Ni)	50.0 – 55.0	Base element, provides austenitic stability and SCC resistance
Chromium (Cr)	17.0 – 21.0	Oxidation resistance, forms protective Cr₂O₃ scale
Iron (Fe)	Balance	Cost-effective base element, participates in solid-solution strengthening
Niobium + Tantalum (Nb+Ta)	4.75 – 5.50	Primary γ″ strengthening phase (Ni₃Nb), key differentiator
Molybdenum (Mo)	2.80 – 3.30	Solid-solution strengthening, creep resistance
Titanium (Ti)	0.65 – 1.15	γ′ phase formation (Ni₃Al,Ti), secondary strengthening
Aluminum (Al)	0.20 – 0.80	γ′ phase formation, oxidation resistance
Cobalt (Co)	≤ 1.00	Residual element, contributes to solid-solution strengthening
Carbon (C)	≤ 0.08	Carbide former, grain boundary strengthening
Manganese (Mn)	≤ 0.35	Deoxidizer
Silicon (Si)	≤ 0.35	Deoxidizer
Phosphorus (P)	≤ 0.015	Impurity, tightly controlled
Sulfur (S)	≤ 0.015	Impurity, tightly controlled
Boron (B)	≤ 0.006	Grain boundary strengthening, improves creep resistance
Copper (Cu)	≤ 0.30	Residual element

Mechanical Properties

Room Temperature Properties (Aged Condition)

Property	ASTM Requirement	Typical Achieved
Tensile Strength	≥ 1,240 MPa (180 ksi)	1,275 – 1,380 MPa
Yield Strength (0.2% Offset)	≥ 1,034 MPa (150 ksi)	1,030 – 1,170 MPa
Elongation	≥ 12%	12 – 25%
Hardness	—	36 – 44 HRC
Density	8.19 g/cm³
Melting Range	1,260 – 1,340°C
Elastic Modulus	200 GPa at 20°C
Thermal Conductivity	11.4 W/m·K at 20°C
Magnetic Permeability	Non-magnetic

High-Temperature Mechanical Properties

Temperature °C	Tensile Strength (MPa)	Yield Strength (MPa)
20	1,275 – 1,380	1,030 – 1,170
540	1,150	960
650	1,030 – 1,100	850 – 860
700	—	700

Creep-Rupture Strength

Temperature	Stress (MPa)	Rupture Life
650°C	690	> 100 hours
700°C	620	> 100 hours
650°C	586	> 1,000 hours

Key Advantages of Inconel 718

Precipitation Hardening Mechanism – Inconel 718 derives its strength from the precipitation of γ″ (Ni₃Nb) and γ′ (Ni₃(Al,Ti)) phases during aging heat treatment. The γ″ phase is the primary strengthening mechanism, with volume fraction reaching approximately 12-20%. Unlike many other precipitation-hardening alloys, Inconel 718 has a slower aging response, providing a wide processing window and excellent weldability.

Exceptional High-Temperature Strength – Inconel 718 maintains high tensile, fatigue, and creep-rupture strength up to 700°C (1300°F). At 650°C, its yield strength surpasses that of all other wrought superalloys. The alloy exhibits excellent creep resistance in the 650-700°C range, making it the preferred material for long-term high-temperature service.

Excellent Weldability – Inconel 718 is widely regarded as the most weldable nickel-based superalloy. Unlike many precipitation-hardening alloys that are susceptible to strain-age cracking, Inconel 718 can be welded in both annealed and aged conditions without significant risk. The alloy exhibits excellent resistance to weld-induced cracking and can be welded using TIG, MIG, electron beam, and laser welding processes. Welded joints typically achieve near-parent metal strength with simple post-weld aging treatment.

Wide Temperature Capability – The alloy maintains stable properties from cryogenic temperatures (-253°C) up to 700°C. At cryogenic temperatures, strength and toughness increase without ductile-to-brittle transition, making it suitable for liquid hydrogen and liquid oxygen systems. This wide operating range is rare among superalloys.

Good Corrosion Resistance – With 17-21% chromium content, Inconel 718 provides excellent resistance to oxidation, pitting, and stress corrosion cracking in diverse environments. The alloy performs well in seawater, salt spray, acidic media, and high-temperature oxidizing atmospheres. Its low cobalt content (≤1.0%) makes it suitable for nuclear applications where radiation activation is a concern.

Excellent Fabricability – Inconel 718 can be hot worked at 900-1150°C and cold worked using standard techniques. Cold working significantly increases strength while maintaining adequate ductility. The alloy is machinable using carbide or ceramic tooling with appropriate parameters.

Inconel 718 vs Inconel 625: Comparison

Property	Inconel 718	Inconel 625
Strengthening Mechanism	Precipitation hardening (γ″ + γ′). Requires heat treatment	Solid-solution strengthening (no heat treatment required)
Tensile Strength	≥ 1,240 MPa (180 ksi)	~690 MPa (100 ksi)
Temperature Capability	-253°C to 700°C	Cryogenic to 982°C (1800°F)
Iron Content	~15-20%	≤5%
Niobium Content	4.75-5.50% (critical for γ″)	3.15-4.15%
Molybdenum Content	2.80-3.30%	8-10% (pitting resistance)
Key Differentiator	Highest strength, weldability, fatigue resistance	Excellent corrosion resistance in seawater and acids
Preferred For	Aerospace turbine disks, jet engines, rocket motors, HPHT oil and gas, fasteners	Marine environments, seawater piping, chemical processing, FGD

When to choose Inconel 718: Applications requiring maximum high-temperature strength, creep resistance, and fatigue performance up to 700°C where corrosion resistance is secondary. Typical: turbine components, rocket engines, HPHT oil and gas.

When to choose Inconel 625: Applications requiring excellent corrosion resistance in seawater, acids, and high-chloride environments where moderate strength is acceptable. Typical: marine piping, chemical processing, FGD systems.

Heat Treatment of Inconel 718

Inconel 718 requires a specific heat treatment sequence to achieve optimal mechanical properties:

Standard Heat Treatment:

1. Solution Anneal: 927-1010°C (1700-1850°F) for 1 hour, rapid cooling (water or oil quench)

Purpose: Dissolve all precipitates, achieve single-phase austenite

2. First Stage Aging: 718°C (1325°F) for 8 hours, furnace cool to 620°C

3. Second Stage Aging: 620°C (1150°F) for 8-10 hours, total aging time 18 hours, air cool

Purpose: Precipitate γ″ and γ′ for maximum strength and creep resistance

Alternative Heat Treatment (For Improved Stress Relaxation):

Solution Anneal: 1038-1066°C (1900-1950°F), rapid cooling
First Stage Aging: 760°C (1400°F) for 10 hours, furnace cool to 650°C
Second Stage Aging: 650°C (1200°F) for 20 hours, air cool

Key Considerations:

The γ″ phase (Ni₃Nb) is the primary strengthening phase and is metastable. Above 650°C, it transforms to stable δ phase (Ni₃Nb, orthorhombic) causing strength reduction
Prolonged exposure at 600-800°C can precipitate δ phase at grain boundaries, reducing toughness
Cooling rate after solution annealing must be rapid to suppress harmful precipitation

Available Product Forms from Womic Steel

Seamless Pipe & Seamless Tube

Outside Diameter: 6mm – 350mm (1/4" – 14")
Wall Thickness: 1mm – 40mm (Schedule 5S to XXS)
Length: Up to 12m
Standards: ASTM B983, AMS 5589, AMS 5590
Applications: Heat exchanger tubes, high-pressure fluid systems, aerospace tubing, instrumentation lines
Keywords: Inconel 718 seamless pipe, UNS N07718 tube, 2.4668 seamless tube, GH4169 pipe, high strength nickel tube

Welded Pipe

Outside Diameter: 50mm – 1200mm (2" – 48")
Wall Thickness: 1.0mm – 30mm
Standards: ASTM B904, ASTM B906
Applications: Large-diameter process piping, heat exchanger shells, structural applications
Keywords: Inconel 718 welded pipe, alloy 718 ERW pipe, large diameter nickel pipe

Steel Bar (Round, Square, Flat, Hex)

Diameter: 5mm – 600mm
Surface: Hot rolled, Bright (cold drawn), Ground, Turned, Peeled
Standards: ASTM B637, AMS 5662 (Solution Annealed), AMS 5663 (Aged), AMS 5664
Applications: Turbine disks, shafts, valve stems, fasteners, bolts, studs, machining stock, springs
Keywords: Inconel 718 round bar, UNS N07718 bar, GH4169 bar, nickel alloy bar, high strength superalloy bar

Steel Plate & Sheet

Thickness: 0.5mm – 150mm
Width: Up to 2500mm
Standards: ASTM B670, AMS 5596, AMS 5597
Applications: Pressure vessel shells, structural components, flange blanks, rocket motor components
Keywords: Inconel 718 plate, alloy 718 sheet, GH4169 plate, high temperature alloy sheet

Steel Coil & Strip

Thickness: 0.5mm – 6.0mm
Width: 10mm – 1500mm
Standards: ASTM B670, AMS 5596
Applications: Formed components, bellows, expansion joints, stampings, thin-walled tubing
Keywords: Inconel 718 coil, nickel alloy strip, GH4169 coil

Flanges & Pipe Fittings

Types: Weld Neck, Slip-On, Blind, Socket Weld, Threaded, Lap Joint
Sizes: 1/2" – 48" (DN15 – DN1200)
Pressure Rating: Class 150 – Class 2500
Standards: ASTM B637, ASME B16.5, ASME B16.47
Keywords: Inconel 718 flanges, alloy 718 weld neck flange, N07718 fittings

Forgings

Types: Rings, disks, shafts, valve bodies, turbine components, custom geometries
Weight: Up to 5,000kg per piece
Standards: ASTM B637, AMS 5662, AMS 5663, AMS 5664
Applications: Turbine disks, compressor disks, engine casings, rocket motor components
Keywords: Inconel 718 forging, GH4169 forged ring, turbine disk forging

Inconel 718 Valves

Types: Ball valves, Gate valves, Globe valves, Check valves
Sizes: 1/2" – 24" (DN15 – DN600)
Pressure Rating: Class 150 – Class 2500
Temperature Range: -253°C to +700°C
Design Standards: API 608, API 6D, ASME B16.34
Seating: Stellite hardfaced, RPTFE, PEEK
Actuation: Manual, Pneumatic, Electric
Applications: HPHT oil and gas, high-temperature process isolation, turbine control systems, nuclear service
Keywords: Inconel 718 ball valve, alloy 718 gate valve, high temperature nickel valve

Wire & Fasteners

Wire Diameter: 0.5mm – 12mm
Fasteners: Bolts, nuts, studs, screws, washers, springs
Standards: ASTM B637, AMS 5662, AMS 5663, AMS 5832
Keywords: Inconel 718 wire, GH4169 fasteners, nickel alloy bolts, high temperature springs

Applications of Inconel 718

Aerospace – Inconel 718 is the industry standard for jet engine components including high-pressure compressor disks, turbine disks, turbine shafts, compressor blades, combustion chambers, afterburner components, and engine casings. The alloy is also used for rocket motor components, thrust chambers, turbopumps, and spacecraft structures. It is the most widely used superalloy in aerospace applications.

Energy and Power Generation – The alloy is extensively used in gas turbine components including turbine disks, blades, transition ducts, and fasteners for both land-based and marine gas turbines. Its creep resistance at 650-700°C makes it suitable for long-term service in power generation equipment.

Nuclear Industry – Inconel 718 is used in reactor pressure vessels, steam generator tubing, control rod drive mechanisms, and core internals. Its low cobalt content (≤1.0%) minimizes radioactive activation, making it suitable for nuclear applications.

Oil and Gas – The alloy is specified for high-pressure high-temperature (HPHT) wellhead equipment, downhole tubular components, tubing hangers, valves, pumps, and subsea equipment in sour service environments. Inconel 718 resists sulfide stress cracking and hydrogen induced cracking in H₂S-containing environments.

High-Temperature Fasteners – Inconel 718 is the standard material for high-temperature fasteners including turbine bolts, studs, nuts, and screws operating at temperatures up to 700°C. Its resistance to stress relaxation ensures long-term joint integrity.

Cryogenic Applications – The alloy maintains exceptional toughness at cryogenic temperatures down to -253°C, making it suitable for liquid hydrogen and liquid oxygen storage tanks and systems.

Standards & Specifications

Product Form	ASTM Standard	ASME Standard	AMS Standard
Seamless Pipe/Tube	ASTM B983	—	AMS 5589, AMS 5590
Bar/Rod	ASTM B637	ASME SB-637	AMS 5662, AMS 5663, AMS 5664
Plate/Sheet/Strip	ASTM B670	—	AMS 5596, AMS 5597
Forgings	ASTM B637	ASME SB-637	AMS 5662, AMS 5663, AMS 5664
Wire	ASTM B637	—	AMS 5832, AMS 5962
UNS Designation	N07718
Werkstoff Number	2.4668
DIN Designation	NiCr19Fe19NbMo
Chinese Equivalent	GH4169 / GH169

Frequently Asked Questions

Q: What is the difference between Inconel 718 and Inconel 625?

A: Inconel 718 excels in high-temperature strength (up to 700°C) through precipitation hardening (γ″ and γ′ phases), while Inconel 625 excels in corrosion resistance (seawater, acids) through solid-solution strengthening. Choose 718 for strength-critical applications; choose 625 for corrosion-critical environments. The density of 718 is 8.19 g/cm³, the density of 625 is 8.44 g/cm³.

Q: What is the maximum service temperature for Inconel 718?

A: Inconel 718 is rated for long-term service up to 700°C (1300°F). Short-term excursions up to 980°C (1800°F) are possible for oxidation resistance, but strength decreases significantly above 700°C.

Q: Is Inconel 718 readily weldable?

A: Yes. Inconel 718 is widely regarded as the most weldable nickel-based superalloy. It exhibits excellent resistance to strain-age cracking and can be welded in both annealed and aged conditions using TIG, MIG, electron beam, and laser welding methods. It is considered suitable for welding without post-weld cracking issues.

Q: What are the Chinese equivalents of Inconel 718?

A: The Chinese equivalents are GH4169 and GH169 (GB/T 14992). These grades conform to Chinese national standards for high-temperature alloys.

Q: What product forms are available for Inconel 718?

A: Womic Steel supplies Inconel 718 in seamless pipe, welded pipe, steel bar, steel plate, steel coil, strip, flanges, fittings, forgings, wire, springs, fasteners, and valves. Complete product specifications and size ranges are listed in the product availability section.

Q: Does Inconel 718 require heat treatment?

A: Yes. Inconel 718 achieves its exceptional strength through precipitation hardening heat treatment. The standard process is solution annealing (927-1010°C) followed by double-aging (718°C for 8 hours, furnace cool to 620°C, hold 8-10 hours).

Q: Is Inconel 718 magnetic?

A: No. Inconel 718 is non-magnetic in all conditions, making it suitable for applications where magnetic interference must be avoided.

Q: What is the fatigue performance of Inconel 718?

A: Inconel 718 exhibits outstanding fatigue resistance at high temperatures. Its high-cycle fatigue (HCF) limit at 650°C exceeds 400 MPa, making it suitable for applications subject to cyclic loading and thermal fatigue.

Contact Womic Steel

Website: www.womicsteel.com

E-mail: sales@womicsteel.com

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Womic Steel – Your reliable partner for Inconel 718 (UNS N07718, GH4169) seamless pipe, welded pipe, steel bar, steel plate, steel coil, flanges, fittings, forgings, valves, fasteners, and springs for aerospace, gas turbine, nuclear, and HPHT oil and gas applications worldwide. Full traceability, classification society certification (CCS, ABS, BV, DNV, LR), and EN 10204 Type 3.1/3.2 documentation available.