Inconel 718 (UNS N07718) Technical Guide | High-Temperature Strength & Aerospace Engineering

Inconel 718 (UNS N07718): High-Temperature Strength for Aerospace and Critical Energy Service | Hangbo Alloy

Introduction

Inconel 718, UNS N07718, is the most widely produced and most commercially important precipitation-hardenable nickel-chromium-iron alloy in existence. It accounts for roughly half of all nickel-alloy production worldwide and the majority of superalloy tonnage consumed by the aerospace industry. Its dominance is explained by an unusual combination of properties: high tensile, creep, and fatigue strength from cryogenic temperatures up to approximately 700 °C; good corrosion and oxidation resistance; and — uniquely among high-strength superalloys — excellent weldability and forgeability made possible by a deliberately slow age-hardening response.

Inconel 718 was developed by H. L. Eiselstein of International Nickel in the late 1950s specifically to provide a high-strength alloy that could be welded without the strain-age cracking that plagued other age-hardenable superalloys. That design objective was achieved so successfully that the alloy remains the backbone of gas-turbine rotating hardware — discs, blades, shafts, casings, and fasteners — in commercial and military aero-engines, as well as in land-based power turbines, rocket engines, nuclear reactors, and high-performance tooling.

Hangbo Alloy manufactures Inconel 718 in bar, billet, plate, sheet, strip, pipe, and forged shapes to ASTM B637 and AMS 5662, backed by full melt-to-product traceability. This article examines the metallurgy, mechanical behavior, corrosion resistance, fabrication, and applications of Inconel 718 from an engineering standpoint.

Chemical Composition and Strengthening Mechanism

The nominal composition of Inconel 718 is balanced to achieve age hardening through two intermetallic phases while suppressing harmful grain-boundary reactions.

The precipitation-hardening response is dominated by the metastable body-centered-tetragonal phase gamma double-prime (γ″), Ni₃Nb, which precipitates as fine, coherent disks on the {100} planes of the austenitic matrix. Gamma double-prime provides an exceptional hardening increment but is metastable: prolonged exposure above about 650 °C slowly transforms it to the stable orthorhombic delta phase (δ), Ni₃Nb, which is non-hardening. This metallurgical ceiling sets the alloy's practical maximum service temperature near 650–700 °C, beyond which overaging reduces strength. A secondary contribution comes from the gamma prime phase (γ′), Ni₃(Al,Ti), which is more thermally stable and provides some strengthening at the highest service temperatures.

The niobium addition is thus the heart of the alloy system. It is also the reason for the sluggish aging kinetics: γ″ precipitates slowly because niobium diffuses slowly in nickel. This deliberate design means parts can be welded in the solution-treated condition and aged afterward, or welded directly in the aged condition, without the near-instantaneous hardening — and associated strain-age cracking — typical of gamma-prime alloys such as Waspaloy, Rene 41, or Inconel X-750.

Boron and carbon are controlled to refine grain-boundary carbide distributions, which improves rupture ductility. Chromium provides oxidation and corrosion resistance, while molybdenum and iron contribute solid-solution strengthening and lower raw-material cost.

Heat Treatment Schedules

Inconel 718 derives its properties from a sequence of solution treatment and two-step aging. Common schedules include:

The classic AMS 5662 schedule — 718 °C for 8 hours, furnace cool at 55 °C/h to 621 °C, hold 8 hours, air cool — produces the standard high-strength condition. Because age hardening is slow, large forgings can be cooled and machined in the solution-treated condition without cracking, a manufacturing advantage unique to this alloy.

Mechanical Properties at Room and Elevated Temperature

The room-temperature minimum properties for solution-annealed-and-aged bar per ASTM B637 are shown below, together with representative physical data.

Elevated-Temperature Strength

What distinguishes Inconel 718 from solid-solution alloys such as Inconel 625 is retention of strength at temperature. The alloy is routinely specified for continuous service to 650 °C and is usable, with appropriate design allowables, to 700 °C for short durations. At 650 °C the yield strength remains above 860 MPa in the fully aged condition — roughly double that of 625 at the same temperature. Tensile data at representative temperatures illustrate this retention:

Creep and stress-rupture strength are excellent: in the standard condition the alloy supports approximately 690 MPa for 100 h at 650 °C in smooth-bar rupture testing. Fatigue performance, both low-cycle (LCF, thermally driven) and high-cycle (HCF), is outstanding and is the primary reason turbine-disc alloys are forged to fine, uniform grain sizes — typically ASTM 6–8 for discs — because fatigue-crack initiation resistance improves as grain size decreases.

Low-Temperature Toughness

Inconel 718 does not exhibit a ductile-to-brittle transition. Its strength and toughness increase as temperature falls, making it a leading candidate for cryogenic hardware: rocket-engine turbopumps, cryogenic storage, and liquefied-gas service operate confidently to −253 °C.

Corrosion and Oxidation Resistance

Chromium at 17–21 % gives Inconel 718 good resistance to oxidation and hot corrosion up to about 900 °C for non-load-bearing, short-term exposure, and useful scaling resistance through the engine operating range. It resists aqueous corrosion better than most stainless steels in many media, though it is not a substitute for the high-molybdenum corrosion alloys (625, C-276) in severe chemical or seawater chloride service. The alloy is, however, notably resistant to chloride stress-corrosion cracking, and aged hardware shows good resistance in marine and sour (H₂S-containing) environments when properly specified. For aerospace, the alloy is routinely coated or aluminized only when surface temperatures exceed its intrinsic oxidation capability.

Weldability — The Defining Advantage

Inconel 718 can be joined by essentially every fusion and solid-state welding process: GTAW, GMAW, electron-beam (EB), laser, plasma arc, and inertia/friction welding. Matching filler ERNiFeCr-2 (AWS A5.14) is standard. Two characteristics deserve emphasis:

1. Resistance to strain-age cracking. Because hardening is sluggish, welded components can be aged without the dangerous post-weld cracking seen in rapidly hardening alloys. This permits repair welding of service-aged turbine hardware — a routine, economically vital operation in engine overhaul. 2. Preservation of joint strength. EB and laser welds of thin sections in the aged condition can develop joint efficiencies approaching 100 % after local aging. For the highest-quality welds, solution treatment before welding and full aging after welding is recommended to restore uniform properties across the joint.

For dissimilar joints to austenitic stainless steels or low-alloy steels, the alloy's intermediate expansion coefficient and metallurgical compatibility minimize residual stress and carbide migration problems.

Hot and Cold Working, Machining

Inconel 718 is forged over 927–1120 °C with a preferred hot-working window around 980–1060 °C for fine-grain control. It can be cold formed only in the solution-treated condition, and work hardens rapidly. Machining of the aged alloy is demanding: its combination of high strength, low thermal conductivity, and work-hardening tendency requires rigid tooling, positive rake angles, carbide or ceramic inserts, and copious coolant. Modern high-efficiency machining with coated carbide and ceramic (whisker-reinforced alumina) tooling at high speeds has made component manufacturing far more economical. Electrical discharge machining (EDM) is widely used for cooling holes in turbine hardware.

Product Forms and Standards

Applications

In aero-engines, Inconel 718 carries the intermediate stages of the compressor and the forward stages of the turbine where temperatures range from 400 to 650 °C — hot enough to defeat titanium and aluminum alloys but cool enough to respect the γ″ stability limit. Its combination of manufacturability, damage tolerance, and cost has kept it competitive even as more expensive powder-metallurgy alloys penetrate the hottest disc stages.

Why Choose Hangbo Alloy for Inconel 718

Hangbo Alloy operates a complete integrated route for Inconel 718: VIM (vacuum induction melting) followed by VAR (vacuum arc remelting) or ESR (electroslag remelting) for aerospace-grade cleanliness, controlled forging, and precision machining of test specimens. Every heat is verified by chemical analysis, ultrasonic inspection per AMS 2634 or ASTM E2375, and mechanical testing. Hangbo Alloy supplies forged bar and billet to AMS 5662/AMS 5663, and coordinates customer and third-party inspection to NACE, aerospace primes, and classification-society requirements. Through nickel-alloy.com, Hangbo Alloy exports Inconel 718 to engine manufacturers, MRO shops, and oilfield service companies across six continents.

Technical FAQ

Q1: What is the maximum continuous service temperature of Inconel 718? About 650–700 °C. Above 650 °C the metastable γ″ phase slowly transforms to δ phase, reducing strength with time. For sustained service at higher temperatures, use Waspaloy, Rene 41, or Inconel 625 for oxidation-dominated duty.

Q2: Why is Inconel 718 preferred over other superalloys for turbine discs? It offers the best balance of strength, fatigue resistance, forgeability, weldability, and cost. The slow aging response prevents cracking during manufacture, enabling large, complex, fine-grained forgings that other age-hardenable alloys cannot produce reliably.

Q3: Is Inconel 718 weldable in the aged condition? Yes — uniquely among high-strength superalloys, it can be welded in the fully aged condition and locally aged afterward with low risk of strain-age cracking. This underpins repair welding of service-run turbine components.

Q4: Does Inconel 718 require vacuum melting? For aerospace rotating hardware, VIM+VAR (or VIM+ESR) is required to achieve the low inclusion and segregation levels demanded by fatigue-critical applications. Hangbo Alloy supplies vacuum-remelted grades as standard for such service.

Q5: What causes delta phase, and is it harmful? Delta phase (Ni₃Nb) forms during exposure above ~650 °C or during excessive hot working. A controlled amount at grain boundaries can pin grain growth, but large amounts consume niobium and lower strength and ductility.

Q6: Can Inconel 718 be used at cryogenic temperatures? Yes. It has no ductile-to-brittle transition and gains strength at low temperature, making it standard for rocket-engine turbopumps and cryogenic systems.

Q7: What filler metal is used for welding Inconel 718 to itself? AWS A5.14 ERNiFeCr-2 (matching composition) is standard; for dissimilar joints to stainless steel, the same filler is commonly used.

Q8: How is Inconel 718 aged after welding? Solution treatment at 941–1010 °C followed by the standard 718 °C/8 h + 621 °C/8 h age restores near-parent-metal properties in the joint.

Q9: Is Inconel 718 suitable for seawater service? It has good resistance and immunity to chloride SCC, but for seawater or chemical service demanding pitting/crevice immunity, Inconel 625 or Hastelloy C-276 are the preferred Hangbo Alloy offerings.

Q10: What forms and sizes does Hangbo Alloy supply? Bar and billet from 10 mm to 500 mm diameter, plate and sheet, strip, pipe, forgings and rings up to large turbine-disc dimensions, all with full traceability and inspection documentation.