Inconel 718: Composition, Properties, and Hardness
Inconel 718 is a nickel-based superalloy engineered to perform under extreme mechanical stress, elevated temperatures, and corrosive environments. Inconel 718 belongs to a class of high-performance alloys where nickel content exceeds 50%, giving the material its foundation for thermal stability and oxidation resistance. The alloy achieves its exceptional strength through a precipitation hardening mechanism involving niobium and molybdenum, which form coherent precipitate phases within the nickel matrix. Aerospace turbine components, rocket engine parts, and deep-well drilling equipment all depend on the alloy's ability to retain structural integrity at temperatures reaching 1300°F (704°C).
Inconel 718 stands apart from standard engineering alloys because of its unique combination of high tensile strength, creep resistance, and outstanding weldability without post-weld cracking. The alloy's chemical composition is tightly controlled, with nickel and iron forming the base, while chromium provides corrosion resistance and niobium drives age-hardening response. Industries from oil and gas to nuclear power generation rely on the material's long-term reliability in demanding service conditions. Xometry supplies Inconel 718 in multiple forms, including bar stock, sheet, and machined components, supporting precision manufacturing needs across critical engineering sectors.
What Is Inconel 718?
Inconel 718 is a precipitation-hardened nickel-chromium superalloy composed of nickel (50 to 55%), chromium (17 to 21%), and iron (balance), with significant additions of niobium, molybdenum, titanium, and aluminum. The alloy was originally developed in the late 1950s and early 1960s by International Nickel Company to meet the growing demands of jet engine manufacturing, where conventional stainless steels and titanium alloys could not sustain reliable performance at high operating temperatures.
The material's defining characteristic is its ability to maintain tensile strength above 180 ksi (1241 MPa) even after prolonged exposure to temperatures up to 1300°F (704°C). Unlike solid-solution strengthened alloys, Inconel 718 derives its strength from gamma double prime (γ″) precipitates, primarily Ni₃Nb, which form during a controlled aging heat treatment process. The γ″ phase accounts for the majority of the alloy's age-hardening response, making it one of the most widely produced nickel superalloys in the world. Xometry machines and supplies Inconel 718 components to tight tolerances, meeting aerospace and defense procurement standards.
What Makes Inconel 718 a Nickel-Based Superalloy?
Inconel 718 is a nickel-based superalloy because nickel constitutes the largest single element in its composition, ranging from 50 to 55% by weight, and the alloy retains mechanical integrity at temperatures where most ferrous and aluminum alloys fail structurally. Nickel provides the face-centered cubic (FCC) austenitic matrix, which resists phase transformation at high temperatures and serves as the host lattice for strengthening precipitates. Chromium, present at 17 to 21%, forms a protective Cr₂O₃ oxide layer on the surface, providing oxidation and hot corrosion resistance at elevated service temperatures.
The alloy's precipitation hardening mechanism distinguishes it from simple solid-solution alloys. During aging heat treatment at temperatures from 1325°F to 1375°F (718°C to 746°C), niobium and titanium combine with nickel to form gamma double prime (γ″) precipitates (Ni₃Nb) and gamma prime (γ′) precipitates (Ni₃(Al,Ti)) within the FCC matrix. The γ″ phase contributes approximately 70% of the alloy's total strengthening response. Molybdenum additions at 2.8 to 3.3% provide additional solid-solution strengthening, raising resistance to creep deformation. The interaction among nickel, chromium, niobium, and molybdenum produces a superalloy capable of achieving yield strengths around 150 ksi (1034 MPa) in common aged conditions.
What Are the Key Characteristics of Inconel 718?
The key characteristics of Inconel 718 are listed below.
- Exceptional Mechanical Strength: Inconel 718 achieves an ultimate tensile strength of up to 185 ksi (1276 MPa) in the aged condition, making it one of the strongest precipitation-hardened nickel alloys in industrial use. The strength comes from γ″ precipitates distributed coherently within the nickel matrix, resisting dislocation movement under applied load. Aerospace turbine discs and rocket motor cases are common structural applications that depend on the material's load-bearing capacity.
- Age-Hardenable: The alloy responds to a two-stage aging heat treatment, first at 1325°F (718°C) for 8 hours, then at 1150°F (621°C) for 8 hours, which precipitates the γ″ and γ′ phases responsible for hardening. The hardness level after aging reaches 36 to 40 HRC, compared to approximately 25 HRC in the annealed condition. The process allows manufacturers to tailor mechanical properties to specific application requirements.
- Corrosion and Oxidation Resistance: Chromium content at 17 to 21% creates a self-repairing oxide layer that protects the alloy in oxidizing and mildly reducing environments. The alloy withstands exposure to chloride-bearing media, sulfurous gases, and acidic process streams encountered in chemical processing and oil and gas service. Resistance to pitting and crevice corrosion exceeds that of standard austenitic stainless steels.
- Excellent Weldability: Inconel 718 exhibits minimal strain-age cracking during welding, a problem common in other precipitation-hardened superalloys, because the γ″ phase precipitates slowly enough to avoid stress buildup in the heat-affected zone. Gas tungsten arc welding (GTAW) and electron beam welding produce sound joints without pre-heat requirements.
- High Temperature Stability: The alloy retains meaningful tensile strength up to 1300°F (704°C) and resists creep deformation at sustained elevated temperatures, with a creep rupture life exceeding 23 hours at 1200°F (649°C) under 100 ksi (689 MPa) stress.
- Hard to Machine: Inconel 718 work-hardens rapidly during cutting operations, generating cutting forces 2 to 3 times greater than those required for machining austenitic stainless steel. Carbide tooling with sharp cutting edges, low feed rates, and high-pressure coolant delivery is necessary to control tool wear and achieve an acceptable surface finish.
Can Inconel 718 Withstand Extreme Temperatures?
Yes, Inconel 718 can withstand extreme temperatures, maintaining structural integrity and meaningful mechanical properties at continuous service temperatures up to 1300°F (704°C) and short-term exposures approaching 1800°F (982°C). The alloy's face-centered cubic matrix resists phase transformation even under prolonged thermal cycling, and the “γ″ provides strengthening below roughly 1200–1300°F, but prolonged exposure can cause coarsening or phase changes that reduce strength.
Jet engine turbine discs operate continuously above 1000°F (538°C) and rely on Inconel 718's creep resistance to prevent dimensional distortion under centrifugal loading at rotational speeds exceeding 10,000 RPM. Rocket engine combustion chamber liners fabricated from the alloy endure thermal shock conditions where surface temperatures change by more than 1000°F (538°C) within milliseconds during ignition sequences. In gas turbine applications, the material's resistance to oxidation and thermal fatigue allows service intervals of several thousand hours without significant property degradation. Inconel 718 retains toughness at cryogenic temperatures as low as −423°F (−253°F), making it a material of choice for liquid hydrogen fuel system components in space launch vehicles.
What Is the Chemical Composition of Inconel 718?
The chemical composition of Inconel 718 is governed by AMS 5596, AMS 5662, and ASTM B637 specifications, which define allowable ranges for each alloying element. Tight compositional control is necessary because minor variations in niobium, titanium, or aluminum content directly affect the volume fraction of γ″ and γ′ precipitates and, therefore, the final mechanical properties after heat treatment.
The chemical composition of Inconel 718 is shown in the table below.
| Element | Percentage (%) | Function |
|---|---|---|
Element Nickel (Ni) | Percentage (%) 50.0 to 55.0 | Function The primary matrix element provides the FCC crystal structure and high temperature stability. |
Element Chromium (Cr) | Percentage (%) 17.0 to 21.0 | Function Forms a Cr₂O₃ oxide layer and supplies oxidation and hot corrosion resistance. |
Element Iron (Fe) | Percentage (%) Balance (~18.5) | Function A cost-reducing filler element contributes to matrix solid solution strengthening. |
Element Niobium + Tantalum (Nb+Ta) | Percentage (%) 4.75 to 5.50 | Function Principal γ″ precipitate former (Ni₃Nb) serves as the primary source of age hardening response. |
Element Molybdenum (Mo) | Percentage (%) 2.80 to 3.30 | Function Solid solution strengthener raises creep resistance and corrosion resistance. |
Element Titanium (Ti) | Percentage (%) 0.65 to 1.15 | Function γ′ precipitate former (Ni₃Ti) acts as a secondary age hardening contributor. |
Element Aluminum (Al) | Percentage (%) 0.20 to 0.80 | Function γ′ precipitate former functions as a minor deoxidizer during melting. |
Element Cobalt (Co) | Percentage (%) ≤ 1.00 | Function Solid solution strengthener stabilizes the microstructure at elevated temperature. |
Element Carbon (C) | Percentage (%) ≤ 0.08 | Function Carbide former supports grain boundary strengthening at trace levels. |
Element Manganese (Mn) | Percentage (%) ≤ 0.35 | Function Deoxidizer remains at low levels to preserve corrosion resistance. |
Element Silicon (Si) | Percentage (%) ≤ 0.35 | Function Deoxidizer provides minor support to oxidation resistance. |
Element Phosphorus (P) | Percentage (%) ≤ 0.015 | Function Controlled impurity requires limitation since excess reduces hot workability. |
Element Sulfur (S) | Percentage (%) ≤ 0.015 | Function Controlled impurity remains minimized to prevent hot cracking during processing. |
Element Boron (B) | Percentage (%) ≤ 0.006 | Function Grain boundary strengthener improves stress rupture ductility. |
Element Copper (Cu) | Percentage (%) ≤ 0.30 | Function Impurity element stays low to maintain corrosion resistance in reducing acids. |
How Does the Composition of Inconel 718 Affect its Performance?
The composition of Inconel 718 affects its performance through the attributes the alloy exhibits in service, from tensile strength to corrosion resistance to weldability. Niobium, at 4.75 to 5.50%, is the most critical alloying addition because it controls the formation of Ni₃Nb (γ″) precipitates, which account for the majority of the alloy's age-hardened strength. A reduction in niobium content below specification limits decreases the volume fraction of γ″, lowering yield strength by as much as 20%. Chromium at 17 to 21% governs the alloy's ability to resist oxidation and corrosion.
A continuous and adherent Cr₂O₃ oxide scale forms on the surface during high-temperature exposure at chromium concentrations above 17%, limiting oxygen diffusion into the metal. Molybdenum additions at 2.8 to 3.3% strengthen the nickel matrix through solid-solution hardening and raise resistance to pitting corrosion in chloride environments. The balance among titanium, aluminum, and niobium determines the ratio of γ′ to γ″ precipitates after aging. Higher titanium and aluminum relative to niobium shifts precipitation toward γ′, which is more stable at temperatures above 1200°F (649°C) but contributes less total strengthening at room temperature. The controlled compositional balance in Inconel 718 is what enables precipitation hardening to produce a superalloy with both high ambient-temperature strength and reliable elevated-temperature performance.
What Are the Material Properties of Inconel 718?
The material properties of Inconel 718 span mechanical, physical, and thermal characteristics that collectively define its suitability for high-performance engineering applications. Mechanical-property values generally correspond to common aged conditions of Inconel 718 unless otherwise noted.
The Material properties of Inconel 718 are shown in the table below.
| Property | Value | Description |
|---|---|---|
Property Ultimate Tensile Strength | Value 185 ksi (1276 MPa) | Description The maximum stress the alloy sustains before fracture in the aged condition |
Property 0.2% Yield Strength | Value 150 ksi (1034 MPa) | Description Stress at which 0.2% permanent plastic deformation occurs |
Property Elongation at Break | Value 12% | Description Measure of ductility: percentage of gauge length increase before fracture |
Property Reduction in Area | Value 15% | Description Cross-sectional area reduction at the fracture point: an indicator of toughness |
Property Hardness (Rockwell C) | Value 36 to 40 HRC | Description Surface hardness in the double-aged condition |
Property Elastic Modulus | Value 29.0 Msi (200 GPa) | Description Stiffness of the material under elastic loading |
Property Thermal Conductivity | Value 11.4 W/m·K | Description Rate of heat transfer through the alloy at room temperature |
Property Coefficient of Thermal Expansion | Value 13.0 µm/m·°C | Description Dimensional change per unit temperature rise (70°F to 1200°F range) |
Property Density | Value 0.296 lb/in³ (8.19 g/cm³) | Description Mass per unit volume; relevant to weight-critical aerospace design |
Property Melting Range | Value 2300 to 2437 °F (1260 to 1336°C) | Description Solidus to liquidus temperature range |
Property Specific Heat Capacity | Value 435 J/kg·K | Description Energy required to raise 1 kg of material by 1°C |
Property Electrical Resistivity | Value 1.25 µΩ·m | Description Resistance to electrical current flow at room temperature |
Property Fatigue Strength (10⁸ cycles) | Value 100 ksi (690 MPa) | Description Endurance limit under fully reversed cyclic loading |
Property Stress Rupture Life at 1200°F / 100 ksi | Value 23+ hours | Description Minimum time to rupture under sustained elevated-temperature stress |
The mechanical properties of Inconel 718 are shown in the table below.
| Property | Value | Description |
|---|---|---|
Property Ultimate Tensile Strength | Value 185 ksi (1276 MPa) | Description Peak load bearing capacity before fracture ranks among the highest of any weldable nickel superalloy. |
Property 0.2% Yield Strength | Value 150 ksi (1034 MPa) | Description The onset of permanent deformation defines design limits, where turbine discs operate at stresses below the yield threshold. |
Property Elastic Modulus | Value 29.0 Msi (200 GPa) | Description Defines rigidity under elastic load, with values comparable to low alloy steels and higher than those of titanium alloys. |
Property Elongation at Break | Value 12% | Description Confirms adequate ductility for complex formed shapes and impact-loaded structures. |
Property Fatigue Strength (10⁸ cycles) | Value 100 ksi (690 MPa) | Description Endurance limit under cyclic loading supports jet engine rotating parts with required values above 80 ksi. |
Property Creep Rupture Life (1200°F, 100 ksi) | Value 23+ hours | Description Time to failure under constant elevated temperature load serves as the governing design criterion for turbine components. |
Property Charpy Impact Energy | Value 20 ft·lbf (27 J) | Description Energy absorbed during fracture reflects notch toughness at ambient temperature. |
Property Fracture Toughness (K₁c) | Value 88 ksi·√in (97 MPa·√m) | Description Resistance to crack propagation limits failure growth from pre-existing flaws. |
The properties important in Inconel 718 are shown in the table below.
| Property | Value | Description |
|---|---|---|
Property Density (Inconel 718 density) | Value 8.19 g/cm³ (0.296 lb/in³) | Description Mass per unit volume shows moderate density relative to steels and higher density than titanium. |
Property Melting Point (Inconel melting point) | Value 2300 to 2437 °F (1260 to 1336°C) | Description The solidus to liquidus range governs casting and welding process parameters. |
Property Coefficient of Thermal Expansion | Value 13.0 µm/m·°C | Description Measured from 70°F to 1200°F (21°C to 649°C), which affects fit and clearance at temperature. |
Property Thermal Conductivity | Value 11.4 W/m·K | Description Relatively low conductivity drives heat management decisions in turbine blade design. |
Property Specific Heat Capacity | Value 435 J/kg·K | Description Energy storage per unit mass per degree supports thermal transient analysis. |
Property Electrical Resistivity | Value 1.25 µΩ·m | Description High resistivity limits electrical discharge machining efficiency. |
Property Magnetic Permeability | Value ~1.001 dimensionless | Description Effectively non-magnetic in the annealed and aged conditions. |
Property Poisson's Ratio | Value 0.284 dimensionless | Description Lateral-to-axial strain ratio under uniaxial stress. |
What Is the Density of Inconel 718?
The density of Inconel 718 measures 8.19 g/cm³ (0.296 lb/in³) at room temperature, a value that remains essentially constant across the alloy's service temperature range up to 1300°F (704°C). The density reflects the combined atomic weights and packing efficiency of its constituent elements, particularly nickel (8.9 g/cm³), iron (7.87 g/cm³), chromium (7.19 g/cm³), and niobium (8.57 g/cm³), integrated within the face-centered cubic crystal lattice.
The density of Inconel 718 directly enters specific strength calculations in aerospace design, defined as tensile strength divided by density. With a tensile strength of 185 ksi (1276 MPa) and a density of 8.19 g/cm³, the alloy achieves a specific strength of approximately 155 kN·m/kg in the aged condition. Rotating turbine disc designs account for density in centrifugal stress calculations, where higher density at the same geometry increases hoop stresses at operating speed. The density value of 8.19 g/cm³ is consistent across AMS 5596, AMS 5662, and AMS 5664 product forms, making it a reliable input for structural finite element analysis of Inconel 718 components.
How Does Inconel 718 Density Compare to Other Nickel Alloys?
Inconel 718 density compared to other Nickel Alloys through direct material comparison shows that Inconel 718 has a density of approximately 8.19 g/cm³. Inconel 718 places in the mid-range among precipitation-hardened and solid-solution nickel alloys. Hastelloy C-276 has a density of 8.89 g/cm³, approximately 8.5% denser than Inconel 718, due to its high molybdenum content of 15 to 17%. Nimonic 80A, a γ′-strengthened alloy, measures 8.19 g/cm³, essentially identical to Inconel 718. Waspaloy carries a density of 8.19 g/cm³ as well, while René 41 is slightly denser at 8.25 g/cm³.
The practical implication of density differences in nickel alloys becomes apparent in rotating component design. A turbine disc fabricated from Hastelloy C-276 at the same geometry as an Inconel 718 disc would generate approximately 8.5% greater centrifugal stress at the same rotational speed, requiring either a weight penalty or a geometry change to maintain equivalent safety factors. For weight-sensitive aerospace structures where the strength-to-density ratio governs material selection, Inconel 718 outperforms denser alloys. Xometry offers machined components from qualified nickel alloys, including Inconel 718, with density values certified to specification.
Is Inconel 718 Heavier than Stainless Steel?
Yes, Inconel 718 is heavier than most stainless steel grades. Inconel 718 has a density of 8.19 g/cm³, while 304 stainless steel measures 7.93 g/cm³, 316 stainless steel measures 7.99 g/cm³, and 17-4 PH stainless steel measures 7.78 g/cm³. The density difference from Inconel 718 to 304 stainless steel is approximately 3.3%, and from Inconel 718 to 17-4 PH, the difference increases to approximately 5.3%.
The density gap can have consequences for weight-sensitive engineering design. A component machined from Inconel 718 at the same geometry as one made from 304 stainless steel weighs roughly 3.3% more, which matters in airframe and rotating machinery weight budgets. The trade-off is substantial: Inconel 718's tensile strength of 185 ksi exceeds 304 stainless steel's 73 ksi by more than 150%, meaning the alloy delivers far greater load capacity per unit cross-section despite the weight penalty. Designers substituting Inconel 718 for stainless steel in high-temperature applications typically reduce cross-sectional area to offset weight, achieving equivalent or superior structural performance at similar mass.
What Is the Hardness of Inconel 718?
The hardness of Inconel 718 ranges from 36 to 40 HRC (approximately 350 to 390 HV) in the fully aged condition following solution annealing and double aging per AMS 2770. In the annealed condition without aging, hardness drops to approximately 24 to 26 HRC (250 to 270 HV), reflecting the absence of γ″ precipitate strengthening. The Brinell hardness of aged Inconel 718 falls in the range of 331 to 401 HB.
The hardness level correlates with machinability difficulty. The alloy generates high cutting forces and rapid tool wear rates at 36 to 40 HRC, requiring carbide or ceramic tooling with sharp geometries and aggressive coolant strategies. Cutting speeds for turning operations on aged Inconel 718 are held at 40 to 80 surface feet per minute (sfm) to control tool life. 17-4 PH stainless steel at H900 condition reaches approximately 43 HRC, while 316 stainless steel in the annealed condition measures only 79 HRB. The hardness of Inconel 718 in the aged condition establishes its wear resistance in sliding contact applications such as bearing journals and fastener threads.
How Does Heat Treatment Affect Inconel 718 Hardness?
Heat treatment affects Inconel 718 hardness by governing the nucleation, growth, and volume fraction of γ″ and γ′ precipitates within the nickel matrix. In the solution-annealed condition at 1750°F to 1800°F (955°C to 982°C), precipitates dissolve into solid solution, producing a soft, ductile microstructure with hardness near 24 to 26 HRC. Rapid cooling after solution annealing retains the supersaturated solid solution, preparing the matrix for subsequent precipitation hardening.
The double-aging heat treatment sequence prescribed by AMS 2770 begins at 1325°F (718°C) for 8 hours, followed by furnace cooling at 100°F/hour (56°C/hour) to 1150°F (621°C), then held for an additional 8 hours before air cooling. The first aging stage nucleates fine, coherent γ″ precipitates at elevated density. The second, lower-temperature stage grows the precipitates to their optimal size for maximum dislocation interaction. The combined effect raises hardness from approximately 25 HRC in the annealed state to 36 to 40 HRC after completion of both aging stages. Skipping the second aging stage produces hardness near 30 to 32 HRC, confirming that the heat treatment stages contribute independently to the final hardness level.
Can Inconel 718 Hardness be Increased Through Aging?
Yes, the hardness of Inconel 718 is increased through aging heat treatment, which precipitates strengthening phases that impede dislocation movement within the nickel matrix. The alloy holds a hardness of 24 to 26 HRC in the solution-annealed condition. After the complete double-aging cycle, hardness rises to 36 to 40 HRC, representing an increase of approximately 10 to 16 HRC points attributable entirely to precipitate formation.
The primary aging stage at 1325°F (718°C) drives nucleation of γ″ precipitates (Ni₃Nb), which are metastable disc-shaped particles approximately 20 to 30 nanometers in diameter that create coherency strains in the matrix. The secondary aging stage at 1150°F (621°C) grows the γ″ particles to their optimal size and simultaneously precipitates γ′ (Ni₃(Al,Ti)) at grain boundaries and within grains. Aging temperature above 1400°F (760°C) causes γ″ to transform to the incoherent delta (δ) phase (Ni₃Nb), which does not contribute to hardening and instead reduces strength. An aging time of 8 hours at each stage represents the industry-standard duration for producing peak hardness without precipitate overaging, as validated by the aging response documented in AMS 2770.
How Does Inconel 718 Compare to Other Inconel Alloys?
Inconel 718 compares to other Inconel Alloys through mechanical strength, heat treatment response, and service temperature capability, showing a clear distinction in performance behavior across the Inconel family. Inconel 718 occupies a specific performance position among nickel superalloys, distinguished primarily by its reliance on γ″ precipitation hardening through niobium, while other Inconel alloys derive strength through different mechanisms or compositional strategies. The comparison is most meaningful in terms of tensile strength, corrosion resistance, temperature capability, and weldability, as each alloy trades off properties to serve distinct application requirements.
Inconel 718 achieves a tensile strength of 185 ksi (1276 MPa) in the aged condition, the highest of the major Inconel alloys. Inconel 625 achieves 130 ksi (896 MPa) in the annealed condition through solid-solution strengthening, with no age-hardening response comparable to Inconel 718. Inconel 600 reaches only 80 to 100 ksi (552 to 689 MPa), reflecting its simpler binary nickel-chromium composition optimized for corrosion resistance rather than mechanical strength. Inconel X-750, a γ′-strengthened alloy, reaches 155 to 170 ksi (1069 to 1172 MPa) after aging but shows susceptibility to strain-age cracking during welding, a limitation that Inconel 718 largely avoids. Xometry machines all major Inconel alloy grades and supports material selection decisions based on application-specific property requirements.
Inconel 718 is a precipitation-hardened nickel-based superalloy designed for extreme environments requiring high strength, thermal stability, and corrosion resistance. Its nickel-chromium matrix, strengthened by niobium-based γ″ precipitates, allows it to retain tensile strength, creep resistance, and fatigue performance at temperatures up to about 1300°F. The alloy is widely used in aerospace, power generation, marine, chemical processing, and oil and gas applications where reliability is critical. It is valued for its weldability, oxidation resistance, and strong age-hardening response, though it remains difficult to machine because of rapid work hardening and high cutting forces. Its ability to perform in cryogenic, corrosive, and high-temperature conditions makes Inconel 718 one of the most important nickel superalloys in modern engineering.
What Is the Difference Between Inconel 718 and Inconel 625?
Inconel 718 and Inconel 625 serve different primary purposes: 718 is engineered for maximum strength through precipitation hardening, while 625 is engineered for maximum corrosion resistance and weldability through solid-solution strengthening. The absence of a meaningful aging response in Inconel 625 limits its strength ceiling but expands its applicability in highly corrosive and cryogenic service environments.
The difference between Inconel 718 and Inconel 625 is shown in the table below.
| Property | Inconel 718 | Inconel 625 |
|---|---|---|
Property Nickel Content | Inconel 718 50 to 55% | Inconel 625 58% minimum |
Property Chromium Content | Inconel 718 17 to 21% | Inconel 625 20 to 23% |
Property Niobium Content | Inconel 718 4.75 to 5.50% | Inconel 625 3.15 to 4.15% |
Property Molybdenum Content | Inconel 718 2.80 to 3.30% | Inconel 625 8.0 to 10.0% |
Property Iron Content | Inconel 718 Balance (~18.5%) | Inconel 625 ≤ 5.0% |
Property Strengthening Mechanism | Inconel 718 Precipitation hardening (γ″) | Inconel 625 Solid-solution strengthening |
Property Ultimate Tensile Strength | Inconel 718 185 ksi (1276 MPa) aged | Inconel 625 130 ksi (896 MPa) annealed |
Property Yield Strength | Inconel 718 150 ksi (1034 MPa) | Inconel 625 60 ksi (414 MPa) annealed |
Property Max Service Temperature | Inconel 718 1300°F (704°C) | Inconel 625 1800°F (982°C) |
Property Weldability | Inconel 718 Excellent; low-strain-age cracking | Inconel 625 Excellent weldability; many applications do not require post-weld heat treatment |
Property Corrosion Resistance | Inconel 718 High | Inconel 625 Generally superior corrosion resistance, particularly in seawater, chloride environments, and many chemical-processing systems |
Property Precipitation Hardening Response | Inconel 718 Strong γ″ hardening | Inconel 625 Negligible, not age-hardenable |
Property Primary Applications | Inconel 718 Turbine discs, fasteners, downhole tools | Inconel 625 Marine components, chemical reactor vessels, aerospace exhaust |
What Is the Difference Between Inconel 718 and Inconel 600?
Inconel 718 and Inconel 600 occupy opposite ends of the nickel superalloy spectrum. Inconel 718 prioritizes mechanical strength through precipitation hardening, while Inconel 600 prioritizes resistance to oxidation and reducing atmospheres through a simplified nickel-chromium-iron composition developed in the 1930s.
The difference between Inconel 718 and Inconel 600 is shown in the table below.
| Property | Inconel 718 | Inconel 600 |
|---|---|---|
Property Nickel Content | Inconel 718 50 to 55% | Inconel 600 72% minimum |
Property Chromium Content | Inconel 718 17 to 21% | Inconel 600 14 to 17% |
Property Iron Content | Inconel 718 Balance (~18.5%) | Inconel 600 6 to 10% |
Property Strengthening Mechanism | Inconel 718 Precipitation hardening (γ″ + γ′) | Inconel 600 Solid-solution only |
Property Ultimate Tensile Strength | Inconel 718 185 ksi (1276 MPa) aged | Inconel 600 80 to 100 ksi (552 to 689 MPa) |
Property Yield Strength | Inconel 718 150 ksi (1034 MPa) | Inconel 600 25 to 45 ksi (172 to 310 MPa) |
Property Hardness (aged/annealed) | Inconel 718 36 to 40 HRC | Inconel 600 65 to 90 HRB |
Property Max Service Temperature | Inconel 718 1300°F (704°C) | Inconel 600 2000°F (1093°C) in an oxidizing atmosphere |
Property Corrosion Resistance | Inconel 718 High in chlorides and acids | Inconel 600 Excellent in reducing and mildly oxidizing environments |
Property Weldability | Inconel 718 Excellent | Inconel 600 Good, prone to sensitization without post-weld anneal |
Property Use Cases | Inconel 718 Turbine discs, aerospace fasteners, downhole tools | Inconel 600 Heat treating equipment, nuclear reactor components, and chemical plant piping |
Property Relative Cost | Inconel 718 Higher | Inconel 600 Lower |
Is Inconel 718 Stronger than Inconel X-750?
Yes, Inconel 718 is stronger than Inconel X-750 in terms of ultimate tensile strength and yield strength in their respective aged conditions. Inconel 718 reaches an ultimate tensile strength of 185 ksi (1276 MPa) and a yield strength of 150 ksi (1034 MPa) after double aging per AMS 2770. Inconel X-750 achieves an ultimate tensile strength of 155 to 170 ksi (1069 to 1172 MPa) and a yield strength of 100 to 130 ksi (689 to 896 MPa), depending on the aging treatment
applied.
The difference originates in the heat treatment response of each alloy. Inconel 718 hardens primarily through γ″ precipitates (Ni₃Nb), which form rapidly and in high volume fraction during aging, producing greater total strengthening. Inconel X-750 is strengthened primarily through γ′ precipitates (Ni₃(Al, Ti)), which provide less total strengthening per unit volume fraction but remain stable at temperatures above 1200°F (649°C) where γ″ begins to dissolve. The Inconel X-750 retains better strength at temperatures from 1300°F to 1500°F (704°C to 816°C), making it preferable for applications at the upper end of the nickel superalloy temperature range, while Inconel 718 dominates in applications where maximum ambient and moderately elevated-temperature strength is the governing criterion.
What Are the Common Applications of Inconel 718?
The common applications of Inconel 718 are listed below.
- Applications in Aerospace Industry: Inconel 718 is a nickel-based superalloy widely used in aerospace for its exceptional resistance to high temperatures and mechanical stress. Jet engine components (turbine blades, compressor discs, and combustion chambers) rely on the alloy to maintain structural integrity at temperatures reaching 1,300°F (704°C). The material retains tensile strength up to 220,000 psi, making it a standard choice for aircraft fasteners and structural airframe parts.
- Applications in Marine and Offshore Engineering: Inconel 718 performs reliably in saltwater environments due to its resistance to chloride-induced stress corrosion cracking. Subsea components (valve bodies, pump shafts, and propeller shafts) are fabricated from the alloy to withstand pressures exceeding 15,000 psi. Its corrosion resistance in seawater temperatures ranging from 35°F to 300°F (2°C to 149°C) makes it a preferred material for offshore drilling hardware.
- Applications in Chemical Processing Industry: Inconel 718 resists degradation when exposed to aggressive chemicals (sulfuric acid, nitric acid, and hydrochloric acid) at elevated temperatures. Reactors, heat exchangers, and pressure vessels operating at temperatures up to 1,200°F (649°C) are commonly constructed from the alloy. The material's oxidation resistance and creep strength allow it to maintain dimensional stability under prolonged chemical exposure.
- Applications in Oil and Gas Industry: Inconel 718 is used extensively in downhole tooling and wellhead components due to its resistance to hydrogen embrittlement and sulfide stress cracking. Completion tools (packers, hangers, and safety valves) are manufactured from the alloy to operate in sour gas environments containing hydrogen sulfide concentrations above 0.05 psi partial pressure. The alloy meets NACE MR0175 standards, qualifying it for use in highly corrosive hydrocarbon extraction environments.
- Applications in Power Generation: Inconel 718 is used in gas turbines and nuclear reactors where components face extreme thermal cycling and radiation exposure. Turbine discs, rotor blades, and exhaust systems fabricated from the alloy operate at sustained temperatures from 900°F to 1,300°F (482°C to 704°C). Its fatigue strength of approximately 100,000 psi at elevated temperatures supports long-term reliability in continuous power generation equipment.
- Applications in Pollution Control and Environmental Systems: Inconel 718 is found in flue gas desulfurization systems and industrial scrubbers that handle corrosive byproducts from combustion processes. Components (dampers, ducting, and spray nozzles) exposed to sulfur dioxide concentrations and acidic condensates rely on the alloy's corrosion resistance at temperatures up to 1,100°F (593°C). The material's durability in oxidizing and reducing atmospheres makes it a dependable choice for emission control infrastructure in power plants and industrial facilities.
- Applications in Automotive and High-Performance Exhaust Systems: Inconel 718 is used in high-performance and motorsport applications where exhaust components are exposed to temperatures exceeding 1,200°F (649°C). Turbocharger housings, exhaust manifolds, and exhaust valves in racing engines are fabricated from the alloy to resist thermal fatigue and oxidation under repeated heat cycling. The material's yield strength of 150,000 psi at elevated temperatures provides a structural advantage over stainless steel in extreme-duty exhaust applications.
1. Applications in Aerospace Industry
Aerospace stands as the primary application sector for Inconel 718 due to its high strength, weldability, and stable performance above 1000°F (538°C). Turbine discs operate above 10,000 RPM under stresses near 100 ksi, supported by tensile strength of 185 ksi and fatigue endurance of 100 ksi at 10⁸ cycles. Aircraft components across platforms (Boeing 737, Airbus A320, F-35 Lightning II) incorporate Inconel 718 for structural reliability in combustion chambers, compressor systems, and exhaust assemblies. Rocket engines use Inconel 718 in combustion chambers, injector plates, and turbopumps due to its strength under extreme temperature variation. Space shuttle turbopumps demonstrated performance under simultaneous exposure to cryogenic liquid hydrogen at −423°F and combustion gases above 1200°F, confirming durability across extreme thermal conditions.
2. Applications in Marine and Offshore Engineering
Marine and offshore applications rely on Inconel 718 for strength under extreme pressure and resistance to seawater corrosion. Subsea components operate beyond 10,000 feet with pressures above 4500 psi, where Inconel 718 maintains structural integrity and resists chloride-induced stress corrosion cracking up to 140°F. Naval and offshore systems (riser connectors, mooring chains, propeller shafts) use Inconel 718 under tensile loads above 100 ksi in continuous seawater exposure. Offshore platforms in regions (North Sea, Gulf of Mexico) apply Inconel 718 in hydrogen sulfide environments; compliance with NACE MR0175/ISO 15156 supports sour-service use at hardness levels up to 40 HRC when properly processed. Splash zone components benefit from fatigue resistance and stable oxide protection under cyclic wave loading and atmospheric corrosion.
3. Applications in Chemical Processing Industry
Chemical processing applications rely on Inconel 718 for corrosion resistance and mechanical stability under aggressive chemical exposure and wide temperature ranges. Inconel 718 withstands nitric acid up to 200°F, phosphoric acid systems, and mixed acid streams due to chromium content of 17 to 21%, forming a stable passive oxide layer, while molybdenum at 2.8 to 3.3% improves resistance in reducing and chloride-contaminated environments. Industrial components (reactor vessels, heat exchangers, agitator shafts, pump impellers) operate from −100°F to 1200°F without loss of performance. Inconel 718 can support thinner wall sections in some designs due to high yield strength, often around 150 ksi or higher, depending on heat treatment and product form. Petrochemical equipment (distillation internals, condenser tubes, valve trim) benefits from long-term durability and resistance to corrosion-related failure.
4. Applications in Oil and Gas Industry
Oil and gas applications rely on Inconel 718 for strength and corrosion resistance under high pressure, high temperature, and sour service conditions. Inconel 718 meets NACE MR0175/ISO 15156 requirements at a maximum hardness of 40 HRC, supporting use in completion tools, packers, and safety valves exposed to H₂S-bearing fluids. Downhole systems operate near 350°F and above 15,000 psi, where Inconel 718 maintains dimensional stability during thermal and mechanical cycling. Components (coiled tubing connectors, perforating guns, lock mandrels) endure repeated stress, supported by fatigue resistance near 100 ksi at 10⁸ cycles. Xometry produces Inconel 718 parts to API tolerances for oil and gas procurement, ensuring precision and reliability in high-performance environments.
5. Applications in Power Generation
Power generation applications use Inconel 718 for long-term performance under high temperature and mechanical loading. Gas turbines operate at 1200°F to 1400°F, where Inconel 718 maintains creep resistance and dimensional stability across service intervals near 25,000 hours. Nuclear systems apply Inconel 718 in reactor internals exposed to neutron radiation, pressurized water near 600°F, and boric acid environments, where resistance to stress corrosion cracking supports safety-critical performance under ASME Code Case N-60. Steam turbines in fossil-fuel plants use Inconel 718 for blade rings, spacer rings, and bolting due to tensile strength retention and oxidation resistance. Power generation represents a major non-aerospace sector for Inconel 718 wrought materials.
6. Applications in Pollution Control and Environmental Systems
Pollution control systems rely on Inconel 718 for durability under high temperature and corrosive chemical exposure. Flue gas desulfurization scrubbers operate at 140°F to 200°F in sulfuric acid slurries, where Inconel 718 resists chloride pitting and acid attack that damages 316L stainless steel. Selective catalytic reduction systems may use high-temperature nickel alloys where thermal stability and oxidation resistance are required. Incinerator afterburner chambers exceed 1800°F, where Inconel 718 supports structural brackets and heat shields during short-term thermal spikes. Environmental regulations across the United States and the European Union drive continued use of high-performance alloys in emission control equipment.
7. Applications in Automotive and High-Performance Exhaust Systems
Automotive and motorsport systems use Inconel 718 for extreme temperature, pressure, and rotational demands that exceed stainless steel limits. Turbocharger turbine wheels can operate at rotational speeds exceeding 100,000 RPM and experience very high exhaust gas temperatures approaching 1700°F in extreme applications where Inconel 718 maintains strength and oxidation resistance. Exhaust manifolds in endurance racing withstand repeated thermal cycles without cracking, where Inconel 718 offers improved thermal fatigue and high-temperature strength compared with common stainless steel exhaust alloys such as 304 or 321. High-performance applications (Formula racing, Le Mans prototypes, motorcycle exhaust systems, jet-powered land vehicles) rely on Inconel 718 for durability and thermal fatigue resistance. Xometry produces precision Inconel 718 turbocharger components and exhaust fittings for motorsport and aerospace ground support use.
Can Inconel 718 be Used in Corrosive Environments?
Yes, Inconel 718 can be used in corrosive environments, including selected seawater, sour-service, acidic, and high-temperature oxidizing environments when properly specified and qualified. The alloy's corrosion resistance originates from its chromium content at 17 to 21%, which forms a self-repairing Cr₂O₃ passive oxide layer that limits the rate of electrochemical attack at the metal surface. Molybdenum additions at 2.8 to 3.3% enhance resistance to pitting and crevice corrosion in chloride-containing solutions, raising the critical pitting temperature (CPT) above that of standard austenitic stainless steels.
Inconel 718 resists chloride stress corrosion cracking at temperatures up to 140°F (60°C) in seawater service, a failure mode responsible for catastrophic fracture of 300-series stainless steel components in offshore applications. In sour oil and gas service per NACE MR0175, the alloy may qualify for sour service under NACE MR0175 / ISO 15156 when hardness, heat treatment, and environmental limits meet the standard. In chemical processing, the material withstands nitric acid concentrations up to 50% at temperatures below 160°F (71°C) and phosphoric acid environments across wide concentration ranges. Xometry fabricates Inconel 718 components for corrosive service applications and supports corrosive environment material qualification documentation for oil and gas, marine, and chemical processing programs.
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