Hangbo Alloy Group
Shanghai · Est. 2012
310S (UNS S31008, ASTM A240) vs Inconel 718 (UNS N07718, AMS 5662) — a head-to-head technical comparison of chemical composition, mechanical properties, oxidation resistance, creep strength, weldability, and cost for high-temperature material selection.
When engineers need a material that can handle high temperatures, the conversation usually comes down to a fundamental question: do you need oxidation resistance or mechanical strength? That is essentially the choice between 310S and Inconel 718 — two alloys that are both classified as "high-temperature" but serve completely different purposes.
310S (UNS S31008) is an austenitic stainless steel specified under ASTM A240, known for its outstanding oxidation resistance up to 1100°C and relatively low cost. It is the workhorse of furnace and heat exchanger construction worldwide. Inconel 718 (UNS N07718), specified under AMS 5662 and ASTM B637, is a precipitation-hardened nickel-based superalloy that maintains exceptional mechanical strength up to 700°C. It is the most widely used superalloy in aircraft gas turbine engines.
These two alloys are not really competitors — they are complementary tools in the high-temperature engineer's toolkit. But if you are selecting between them for a specific application, you need to understand exactly where each one excels and where it falls short. This article provides a comprehensive, data-driven comparison based on ASTM and AMS standard data.
The fundamental difference between these two alloys starts at the chemistry. 310S is an iron-based austenitic stainless steel — iron is the matrix, with chromium and nickel providing oxidation resistance and austenite stability. Inconel 718 is a nickel-based superalloy — nickel is the matrix, with a carefully balanced cocktail of niobium, molybdenum, titanium, and aluminum that form strengthening precipitates during heat treatment.
| Element | 310S (ASTM A240) | Inconel 718 (AMS 5662) | Key Difference |
|---|---|---|---|
| Nickel (Ni) | 19.0 – 22.0% | 50.0 – 55.0% | 718 has 2.5x more Ni |
| Chromium (Cr) | 24.0 – 26.0% | 17.0 – 21.0% | 310S has higher Cr for oxidation |
| Iron (Fe) | Balance | Balance (~18%) | 310S is Fe-based; 718 is Ni-based |
| Niobium (Nb+Ta) | — | 4.75 – 5.50% | 718 only; forms γ'' phase |
| Molybdenum (Mo) | — | 2.80 – 3.30% | 718 only; solid-solution + γ'' |
| Titanium (Ti) | — | 0.65 – 1.15% | 718 only; forms γ' phase |
| Aluminum (Al) | — | 0.20 – 0.80% | 718 only; forms γ' phase |
| Carbon (C) | ≤ 0.08% | ≤ 0.08% | Both low-carbon |
| Manganese (Mn) | ≤ 2.00% | ≤ 0.35% | 310S allows more Mn |
| Silicon (Si) | ≤ 1.00% | ≤ 0.35% | 310S allows more Si |
| Phosphorus (P) | ≤ 0.045% | ≤ 0.015% | 718 tighter P control |
| Sulfur (S) | ≤ 0.030% | ≤ 0.015% | 718 tighter S control |
| Cobalt (Co) | — | ≤ 1.0% | 718 limits Co |
| Boron (B) | — | ≤ 0.006% | 718 only; grain boundary |
The composition tells the whole story. 310S is a straightforward Cr-Ni austenitic alloy — its chemistry is simple, its production is well-established, and its cost is moderate. Inconel 718's chemistry is far more complex, with niobium as the star player. Niobium forms the gamma-double-prime (Ni3Nb) phase, which is the primary strengthening mechanism in Inconel 718 and is largely responsible for its extraordinary strength. The tighter control on phosphorus, sulfur, and other trace elements in AMS specifications reflects the higher quality requirements for aerospace applications.
| Property | 310S (ASTM A240) | Inconel 718 (AMS 5662) |
|---|---|---|
| Density | 7.98 g/cm³ (0.288 lb/in³) | 8.19 g/cm³ (0.296 lb/in³) |
| Melting Range | 1400 – 1450°C (2550 – 2640°F) | 1260 – 1336°C (2300 – 2437°F) |
| Modulus of Elasticity (RT) | 193 GPa (28.0 × 10⁶ psi) | 200 GPa (29.0 × 10⁶ psi) |
| Modulus of Elasticity (540°C) | ~155 GPa | ~170 GPa |
| Mean CTE (20–100°C) | 15.9 µm/m·°C | 13.0 µm/m·°C |
| Mean CTE (20–800°C) | 18.7 µm/m·°C | 14.7 µm/m·°C |
| Thermal Conductivity (RT) | 14.2 W/m·K | 11.4 W/m·K |
| Thermal Conductivity (800°C) | 21.5 W/m·K | 22.6 W/m·K |
| Specific Heat (RT) | 502 J/kg·K | 435 J/kg·K |
| Electrical Resistivity (RT) | 0.78 µΩ·m | 1.25 µΩ·m |
A few observations worth noting: 310S has a notably higher thermal expansion coefficient than Inconel 718 (18.7 vs 14.7 µm/m·°C at 800°C). This means 310S components will grow more when heated — a critical factor in tight-clearance designs and thermal cycling applications. 310S also has better thermal conductivity at room temperature, which helps with heat transfer in heat exchanger applications but can lead to faster heat loss in furnace structures. Inconel 718's lower and more stable expansion behavior is one reason it is preferred for precision aerospace components.
This is where the comparison gets dramatic. The strength difference between these two alloys is enormous — not because 310S is a bad material, but because it is not designed for strength. 310S is an annealed austenitic stainless steel that gets its properties from solid-solution strengthening alone. Inconel 718, after precipitation hardening, achieves strength levels that approach those of tool steels.
| Property | 310S (Annealed, ASTM A240) | Inconel 718 (Aged, AMS 5662) | Ratio |
|---|---|---|---|
| Tensile Strength | ≥ 515 MPa (75 ksi) | ≥ 1241 MPa (180 ksi) | 2.4x |
| Yield Strength (0.2%) | ≥ 205 MPa (30 ksi) | ≥ 1034 MPa (150 ksi) | 5.0x |
| Elongation in 50 mm | ≥ 40% | ≥ 12% | 310S is 3.3x more ductile |
| Hardness | ≤ 95 HRB (~200 HB) | 331 – 415 HB | 718 much harder |
The yield strength of Inconel 718 is five times that of 310S. That is not a typo — five times. Inconel 718 achieves this through precipitation hardening: aging at 720°C followed by 620°C produces a bimodal distribution of gamma-prime (Ni3(Al,Ti)) and gamma-double-prime (Ni3Nb) precipitates that effectively block dislocation movement. 310S, in contrast, has no such mechanism available.
| Temperature | 310S Tensile (MPa) | 718 Tensile (MPa) | 310S Yield (MPa) | 718 Yield (MPa) |
|---|---|---|---|---|
| 21°C (RT) | 515 | 1241 | 205 | 1034 |
| 540°C | ~390 | ~1100 | ~150 | ~950 |
| 650°C | ~310 | ~1000 | ~130 | ~900 |
| 800°C | ~180 | ~500 | ~100 | ~400 |
| 1000°C | ~60 | ~100 | ~35 | ~70 |
Even at 800°C, Inconel 718 maintains yield strength of approximately 400 MPa — roughly double what 310S achieves at room temperature. However, by 1000°C, both alloys have lost most of their mechanical strength, and the comparison shifts to oxidation resistance.
Here is where 310S fights back. While Inconel 718 wins the strength contest, 310S is the clear winner in oxidation resistance at extreme temperatures. The reason is simple: 310S has 24-26% chromium, which forms a stable Cr2O3 protective scale that remains effective up to approximately 1100°C. Inconel 718's lower chromium content (17-21%) means its protective scale is less robust at these extreme temperatures.
| Oxidation Parameter | 310S | Inconel 718 |
|---|---|---|
| Continuous Service (Oxidation) | Up to 1100°C (2010°F) | Up to ~980°C (1795°F) |
| Intermittent Service | Up to 1150°C (2100°F) | Up to ~1050°C (1920°F) |
| Protective Oxide Formed | Cr2O3 | Cr2O3 + NiO + Al2O3 |
| Scale Spallation Resistance | Good | Moderate |
| Carburization Resistance | Good | Moderate |
| Sulfidation Resistance | Moderate | Good (higher Ni helps) |
For furnace parts, burner nozzles, heat treatment baskets, and other components that spend their lives at 800-1100°C with minimal mechanical load, 310S is the superior choice. Its thicker, more stable Cr2O3 scale provides long-term protection with minimal material loss. Inconel 718, while it can survive short-term high-temperature excursions, is not optimized for continuous service above 950°C.
Creep resistance — the ability to resist slow deformation under sustained load at high temperature — is another area where Inconel 718 dominates, but only within its effective temperature range.
| Temperature | 310S 1000-h Rupture (MPa) | 718 1000-h Rupture (MPa) | Advantage |
|---|---|---|---|
| 540°C | ~100 | ~700 | 718 is 7x stronger |
| 650°C | ~50 | ~400 | 718 is 8x stronger |
| 760°C | ~20 | ~100 | 718 is 5x stronger |
| 870°C | ~8 | ~30 | 718 is ~4x stronger |
For any application that carries significant mechanical load at 540-760°C (gas turbine discs, pressure vessels, structural supports), Inconel 718 is in a completely different league. Its gamma-prime and gamma-double-prime precipitates effectively pin dislocations and resist grain boundary sliding, the primary creep mechanism. 310S, being solid-solution strengthened only, has no such defense and creeps relatively quickly under sustained load at elevated temperatures.
| Welding Parameter | 310S | Inconel 718 |
|---|---|---|
| Overall Weldability | Good Easier | Fair to Good |
| Recommended Process | TIG, MIG, SMAW | TIG, EBW, friction welding |
| Filler Metal | ER310, ER308 (dissimilar) | ERNiFeCr-2 (Inconel 718) |
| Preheat Required | Generally no | No (solution annealed condition) |
| Post-Weld Heat Treatment | Not typically required | Required (solution + aging) |
| Hot Cracking Risk | Moderate (control dilution) | Low (microfissuring concern) |
| Strain-Age Cracking | Not applicable | Low (better than other γ' alloys) |
310S is a straightforward stainless steel to weld — standard processes, common filler metals, minimal post-weld treatment. The main concern with 310S is avoiding sensitization (chromium carbide precipitation at grain boundaries) by keeping interpass temperatures controlled and using low-carbon filler metals. Inconel 718 is one of the better-welded nickel superalloys, but it requires more attention: the component should be in the solution-annealed condition before welding, heat input must be controlled, and a full post-weld solution + aging treatment is needed to restore precipitation-hardened properties.
| Application Area | 310S | Inconel 718 |
|---|---|---|
| Furnace Components (800-1100°C) | Excellent Preferred | Over-specified |
| Heat Exchanger Tubes | Excellent Preferred | Over-specified |
| Gas Turbine Discs (to 650°C) | Inadequate strength | Excellent Preferred |
| Aerospace Engine Parts (to 700°C) | Inadequate | Excellent Preferred |
| Cryogenic Components | Good (non-magnetic) | Excellent Preferred |
| Petrochemical Furnace Tubes | Good Common | Used (high stress areas) |
| Nuclear Components | Limited use | Good Used |
| High-Strength Fasteners (to 650°C) | Inadequate | Excellent Preferred |
| Burner Nozzles & Radiant Tubes | Excellent Preferred | Over-specified |
| Cryogenic Storage Tanks | Good | Excellent Used |
| Cost Factor | 310S | Inconel 718 |
|---|---|---|
| Relative Material Cost (per kg) | 1.0 × (baseline) | 3.0 – 5.0 × 310S cheaper |
| Raw Material Drivers | Cr 24-26%, Ni 19-22% | Ni 50-55%, Nb, Mo, Ti, Al |
| Heat Treatment Cost | Minimal (annealing only) | Significant (solution + aging) |
| Machining Cost | Moderate (work-hardens) | High (very hard after aging) |
| Availability | Widely available Better | Available (aerospace supply chain) |
| Lead Time | Short (stock items) | Medium to long (certified material) |
The cost difference is significant. Inconel 718's higher nickel content, the addition of expensive niobium and molybdenum, the required precipitation-hardening heat treatment, and the more stringent aerospace quality assurance (AMS specifications, full traceability, material certifications) all contribute to a material cost that is typically 3-5 times that of 310S. When machining and heat treatment costs are factored in, the total component cost difference can be even larger.
Here is the bottom line, broken down by application type:
| Property | 310S (UNS S31008) | Inconel 718 (UNS N07718) | Winner |
|---|---|---|---|
| Standard | ASTM A240 | AMS 5662 / ASTM B637 | — |
| Alloy Type | Austenitic Stainless Steel | Ni-Based Precipitation-Hardened | — |
| Density (g/cm³) | 7.98 | 8.19 | 310S (lighter) + |
| Melting Range (°C) | 1400-1450 | 1260-1336 | 310S (higher) + |
| RT Yield Strength (MPa) | ≥ 205 | ≥ 1034 | 718 (5x) + |
| RT Tensile Strength (MPa) | ≥ 515 | ≥ 1241 | 718 (2.4x) + |
| Elongation (%) | ≥ 40 | ≥ 12 | 310S (ductile) + |
| Max Oxidation Temp (°C) | 1100 | ~980 | 310S + |
| Creep at 650°C | Low | Excellent | 718 + |
| Weldability | Good | Fair to Good | 310S + |
| Relative Cost | 1.0 × | 3-5 × | 310S + |
| Availability | Excellent | Good | 310S + |
Hangbo Alloy Group is an ISO 9001 certified manufacturer and supplier of 310S stainless steel and Inconel 718 nickel superalloy products. We stock a full range of plate, bar, pipe, tube, sheet, strip, and forgings in standard and custom dimensions. All material shipped with full mill test certificates (MTC) per ASTM/AMS standards.
Response within 24 hours • ISO 9001:2015 Certified • Worldwide Shipping
310S can operate continuously up to approximately 1100°C (2010°F) in oxidizing atmospheres per ASTM A240, and up to 1150°C for intermittent exposure. Inconel 718 is rated for structural load-bearing service up to approximately 650-700°C (1200-1300°F) per AMS 5662, though it can withstand short-term oxidation exposure up to ~1200°C. The key distinction: 310S excels in unloaded high-temperature environments (furnace parts, heat exchangers), while Inconel 718 maintains far superior mechanical strength under load at temperatures up to 700°C.
310S (UNS S31008, ASTM A240) is an austenitic stainless steel: Cr 24-26%, Ni 19-22%, C max 0.08%, Fe balance. Inconel 718 (UNS N07718, AMS 5662) is a nickel-based precipitation-hardened superalloy: Ni 50-55%, Cr 17-21%, Fe balance, Nb 4.75-5.50%, Mo 2.8-3.3%, Ti 0.65-1.15%, Al 0.20-0.80%. The critical difference is that Inconel 718 contains niobium, molybdenum, titanium, and aluminum for gamma-prime and gamma-double-prime precipitation strengthening, while 310S is purely solid-solution strengthened by chromium and nickel in the austenitic matrix.
Inconel 718 is dramatically stronger than 310S at all temperatures. At room temperature, aged Inconel 718 achieves yield strength of 1034+ MPa vs only 205 MPa for annealed 310S — a 5:1 ratio. At 650°C, Inconel 718 retains yield strength of approximately 900 MPa, while 310S drops to roughly 130 MPa. Even at 800°C, Inconel 718 outperforms 310S in load-bearing capability. However, 310S has better oxidation resistance above 1000°C because its high chromium content (24-26%) forms a more stable Cr2O3 protective scale.
310S is covered under ASTM A240/A240M (plate, sheet, strip), ASTM A276 (bar), ASTM A479 (bar for pressure vessel), and EN 10095 (European heat-resistant steels). It is designated UNS S31008. Inconel 718 is covered under AMS 5662 (bars, forgings, rings), AMS 5663, AMS 5664 (high-temperature processing), ASTM B637 (bars and forgings), and is designated UNS N07718 / W.Nr. 2.4668. Both alloys are widely specified in ASME, API, and NACE standards for their respective application domains.
310S is generally easier to weld than Inconel 718. 310S is an austenitic stainless steel with good weldability using standard TIG, MIG, and SMAW processes with matching ER310 filler. The main concern is avoiding sensitization through proper interpass temperature control. Inconel 718, while considered one of the better-weldable nickel superalloys, requires more careful procedures: solution annealing before welding, controlled heat input, and post-weld aging treatment to restore precipitation-hardened properties. Both are weldable, but 310S welding is simpler and more forgiving.
Inconel 718 is typically 3 to 5 times more expensive than 310S per kilogram. The cost difference reflects the nickel content (50-55% vs 19-22%), the addition of expensive elements like niobium and molybdenum, the complex precipitation-hardening heat treatment required for Inconel 718, and the more demanding quality assurance for aerospace-grade material. For applications where 310S meets the requirements, it is the more economical choice. Inconel 718 is justified when its superior high-temperature mechanical strength and creep resistance are essential.
For furnace components such as heat treatment baskets, radiant tubes, and burner parts, 310S is usually the better choice. Furnace applications typically involve high temperatures (800-1100°C) with relatively low mechanical loads, where 310S's excellent oxidation resistance and lower cost are advantageous. Inconel 718 is over-specified for these non-structural high-temperature applications. However, if the furnace component also bears significant mechanical load at elevated temperature (such as a load-bearing support beam operating above 650°C), Inconel 718 may be warranted.
Inconel 718 is overwhelmingly the preferred choice for aerospace gas turbine components. It is the most widely used nickel superalloy in aircraft engines, accounting for over 30% of total engine weight in modern designs. Its precipitation-hardened strength (yield 1034+ MPa), excellent creep resistance up to 700°C, fatigue resistance, and good weldability make it ideal for turbine discs, compressor cases, engine mounts, and fasteners. 310S is not used in aerospace gas turbine hot sections due to its insufficient high-temperature mechanical strength.
310S has a density of 7.98 g/cm³ (0.288 lb/in³), while Inconel 718 has a density of 8.19 g/cm³ (0.296 lb/in³). Inconel 718 is approximately 2.6% denser than 310S due to its higher nickel content and the presence of heavy elements like niobium and molybdenum. This small density difference is rarely a decisive factor in material selection between these two alloys.
310S has a melting range of approximately 1400-1450°C (2550-2640°F), which is higher than Inconel 718's melting range of 1260-1336°C (2300-2437°F). Despite its lower melting point, Inconel 718 maintains much better mechanical strength at elevated temperatures due to its gamma-prime and gamma-double-prime precipitation strengthening. The higher melting point of 310S contributes to its excellent dimensional stability in high-temperature oxidation environments.
No. 310S is an austenitic stainless steel that cannot be strengthened by heat treatment. It is supplied in the annealed condition and its strength is determined by its solid-solution composition and grain size. Cold working can increase its strength but at the cost of ductility, and the strengthening effect is lost at elevated temperatures. Inconel 718, by contrast, achieves its remarkable strength through precipitation hardening (aging at 720-760°C), which produces gamma-prime and gamma-double-prime precipitates that provide dislocation strengthening. This fundamental metallurgical difference is why Inconel 718 is 5 times stronger than 310S at room temperature.
310S is available in a wide range of product forms: plate (ASTM A240), sheet, strip, bar (ASTM A276, A479), seamless and welded pipe/tube (ASTM A312, A213), and forgings. It is one of the most readily available heat-resistant stainless grades globally. Inconel 718 is available as bar and forging (AMS 5662, ASTM B637), sheet and plate (AMS 5596), wire (AMS 5832), and investment castings (AMS 5383). Both alloys are available from Hangbo Alloy Group in standard and custom dimensions.