In 25 years of forging high-temperature superalloys, we have been asked this question more often than any other: “Should I use Grade 660 or Inconel 718?” This guide gives you the complete answer — metallurgy, real-world cost, machinability, and exactly when each alloy wins.
Both ASTM A453 Grade 660 (Alloy A286) and Inconel 718 are precipitation-hardenable superalloys capable of performing where carbon steel fails and stainless steel gives up. But they serve different performance envelopes, are manufactured differently, and are priced worlds apart. Specifying the wrong one means either paying 4–6× too much, or choosing a material that softens at the temperatures your application demands.
Why This Comparison Actually Matters
Grade 660 (UNS S66286) and Inconel 718 (UNS N07718) are often grouped together in procurement databases as “precipitation-hardened superalloys.” That categorization is technically accurate and practically useless. It is like comparing a heavy-duty industrial boiler pump and a jet engine turbine because both move fluid under pressure.
The real distinction comes down to three factors: base metal cost, temperature ceiling, and fabrication behavior. Grade 660 is iron-based — roughly 54% iron — making it significantly more affordable than nickel-based IN-718. But that iron foundation also limits its peak service temperature to 700°C. Inconel 718’s nickel-rich matrix holds its strength to nearly 980°C — some 280°C higher.
For applications between 400°C and 700°C — which describes the majority of industrial turbine hardware, pressure vessel bolting, and oil-and-gas valve work — Grade 660 delivers equivalent performance at 20–25% of the cost of Inconel 718. Above 700°C, the calculation shifts decisively in Inconel 718’s favor.
At least 60% of the projects we see that specify Inconel 718 could have used Grade 660 with identical performance results. The over-specification typically traces back to a conservative engineering call made without a formal review of actual service temperatures and ASTM A453 stress rupture data.
Alloy DNA: Composition & Origin
Both alloys achieve strength through precipitation hardening — a heat treatment that causes fine intermetallic particles to form within the crystal structure. But the hardening phases and the matrix surrounding them are fundamentally different.
| Element / Property | Grade 660 (A286) — UNS S66286 | Inconel 718 — UNS N07718 |
|---|---|---|
| Base Metal | Iron (~54% Fe) | Nickel (~52% Ni) |
| Nickel (Ni) | 24–27% | 50–55% |
| Chromium (Cr) | 13.5–16% | 17–21% |
| Titanium (Ti) | 1.90–2.35% | 0.65–1.15% |
| Niobium (Nb) | Not present | 4.75–5.50% (key differentiator) |
| Molybdenum (Mo) | 1.00–1.50% | 2.80–3.30% |
| Iron (Fe) | Balance (~54%) — cost advantage | 17–19% max |
| Primary Hardening Phase | γ′ — Ni&sub3;(Ti,Al) | γ″ — Ni&sub3;Nb (+ secondary γ′) |
| Magnetic in service? | No — fully austenitic | No — fully austenitic |
Grade 660 strengthens via gamma-prime (γ′) precipitates — Ni&sub3;(Ti,Al) particles that form when aged at 720°C after solution treatment. These precipitates coarsen above approximately 700°C, causing gradual strength loss. This coarsening temperature is the fundamental service ceiling of Grade 660.
Inconel 718 strengthens primarily via gamma-double-prime (γ″) precipitates — Ni&sub3;Nb particles enabled by its high niobium content. IN-718’s nickel-rich matrix extends useful service to 980°C, but requires a more complex double-aging heat treatment cycle.
Grade 660 (Alloy A286) was engineered in the 1950s to match the thermal expansion coefficient of austenitic stainless steels. At 16.6 µm/m·°C, it expands at virtually the same rate as 316L SS (16.5 µm/m·°C) — preventing joint loosening during thermal cycling. Inconel 718 expands at 13.0 µm/m·°C, a different rate that causes differential movement between bolt and flange.
Temperature Performance: Where Each Alloy Lives
Temperature capability is the primary axis on which these alloys diverge. The ranges below represent sustained service temperatures under load — not brief excursions, but conditions the part must reliably maintain for thousands of operating hours.
Grade 660 does not fail abruptly at 701°C. The γ′ precipitates coarsen gradually above 700°C, causing progressive loss of tensile strength and creep resistance. Engineers designing for peak temperatures near 650–700°C should use ASTM A453 stress rupture test data (100 hours at 650°C under specified load) — not room-temperature tensile values.
At the cryogenic end, Grade 660 Class D is widely used for LNG equipment to −196°C (Charpy impact ≥70 J required). For liquid hydrogen applications to −253°C (space launch systems), Inconel 718 is the appropriate choice.
Inconel 718 accounts for approximately 34% of total alloy weight in a typical turbofan jet engine. Its reliable strength retention to 980°C and consistent behavior under AMS-specified double-aging makes it the standard engineering choice for any aerospace or gas turbine hot-section application above 700°C. No iron-based alloy competes at those temperatures.
Mechanical Properties Side by Side
| Mechanical Property | Grade 660 — Class B | Inconel 718 (AMS 5662, aged) |
|---|---|---|
| Tensile Strength (min, RT) | 1035 MPa (150 ksi) | 1275 MPa (185 ksi) Stronger |
| Yield Strength (0.2%, min) | 759 MPa (110 ksi) | 1034 MPa (150 ksi) Higher |
| Elongation at Break (min) | 25% More Ductile | 12% |
| Reduction of Area (min) | 40% More Ductile | 15% |
| Hardness Range | 30–36 HRC | 36–44 HRC |
| Density | 7.94 g/cm³ Lighter | 8.19 g/cm³ |
| Elastic Modulus (RT) | 200 GPa | 200 GPa Equal |
| Thermal Expansion (20–600°C) | 16.6 µm/m·°C Matches 316SS | 13.0 µm/m·°C |
| Stress Rupture Certification | 100h @ 650°C — ASTM required | Per AMS spec (varies) |
| Fatigue Strength (RT) | ~550 MPa | ~700 MPa Higher |
| Charpy Impact (−100°C, Class B) | ≥40 J Specified | ~80–120 J |
| Non-magnetic in service | Yes — austenitic | Yes — austenitic Both |
Grade 660 is the more ductile alloy by a wide margin. Elongation of 25% (vs. 12% for IN-718) and reduction of area of 40% (vs. 15%) mean Grade 660 tolerates overload conditions and stress concentrations more gracefully — a genuine engineering advantage for pressure vessel bolting where controlled plastic deformation may be preferable to brittle fracture.
Machinability & Forging Behavior
Both alloys require carbide tooling, slow cutting speeds, and high coolant flow compared to stainless steels. But the gap between them in practice directly affects part cost and delivery lead times.
For every 10 hours of CNC machining time on a Grade 660 part, expect 16–22 hours on a geometrically equivalent Inconel 718 part. That gap alone materially shifts project economics on large or complex forgings.— Jiangsu Liangyi Engineering Team, based on 25 years of production records
| Manufacturing Factor | Grade 660 | Inconel 718 |
|---|---|---|
| Hot Forging Window | 980–1150°C (wider) | 954–1120°C |
| Work Hardening Rate | Moderate — fewer reheats | High — more reheats required |
| CNC Turning Speed | 15–25 m/min | 8–15 m/min |
| Relative Machining Cost | 1× baseline | 1.6–2.2× Grade 660 |
| Heat Treatment Process | Solution anneal + single age | Solution + double age |
| Typical Forging Lead Time | 4–6 weeks | 6–10 weeks |
We supply Grade 660 forgings in solution-treated (pre-aged) condition for customers who weld subassemblies before final precipitation hardening. This allows the complete welded assembly to undergo a single simultaneous aging cycle — producing a uniform microstructure across all joined parts. For full dimensional options, Class A/B/C/D heat treatment details, and available shapes, see our ASTM A453 Grade 660 forging specifications and dimensions. Contact our engineering team to discuss the pre-aged supply option.
Corrosion & Oxidation Resistance
| Service Environment | Grade 660 Performance | Inconel 718 Performance |
|---|---|---|
| High-Temp Oxidation (below 700°C) | Excellent | Excellent — both equal |
| High-Temp Oxidation (700–980°C) | Not suitable — softens | Excellent |
| Seawater / Chloride Environments | Good | Superior |
| H&sub2;S Sour Service (NACE MR0175) | Class B ≤35 HRC ✓ Compliant | Possible with hardness verification |
| ASME Pressure Code Applications | ASME SA453 — direct adoption | No ASME pressure vessel code version |
| Non-magnetic requirement | Both qualify — austenitic | Both qualify — austenitic |
For NACE MR0175 / ISO 15156 sour-service applications, Grade 660 Class B is the well-established specification path. Final part hardness must be tested and confirmed at ≤35 HRC — within the Class B range, but must be explicitly documented. Grade 660 Class D is excluded from sour service because its direct-aging process may produce hardness above the NACE limit.
Cost & Supply Chain Reality
The cost economics are driven by one fundamental factor: nickel content. Inconel 718 contains approximately 52% nickel. Grade 660 contains 24–27% nickel. Doubling nickel content roughly doubles the ingot cost before any processing premium is applied. When machining time, heat treatment complexity, and lead time are factored in, the total cost difference becomes 4–6×.
Nickel prices can swing 30–50% within a single year (as demonstrated in March 2022, when LME nickel prices doubled in 24 hours). Grade 660’s lower nickel content makes it significantly less exposed to these commodity price shocks — a meaningful supply chain resilience advantage on long-cycle capital projects.
Application Fit by Industry
Grade 660 (ASTM A453 / Alloy A286) — Primary Industrial Applications
Inconel 718 — Primary Aerospace & Extreme-Temperature Applications
Decision Guide: Which to Specify
- Operating temperature stays below 700°C sustainably
- Application falls under ASTM or ASME pressure codes
- Budget is real — 4–6× cost premium not justifiable
- Bolting into austenitic stainless steel flanges (matched expansion)
- NACE MR0175 sour service required (Class B, verify ≤35 HRC)
- Cryogenic service to −196°C (Class D for LNG)
- 100-hour stress rupture certification required by spec
- Power generation, oil & gas, petrochemical, nuclear auxiliary
- Operating temperature exceeds 700°C under sustained load
- Application governed by AMS aerospace specifications
- Maximum strength-to-weight ratio is essential
- Jet engine or defense OEM drawing explicitly specifies IN-718
- Hot chloride or high-sulfur gases above 700°C
- High cyclic fatigue loading — 700 MPa fatigue strength needed
- Budget supports 4–6× total part cost premium
Grade 660 is rated to 700°C and is widely used across this range in thermal power plants and industrial gas turbines. The key variable is loading type: static sustained load (where ASTM A453 Class A, B, or C with 100-hour stress rupture certification is appropriate) versus cyclic fatigue loading (where Inconel 718’s higher fatigue strength may justify the cost premium). Our engineering team provides complimentary application review for projects in this range — contact us before finalizing your specification.
The Forger’s Verdict
After 25 years of producing both alloys for clients across more than 50 countries, our assessment is this: Grade 660 is the correct answer for the majority of industrial high-temperature applications. It is not a compromise — it is a precisely engineered alloy purpose-built for exactly the service conditions that define most power generation, oil and gas, petrochemical, and process industry work.
Inconel 718 is genuinely irreplaceable for aerospace, defense, and any application with sustained temperatures above 700°C. Specifying it below 700°C for cost-sensitive industrial work is one of the most common and expensive over-engineering decisions we observe. The performance difference between Grade 660 Class B and Inconel 718 at 500°C is negligible. The cost difference is 4–6×.
| Decision Factor | Winner | Engineering Notes |
|---|---|---|
| Temperature Ceiling | Inconel 718 980°C | 280°C advantage over Grade 660 in sustained service |
| Tensile Strength (RT) | Inconel 718 1275 MPa | ~23% stronger at room temperature |
| Ductility / Elongation | Grade 660 25% | More forgiving under overload; ideal for pressure vessel bolting |
| Raw Material Cost | Grade 660 3–4× Cheaper | Lower nickel content is the primary cost driver |
| Total Part Cost | Grade 660 4–6× Cheaper | Machining + heat treatment + lead time compound the gap |
| Machinability | Grade 660 1.6–2.2× Faster | Direct impact on CNC cycle time and tooling cost |
| Thermal Expansion Match (316SS) | Grade 660 16.6 µm/m·°C | Critical for stainless flange bolting |
| NACE MR0175 Sour Service | Grade 660 Class B ≤35 HRC | Well-defined compliance pathway per ASTM A453 |
| ASME Pressure Equipment Code | Grade 660 SA453 | ASME SA453 adopted verbatim from ASTM A453 |
| Cryogenic Toughness | Both Qualify Tie | Class D for LNG (−196°C); IN-718 for liquid hydrogen (−253°C) |
| Non-magnetic Property | Both Qualify Tie | Both fully austenitic; Grade 660 achieves this at much lower cost |
| Aerospace / AMS Certification | Inconel 718 AMS 5662/63 | Industry-standard AMS traceability for aerospace and defense |
| Best Value Below 700°C | Grade 660 Clear Winner | Equivalent performance at 20–25% of Inconel 718 total part cost |
Grade 660 vs. Inconel 718 — Common Questions Answered
What is the difference between ASTM A453 Grade 660 and Inconel 718?
ASTM A453 Grade 660 (Alloy A286, UNS S66286) is an iron-based precipitation-hardenable superalloy with approximately 54% iron and 24–27% nickel, rated for service up to 700°C. Inconel 718 (UNS N07718) is a nickel-based superalloy with approximately 52% nickel, rated for sustained service up to 980°C. Grade 660 costs roughly 4–6× less than Inconel 718 in finished forging form and machines 1.6–2.2× faster, making it the standard choice for all industrial applications below 700°C.
Can ASTM A453 Grade 660 replace Inconel 718?
Yes, for the majority of industrial applications operating below 700°C, Grade 660 can directly replace Inconel 718 with equivalent performance at 20–25% of total part cost. The substitution is well-established in power generation, oil and gas, petrochemical, and process industry applications. For aerospace hardware above 700°C, or where a specification explicitly requires Inconel 718 (UNS N07718), substitution requires engineering review and re-qualification.
What is the maximum operating temperature of ASTM A453 Grade 660?
The maximum sustained service temperature of ASTM A453 Grade 660 is 700°C (1292°F). Above this temperature, the gamma-prime (γ′) precipitates begin to coarsen, resulting in progressive loss of tensile and creep strength. For applications in the 650–700°C range, ASTM A453 mandates a 100-hour stress rupture test at 650°C to certify sustained performance — room-temperature tensile values alone are insufficient for high-temperature design. Grade 660 also performs excellently at cryogenic temperatures down to −196°C in Class D condition.
How much cheaper is Grade 660 compared to Inconel 718?
ASTM A453 Grade 660 forgings are typically 4–6× less expensive than equivalent Inconel 718 forgings in total part cost. The difference compounds from: (1) raw material — Grade 660 contains 24–27% nickel vs 50–55% in Inconel 718; (2) machining — Grade 660 machines 1.6–2.2× faster; (3) heat treatment — Grade 660 uses a single-aging cycle vs Inconel 718’s double-aging protocol. Lead times are also shorter: 4–6 weeks for Grade 660 vs 6–10 weeks for Inconel 718.
Does ASTM A453 Grade 660 meet NACE MR0175 sour service requirements?
Yes. ASTM A453 Grade 660 Class B meets NACE MR0175 / ISO 15156 sour service requirements, provided the final part hardness is tested and confirmed at or below 35 HRC (≤331 BHN). This must be explicitly measured and documented — not assumed. We include hardness testing for every heat-treated batch and provide NACE compliance documentation alongside the standard EN 10204 3.1 Mill Test Certificate. Grade 660 Class D is excluded from NACE sour service because direct-aging without solution treatment may produce hardness above the NACE limit.
What are the ASTM A453 Grade 660 Class A, B, C, and D differences?
Class A (≥1000 MPa tensile): standard solution treatment + aging, general high-temperature service, stress rupture certified. Class B (≥1035 MPa tensile): same cycle with tighter requirements, Charpy impact ≥40 J at −100°C, NACE MR0175 compliant at ≤35 HRC; the most widely specified class for turbine and oil & gas applications. Class C (≥1100 MPa tensile): higher solution temperature for increased strength, stress rupture certified. Class D (≥1170 MPa tensile): direct aging without solution treatment, highest strength, Charpy ≥70 J at −196°C for cryogenic use, but no 100-hour stress rupture test is required or provided per ASTM A453.
Is Inconel 718 magnetic?
No. Inconel 718 (UNS N07718) is non-magnetic — it is a fully austenitic alloy with magnetic permeability near 1.001. ASTM A453 Grade 660 (UNS S66286) is also non-magnetic and fully austenitic, with magnetic permeability ≤1.02. Both alloys qualify for non-magnetic applications including nuclear power generator retaining rings and precision measurement equipment. Grade 660 achieves equivalent non-magnetic performance at significantly lower cost.
Who manufactures ASTM A453 Grade 660 forgings in China?
Jiangsu Liangyi Co., Limited (jnmtforgedparts.com) is an ISO 9001:2015 certified manufacturer of ASTM A453 Grade 660 forgings located in Jiangyin City, Jiangsu Province, China. Established with 25+ years of superalloy forging experience, operating 2,000–6,300 ton hydraulic presses and 5-meter ring rolling machines. We produce Grade 660 forgings from 30 kg to 30,000 kg in Class A, B, C, and D, with full dimensional specifications and EN 10204 3.1 Mill Test Certificates supplied on every order. Independent third-party inspection can be arranged upon customer request at an additional cost. Contact: sales@jnmtforgedparts.com · +86-13585067993
Need a Grade 660 Quote or Material Selection Advice?
Our engineering team reviews your drawings, confirms class suitability, advises on NACE compliance, and provides competitive quotations. ISO 9001:2015 certified. EN 10204 3.1 MTC standard. Ships to 50+ countries.