Comparison Between ASTM A453 Gr.660 Alloy and Similar High-Temperature Alloy Inconel 718

Choosing between ASTM A453 Gr.660 (A-286) and Inconel 718 for high-temperature bolting comes down to temperature, load and environment. Both are precipitation-hardened superalloys for hot, pressurized equipment — turbine casings, boiler shells and pipeline connections — and the usual joint seal in such service is a high-temperature flange gaskets.

Key Differences at a Glance

Four points separate the two alloys in everyday engineering practice.

• Service temperature: Gr.660 keeps oxidation and creep resistance up to 704 °C, while Inconel 718 is strongest below about 650 °C, where its γ'' strengthening phase is stable.
• Strength: Inconel 718 reaches 1000–1400 MPa tensile at room temperature, against ≥895 MPa for Gr.660 (Class D up to 725 MPa yield).
• Corrosion: Inconel 718 resists chloride stress corrosion cracking and handles seawater and H₂S, whereas Gr.660 behaves like 316/316L stainless steel in water.
• Cost: Gr.660 is iron-based with 24–27% nickel and no niobium, while Inconel 718 carries 50–55% nickel plus niobium and costs several times more.

Core Property Comparison

The table below summarizes the properties that drive material selection for high-temperature fasteners.

Table 1. Typical values as listed in the source comparison; confirm exact figures against the material certificate.

ASTM A453 Gr.660 stud bolt

Temperature, Corrosion and Fabrication

Numbers only tell part of the story; the phase behavior behind them decides where each alloy works.

• Temperature window: below 650 °C Inconel 718 has unmatched strength and creep resistance; above 650–700 °C its γ'' phase coarsens or dissolves and strength falls quickly. Gr.660 keeps oxidation and creep resistance up to 704 °C and fits static bolting such as steam turbine bolts and pipe flanges on boilers.
• Corrosion: Inconel 718 performs in oxidizing and reducing acids, seawater and H₂S-bearing media, with strong resistance to chloride stress corrosion cracking — useful offshore and in petrochemical plants. Gr.660 covers most industrial atmospheres and water, comparable to 316 stainless steel.
• Machining and forming: Gr.660 keeps good plasticity in the solution-treated state and machines with standard tooling. Inconel 718 work-hardens, generates high cutting forces and wears tools quickly, although its hot and cold workability beats most superalloys.
• Welding: Inconel 718 welds well on large or complex parts. Gr.660 is weldable too, but the joint usually needs post-weld aging to restore mechanical properties.

Cost follows composition. Gr.660 uses an iron-based matrix with 24–27% nickel and no niobium, so it is the economical choice for hot, moderately loaded joints. Inconel 718, with 50–55% nickel and niobium, is often several times more expensive, and that premium is justified only where stress, thermal cycling or aggressive media leave no margin.

Frequently Asked Questions

What is Inconel 718?

Inconel 718 is a precipitation-hardened nickel-based superalloy strengthened mainly by the γ'' phase. Below 650 °C it shows the highest yield strength among wrought superalloys, which is why it is used for high-stress fasteners such as turbine disc bolts.

What is the maximum service temperature of ASTM A453 Gr.660 bolts?

Gr.660 (A-286) is designed to keep oxidation and creep resistance up to about 704 °C. Below 650 °C it also holds high yield and creep strength, which is why it is common in steam turbine and boiler bolting.

Can ASTM A453 Gr.660 be welded?

Yes. Gr.660 is weldable, but the joint usually needs post-weld aging to restore mechanical properties. Inconel 718 welds more easily and is preferred for large or complex welded structures.

Why does Inconel 718 cost more than Gr.660?

Composition drives the price. Gr.660 is iron-based with 24–27% nickel and no niobium, while Inconel 718 contains 50–55% nickel plus niobium and is often several times the price of Gr.660.