Inconel 718 vs Inconel 625: Which Superalloy Forging Is Right for Your Application?

Two Industry-Dominant Nickel Superalloys — and Why the Choice Still Confuses Engineers

Inconel 718 (UNS N07718 / DIN 2.4668) and Inconel 625 (UNS N06625 / DIN 2.4856) are the two most widely procured nickel superalloy forgings in global heavy industry. Both originate from nickel-chromium base systems, both outperform stainless steels in extreme environments, and both appear on the same project material requisitions — across oil & gas, aerospace, nuclear power, marine engineering, and chemical processing.

Yet specifying the wrong alloy can compromise component service life, trigger costly replacements, or cause a non-conformance at final inspection. This guide provides a complete engineering comparison across all parameters relevant to alloy selection, based on our 27+ years of manufacturing custom forgings for customers across Europe, North America, the Middle East, Southeast Asia, and Australia.

Chemical Composition: Why 718 and 625 Behave So Differently

The performance gap between Inconel 718 and Inconel 625 flows directly from their chemistry. Three elements explain almost all of the key differences: molybdenum (corrosion resistance), niobium (precipitation hardening), and iron (cost vs. chloride performance).

Why Inconel 718 Achieves Exceptional Strength

Inconel 718 was engineered specifically for precipitation hardening. After solution annealing at 954–982 °C, a two-stage aging cycle — 8 hours at 718 °C then 8 hours at 621 °C — causes niobium to form coherent γ″ (Ni₃Nb) disc-shaped precipitates within the nickel matrix. These nanoscale particles block dislocation movement, producing a yield strength exceeding 1,034 MPa. A secondary γ′ (Ni₃(Al,Ti)) phase contributes additional strength. No other commercially forged nickel alloy achieves this strength level below 700 °C at comparable cost.

Why Inconel 625 Has Superior Corrosion Resistance

Inconel 625 contains 8–10% molybdenum — approximately three times the level in Alloy 718. Molybdenum is the primary driver of pitting and crevice corrosion resistance, quantified by the Pitting Resistance Equivalent Number: PREN = %Cr + 3.3×%Mo. Alloy 625 achieves a PREN above 50, placing it among the most corrosion-resistant forgeable alloys. Its low iron content (≤5%) eliminates a source of galvanic weakness in chloride-containing environments, and its higher chromium level (20–23%) provides better oxidation resistance above 700 °C.

Mechanical Property Comparison: Strength, Hardness & Toughness

The following data represents minimum mechanical property requirements for forgings in their optimum heat-treated condition, per ASTM B637 for Alloy 718 aged per AMS 5663, and ASTM B443 / AMS 5599 for Alloy 625 in the annealed condition at room temperature (21 °C).

Operating Temperature: Where Each Alloy Thrives and Where It Fails

Corrosion Resistance by Media: Chlorides, Sour Gas, Seawater & Acids

Both alloys vastly outperform austenitic stainless steels (316L, 317L, 904L) and duplex grades in severe corrosive environments. However, the performance difference between them is significant and in most corrosive media, Inconel 625 holds a clear advantage.

For subsea hardware, offshore structural components, and chemical process equipment where corrosion is the primary failure mode, Inconel 625 is the engineering choice. For wellhead structural components, downhole tools, and rotating machinery where mechanical load dominates in a sour environment, Alloy 718's strength-to-cost advantage makes it the global industry standard — provided NACE MR0175 compliance is confirmed.

Applicable Standards and Certification Requirements

Weldability, Machinability & Forgeability Compared

Weldability

Inconel 625 is among the most weldable of all nickel superalloys. Its solid-solution microstructure allows welding in the annealed condition without subsequent post-weld heat treatment (PWHT), and it exhibits minimal susceptibility to heat-affected zone (HAZ) cracking. Its corrosion resistance is so high that ERNiCrMo-3 (Alloy 625) filler wire is routinely used to clad carbon steel components for offshore service.

Inconel 718 was specifically engineered with slow γ″ precipitation kinetics to minimize post-weld age cracking — a critical failure mode in other precipitation-hardening nickel alloys such as Waspaloy and René 41. Welding in the solution-annealed condition avoids HAZ cracking during manufacture. However, to restore ≥1,034 MPa mechanical properties in welded structural joints, a complete re-solution-anneal at 954–982 °C followed by the double-aging cycle is required, adding 2–5 days of lead time per heat treatment cycle.

Machinability

Both alloys exhibit rapid work hardening during cutting operations, requiring lower surface speeds, sharper carbide or ceramic tooling, and continuous flood coolant compared to stainless steels. Aged Inconel 718 (HRC 36–47) is significantly harder than solution-annealed Inconel 625 (HRC 20–30) and requires more conservative machining parameters. For complex-profile forgings such as impellers, pump stages, or valve bodies, requesting semi-finished or rough-machined delivery condition from your forging supplier can reduce total machining costs by 20–35% by eliminating excess stock removal.

Forgeability

Both alloys are hot-worked in the 980–1,180 °C range. Inconel 718 has a somewhat wider forging window and better die-fill response, due in part to its higher iron content and more accommodating precipitation kinetics. Inconel 625 requires stricter upper-temperature control to prevent grain growth in large cross-sections — a concern for forgings above 500 mm diameter where through-thickness mechanical property uniformity must be verified by transverse testing. Both alloys require melting via VIM+VAR or VIM+ESR+VAR triple-melt processes for aerospace and nuclear quality forgings.

Real-World Application Mapping: Which Alloy Is Used Where?

Decision Matrix: Select the Correct Alloy in Under 2 Minutes

Match your primary design driver to the recommended forging alloy using the matrix below. Where multiple rows apply to your application, the row with the strongest constraint (typically temperature or corrosion environment) takes precedence.

Cost, Lead Time & Total Cost of Ownership

Purchase Price per Kilogram

Inconel 718 forgings are typically 15–25% less expensive per kilogram than equivalent Inconel 625 forgings of the same geometry and delivery condition. The primary driver is molybdenum price: at 8–10% Mo content, Alloy 625 contains approximately 3× more of this expensive alloying element. For large-volume orders — structural wellhead campaigns, bulk fastener forgings — this per-kilogram differential is significant in total project cost.

Total Cost of Ownership

In marine and chemically corrosive service, Inconel 625 components typically require no protective coatings, no cathodic protection management, and have longer inspection intervals than Alloy 718 components. Over a 15–25 year asset life in offshore or chemical process service, the lower maintenance and replacement costs of 625 can fully recover — and in severe environments exceed — the higher initial purchase price.

Production Lead Times

Both alloys are available from qualified stock billets in standard round bar, flat bar, and ring configurations for rapid shipment. For a full overview of sizes, weights, and shape options, see our custom Inconel 718 forging solutions. Full custom forgings — open-die forgings, seamless rolled rings, machined components — follow the same production flow: billet selection → heating → forging press → heat treatment → UT inspection (ASTM A388 or EN10228) → machining → dimensional inspection → EN 10204 3.1/3.2 certification. Custom orders with third-party witness inspection (BV, SGS, TÜV) typically complete in 6–10 weeks from drawing approval.

How to Choose Between Inconel 718 and 625 Forgings: 6 Steps

Follow this structured selection process to specify the correct nickel superalloy forging for your application without ambiguity.

Frequently Asked Questions: Inconel 718 vs 625 Forgings

Summary: The Right Superalloy Forging for Your Application

Inconel 718 and Inconel 625 are not interchangeable — they are complementary alloys engineered for fundamentally different primary failure modes. Alloy 718 is the world's most widely specified high-strength nickel superalloy forging for structural service below 700 °C, providing 3.75× higher yield strength and twice the fatigue resistance of Alloy 625 at 15–25% lower cost. Alloy 625 is the first choice where corrosion is the primary failure risk: seawater immersion, chloride pitting, mineral acid processing, high-temperature oxidation above 700 °C, or wherever on-site weld repair without post-weld heat treatment is a constraint.

When both strength and corrosion resistance are simultaneously critical in a single component — such as wellhead valve bodies in deepwater sour service — the proven engineering solution is a bimetallic design: Alloy 718 structural forging with Alloy 625 or Alloy C-276 corrosion-resistant overlay weld cladding on exposed surfaces. This approach is standard practice in premium wellhead equipment and deepwater subsea hardware.

At Jiangsu Liangyi Co., Limited, we manufacture both Inconel 718 and Inconel 625 forgings — from qualified billet through open-die forging, seamless ring rolling, heat treatment, and full non-destructive testing — under ISO 9001:2015 quality management with EN 10204 3.1/3.2 certification as standard. Third-party inspection by BV, SGS, and TÜV is available on every order. For full size ranges, delivery conditions, and specifications on Alloy 718 forged bars, rings, and custom components, visit our product page to request a quotation.