Inconel 625 vs Inconel 718 High-Temperature vs High-Strength Nickel Superalloys

Inconel 625 and Inconel 718 are the world's two most widely used nickel-base superalloys, together accounting for a substantial share of global superalloy consumption. Despite sharing the Inconel® family name, they are engineered for fundamentally different primary functions: Inconel 625 prioritizes corrosion and oxidation resistance at extreme temperatures, while Inconel 718 prioritizes exceptional mechanical strength under demanding structural loads.

This report provides engineers, procurement specialists, and technical buyers with a rigorous, data-driven comparison spanning chemical composition, mechanical and physical properties, corrosion behavior, weldability, industry applications, and total cost of ownership. A definitive selection matrix is included to accelerate alloy decision-making.

What Are Nickel Superalloys?

Nickel superalloys are a class of high-performance metallic materials engineered to retain mechanical strength, surface stability, and corrosion resistance at temperatures that would cause conventional steels and aluminum alloys to fail entirely. They are the backbone of modern aerospace, energy, chemical processing, and deep-sea engineering.

The term 'Inconel' is a registered trademark of Special Metals Corporation (now part of PCC, Precision Castparts Corp.), applied to a family of austenitic nickel-chromium superalloys. Within this family, grade 625 and grade 718 have dominated global consumption for decades owing to their exceptional and complementary property profiles.

Chemical Composition

The performance differences between 625 and 718 trace directly to their distinct chemical blueprints. Understanding composition is the first step to understanding behavior.

Inconel 625 contains significantly more molybdenum (8–10%) than 718 (2.8–3.3%). Mo is the primary driver of pitting and crevice corrosion resistance in chloride environments, giving 625 its superior seawater performance.

Inconel 718 uses a higher niobium content (4.75–5.5%) to form the Ni3Nb gamma-double-prime (γ'') precipitate phase - the source of its extraordinary precipitation-hardening response.

The iron balance in 718 (vs. a nickel minimum in 625) reflects its cost-optimized origin as an aerospace structural alloy where strength, not ultimate corrosion resistance, was the design target.

Both alloys are fully austenitic (FCC crystal structure) and non-magnetic in the annealed state.

Mechanical Properties

Mechanical properties are the most critical differentiator for structural and rotating-machinery applications. The data below reflects typical values for the most common industrial product conditions.

Inconel 718 aged achieves more than twice the tensile and yield strength of annealed 625 - making it the clear choice for structural, load-bearing, or fatigue-critical components.

Inconel 625 retains outstanding ductility (>40% elongation) even in the annealed state, critical for forming, cold-working, and applications subject to impact loading.

The maximum service temperature of 625 (982°C continuous) is nearly 280°C higher than 718 (704°C), reflecting the superior oxidation resistance conferred by its higher chromium and molybdenum content.

Corrosion Resistance

Corrosion resistance is the primary strength of Inconel 625 and a significant (though secondary) capability of Inconel 718. Both outperform stainless steels in most aggressive environments, but their relative superiority varies dramatically by environment type.

High molybdenum (8–10%) raises the PREN from ~38 (718) to ~52 (625), placing it above the critical threshold for reliable resistance to pitting in seawater.

625 is immune to chloride stress-corrosion cracking (SCC) that attacks austenitic stainless steels and poses risk to 718 in sensitized conditions.

625 is used as the NACE MR0175/ISO 15156 reference material for sour-service (H₂S + CO₂) environments in oil and gas.

Physical Properties

Physical properties govern thermal management, dimensional stability, and component weight - important for system design even when they are secondary to mechanical performance.

Both alloys have broadly similar physical properties, as expected from alloys sharing a predominantly nickel-austenite matrix. The modest density difference (8.44 vs 8.19 g/cm³) slightly favors 718 in weight-sensitive aerospace structural applications.

Weldability and Fabrication

Fabricability - the ease with which an alloy can be welded, formed, and machined - has a direct impact on total manufacturing cost and field repairability. This is an area of meaningful practical difference between 625 and 718.

Fabrication Cost Implications

Inconel 625 is significantly easier to weld, negating the need for expensive post-weld heat treatment (PWHT) in most applications. This translates to lower fabrication lead times and costs for welded assemblies.

Inconel 718's requirement for PWHT (and the associated controlled-atmosphere furnaces) adds cost and complexity, but the resulting precipitation-hardened microstructure delivers the strength levels that justify this investment in aerospace and high-load applications.

Machining both alloys is challenging due to work hardening; sharp tooling, low feeds, and flood coolant are essential. 625 machines slightly more easily than 718 in the aged condition.

Frequently Asked Questions

No. While 625 has excellent high-temperature corrosion resistance, its annealed yield strength (~517 MPa) is less than half that of aged 718 (~1,170 MPa). A turbine disc spinning at tens of thousands of RPM requires the precipitation-hardened microstructure of 718 to resist centrifugal stresses. 625 would fail by yielding and fatigue at engine-operating stress levels.

718 is not the preferred choice for continuous seawater immersion. Its PREN (~38) falls below the >40 threshold generally considered safe for crevice corrosion resistance in stagnant seawater. In flowing seawater or with cathodic protection, 718 may perform acceptably, but Inconel 625 (PREN ~52) is the engineered solution for offshore applications per NACE/ISO standards.

No. In their optimum heat-treated conditions, Inconel 718 is approximately 48% stronger in tensile strength and over 100% stronger in yield strength. 625 can be strengthened by cold work, but cannot match the precipitation-hardened properties of 718 by any heat treatment.

Both alloys are established in additive manufacturing via laser powder bed fusion (LPBF) and directed energy deposition (DED). Inconel 718 has the larger published dataset and broader process qualification history, particularly in aerospace. Inconel 625 is preferred for AM parts requiring corrosion resistance as the primary function. The choice mirrors conventional applications: 625 for corrosion-critical, 718 for strength-critical AM components.

Yes. Several alloys occupy intermediate positions. Inconel 625 Plus (UNS N07716) adds age-hardening capability to a 625-like base composition, achieving yield strengths of ~827 MPa - useful when both corrosion resistance and higher strength are needed. Alloy 725 (UNS N07725) is another age-hardenable nickel-chromium-molybdenum alloy qualified for sour-service under NACE MR0175. Consult a materials engineer when standard 625 or 718 does not fully meet dual corrosion/strength requirements.

Conclusion

Inconel 625 and Inconel 718 are complementary, not competing, solutions to different engineering challenges. No single alloy is universally superior; the 'best' alloy is always the one optimally matched to the specific combination of temperature, stress, corrosion environment, fabrication method, and cost constraints of your application.

In summary:

Inconel 625 is the world-leading alloy for corrosion resistance and high-temperature oxidation stability, at the cost of lower mechanical strength. It is the default choice for chemical processing, offshore, marine, and any application where corrosion is the dominant failure mode.

Inconel 718 is the world's most widely used superalloy for high-strength structural applications, particularly in aerospace propulsion and rotating machinery. Its precipitation-hardened microstructure delivers mechanical properties that cannot be achieved by 625 under any condition.

Where both corrosion resistance and elevated strength are required simultaneously, intermediate alloys or engineering trade-off analysis should be conducted with a qualified materials engineer.