Inconel 625 vs 718

Inconel 625 and Inconel 718 are both nickel-based superalloys routinely specified for high-consequence aerospace and defense hardware. They share enough surface-level similarity—both are “Inconel,” both are corrosion-resistant, both are used in extreme environments—that procurement and engineering teams often treat alloy selection as a routine decision. It is not. Choosing between 625 and 718 affects manufacturability, qualification, cost, lead time, and long-term supportability in ways that are only visible when you trace the implications through real manufacturing workflows: additive manufacturing (AM) via powder bed fusion (PBF / DMLS / SLM), PM-HIP densification, CNC machining, post-processing, and inspection under AS9100, NADCAP, ITAR, and DFARS requirements.

This guide compares 625 and 718 from a manufacturing and procurement perspective—not just datasheet properties—so you can specify the right alloy for the right application and avoid costly mid-program changes.

Composition and strengthening mechanism

The fundamental difference between 625 and 718 is how they get their strength.

Inconel 625 is primarily a solid-solution-strengthened alloy. Its strength comes from the dissolution of molybdenum and niobium in the nickel-chromium matrix. 625 does not require precipitation hardening to achieve its design properties, which simplifies thermal processing and makes it more forgiving in manufacturing.

Inconel 718 is a precipitation-hardened alloy. Its high strength comes from the controlled precipitation of gamma-prime (γ′) and gamma-double-prime (γ″) phases during aging heat treatments. This means 718 is highly sensitive to thermal history: improper heat treatment, uncontrolled cooling, or incorrect HIP/aging sequences can produce out-of-spec mechanical properties that are difficult or impossible to recover.

Why this matters for manufacturing: 625 is more tolerant of process variation. 718 demands tighter process control at every thermal step—stress relief, HIP, solution treat, and aging—because each step directly affects phase precipitation and final properties. For AM programs, this distinction is critical: PBF build thermal history, post-build stress relief, HIP parameters, and aging cycles must all be qualified and controlled as an integrated sequence.

Mechanical properties

At room temperature, 718 is meaningfully stronger than 625:

Inconel 625 (annealed): typical yield strength ~60–75 ksi; tensile strength ~120–145 ksi. Good ductility and toughness across a wide temperature range. Strength is relatively stable and predictable because it does not depend on precipitation hardening.

Inconel 718 (solution treated + aged): typical yield strength ~150–175 ksi; tensile strength ~180–200+ ksi. Significantly higher strength, but properties are highly dependent on correct aging. Ductility is lower than 625, especially at elevated temperatures approaching 1200°F (650°C).

Temperature limits: 625 retains useful properties to approximately 1800°F (982°C) for some applications, though strength drops at elevated temperatures. 718 is typically limited to about 1200–1300°F (650–704°C) for sustained service because the strengthening precipitates become unstable above that range (overaging/coarsening of γ″).

Procurement implication: if the application requires high static or fatigue strength at moderate temperatures (≤ 1200°F), 718 is usually the stronger candidate. If the application is dominated by corrosion resistance, weldability, or higher-temperature exposure (above 1200°F), 625 is often better suited. But the decision is rarely this simple in real programs—cost, manufacturability, and qualification timelines usually dominate.

Corrosion and environmental resistance

Both alloys are corrosion-resistant, but 625 has a meaningful advantage in aggressive environments.

Inconel 625 contains higher molybdenum (~8–10%) and chromium (~20–23%), giving it excellent resistance to pitting, crevice corrosion, and chloride stress-corrosion cracking. It is widely used in marine, chemical processing, and subsea applications where prolonged exposure to seawater, acids, or mixed-gas environments is expected.

Inconel 718 is corrosion-resistant in many aerospace environments (hot gas paths, fuel, hydraulic fluid exposure) but is not typically specified for prolonged marine or chemical immersion service. Its corrosion resistance is adequate for most flight hardware and propulsion applications but is secondary to its strength.

Engineering decision point: if the part will see sustained exposure to seawater, chlorides, or reducing acids, favor 625. If the environment is primarily hot gas or dry oxidation with moderate corrosion exposure, 718 is usually acceptable.

Additive manufacturing considerations

Both 625 and 718 are well-established PBF alloys, but they differ in how much post-build processing is required and how sensitive the final properties are to build and thermal parameters.

Inconel 625 in AM:
625 is among the most forgiving superalloys for PBF. As-built parts typically exhibit good density, and the alloy’s solid-solution nature means that as-built or stress-relieved properties are often within a useful range even without aging. HIP is commonly applied to close residual porosity and improve fatigue life, but the property window is wide. Post-HIP, parts can be annealed and used without complex multi-step aging.

Inconel 718 in AM:
718 prints well in PBF, and powder is widely available, but the as-built microstructure is not in a usable condition for most structural applications. Parts must go through a controlled sequence: stress relief → HIP (commonly) → solution treatment → double aging. Each step must be tightly controlled because the precipitation response is sensitive to prior thermal history, including build thermal cycles and stress-relief temperature. Deviation at any step can produce low ductility, unexpected hardness, or failure to meet minimum tensile requirements.

Practical AM takeaway: 625 is a lower-risk alloy for AM programs with compressed schedules or limited thermal processing experience. 718 offers higher performance but requires a fully qualified thermal route. For first-article or rapid prototyping programs, this difference can be schedule-critical.

HIP and thermal processing

HIP (Hot Isostatic Pressing) is commonly specified for both alloys in AM and PM-HIP applications, but the thermal processing complexity differs substantially.

625 HIP: a typical HIP cycle for AM 625 is relatively straightforward—high temperature, high pressure, controlled cooling. Post-HIP, a simple anneal (or the HIP cycle itself may serve as the anneal, depending on the specification) brings the part to its final condition. There is no aging step. This simplifies furnace scheduling, reduces cycle time, and limits the number of process variables that must be controlled and documented.

718 HIP + heat treat: a typical thermal route for AM 718 involves HIP at specific temperature/pressure/time, followed by solution treatment at a defined temperature, then a two-step aging sequence (e.g., 1325°F + 1150°F for specific hold times). Each step has tight windows, and the sequence matters: HIP before solution treat, solution before age, with controlled cooling rates between steps. If any step is out of spec, mechanical properties can fall outside allowables, and rework options are limited.

Cost and schedule impact: 718 thermal processing is more expensive and takes longer because of the multi-step sequence, tighter controls, and higher risk of non-conformance. For programs with NADCAP requirements on heat treating, each step must be performed under accredited conditions with full documentation—multiplying the certification burden.

Machining and post-processing

Both alloys are difficult to machine compared to steel or aluminum, but 718 in the aged (hardened) condition presents additional challenges.

625 machining: in the annealed condition, 625 machines reasonably well with carbide tooling. It work-hardens, so consistent chip load and sharp tools are important. For AM parts, machining typically occurs after HIP/anneal, and the material condition is predictable.

718 machining: in the solution-treated-and-aged condition, 718 is significantly harder (typically 36–45 HRC). This increases tool wear, requires lower speeds, and can demand more rigid fixturing and 5-axis strategies to maintain tolerance on complex AM geometries. Some programs rough-machine 718 after solution treat (before aging) and finish-machine after aging—but this adds setups and handling.

Surface finishing: both alloys respond to abrasive flow machining, bead blasting, and electrochemical methods for internal passage finishing in AM parts. The higher hardness of aged 718 can make internal finishing slightly more difficult.

Procurement note: when requesting quotes for 718 AM parts, ask the supplier about their machining sequence relative to heat treatment. The answer affects cost, lead time, and risk.

Inspection and qualification

Inspection requirements are generally similar for both alloys in aerospace applications, but the qualification burden for 718 is higher because mechanical properties must be validated against tighter minimums.

Common inspection for both: CMM dimensional inspection, NDE (penetrant, radiography, CT scanning as specified), surface finish measurement, and material traceability verification (powder lot, build ID, heat treat records).

Additional 718 requirements: tensile testing of witness coupons (often required per build or per HIP/heat treat batch) to verify yield, tensile, elongation, and reduction of area meet minimums. Hardness testing is sometimes used as a screening check. If coupons fail, the entire batch may need to be dispositioned—driving scrap or additional testing.

625 qualification: tensile properties are typically easier to meet because the allowable window is wider and less sensitive to process variation. Some programs require coupon testing for 625 as well, but the pass rate is generally higher and the risk of out-of-spec results is lower.

FAI and documentation: both alloys require full certification packs (CoC, material certs, process certs, inspection reports, FAI per AS9102 when applicable). The documentation burden is comparable, but 718 programs often require more detailed heat treat records and coupon data.

Cost and availability

625 and 718 are both widely available in powder form for AM and in wrought/bar stock for traditional machining. However, cost and availability differ in practice.

Powder cost: 718 powder is typically less expensive per kilogram than 625 due to higher production volume and broader demand. However, the total cost of a finished part in 718 is often higher because of the more complex thermal processing, tighter machining tolerances in the aged condition, and higher qualification burden.

Lead time: 718 lead times are generally longer because of the multi-step thermal route and the need for coupon testing. For AM programs, the print time is similar, but post-processing adds days to weeks for 718 compared to 625.

DFARS and traceability: both alloys fall under specialty metals requirements when used on DoD contracts. Suppliers must demonstrate compliant sourcing (melt source in a qualifying country) and maintain full heat/lot traceability. Powder traceability—from atomizer to build to finished part—is required for both.

When to specify 625 vs 718

Choose 625 when:

• Corrosion resistance is a primary driver (marine, chemical, subsea, mixed-gas environments).
• The application requires good strength but not the highest possible static strength.
• Weldability and repairability are important (625 is more weld-friendly than aged 718).
• Schedule and qualification risk need to be minimized—625’s simpler thermal processing reduces variables.
• The operating temperature exceeds 1200°F for sustained service.

Choose 718 when:

• High static and fatigue strength is required at moderate temperatures (≤ 1200°F).
• The part is load-critical and the design relies on yield/tensile minimums that only 718 can meet.
• The thermal processing infrastructure is mature—qualified HIP, solution, and aging cycles are already established.
• Cost of powder is a significant fraction of total program cost and the lower $/kg of 718 powder matters at volume.
• Existing program specs or customer requirements mandate 718 (common in propulsion and structural applications).

Key RFQ practice: when issuing an RFQ, state the alloy, governing specification, required mechanical minimums, thermal processing requirements, and inspection/qualification expectations. If you are open to both alloys, ask the supplier to quote both and provide a brief technical comparison for your application. This gives your engineering team data to make the final call based on real manufacturing costs and timelines—not just datasheet numbers.

Summary

625 and 718 are both capable, well-established superalloys with extensive aerospace and defense pedigree. The right choice depends on the intersection of application requirements (strength, temperature, corrosion), manufacturing complexity (thermal processing, machining, qualification), and program constraints (schedule, cost, risk tolerance). For AM programs, the thermal processing difference between the two alloys is the single most consequential factor—it drives cost, lead time, qualification risk, and supply chain complexity.

When evaluating suppliers for either alloy, prioritize those who can demonstrate end-to-end workflow control: powder traceability, controlled build parameters, qualified HIP and heat treatment, precision machining, and a complete certification pack that ties material, process, and inspection records together. For 718, pay particular attention to the supplier’s thermal processing qualification and coupon testing track record. For 625, focus on density verification (especially in AM) and corrosion-relevant testing if the application demands it.

Whichever alloy you specify, lock down traceability, HIP/heat treat control, machining strategy, and NDE early—before first article.

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