Side-by-Side Comparison: Inconel 718 vs 625 at a Glance

This table compares UNS N07718 (Inconel 718) against UNS N06625 (Inconel 625). Both are nickel-based superalloys, but their strengthening mechanisms and target applications are fundamentally different. Values from ASTM B564 (forgings) and AMS 5662/5666.

Detailed Comparison: Why 718 and 625 Are Not Interchangeable

1. Strengthening Mechanisms — The Fundamental Difference

The entire selection decision between 718 and 625 hinges on how each alloy achieves its strength. 718 relies on precipitation hardening: the combination of 4.75-5.50% Nb, 0.20-0.80% Al, and 0.65-1.15% Ti allows the formation of gamma-prime (γ', Ni₃Al) and gamma-double-prime (γ'', Ni₃Nb) precipitates during a two-stage aging heat treatment (720°C for 8 hours, furnace cool to 620°C, hold 8 hours, air cool). After aging, 718 achieves a minimum yield strength of 1034 MPa — roughly 2.5× the annealed condition.

625 achieves its strength through solid solution strengthening — molybdenum atoms (8-10%) and niobium atoms (3.15-4.15%) dissolved in the nickel matrix distort the crystal lattice and impede dislocation motion. This mechanism provides a stable 414 MPa yield at all temperatures up to its maximum service rating but cannot be boosted through heat treatment. 625 is not age-hardenable — if an application demands 800+ MPa yield from a nickel alloy, 718 is the answer, not 625.

2. Temperature Capability — 625 Outperforms 718 at the High End

At first glance, 718 looks competitive on temperature: it retains useful strength to 700°C (1300°F) in the aged condition. But this is misleading for two reasons. First, the γ'' phase that provides 718's strength is metastable above 650°C — prolonged exposure causes γ'' to transform to the stable but incoherent delta (δ) phase, which does not contribute to strengthening. Second, 718's lower chromium content (17-21% vs 20-23% for 625) means its oxidation resistance is inferior at temperatures above 800°C.

625, with 58% minimum nickel and 20-23% chromium, maintains both strength and oxidation resistance to 980°C (1800°F) — fully 280°C higher than 718's practical ceiling. For furnace hardware, combustion liners, and heat treatment fixtures, 625 is the clear choice. For gas turbine discs operating at 650°C where strength and fatigue resistance govern, 718 dominates.

3. Corrosion Resistance — 625's Molybdenum Advantage is Decisive

The PREN numbers tell the story: 718 PREN ≈ 30 (based on 3% Mo) versus 625 PREN ≈ 52 (based on 9% Mo). This 22-point gap is massive — it's the difference between a material that handles seawater with caution and one that handles seawater with confidence.

The takeaway: if someone specifies 718 for a corrosion application — a chemical reactor, a marine exhaust system, a sour gas wellhead — redirect them to 625. The 2.5× strength advantage of 718 is worthless if the fitting pits through in 18 months. We at HT PIPE have intervened on at least 3 quotations in the last year where the buyer specified 718 fittings for chloride-containing process streams. In each case, the engineer was selecting "the strongest Inconel" without checking the corrosion data. 625 solved their problem at comparable material cost.

4. Application Scenarios — Two Distinct Worlds

5. Machinability and Weldability — Both Are Difficult, in Different Ways

Both 718 and 625 are notoriously difficult to machine — they work-harden rapidly, generate extreme cutting temperatures, and destroy tooling if feeds and speeds are not carefully controlled. 718 in the aged condition (40+ HRC) is harder to machine than solution-annealed 625. The practical advice from our machine shop: rough machine 718 in the solution-annealed condition (below 30 HRC), then age-harden, then finish machine. Attempting to rough machine aged 718 at 42 HRC will consume carbide inserts at an alarming rate.

For welding, both alloys use matching filler metals: ERNiFeCr-2 (FM 718) for 718, ERNiCrMo-3 (FM 625) for 625. 718 requires post-weld aging to restore the full strength of the precipitation-hardened condition — a welded 718 fitting in the as-welded condition has only ~415 MPa yield in the weld zone, not 1034 MPa. Full post-weld solution anneal + aging is recommended for critical applications. 625, as a solid-solution alloy, does not require post-weld heat treatment for strength recovery, though solution annealing at 1095-1175°C may be specified to restore maximum corrosion resistance.

HT PIPE's Experience with 718 and 625

Nickel alloy fittings are a specialized segment of our product line, representing roughly 2% of total volume but a much higher percentage of revenue due to the material value. Our 625 shipments are 10× the volume of our 718 shipments — the market for corrosion-resistant nickel alloy pipe fittings dwarfs the market for high-strength nickel alloy components in the piping industry.

We supplied specialty 718 hex bolts and stud bolts to an aerospace ground support equipment manufacturer in 2024. The application required M16 and M20 bolting with minimum 1034 MPa yield for hydrogen service test stands. The bolts were machined from 718 bar stock, solution annealed, aged per AMS 5663, and shipped with full mechanical test reports including tensile, hardness, and grain size per ASTM E112. This is not our typical pipe fitting work, but it demonstrates the range of 718 applications in high-strength fastening.

For 625, our typical orders are butt weld fittings (elbows, tees, reducers) in sizes 1/2" through 4" for chemical plant and marine SCR system fabricators. We stock 625 fittings in limited sizes and can source larger items with 8-12 week lead time. All 625 fittings ship with EN 10204 3.2 certification per ASTM B564 and B366.

Frequently Asked Questions

Q1: Can 718 be used in place of 625 for corrosion applications?

No. 718 contains only 2.8-3.3% Mo versus 8-10% Mo in 625. The PREN gap (30 vs 52) means 718 has roughly the pitting resistance of 317L stainless steel — better than 316L but far below 625. Using 718 in a chloride-rich chemical process environment will result in pitting corrosion at a rate comparable to a mid-grade stainless steel, wasting the nickel alloy premium entirely. If corrosion is the selection driver, 625 is the correct choice.

Q2: Can 625 be heat treated to match 718's strength?

No. 625 is fundamentally not age-hardenable. Its strengthening comes from molybdenum and niobium in solid solution, not from precipitate-forming elements. 625 lacks the critical Al and Ti levels (0.4% max each) needed to form gamma-prime precipitates. The maximum yield strength achievable from 625 is approximately 550 MPa through cold working, and even cold-worked 625 cannot approach the 1034 MPa minimum of aged 718. If 1000+ MPa yield is required from a corrosion-resistant alloy, consider MP35N or aged C-276 — but both carry significant cost and availability penalties.

Q3: Why does 718 lose strength above 650°C even though it starts stronger?

The gamma-double-prime (γ'', Ni₃Nb) precipitates that give 718 its strength are metastable. Above 650°C, γ'' transforms to the equilibrium delta (δ) phase, which is incoherent with the matrix and does not impede dislocation motion. This transformation is time-dependent — at 700°C, significant strength loss occurs within 100-200 hours. 625, having no strengthening precipitates to degrade, maintains stable properties to 980°C. This is why 718 is limited to the cold section of gas turbines (compressor discs) while 625 can be used in combustion liners.

Q4: What filler metal is used for welding 718 to 625?

Use ERNiCrMo-3 (Inconel 625 filler, AWS A5.14 ERNiCrMo-3). The high molybdenum in the 625 filler compensates for dilution from the 718 side and keeps the weld deposit above the corrosion resistance threshold of the weaker base metal. Do NOT use ERNiFeCr-2 (718 filler) — the lower Mo content creates a corrosion weak point at the weld. Post-weld aging may be required if the 718 side of the joint requires full aged strength properties.

Q5: Which is more expensive, 718 or 625?

Raw material costs are comparable — both are nickel-based with similar nickel content (50-58%). 718 is typically 5-10% more expensive for finished fittings due to the additional aging heat treatment and the tighter compositional controls on Al and Ti. However, in the context of a piping project, the material cost difference is negligible compared to the engineering cost of selecting the wrong alloy. A $50/fitting premium for 718 over 625 is irrelevant when the wrong choice causes a $500,000 process shutdown.

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