Inconel 625 vs 718: What’s the Difference and Which Should You Choose?
When sourcing nickel-based superalloys, Inconel 625 and Inconel 718 are two of the most frequently specified grades. Both belong to the Inconel family, both handle elevated temperatures, and both outperform standard stainless steels in demanding environments — yet they are fundamentally different alloys designed for different jobs.
Choosing the wrong one leads to either over-engineering (and overspending) or underperformance in the field. This guide breaks down every key difference so you can make a confident material decision.
Quick Comparison: Inconel 625 vs 718 at a Glance
| Property | Inconel 625 (UNS N06625) | Inconel 718 (UNS N07718) |
|---|---|---|
| Strengthening mechanism | Solid solution strengthening | Precipitation hardening (γ’ + γ”) |
| Tensile strength | 827–930 MPa | 1,240–1,550 MPa |
| Yield strength (0.2%) | 414–620 MPa | 760–1,310 MPa |
| Elongation | 30–40% | 12–30% |
| Max service temperature | ~982°C | ~650°C (continuous) |
| Corrosion resistance | Excellent (especially Cl⁻) | Good (oxidizing environments) |
| Weldability | Excellent | Moderate (requires post-weld heat treatment) |
| Machinability | Moderate | Difficult |
| Typical industry | Chemical, marine, nuclear | Aerospace, oil & gas (downhole) |
Chemical Composition: Where the Differences Begin
The performance gap between 625 and 718 starts at the atomic level.
Inconel 625 (UNS N06625)
Inconel 625 is a nickel-chromium-molybdenum alloy with niobium. Its design philosophy is corrosion resistance through solid-solution strengthening:
| Element | Content (wt%) | Role |
|---|---|---|
| Nickel (Ni) | ≥58.0 | Base matrix, corrosion resistance |
| Chromium (Cr) | 20.0–23.0 | Oxidation and corrosion resistance |
| Molybdenum (Mo) | 8.0–10.0 | Pitting and crevice corrosion resistance |
| Niobium (Nb) | 3.15–4.15 | Solid solution strengthening |
| Iron (Fe) | ≤5.0 | Filler |
| Carbon (C) | ≤0.10 | — |
The high molybdenum content (8–10%) is the defining feature of 625. It is responsible for the alloy’s outstanding resistance to chloride-induced pitting and crevice corrosion — a critical advantage in seawater and acidic chemical environments.
Inconel 718 (UNS N07718)
Inconel 718 is a nickel-chromium alloy with iron, niobium, titanium, and aluminum. Its design philosophy is maximum strength through precipitation hardening:
| Element | Content (wt%) | Role |
|---|---|---|
| Nickel (Ni) | 50.0–55.0 | Base matrix |
| Chromium (Cr) | 17.0–21.0 | Oxidation and corrosion resistance |
| Iron (Fe) | Balance (~17–21) | Cost reduction, structural filler |
| Niobium (Nb) | 4.75–5.50 | Forms γ” phase (Ni₃Nb) — primary strengthener |
| Molybdenum (Mo) | 2.8–3.3 | Solid solution strengthening |
| Titanium (Ti) | 0.65–1.15 | Forms γ’ phase (Ni₃Al/Ti) |
| Aluminum (Al) | 0.2–0.8 | Forms γ’ phase |
718’s high niobium combined with titanium and aluminum allows it to form two strengthening precipitate phases (γ” and γ’) through heat treatment. This is what gives Inconel 718 its extraordinary strength — roughly double that of 625 in the aged condition.
Key difference in a sentence: 625 uses molybdenum for corrosion resistance; 718 uses niobium + Ti + Al for extreme strength.
Mechanical Properties: 718 Is Significantly Stronger
This is the most dramatic difference between the two alloys.
| Mechanical Property | Inconel 625 (annealed) | Inconel 718 (aged) |
|---|---|---|
| Tensile strength (UTS) | 827–930 MPa | 1,240–1,550 MPa |
| Yield strength (0.2% offset) | 414–620 MPa | 760–1,310 MPa |
| Elongation | 30–40% | 12–30% |
| Hardness | ~170–260 HB | ~250–360 HB |
| Density | 8.44 g/cm³ | 8.19–8.25 g/cm³ |
| Elastic modulus | ~205 GPa | ~205 GPa |
What this means in practice:
- Inconel 718’s tensile strength is approximately 55–65% higher than 625’s in its typical heat-treated condition.
- For structural applications where weight matters — turbine disks, fasteners, pressure vessels under high mechanical load — 718 is the clear choice.
- Inconel 625’s higher elongation (30–40%) means it absorbs more deformation before fracture, making it preferable where flexibility or impact resistance is required.
High-Temperature Performance: Different Temperature Windows
Both alloys handle high heat, but their useful temperature ranges differ significantly.
Inconel 625
- Continuous service: up to approximately 816°C
- Peak/short-term: up to 982°C
- Maintains good oxidation resistance across an exceptionally wide range: from cryogenic temperatures (-196°C) to near 1,000°C
- Does not rely on precipitate phases, so there is no concern about overaging or precipitate dissolution at elevated temperatures
Inconel 718
- Optimal range for strength: 300°C to 650°C
- Continuous service limit: approximately 650°C
- Above 650°C, the γ” precipitate phase begins to coarsen and transform to the less effective δ phase, causing a progressive loss of strength
- Best suited for applications demanding high strength at moderate-to-elevated temperatures, not extreme heat exposure
Practical takeaway: If your application operates above 700°C, Inconel 625 is the safer choice. If the design temperature stays below 650°C and strength is paramount, Inconel 718 is superior.
Corrosion Resistance: 625 Has a Clear Advantage
This is where Inconel 625 truly distinguishes itself.
Inconel 625 Corrosion Performance
- Resists pitting corrosion and crevice corrosion in chloride-containing environments — a direct result of high Mo (8–10%)
- Excellent resistance to organic and inorganic acids (hydrochloric, sulfuric, phosphoric, nitric)
- Resistant to stress corrosion cracking (SCC) in chloride environments
- Performs well in seawater, flue gas desulfurization (FGD) systems, and nuclear cooling circuits
Inconel 718 Corrosion Performance
- Provides good general corrosion resistance in oxidizing environments
- Adequate for mild acids and elevated temperature oxidation
- Not recommended for environments with high chloride concentrations or strongly reducing acids, where its lower Mo content leaves it vulnerable to pitting
Summary: For chemical processing, marine, or any chloride-heavy application, Inconel 625 is the correct choice. Inconel 718’s corrosion resistance is adequate for aerospace and downhole environments but is not its primary design objective.
Weldability and Fabrication: 625 Is Much Easier to Work With
Inconel 625
Inconel 625 is one of the most weldable nickel alloys available:
- Joins readily using TIG (GTAW), MIG (GMAW), and plasma arc welding
- Recommended filler metal: ERNiCrMo-3 (matching composition)
- No post-weld heat treatment (PWHT) required for most applications
- Very low susceptibility to heat-affected zone (HAZ) cracking
- Also widely used as a weld overlay / cladding material to protect carbon steel and low-alloy steel from corrosion
Inconel 718
Inconel 718 can be welded, but the process is more demanding:
- The alloy was specifically developed with “slow aging kinetics” in mind — meaning the γ” strengthening phase forms slowly enough that welding does not immediately cause strain-age cracking
- However, post-weld solution annealing and aging heat treatment is typically required to restore full mechanical properties
- Strict control of heat input, preheat, and interpass temperature is essential
- More susceptible to micro-segregation and weld solidification issues compared to 625
For projects requiring extensive welding or field repair, Inconel 625 is strongly preferred.
Typical Applications Side by Side
When to Specify Inconel 625
| Industry | Application |
|---|---|
| Chemical processing | Heat exchangers, reaction vessels, acid piping systems |
| Marine & offshore | Seawater piping, submarine cables, marine hardware |
| Oil & gas (surface) | Flare stacks, sour gas scrubbers, FGD systems |
| Nuclear | Reactor core components, spent fuel storage equipment |
| Environmental | Waste incineration liners, flue gas scrubbers |
| Aerospace | Exhaust systems, engine nacelles (where corrosion resistance > strength) |
When to Specify Inconel 718
| Industry | Application |
|---|---|
| Aerospace | Turbine disks, compressor blades, engine shafts, structural fasteners |
| Rocket propulsion | Motor casings, thrust chambers |
| Oil & gas (downhole) | High-strength wellhead components, BOP equipment, drill collars |
| Power generation | Steam turbine components, high-strength bolting |
| Cryogenics | Liquid oxygen/hydrogen tanks and fixtures |
Can Inconel 625 and 718 Be Substituted for Each Other?
In most cases, no. They share only about 45% compositional overlap and are designed around entirely different engineering objectives:
- Substituting 625 where 718 is required will result in insufficient mechanical strength
- Substituting 718 where 625 is required may lead to premature corrosion failure, especially in chloride or acidic environments
There is limited crossover only in applications where both corrosion resistance and moderate strength are needed, and neither extreme performance is required — but these cases are uncommon and should always be validated against project specifications.
Frequently Asked Questions
Q1: Which alloy has higher tensile strength — Inconel 625 or 718?
Inconel 718 is significantly stronger. In the fully aged condition, 718 achieves tensile strength of 1,240–1,550 MPa versus 827–930 MPa for 625. This is because 718 relies on precipitation hardening (γ” and γ’ phases), while 625 uses solid solution strengthening alone.
Q2: Which Inconel alloy is more corrosion resistant?
Inconel 625 offers superior corrosion resistance, particularly against pitting, crevice corrosion, and stress corrosion cracking in chloride environments. This is due to its high molybdenum content (8–10%). Inconel 718 has adequate general corrosion resistance but is not designed for aggressive chemical or marine environments.
Q3: Why is Inconel 625 easier to weld than 718?
Inconel 625 requires no post-weld heat treatment in most applications and has very low cracking susceptibility. Inconel 718 typically requires post-weld solution annealing and aging to restore its precipitation-hardened microstructure, making the welding process considerably more complex.
Q4: What is the maximum service temperature for each alloy?
Inconel 625 can be used at temperatures up to approximately 982°C (short term) or 816°C (continuous), with good oxidation resistance throughout. Inconel 718 is optimized for the 300–650°C range; above 650°C, its strengthening precipitates coarsen and the alloy progressively loses strength.
Q5: Which alloy is more expensive?
Pricing varies by product form, certification requirements, and market conditions. Generally, both alloys trade at a significant premium over standard stainless steel (40–40–90/kg range for industrial bar and plate). Aerospace-certified Inconel 718 (AMS 5662/5663, vacuum arc remelted) can carry a premium over standard 625, particularly for critical rotating components.
Summary: How to Choose
| If your priority is… | Choose |
|---|---|
| Maximum strength at 300–650°C | Inconel 718 |
| Corrosion resistance in acids or seawater | Inconel 625 |
| Easy welding and fabrication | Inconel 625 |
| Aerospace structural components | Inconel 718 |
| Chemical plant piping and equipment | Inconel 625 |
| Downhole oil & gas tools (high load) | Inconel 718 |
| Marine or offshore applications | Inconel 625 |
| Operating temperature above 700°C | Inconel 625 |
Both alloys are available from JA Alloy in sheet, plate, pipe, tube, bar, wire, and forged forms. If you need help confirming the right grade for your specific application and operating conditions, contact our technical team for a free consultation.
