Inconel 625 vs Inconel 718: Nickel Alloy Selection Guide
Inconel 625 and Inconel 718 are two of the most widely used nickel-based superalloys in demanding applications. While both offer excellent corrosion resistance and high-temperature performance, they have distinct characteristics that make each suitable for different applications. This guide provides a comprehensive comparison.
Overview of Inconel Alloys
Both alloys are austenitic nickel-chromium alloys strengthened by different mechanisms:
- Inconel 625: Solid solution strengthened ( Nb + Mo + Cr)
- Inconel 718: Precipitation hardened (Ni + Nb + Ti + Al)
Chemical Composition Comparison
Inconel 625 Composition
| Element | Content (%) |
|---|---|
| Nickel | 58.0 min |
| Chromium | 20.0 – 23.0 |
| Niobium (Columbium) | 3.15 – 4.15 |
| Molybdenum | 8.0 – 10.0 |
| Iron | 5.0 max |
| Carbon | 0.10 max |
| Manganese | 0.50 max |
| Silicon | 0.50 max |
| Aluminum | 0.40 max |
| Titanium | 0.40 max |
| Phosphorus | 0.015 max |
| Sulfur | 0.015 max |
Inconel 718 Composition
| Element | Content (%) |
|---|---|
| Nickel | 50.0 – 55.0 |
| Chromium | 17.0 – 21.0 |
| Niobium (Columbium) | 4.75 – 5.50 |
| Molybdenum | 2.80 – 3.30 |
| Iron | Balance (~17%) |
| Carbon | 0.08 max |
| Aluminum | 0.20 – 0.80 |
| Titanium | 0.65 – 1.15 |
| Manganese | 0.35 max |
| Silicon | 0.35 max |
| Cobalt | 1.00 max |
Mechanical Properties Comparison
Room Temperature Properties (Annealed Condition)
| Property | Inconel 625 | Inconel 718 |
|---|---|---|
| Tensile Strength | 760 MPa | 965 MPa (solution treated) |
| Yield Strength (0.2%) | 350 MPa | 550 MPa (solution treated) |
| Elongation | 30% | 30% (solution treated) |
| Hardness | 200-250 HB | 200-250 HB (annealed) |
Age-Hardened Properties (Inconel 718)
| Condition | Tensile Strength | Yield Strength | Elongation |
|---|---|---|---|
| Solution Treated | 965 MPa | 550 MPa | 30% |
| Double Aging | 1375 MPa | 1100 MPa | 12% |
| High Temp Aging | 1200 MPa | 1000 MPa | 15% |
Temperature Performance
Service Temperature Limits
| Alloy | Continuous Service | Intermediate Service | Stress Rupture |
|---|---|---|---|
| Inconel 625 | 982°C (1800°F) | 1093°C (2000°F) | Excellent at high temp |
| Inconel 718 | 700°C (1290°F) | 760°C (1400°F) | Good to 650°C |
Key Temperature Considerations
Inconel 625 Advantages at Temperature:
- Superior oxidation resistance at very high temperatures
- Better creep rupture strength above 650°C
- Maintains strength in thermal cycling
- Excellent for exhaust systems, heat exchangers
Inconel 718 Advantages at Temperature:
- Excellent strength up to 700°C
- Age-hardening provides sustained high strength
- Good fatigue resistance at elevated temperatures
- Widely used in gas turbine components
Corrosion Resistance Comparison
General Corrosion
Inconel 625:
- Excellent resistance to inorganic and organic acids
- Superior chloride ion stress corrosion cracking resistance
- Outstanding pitting and crevice corrosion resistance
- Excellent in seawater (with some limitations)
Inconel 718:
- Good resistance to general corrosion
- Moderate chloride stress corrosion resistance
- Acceptable in seawater (less than 625)
- Suitable for many chemical environments
Specific Environments
| Environment | Inconel 625 | Inconel 718 |
|---|---|---|
| Hydrochloric Acid | Excellent | Good |
| Nitric Acid | Excellent | Good |
| Sulfuric Acid | Excellent | Good |
| Phosphoric Acid | Excellent | Good |
| Seawater | Excellent | Good (limited) |
| High Temp Oxidation | Excellent | Good |
Fabrication and Welding
Inconel 625
- Formability: Good, similar to stainless steel
- Weldability: Excellent, no preheat required
- Welding Wire: ERNiCrMo-3
- Post-weld Heat Treatment: Not required
- Machining: Requires sharp tools, low speeds
Inconel 718
- Formability: More difficult (age-hardened state)
- Weldability: Good, but requires controlled heat input
- Welding Wire: ERNiFeCr-2
- Post-weld Heat Treatment: Recommended (age hardening)
- Machining: More difficult than 625
Applications Comparison
Inconel 625 Applications
- Aerospace exhaust systems
- Flare stacks
- Seawater systems
- Chemical process equipment
- Pollution control equipment
- Wastewater treatment
- Heat exchangers
- Oil and gas production
- Power plant components
Inconel 718 Applications
- Gas turbine components (discs, blades)
- Aircraft structural parts
- Spacecraft components
- Oil and gas wellhead components
- cryogenic tankage
- Fasteners
- Spring materials
- Medical implants
Cost Comparison
Inconel 718 is generally more expensive than Inconel 625 due to:
- Higher niobium content
- More complex processing requirements
- Additional heat treatment steps
- Higher demand in aerospace
Price differential: Typically 10-30% higher for Inconel 718
Selection Guide
| Requirement | Recommended Alloy |
|---|---|
| Very high temperature (>700°C) | Inconel 625 |
| Maximum strength (up to 700°C) | Inconel 718 (aged) |
| Best seawater resistance | Inconel 625 |
| Chloride stress corrosion | Inconel 625 |
| Gas turbine aerospace | Inconel 718 |
| Chemical processing | Inconel 625 |
| Ease of fabrication/welding | Inconel 625 |
| Cost-effective high strength | Inconel 718 (aged) |
| Post-weld no heat treatment | Inconel 625 |
| Cryogenic temperatures | Inconel 718 |
Conclusion
Both Inconel 625 and Inconel 718 are exceptional alloys for demanding applications. Inconel 625 excels in corrosion resistance and high-temperature oxidation service, making it ideal for chemical processing, marine, and exhaust applications. Inconel 718 provides superior strength at elevated temperatures when age-hardened, making it the preferred choice for aerospace gas turbines and high-stress components.
Need nickel alloy materials? Contact Xi’an Coremetal Steel for Inconel 625 and 718 pipe, tube, plate, sheet, and fittings.
Contact: Tracy | Email: tracy@coremetalsteel.com | Phone: +86 18291910632