What Fastener Do I Use for High Heat or Corrosive Environments?
When it comes to selecting the right fastener for demanding applications, material selection can make all the difference. High temperatures, extreme loads, and corrosive environments each present their own challenges, and choosing the right alloy is critical to lasting performance.
Two materials commonly used in these environments are Inconel 625 and Inconel 718. Both are nickel-based superalloys, but they offer different advantages depending on the application. Inconel 718 is known for its exceptional strength at high temperatures, while Inconel 625 offers excellent corrosion resistance in extreme environments.
Omni Fasteners stocks and manufactures fasteners in both alloys, supporting customers in aerospace, defense, energy, and chemical processing.
What Are the Differences Between Inconel 625 and Inconel 718?
Although these alloys share similar characteristics, the biggest difference lies in how they achieve their strength.
Inconel 625 (UNS N06625) uses solid-solution strengthening, with elements like molybdenum and niobium contributing to its mechanical properties. This makes it particularly well suited for applications where corrosion resistance is a primary concern.
Inconel 718 (UNS N07718) achieves higher strength through precipitation hardening, a heat-treating process that greatly improves its mechanical properties. Under typical conditions, Inconel 718 offers approximately 2.5 times the yield strength of annealed Inconel 625, with typical values of 150 ksi versus 60 ksi, respectively.
Choose Inconel 718 when high mechanical strength is the priority, and consider Inconel 625 when corrosion resistance is the primary concern.
Although they have their similarities, these materials are not interchangeable. Understanding operating conditions and performance requirements is key when selecting the appropriate Inconel alloy.
Inconel 625 vs. Inconel 718: Which Alloy Should I Use?
Both alloys perform extremely well in demanding environments, but their strengths become more apparent when comparing mechanical properties, corrosion resistance, and temperature capabilities.
The following comparison highlights typical properties for Inconel 625 in the annealed condition and Inconel 718 in the solution-treated and aged condition. Actual requirements should always be verified against the applicable material specifications and mill certifications.
| Property | Inconel 625 | Inconel 718 |
| UNS number | N06625 | N07718 |
| Strengthening | Solid solution (Mo, Nb) | Precipitation hardened (Nb, Ti, Al) |
| Nickel / chromium / molybdenum (wt %) | 58 min / 20–23 / 8–10 | 50–55 / 17–21 / 2.8–3.3 |
| Niobium (wt %) | 3.15–4.15 | 4.75–5.5 |
| Tensile strength, typical | 120 ksi (827 MPa) | 180–185 ksi (1,240–1,275 MPa) |
| Yield strength, typical | 60 ksi (414 MPa) | 150 ksi (1,035 MPa) |
| Elongation | 30% or more | 12–20% |
| Hardness | ~95 HRB | 36–44 HRC |
| Max service temperature under load | ~1,200°F (650°C) with useful strength; oxidation resistance to ~1,800°F (980°C) | ~1,200–1,300°F (650–700°C); precipitates coarsen above that and strength drops |
| Cryogenic service | Good ductility | Excellent; widely used in cryogenic and LNG fasteners |
| Corrosion resistance | Excellent in seawater, chlorides, sour gas, acids; PREN above 50 | Good general resistance; weaker than 625 in aggressive chlorides and acids |
| Common fastener specs | ASTM B446, AMS 5666, ASTM F468 | ASTM B637, AMS 5662 / AMS 5663, ASTM F468 |
| Relative cost | Higher per pound (more Mo and Ni) | Lower per pound; aging adds processing |
What Is the Difference Between AMS 5662 and AMS 5663?
Both AMS 5662 and AMS 5663 cover Inconel 718, but there is an important difference in how the material is supplied.
AMS 5662 covers Inconel 718 in the solution-treated condition. The material is softer, making it easier to machine or form before undergoing the aging process needed to achieve higher strength.
AMS 5663 covers Inconel 718 that is solution treated and precipitation hardened through aging. This material is supplied in the higher-strength condition.
Both specifications cover the same alloy, they are not automatically interchangeable. If your application calls for AMS 5662 or AMS 5663, make sure the material and heat-treatment condition meet the specified requirements and let us know before ordering.
When Should I Use Inconel 718 Fasteners?
Inconel 718 fasteners are an excellent choice for applications requiring high mechanical strength, especially when operating under heavy loads and elevated temperatures.
With a typical yield strength of approximately 150 ksi the aged condition, Inconel 718 also offers excellent creep resistance and fatigue performance. These properties make it a great choice for high load applications at temperatures up to 1,200°F (650°C), depending on the application.
Common applications for Inconel 718 bolts and fasteners include:
- Aerospace applications, including jet engines, turbine casings, flanges, and high-temperature hardware
- Rocket engines and launch vehicle structural components
- Oil and gas equipment requiring high tensile strength
- Nuclear and power generation equipment
- Cryogenic storage and LNG equipment requiring strength and toughness at extremely low temperatures
One thing to consider when ordering custom Inconel 718 fasteners is the manufacturing process. Inconel 718 can be difficult to machine, particularly in its hardened condition, and may require additional lead time.
If your application requires a special head configuration, thread, diameter, or length, planning ahead can help avoid unnecessary delays.
When Should I Use Inconel 625 Fasteners?
When corrosion resistance is the primary concern, Inconel 625 is often the better choice.
It has a high chromium and molybdenum content and provides excellent resistance to aggressive environments, including seawater, chlorides, and many acids. Inconel 625 also offers strong resistance to pitting, crevice corrosion, and chloride stress-corrosion cracking.
Another advantage is its resistance to oxidation at elevated temperatures. Inconel 625 performs well in certain high-temperature applications approaching 1,800°F (980°C), the maximum temperature depends on the mechanical loads and the operating conditions.
Common applications for Inconel 625 bolts and fasteners include:
- Marine, offshore, and subsea equipment exposed to seawater
- Oil and gas equipment operating in sour-gas environments, subject to NACE MR0175 / ISO 15156 requirements
- Chemical processing equipment exposed to acids and other corrosive substances
- Exhaust systems, furnace equipment, and turbine ducting
- Flue gas treatment and environmental processing equipment
Inconel 625 is also easier to weld and form than many other precipitation-hardened superalloys, making it very useful for fasteners requiring additional fabrication steps.
The biggest trade-off is mechanical strength. Annealed Inconel 625 has a typical yield strength of approximately 60 ksi, compared with 150 ksi for aged Inconel 718.
If your application requires both high strength and corrosion resistance, additional engineering considerations may be necessary. Depending on the application, options could include changes to the fastener size, joint design, or an alternative alloy such as Alloy 725 or Monel K-500.
What Should I Specify When Ordering Inconel Fasteners?
When ordering Inconel bolts, studs, or other fasteners, providing the correct specifications on your prints helps ensure you receive the material your application needs.
Here are some important details to include when requesting a quote:
Material and specification: Identify Inconel 718 (UNS N07718) or Inconel 625 (UNS N06625), along with the applicable material specification. Common specifications include ASTM B637, AMS 5662, and AMS 5663 for Inconel 718, and ASTM B446 or AMS 5666 for Inconel 625. ASTM F468 may also apply to certain fasteners.
Material condition: Specify the required heat treatment. Inconel 718 may be ordered in the solution-treated or solution-treated and aged condition. Inconel 625 is commonly supplied annealed.
Thread requirements: Include thread size, pitch, class of fit, and any manufacturing requirements, such as rolled or cut threads. Thread manufacturing methods can affect fatigue performance, particularly in high load applications.
Coatings and lubricants: Nickel-alloy fasteners can be susceptible to galling during assembly. Depending on your application, silver plating, dry-film lubricants, or other approved anti-galling treatments may be needed. We can supply those too.
Documentation and compliance: Identify requirements for material test reports (MTRs), certificates of conformance (COCs), traceability, and DFARS specialty metals compliance when applicable. Also identify any additional testing that may be required.
Omni Fasteners supports our customers with standard and custom Inconel fasteners, including nuts, bolts, shoulder bolts, studs, and set screws.
Frequently Asked Questions About Inconel 625 and Inconel 718
Q: Is Inconel 718 stronger than Inconel 625?
A: Yes. In aged condition, Inconel 718 offers a typical yield strength of approximately 150 ksi, compared with 60 ksi for annealed Inconel 625. Typical tensile strengths are approximately 180 ksi for Inconel 718 and 120 ksi for Inconel 625. Actual properties depend on material condition and specification.
Q: Which Inconel is more corrosion resistant, 625 or 718?
A: Inconel 625 generally offers better resistance to many aggressive corrosive environments. Its chromium content of approximately 20–23% and molybdenum content of 8–10% provide excellent resistance to seawater, chlorides, and many industrial chemicals.
Q: What is the maximum operating temperature for Inconel 718 fasteners?
A: Inconel 718 is commonly used in high-strength applications at temperatures up to approximately 1,200°F (650°C). The maximum allowable operating temperature depends on the load, exposure time, and applicable engineering requirements.
Q: What is the maximum operating temperature for Inconel 625 fasteners?
A: Inconel 625 offers oxidation resistance at temperatures approaching 1,800°F (980°C). However, applications involving significant mechanical loads may require much lower operating temperatures. Always verify the requirements for your specific application.
Q: Can I substitute Inconel 625 for Inconel 718?
A: Not without an engineering review. Although both are nickel-based superalloys, their mechanical properties, heat-treatment conditions, and corrosion resistance are different. Substituting one for the other can affect the performance and safety of the assembly.
Q: Do Inconel fasteners require a coating?
A: Inconel 625 and Inconel 718 offer inherent corrosion resistance, so additional corrosion-protective coatings may not be necessary. However, anti-galling treatments such as silver plating or approved thread lubricants may be required depending on the application.
Q: What specifications should I use when ordering Inconel fasteners?
A: Common material specifications include ASTM B637, AMS 5662, and AMS 5663 for Inconel 718, and ASTM B446 or AMS 5666 for Inconel 625. ASTM F468 covers certain finished nonferrous bolts and studs. Always verify the material condition and specifications listed on your drawing.
Q: Looking for Inconel 625 or Inconel 718 Fasteners?
A: Selecting the right material is only part of the process. Making sure your fasteners meet the correct specifications, operating conditions, and documentation requirements is just as important.
Omni Fasteners manufactures and supplies standard and custom Inconel 625 and Inconel 718 fasteners for aerospace, defense, energy, and industrial applications.
Whether you need standard bolts, custom studs, or specialty fasteners, our team can help you source the right product for your application.
Contact Omni Fasteners with your drawing or RFQ, and let us help you find the right fastening solution.