Inconel® 718 vs Inconel® 625: A Comparative Insight

Introduction

Inconel® 718 and Inconel® 625 are two of the most renowned high-performance nickel-based superalloys developed by Special Metals Corporation. They are both designed for extreme environments, including high temperatures and corrosive conditions. While they share similar elemental foundations, their unique alloying elements and heat treatment responses result in distinct mechanical and chemical properties. This article provides a detailed comparison to help engineers and materials specialists choose the best alloy for specific applications.

Chemical Composition

Key Differences

• Inconel 718 contains significant amounts of niobium and titanium, enabling it to be precipitation-hardened for high strength.
• Inconel 625 has higher molybdenum and chromium content, which improves corrosion resistance especially in chloride and acidic environments.

Mechanical Properties

Key Differences

• Inconel 718 has much higher strength and hardness, making it suitable for load-bearing, high-stress applications.
• Inconel 625, while softer, offers greater ductility and weldability, making it easier to form and fabricate.

Corrosion Resistance

Inconel 625

• Outstanding resistance to pitting, crevice corrosion, and stress corrosion cracking in chloride environments.
• Performs excellently in seawater, chemical processing, and acidic environments like phosphoric or nitric acid.

Inconel 718

• Good general corrosion and oxidation resistance.
• Less resistant to chloride-induced corrosion compared to 625, especially in welded conditions.

Temperature Resistance

• Inconel 625 remains stable up to 980°C (1800°F), maintaining corrosion resistance under prolonged exposure.
• Inconel 718 can withstand temperatures up to 700°C (1300°F) for structural use, but starts to lose strength at higher temperatures due to aging mechanisms.

Typical Applications

Inconel 718

• Aerospace turbine engines (discs, shafts)
• Nuclear fuel element spacers
• High-strength fasteners and bolts
• Cryogenic tanks and structures

Inconel 625

• Marine exhaust systems, heat exchangers
• Chemical processing plants
• Offshore oil and gas risers and flare stacks
• Pollution control and waste treatment systems

Which One Should You Choose?

Need high strength and creep resistance under mechanical stress? → Choose Inconel 718

Need top-tier corrosion resistance in aggressive environments? → Choose Inconel 625

Conclusion

Both Inconel 718 and Inconel 625 are elite performers in harsh environments. Inconel 718 offers superior strength for load-bearing and high-pressure components, while Inconel 625 excels in corrosion-dominated scenarios. Your choice should be guided by whether mechanical strength or corrosion resistance is the priority in your application.