Inconel 625 vs. 718: A Performance-Based Selection Guide

The Critical Choice

Choosing between Inconel 625 and Inconel 718 is a big challenge for engineers in demanding industries. These are both top nickel-based superalloys, but you can't just swap one for the other. You need to understand their main differences to make the right choice.

The core difference is simple: Inconel 625 works best for fighting corrosion and is easier to work with. Inconel 718, on the other hand, gives you much better strength and hardness at high temperatures, up to about 700°C (1300°F).

This key difference determines where each material is used. You'll find 625 in marine and chemical processing equipment. 718 is mostly used in aerospace parts that must handle heavy stress. This guide will help you compare these materials based on facts. When deciding between Inconel 625 vs 718, think about whether your biggest problem is a harsh environment or mechanical pressure.

At a Glance

This table shows a direct comparison of the most important properties that make Inconel 625 and Inconel 718 different.

Property	Inconel 625	Inconel 718
Primary Strength	Superior Corrosion Resistance	Superior Mechanical Strength & Hardness
Strengthening Mechanism	Solid Solution Hardening	Precipitation Hardening (Age-Hardenable)
Key Alloying Elements	High Mo & Nb	High Fe, Nb, plus Ti & Al
Tensile Strength (Annealed)	~880 MPa / 128 ksi	~1035 MPa / 150 ksi
Yield Strength (Annealed)	~460 MPa / 67 ksi	~725 MPa / 105 ksi
Yield Strength (Aged)	N/A (Not age-hardenable)	~1140 MPa / 165 ksi
Max Service Temperature	~980°C / 1800°F (Low Stress)	~700°C / 1300°F (High Stress)
Weldability	Excellent	Good (Requires specific procedures)

Performance Deep Dive

Chemical Composition

The performance of these alloys comes from their different chemical makeup. Each element plays a specific role in how the material performs.

Inconel 625 has a high Nickel-Chromium (Ni-Cr) base with added Molybdenum (Mo) and Niobium (Nb). These elements mix into the nickel-chrome matrix, making the material stronger and much more resistant to corrosion.

Inconel 718 also has a Ni-Cr base but contains much more Iron (Fe). The big difference is that it also has Titanium (Ti) and Aluminum (Al) along with Niobium (Nb). During heat treatment, these elements form tiny hardening particles. For more details, check these detailed chemical compositions.

Mechanical Strength

The biggest difference between Inconel 625 vs 718 is how strong they are. This comes from how they're strengthened.

When heat-treated, Inconel 718 is much stronger and harder than Inconel 625. It stays stronger from room temperature up to about 650-700°C (1200-1300°F).

This strength comes from tiny, hard particles that form inside the material during heat treatment. These particles block movement within the metal, making it very resistant to bending or breaking under pressure.

Inconel 625 gets its strength differently and isn't as strong as 718 in normal temperature ranges. But 625 keeps its strength better above 700°C. At these higher temperatures, it's often used where corrosion resistance matters more than strength.

Corrosion Resistance

This is where Inconel 625 really shines. Its chemistry is specially designed to stand up to harsh environments.

The high amounts of chromium and molybdenum in Inconel 625 provide excellent protection against many types of corrosion. It resists both general corrosion and focused attacks like pitting, especially in salty environments like seawater. It also works very well in acidic conditions.

While Inconel 718 has good corrosion resistance, it can't match 625. Also, when 718 is heat-treated for maximum strength, it can become vulnerable to cracking in certain environments. This makes 625 the safer choice when chemical attack is your main concern, as shown by recent studies on corrosion performance.

Practical Considerations

Beyond material properties, real-world factors like manufacturing and cost are important for making a good decision.

The Machining Challenge

Both alloys are very difficult to machine. The properties that make them great for extreme conditions also make them hard to cut and shape.

They stay strong even when heated during cutting, create lots of heat at the cutting tool, and quickly harden when touched by cutting tools.

Inconel 718, especially after heat treatment, is generally harder to machine than Inconel 625. Its greater hardness and strength demand more from cutting tools and machines.

Key machining challenges include:
* Rapid Work Hardening: The material's surface hardens during machining, making later cuts harder and wearing out tools faster.
* High Cutting Forces: The material's strength requires strong tooling, powerful machines, and rigid setups to prevent vibration and tool movement.
* Poor Thermal Conductivity: Heat from cutting doesn't move away through the chips well. Instead, it builds up on the cutting tool and workpiece, causing tools to fail early and possibly damaging the part.

To successfully machine these alloys, we recommend using slow cutting speeds. Keep feed rates high and steady. Use very rigid tooling and workholding. Apply plenty of high-pressure coolant to manage heat and clear chips. This topic is often discussed among professional machinists.

Weldability

For welding, there is a clear winner.

Inconel 625 is known for being excellent to weld. It resists cracking during welding and doesn't crack during heat treatment after welding. You can weld it using many standard methods.

Inconel 718, however, can easily crack during or after welding. The elements that make it strong can form brittle areas in the heat-affected zone during welding. To make a reliable joint without cracks, 718 usually must be welded before heat treatment, then heat-treated afterward. This makes the fabrication process more complex and expensive.

Cost Analysis

The cost of a material is more than just its price per pound. You need to consider the total cost of ownership.

For raw materials, Inconel 718 often costs more than Inconel 625. This is because of its more complex composition and the heat treatments it needs.

However, the total cost of a finished part depends heavily on manufacturing. The extreme difficulty of machining Inconel 718 can lead to much higher costs due to longer machining times, more tool wear, and more scrapped parts. In some cases, a part made from the "cheaper" Inconel 625 could actually cost less if the design is complex and needs a lot of machining.

Application Scenarios

The choice between Inconel 625 vs 718 becomes clear when you look at specific applications.

When to Use 625

Choose Inconel 625 when your main concern is fighting corrosive environments, especially when combined with high temperatures and complex fabrication.

• Marine Engineering: Its resistance to salt water corrosion makes it perfect for seawater systems, propeller blades, and underwater connectors.
• Chemical and Petrochemical Processing: Used for reaction vessels, pipes, and valves that handle acidic and corrosive materials.
• Aerospace Exhaust and Ducting Systems: Its good workability and resistance to high-temperature oxidation make it great for engine exhaust parts, bellows, and heat exchanger casings where environment resistance matters more than strength.

When to Use 718

Choose Inconel 718 when you need the highest possible mechanical performance—strength, hardness, and resistance to deformation—at temperatures up to 700°C (1300°F).

• Aerospace and Gas Turbines: This is its main use. It's used for critical, high-stress parts like turbine discs, blades, shafts, and compressor parts in jet engines.
• Oil and Gas Industry: Its high strength makes it perfect for downhole tools, wellhead parts, and high-pressure fasteners used in sour gas environments where strength is crucial.
• Nuclear Applications: Used for structural components and instrumentation in reactors where high strength and good corrosion resistance are needed under extreme conditions.

The Final Verdict

The decision between Inconel 625 and Inconel 718 comes down to choosing between corrosion resistance and mechanical strength.

To make the choice easier, think about the main challenge your component will face.

If Your Priority Is...	The Better Choice Is...	Because...
Corrosion Resistance	Inconel 625	Its high Cr and Mo content offers superior protection against a wide range of corrosive media.
Fabricability & Weldability	Inconel 625	It is far more forgiving to weld and fabricate, reducing manufacturing complexity and cost.
Mechanical Strength & Hardness	Inconel 718	Its age-hardenable nature provides significantly higher strength up to ~700°C (1300°F).
High-Stress Applications	Inconel 718	It is designed to resist deformation and creep under extreme mechanical loads within its service temperature.

By matching the strengths of each superalloy with the specific needs of your application, you can ensure optimal performance, reliability, and long life for your most critical components.