Inconel 625 vs 718: Choosing the Right Superalloy for CNC Machining

This article is part of the CNC Machining Materials Guide: Metals, Plastics & Superalloys on CNX Precision.

Choosing between Inconel 625 vs 718 is one of the most common decisions we see in aerospace and energy machining. Both alloys are nickel-based superalloys. Both resist heat, pressure, and corrosive media. However, they serve different jobs. This guide compares their chemistry, mechanical properties, and machinability. You will also see real-world applications for each alloy. In the end, you can select the right superalloy with confidence.

What Are Inconel 625 and Inconel 718?

Inconel 625 is a solid-solution strengthened alloy. Its matrix contains nickel, chromium, and molybdenum. This composition gives outstanding corrosion resistance. It also keeps useful strength from cryogenic temperatures up to about 980 °C. Moreover, it stays ductile in service. Therefore, marine, chemical, and offshore industries use it widely. Typical parts include heat exchangers, seawater piping, and flue-gas scrubbers. Additionally, 625 welds with relative ease compared with other superalloys.

Inconel 718 is precipitation hardened. Aging produces gamma double-prime phases inside the matrix. Consequently, it delivers much higher tensile and yield strength than 625. Its continuous service ceiling sits near 650 °C. For example, turbine disks, compressor blades, fasteners, and rocket casings rely on 718. When comparing Inconel 625 vs 718, remember that 718 gains its strength through heat treatment. In short, 625 resists corrosion, while 718 resists stress.

Microstructure explains most of the difference. In 625, molybdenum and niobium stay in solid solution. In 718, niobium forms strengthening precipitates during aging. Therefore, 718 is often sold in the solution-treated condition. Many shops machine 718 before aging and finish after heat treatment. Machining the aged condition is harder but avoids distortion on thin sections. Understanding this sequence helps you plan the process window with your supplier.

Material standards guide the purchase. Inconel 625 is covered by AMS 5599 and ASTM B443. Inconel 718 follows AMS 5662 and ASTM B637. These specifications define chemistry, heat treatment, and mechanical properties. Always request certified material with full traceability. In addition, confirm the condition of the bar stock. Solution-treated material machines differently than aged material.

Inconel 625 vs 718: Key Differences

The table below summarizes the main points. Use it whenever you compare Inconel 625 vs 718 for a new project.

Strength alone does not decide the winner. First, review the operating temperature. Second, consider the corrosive media. Third, estimate cyclic loads. Each factor shifts the balance between Inconel 625 vs 718. Also remember that pricing differs by batch, bar size, and heat treatment. Material cost is only one part of the equation. Machining time, tool wear, and finishing also affect the final price.

Where does each alloy shine in practice? Inconel 625 dominates chemical processing, oil-and-gas cladding, and marine fasteners. Inconel 718 dominates gas turbines and cryogenic rocket hardware. In aerospace, 718 accounts for a large share of turbine disk material. In marine engineering, 625 often replaces stainless steel when chlorides cause pitting. Therefore, match the alloy to the environment, not just the drawing tolerance.

Machining Inconel 625 vs 718

Both alloys work-harden quickly. Therefore, machining requires rigid setups and consistent tool engagement. Use carbide inserts with positive rake angles. Alternatively, use ceramic grades for roughing. Keep cutting speeds low. Never let the tool dwell on the surface, because rubbing creates a hardened layer. In addition, choose a heavy enough feed to cut under the work-hardened zone.

When you machine Inconel 625 vs 718, chip control matters. Inconel 625 tends to be gummy in the annealed condition. Inconel 718, after aging, produces short, hard chips. High-pressure coolant helps both cases. It also extends tool life and protects thin-wall features. For turning, use a rigid tailstock and consistent depth of cut. For milling, climb milling reduces heat and improves surface finish.

As a rule, cut Inconel at 10 to 20 percent of the speed used for alloy steel. Feed rates stay moderate, and depth of cut remains steady. Tools with honed edges resist chipping. When roughing, take the full alloy layer in one pass where possible. Finishing passes should be light but never interrupted. These habits keep tolerances stable across a batch.

Threading Inconel also needs attention. Single-point threading with carbide works on simple forms. Thread milling is more stable for larger threads. In addition, slotting creates high edge pressure. Use a high-feed cutter with a small lead angle. These choices reduce deflection and chatter on thin sections.

At CNX Precision, we cut both alloys every week. Our 3/4/5-axis machining centers hold tight tolerances on aerospace brackets, medical instruments, and energy components. We also verify dimensions with CMM reports. In addition, we supply material certificates with every order. Our engineers suggest the best tool paths before production starts.

How to Choose Between the Two Superalloys

Ask three questions before choosing between Inconel 625 vs 718. First, what temperature will the part see? Second, what environment surrounds it? Third, what loads must it carry?

• Choose Inconel 625 for seawater, chemical, and chloride-rich service.
• Choose Inconel 718 for high-strength structural and rotating parts.
• Choose 625 when welded assemblies need crack-free fabrication.
• Choose 718 when fatigue life drives the design.

Application examples clarify the choice. A valve body for a chemical skid may only see 400 °C. Inconel 625 handles it easily and resists pitting. A turbine disk may see 600 °C with high stress. Inconel 718 meets that demand. Therefore, match the alloy to the hottest, most corrosive, or most stressed point in the assembly.

In addition, review the component life cycle. Maintenance cost often outweighs material cost. A stronger alloy may allow thinner walls and lower weight. A corrosion-resistant alloy may eliminate coating and inspection cycles. Therefore, analyze total cost, not just raw material price. When in doubt, send us your drawing. Our engineers will review the material callout and propose the best option.

Also consider the joining method. If the design uses welding, 625 simplifies fabrication. If the design uses mechanical fasteners, 718 provides stronger threaded joints. Furthermore, coating choice interacts with the alloy. Some anti-galling coatings work better on 718. Review the full assembly before finalizing the material.

Frequently Asked Questions

Is Inconel 625 stronger than 718?

No. Inconel 718 is stronger after aging. Its yield strength can exceed 1000 MPa. In comparison, annealed 625 yields near 414 MPa. Therefore, choose 718 for load-bearing and rotating parts.

Can you weld Inconel 625?

Yes. Inconel 625 offers excellent weldability. It resists post-weld cracking and stays ductile. For that reason, fabricators prefer it for complex welded assemblies and repair work.

What is the price difference between Inconel 625 vs 718?

Prices vary with market conditions, bar size, and quantity. Generally, 718 costs more after heat treatment. However, raw material prices fluctuate. Request a quote to compare apples to apples.

Ready to machine a superalloy component? Send your CAD file to CNX Precision. We machine parts in Inconel 625 vs 718 with documented quality and dependable lead times. Get a free quotation today.

For related information, see our guide to cnc machining service and 5-axis cnc machining and cnc milling vs turning and cnc machining tolerances.