Understanding the Differences: Inconel 625 vs Inconel 718

Inconel 625 and Inconel 718 are two high-temperature nickel alloys that share a lot of similarities. Both are members of the Inconel family, renowned for their superb resistance to corrosion and high-temperature performance. Nevertheless, each alloy excels in specific aspects due to their unique chemical compositions and physical properties. This article will dissect the differences between Inconel 625 vs Inconel 718 in terms of their composition, properties, applications, and more.

1. Terminology

Before we proceed, it’s crucial to clarify the nomenclature of these two alloys. The terms ‘Inconel 625’ and ‘alloy 625,’ as well as ‘Inconel 718’ and ‘alloy 718,’ are used interchangeably throughout this article. Both refer to the same respective alloys, with ‘Inconel’ being a registered trademark of the Special Metals Corporation. For legal reasons, most alloy suppliers use the ‘alloy’ prefix.

2. Composition

2.1 Inconel 625

Inconel 625, identified as UNS N06625, is primarily composed of nickel, chromium, and molybdenum, with niobium playing a significant role. It is a solid solution strengthened alloy, meaning its strength is primarily achieved through solid solution mechanisms.

2.2 Inconel 718

On the other hand, Inconel 718, designated as UNS N07718, is a nickel-chromium alloy that also incorporates significant amounts of iron, niobium, and other elements. Unlike Inconel 625, Inconel 718 is a precipitation-strengthened alloy, which means it gains its strength from precipitations or particles that form within the metal’s matrix during the heat treatment process.

3. Mechanical Properties

3.1 Strength and Hardness

One of the most significant differences between Inconel 625 and Inconel 718 lies in their strength. While both alloys are incredibly strong, Inconel 718 outshines Inconel 625 in terms of hardness and tensile strength. This is primarily due to its unique precipitation strengthening mechanism, which allows it to maintain high tensile strength even in extreme conditions.

3.2 Corrosion Resistance

On the flip side, Inconel 625 showcases superior corrosion resistance. Its elevated levels of nickel and chromium, coupled with the presence of molybdenum, make this alloy incredibly resistant to corrosive environments, including seawater and harsh chemicals.

4. Heat Resistance

Both Inconel 625 and Inconel 718 are renowned for their exceptional heat resistance. They can maintain their strength and stability at extremely high temperatures, making them ideal for applications involving high heat. However, Inconel 718 has a slightly higher melting range compared to Inconel 625.

5. Weldability

Inconel 625 and Inconel 718 share excellent weldability. However, Inconel 718 stands out for its superb resistance to post-weld cracking, a common issue faced during the welding process. This characteristic makes it particularly advantageous in applications that involve complex welding procedures.

6. Formability

The formability of both alloys is primarily influenced by their high concentration of nickel. Both Inconel 625 and Inconel 718 can be effortlessly formed and machined into various shapes and sizes, making them suitable for a wide range of applications.

7. Heat Treatment

The heat treatment for these two alloys differs based on their strengthening mechanisms. Inconel 625, being a solid solution strengthened alloy, generally undergoes a solution treatment to achieve optimal performance. In contrast, Inconel 718 typically undergoes an aging treatment following a solution treatment to precipitate the strengthening phase and enhance the alloy’s strength.

8. Applications

Both Inconel 625 and Inconel 718 find extensive applications across diverse industries due to their remarkable properties. However, their specific compositions and properties make each alloy excel in certain fields.

8.1 Inconel 625 Applications

Inconel 625’s exceptional corrosion resistance makes it an ideal choice for the oil and gas industry, particularly for subsea equipment. It also finds use in the aerospace industry, specifically in corrosive environments like exhaust systems, furnace muffles, and engine thrust reversers.

8.2 Inconel 718 Applications

Inconel 718’s incredible heat resistance makes it perfect for high-temperature environments, such as aerospace and automotive engine components. It is also used in high-performance car engines within the motorsport industry.

9. Product Forms

Both Inconel 625 and Inconel 718 are available in a variety of product forms due to their excellent formability and weldability. These include but are not limited to pipes, coils, welding wires, sheets, plates, round bars, and fittings.

10. Standard Specifications

Inconel 625 and Inconel 718 have different standard specifications according to their applications and product forms. For instance, the ASTM B443 standard applies to Inconel 625 plates, sheets, and strips, while the ASTM B670 standard applies to Inconel 718 plates, sheets, and strips.

11. Price Considerations

The price of Inconel 625 and Inconel 718 can vary based on factors like the product’s form, size, and specific grade. However, due to their specialized applications and superior properties, both alloys tend to be more expensive than conventional materials.

12. Conclusion

In summary, while both Inconel 625 and Inconel 718 share several similarities, their unique compositions and properties make each alloy suitable for specific applications. Inconel 625 excels in corrosion resistance, making it a prime choice for applications in corrosive environments, while Inconel 718 is known for its high strength, making it ideal for high-stress environments. Understanding these differences is crucial when selecting the right alloy for your specific application needs.

Table: Comparison Between Inconel 625 and Inconel 718

FeatureInconel 625Inconel 718
CompositionPrimarily nickel, chromium, and molybdenumNickel, chromium, iron, and niobium
Strengthening MechanismSolid Solution StrengtheningPrecipitation Strengthening
Tensile StrengthLower than 718Higher than 625
Corrosion ResistanceHigherLower
Heat ResistanceHighSlightly Higher
WeldabilityExcellentResistant to post-weld cracking
Heat TreatmentSolution TreatmentAging Treatment
ApplicationsOil and Gas, AerospaceAerospace, Automotive
Product FormsVariedVaried

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