Inconel 625 vs 825: A Comprehensive Comparison

In the world of high-performing alloys, Inconel 625 and Incoloy 825 are two names that stand out. These superalloys, known for their remarkable resilience and versatility, are used in a plethora of applications across various industries. This article aims to provide a detailed comparison of Inconel 625 vs 825, exploring their unique properties, chemical compositions, and applications.

1. Basic Overview

Inconel 625 and Incoloy 825 are high-performance alloys predominantly made up of nickel. Developed by Special Metals Corporation, these materials are renowned for their exceptional strength, corrosion resistance, and ability to withstand extreme temperatures. Although they share several similarities, they also exhibit differences that make them suitable for distinct applications.

1.1 Inconel 625

Inconel 625, recognized by the Unified Numbering System (UNS) as N06625 and Werkstoff Number (W.Nr.) as 2.4856, is a solid solution strengthened alloy. It has a high nickel content and includes other elements such as chromium, molybdenum, and niobium. This alloy is known for its high strength, superb fabricability, and outstanding corrosion resistance.

1.2 Incoloy 825

On the other hand, Incoloy 825, identified as UNS N08825 and W.Nr. 2.4858, is a nickel-iron-chromium alloy. It also contains elements like molybdenum, copper, and titanium. This alloy is designed to provide exceptional resistance to various corrosive environments.

2. Chemical Composition

The chemical composition of Inconel 625 and Incoloy 825 significantly influences their properties and performance.

2.1 Inconel 625

Inconel 625 consists predominantly of nickel (58% minimum), followed by chromium (20-23%), molybdenum (8-10%), and niobium (3.15-4.15%). It also contains trace amounts of elements such as iron (5% maximum), manganese (0.50% maximum), and silicon (0.50% maximum).

2.2 Incoloy 825

Incoloy 825, on the other hand, has a higher iron content (22% minimum) and lower nickel content (38-46%). Additionally, it includes chromium (19.5-23.5%), molybdenum (2.5-3.5%), copper (1.5-3%), and titanium (0.6-1.2%).

3. Purity and Corrosion Resistance

The purity and corrosion resistance of these alloys are influenced by their sulfur, manganese, and phosphorus content, as well as their iron, nickel, and molybdenum compositions.

3.1 Purity

The sulfur content in Inconel 625 is strictly controlled as it significantly impacts the alloy’s transient performance and durability. This alloy also contains lower amounts of manganese, which affects the alloy’s durability. Phosphorus in Inconel 625 segregates to form a harmful phase, consuming a considerable amount of niobium and impeding strengthening.

3.2 Corrosion Resistance

Incoloy 825 has a higher iron content, reducing its cost but also decreasing its corrosion resistance. In contrast, the high nickel and molybdenum content in Inconel 625 provides a solid foundation for corrosion resistance.

4. Mechanical Behavior

The mechanical behavior of these alloys is determined by their niobium, tantalum, and molybdenum content.

4.1 Inconel 625

Inconel 625, reinforced with niobium and tantalum, has a stronger solid solution strengthening effect, leading to higher mechanical strength. Further, the larger amount of molybdenum in this alloy also contributes to its strengthening effect.

4.2 Incoloy 825

Incoloy 825, although also a solid solution strengthened alloy, lacks the addition of niobium and tantalum, resulting in lower mechanical strength.

5. Physical Properties

Notable physical properties of these alloys include their density and melting range.

5.1 Inconel 625

Inconel 625 has a density of 8.44 g/cm³ and a melting range of 1290-1350°C.

5.2 Incoloy 825

Incoloy 825, in contrast, has a slightly lower density of 8.14 g/cm³ and a higher melting range of 1370-1400°C.

6. Applications

The applications of these alloys depend on their unique properties. However, it’s worth noting that their application ranges are nearly identical—Incoloy 825 was designed as a cost-effective alternative to Inconel 625.

6.1 Inconel 625

Inconel 625 is typically used in demanding applications such as gas turbines, nuclear reactors, chemical processing equipment, and heat exchangers.

6.2 Incoloy 825

Incoloy 825, meanwhile, is commonly used in less demanding applications like exhaust manifolds and petroleum refining equipment.

7. Tensile Strength

Tensile strength, the ability to resist stretching forces, is a crucial property of these alloys. Inconel 625 exhibits a higher tensile strength of 827 MPa, while Incoloy 825 displays a lower tensile strength of 586 MPa.

8. Yield Strength

Yield strength refers to the stress at which a material begins to deform plastically. Inconel 625 has a yield strength of 414 MPa, while Incoloy 825’s yield strength is measured at 241 MPa.

9. Elongation

Elongation refers to a material’s ability to deform before breaking. Inconel 625 has an elongation of 42.5%, while Incoloy 825 has an elongation of 30%.

10. Price

Although Incoloy 825’s performance is not as robust as Inconel 625, it is less expensive. So, if the performance of Incoloy 825 is adequate for a specific application, it is recommended to use this alloy to reduce costs.

11. Standards

Inconel 625 and Incoloy 825 are produced according to various international standards. These standards guide the production and quality of these materials.

12. Conclusion

In comparing Inconel 625 vs 825, it’s clear that each alloy has its unique strengths and weaknesses. Inconel 625 is known for its high-temperature strength and corrosion resistance, making it ideal for demanding applications. On the other hand, Incoloy 825 is a cost-effective alternative with adequate performance for less demanding applications. The choice between these two superalloys ultimately depends on the specific requirements of your project.

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