A comparison of INCONEL alloys 625 & 725

INCONEL is chosen over INCONEL 625 when exceptionally high strength is required, for instance for high-strength fasteners in the marine industry.

When you’re looking for a superior material for your next project, a high-performance nickel alloy might be just what you need. However, several grades are available in the INCONEL range, any of which could suit your requirements and needs.

By Jan Ward, Corrotherm

The flagship INCONEL alloy is INCONEL alloy 625, one of the most versatile materials ever created. Its outstanding corrosion resistance and high strength lend itself to many applications across a wide range of industries. It’s a hugely popular material of choice that will solve many material problems for design engineers. Its common usage for over 50 years is a testament to its quality. However, if high strength is the priority, INCONEL alloy 725 can be a better fit. It is similar to INCONEL alloy 625, especially in terms of corrosion resistance. However, there are notable differences between the two, which can affect the decision of which to use.
This article compares their main features and properties to help you understand the differences and similarities between these two exceptional materials.

Differences in composition

The reason that INCONEL alloy 625 offers such a wide range of desirable properties lies in its unique make-up. It’s a nickel-heavy alloy, with a minimum 58% of nickel in the blend. It also has chromium (20-23%) and molybdenum (8-10%), plus added niobium (3.15-4.15%).
The niobium combines with molybdenum and distorts the alloy’s atomic matrix, imparting particularly high strength in annealed condition without a specific strengthening heat treatment. The high quantities of nickel and chromium together give the material its incredible corrosion resistance, withstanding highly corrosive elements and environments. INCONEL alloy 725 shares many of these compositional elements – in fact, they are nearly identical. It retains a high quantity of both nickel (55-59%) and chromium (19-22.5%), plus molybdenum (7-9.50%). As such, the material has similar corrosion resistance to INCONEL alloy 625, making it suitable for use in a broad range of highly corrosive environments.
However, there are subtle differences in the overall composition of the two materials. INCONEL alloy 625 contains small amounts of cobalt and silver, which are not present in the mix for INCONEL alloy 725. INCONEL alloy 725 has a higher amount of titanium (1-1.7% as opposed to 0.4% maximum in INCONEL alloy 625), and iron makes up the balance, rather than having a 5% maximum as per INCONEL alloy 625. These small additions and differences allow for micro precipitates to form during the heat-treating process, giving INCONEL alloy 725 its hugely increased strength value.

INCONEL 625 is commonly used in sea-water applications such as submarine propulsion motors.

Comparing strength

INCONEL alloy 625 is a strong nickel alloy. As mentioned, this is down to its composition and the effect of adding niobium. The blend of niobium with molybdenum produces a stiffening effect, negating the need for a precipitation-hardening treatment. For many applications, the high strength inherent in this material is more than adequate. It still outperforms many other nickel alloys in this area. However, there are occasions when even higher strength is required, but not at the sacrifice of the outstanding corrosion resistance. When introduced, INCONEL alloy 625 was marketed as a solid solution strengthened alloy. It was found to be age hardenable, and over time it was subjected to various tests to study the effects of this. These showed that the age-hardening response was improved significantly by the addition of titanium. This led to the creation of a new alloy – what we know as INCONEL alloy 725.
The strength of age-hardened INCONEL alloy 725 is in the region of twice the strength of annealed INCONEL alloy 625. As INCONEL alloy 725 is strengthened through heat treatment, not through cold working, it retains high ductility and toughness. This strength can be imparted to large and non-uniform sections of material that can’t be strengthened by cold work.

Different applications

The choice of which material to specify often comes down to the intended final application of the alloy.
INCONEL alloy 625 is employed in a wide variety of industries, thanks to its versatility. For example, it is commonly used in sea-water applications due to its resistance against localised attack, such as pitting and crevice corrosion. It’s also resistant to chloride-ion stress-corrosion cracking. As such, its applications include propeller blades for gunboats, submarine propulsion motors, exhaust ducts for Navy utility boats, and sheathing for undersea communication cables.
Alloy 625 is also a material of choice in the aerospace industry, where its high strength, high fatigue and excellent weldability are desirable assets. Uses include aircraft ducting systems, engine exhaust systems and fuel lines. Other industry applications include nuclear reactor components and chemical processing plants.
INCONEL alloy 725 is also quite variously used across many industries, but it is chosen over INCONEL alloy 625 when its exceptionally high strength is required. In sour gas collection for example – where exceptional resistance to highly corrosive media is required – INCONEL alloy 725 is specified for items such as hangers, landing nipples, side pocket mandrels and polished bore receptacles.
Like INCONEL alloy 625, INCONEL alloy 725 is also used in the marine industry. For example, it is a good choice for high-strength fasteners, where extra strength is required without sacrificing resistance to corrosion, pitting or crevice attack.

About the author
Jan Ward is the CEO of Corrotherm International, a stockist and supplier of seamless pipes, fittings, flanges and plates in a wide range of nickel and nickel alloys.

For information, visit: www.corrotherm.co.uk

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This Featured Story first appeared in Stainless Steel World May 2021 magazine. To read many more articles like these on an (almost) monthly basis, subscribe to our magazine (available in print and digitally).

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