What is the difference between Inconel 625 and 725?
Composition Differences
The reason INCONEL 625 alloy has such a wide range of desirable properties is its unique makeup. It is a high nickel alloy, with at least 58% nickel in the mix. It also contains chromium (20-23%) and molybdenum (8-10%), as well as added niobium (3.15-4.15%).
Niobium combines with molybdenum and distorts the atomic matrix of the alloy, imparting exceptionally high strength in the annealed condition without the need for specific strengthening heat treatments. The large amounts of nickel and chromium together give the material incredible corrosion resistance, allowing it to withstand highly corrosive elements and environments.
INCONEL 725 alloy shares many of the same constituent elements - in fact, superficially, they are almost identical. It retains large amounts of nickel (55-59%) and chromium (19-22.5%), as well as molybdenum (7-9.50%). As a result, the material has similar corrosion resistance to INCONEL 625 alloy, making it suitable for a wide 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 INCONEL Alloy 725. INCONEL Alloy 725 has a higher titanium content (1-1.7% compared to a maximum of 0.4% for INCONEL Alloy 625), with the remainder made up of iron, compared to a maximum of 5% for INCONEL Alloy 625. These small additions and differences allow for the formation of micro-precipitates during heat treatment, which results in significantly higher strength values for INCONEL Alloy 725.
Strength Comparison
INCONEL 625 alloy is a strong nickel alloy. As mentioned previously, this is due to its composition and the effects of the addition of niobium. The mixture of niobium and molybdenum creates a hardening effect that eliminates the need for precipitation hardening. For many applications, the inherent high strength of this material is more than enough. It still outperforms many other nickel alloys in this area.
However, sometimes even higher strength is required, but not at the expense of excellent corrosion resistance. INCONEL 625 alloy was introduced and sold as a solid solution strengthened alloy. It was found to be age hardenable, and various tests were conducted over time to study its effects. It was found that the addition of titanium significantly improved the age hardening reaction. This led to the creation of a new alloy - what we know as INCONEL 725 alloy (learn more about it in this detailed report).
The strength of age hardened INCONEL 725 alloy is about twice that of annealed INCONEL 625 alloy. Because INCONEL 725 alloy is strengthened by heat treatment rather than cold working, it retains its high ductility and toughness. This strength can be imparted to large and non-uniform sections of material that cannot be strengthened by cold working.
Different Applications
The choice of which material is often determined by the alloy's intended end application.
Due to its versatility, INCONEL Alloy 625 is used in a wide variety of industries. For example, it is often used in seawater applications due to its resistance to localized attack such as pitting and crevice corrosion. It is also resistant to chloride ion stress corrosion cracking. As a result, its applications include propeller blades for gunboats, submarine propulsion motors, exhaust ducting for naval multipurpose vessels, and jacketing for submarine communications cables.
It is also the material of choice for the aerospace industry, where its high strength, high fatigue, and excellent weldability are its desirable advantages. This includes use in aircraft ductwork, engine exhaust systems, and fuel lines. Other industry applications include nuclear reactor components, and it is also widely used in chemical processing plants.
INCONEL Alloy 725 is also widely used in many industries, but it is chosen over INCONEL Alloy 625 when its extreme strength is required. For example, in sour gas gathering-where excellent resistance to highly corrosive media is required-INCONEL Alloy 725 is used in hangers, landing joints, side pocket mandrels, and polished bore sockets, among others.
Like INCONEL Alloy 625, INCONEL Alloy 725 is also used in the marine industry. Here, it is a good choice for products such as high-strength fasteners, where extra strength is needed without sacrificing resistance to corrosion, pitting, or crevice attack.
If you are interested in learning more about INCONEL Alloy 625 or INCONEL Alloy 725, as well as our other product lines, contact an experienced member of our sales team today.
