Inconel 718 vs Inconel 625: Overview
Inconel 718 and Inconel 625 are nickel-chromium base superalloys used in the most demanding industrial applications, including aerospace, automotive and oil and gas. They share many properties of the Inconel alloy family, but their differences determine which one is the right choice for a given end use. Inconel 625 is composed mainly of nickel and chromium with molybdenum and niobium, which give it high tensile strength and corrosion resistance, including resistance to crevice corrosion. Inconel 718 has a similar base composition with small variations, but its defining feature is the ability to be precipitation hardened without succumbing to post-weld cracking.
Key Property Differences
Inconel 625 can maintain its strength in environments up to about 2000°F (1090°C) and provides good oxidation resistance and resistance to carburization. The combination of molybdenum and niobium accounts for its strength in both cryogenic and very high temperature environments, and annealed 625 can be significantly stronger than many other nickel alloys while remaining particularly suited to corrosion resistance, including seawater service.
Inconel 718 resists creep rupture at temperatures up to about 1300°F (704°C). Because it can be precipitation hardened without strain-age cracking, it maintains high tensile strength while also offering high formability. Its weldability is similar in some ways to general stainless steel grades, which simplifies fabrication of complex components.
Formability Compared
The high nickel content of both alloys leads to generally high levels of formability, and both 718 and 625 can be stamped and cold formed. Inconel 718 resistance to post-weld or strain-age cracking during heat treatment makes it particularly suited to forming operations that must preserve high strength in the annealed condition. These alloys can be rolled through a wire mill to produce round, flat and square wire profiles, and they are used to produce honeycombs, seal rings, bellows, springs and high-performance fasteners for extreme conditions.
Neither alloy is commonly deep drawn, so designers should plan stamped or formed geometries. Typical stamped parts include sensor diaphragms for measuring devices that must resist high temperature and high pressure.
Common Uses and Applications
Inconel 625 is widely used for flexible metal hoses, heat exchangers, header bars and marine components because of its considerable strength at elevated temperatures combined with corrosion and oxidation resistance. In the marine sector it serves in downhole drilling applications such as tieback on undersea lines, telecommunications cables in seawater, propeller blades and wire ropes.
Inconel 718 started its life in the aerospace industry and remains dominant there; according to one study, the alloy makes up more than 30 percent of a modern aircraft engine finished component mass. It is also valuable in automobile engines, where components face high stress and high temperatures while requiring the formability that nickel provides.
How to Choose Between 718 and 625
Choose Inconel 718 when the priority is high strength, fatigue and creep resistance at elevated temperatures combined with weldability, such as turbine discs, engine mounts and structural aerospace hardware. Choose Inconel 625 when the priority is corrosion resistance in chloride or seawater environments and strength over a wide temperature range, such as marine hardware, heat exchangers and chemical processing components. For both alloys, confirm the product form, condition and governing specification with the supplier before ordering.
FAQ
Q: Which is stronger, Inconel 718 or Inconel 625?
A: In the annealed condition Inconel 625 can be significantly stronger than many nickel alloys, while Inconel 718 reaches its highest strength after precipitation hardening. The right choice depends on the heat treatment and the service requirement.
Q: Can Inconel 718 be welded without cracking?
A: Inconel 718 is notable for resisting post-weld or strain-age cracking during heat treatment, which makes it easier to weld than many other high-strength superalloys. Proper procedure control is still required.
Q: Is Inconel 625 suitable for seawater?
A: Yes. Inconel 625 has excellent resistance to seawater and chloride environments, which is why it is used in marine hardware, propeller blades, wire ropes and undersea oil and gas equipment.
Q: What temperature limits apply to these alloys?
A: Inconel 625 maintains strength up to about 2000°F (1090°C), while Inconel 718 resists creep rupture up to about 1300°F (704°C); 718 is therefore preferred for high-strength service in the intermediate temperature range.
Q: Are Inconel 718 and 625 available as wire?
A: Yes. Both alloys can be rolled into round, flat and square wire profiles and are used to make springs, bellows, seal rings, honeycombs and fasteners for extreme environments.
Q: Which alloy is better for a heat exchanger?
A: Inconel 625 is usually the better choice for heat exchangers because of its outstanding corrosion resistance at elevated temperatures, especially in chloride or seawater service.
