What is the difference between Inconel 718 and 725?

Inconel 718 and 725 are both nickel-based alloys developed by Special Metal Corporation and marketed within their family of Inconel grades. However, there is a significant difference in their composition which accounts for a significant difference in their physical properties.

Inconel 718 contains 50-55% nickel, 17-21% chromium, 4.7-5.5% niobium, 2.8-3.3% molybdenum plus controlled additions of titanium and aluminium. It achieves its high strength through precipitation strengthening – small additions to its composition result in the formation of fine precipitates during a second heat treatment process, which greatly increases the strength of the alloy.

Inconel 718 was introduced in the 1960s and initially used in the aerospace industry as a high strength grade, with most of that strength retained at high temperature. It was therefore widely used in aerospace applications such as gas turbines and jet engines. Due to its ready availability in the market, Alloy 718 it was taken up by the oil and gas industry. This was associated with wells moving offshore, drilling deeper with higher temperatures, pressures and corrosive contaminants. Other corrosion resistant metals could not match its combination of strength, corrosion resistance or ability to avoid stress corrosion cracking.

Inconel 725 contains 55-59% nickel, 19-22.5% chromium, 7-9% molybdenum, 2.75-4% niobium plus controlled additions of titanium. It achieves almost identical tensile properties through a similar precipitation strengthening process. However, the significantly higher molybdenum content results in even better corrosion resistance than Inconel 718. Whereas the Pitting Resistance Equivalent Number (PREN) for Inconel 718 exceeds 31, it is over 45 for Inconel 725, allowing it to be used in the most aggressive environments.

Langley Alloys stocks Alloy 725 as round bars from 1”-4” diameter (25.4-101.6mm) and Alloy 718 from ½”-10” diameter (12.7-254mm).

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