What is the difference between INCONEL 725 and 718?
Inconel 718 and 725 are both nickel-based alloys developed by Special Metal Corporation and sold under its Inconel grade series. However, there are significant differences in their compositions, which results in significant differences in their physical properties.
Inconel 718 contains 50-55% nickel, 17-21% chromium, 4.7-5.5% niobium, 2.8-3.3% molybdenum and controlled additions of titanium and aluminum. It achieves its high strength through precipitation strengthening – small additions to its composition cause fine precipitates to form during the second heat treatment, greatly increasing the strength of the alloy.
Inconel 718 was introduced in the 1960s and was originally used in the aerospace industry as a high-strength grade that retains most of its strength at elevated temperatures. Therefore, it is widely used in aerospace applications such as gas turbines and jet engines. Since alloy 718 is readily available in the market, it is adopted by the oil and gas industry. This has to do with wells moving offshore and drilling deeper with higher temperatures, pressures and corrosive contaminants. No other corrosion-resistant metal can 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, and controlled additions of titanium. It achieves nearly identical tensile properties through a similar precipitation strengthening process. However, the significantly higher molybdenum content results in better corrosion resistance than Inconel 718. Inconel 718 has a Pitting Resistance Equivalency Number (PREN) of over 31 and Inconel 725 over 45, enabling use in the most corrosive environments.
