Inconel 718 and 725 are both nickel-based alloys marketed within the Inconel family, but a significant difference in composition gives them different physical properties. They reach almost identical strength through precipitation strengthening, while 725 offers markedly better corrosion resistance.

Related: Nickel Alloys

Inconel 718

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

Introduced in the 1960s, Inconel 718 was first used in aerospace as a high-strength grade that retains its strength at high temperature, so it was widely used in gas turbines and jet engines. Because it was readily available, it was then taken up by the oil and gas industry as wells moved offshore and drilling went deeper, into higher temperatures, pressures and corrosive contaminants. Other corrosion-resistant metals could not match its combination of strength, corrosion resistance and freedom from stress corrosion cracking.

Inconel 725

Inconel 725 contains 55-59% nickel, 19-22.5% chromium, 7-9% molybdenum and 2.75-4% niobium, plus controlled additions of titanium. It achieves almost identical tensile properties through a similar precipitation-strengthening process. Its significantly higher molybdenum content gives even better corrosion resistance than 718: the Pitting Resistance Equivalent Number (PREN) exceeds 31 for Inconel 718, but 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 1/2”-10” diameter (12.7-254mm).

Property Inconel 718 Inconel 725
Nickel 50-55% 55-59%
Chromium 17-21% 19-22.5%
Molybdenum 2.8-3.3% 7-9%
Niobium 4.7-5.5% 2.75-4%
PREN >31 >45
Stock size (Langley Alloys) 1/2"-10" 1"-4"