Incoloy 925 was launched in 1982 by Special Metals Corporation as development of Incoloy 825, which was originally developed in 1952 as an alloy for higher temperature applications and with superior acid resistance. It proved particularly cost-effective due to the lower levels of nickel, but its modest mechanical properties limited applications. Note that Incoloy is a registered trademark of ‘Special Metals Corporation’.
In order to increase the strength of the Inconel 825 base alloy, controlled additions of Titanium and Aluminium are made. These form very fine Ni3(Al, Ti) precipitates (gamma prime, ‘ϒ’) throughout the alloy matrix during final heat treatment. This age-hardening greatly increases the hardness and strength of Inconel 925 in a similar way to Monel K500, Inconel 718 and Inconel 725.
As a nickel-based alloy, Incoloy 925 demonstrates good resistance to stress corrosion cracking and sulphide stress cracking in ‘sour’ H2S environments for oil and gas applications and is therefore included by NACE MR0175. As such, common applications comprise down-hole and well-head components, hangers, joints and packers.
The chromium content of Incoloy 925 ensures a good level of general corrosion resistance in oxidising environments, with additional molybdenum to enhance pitting corrosion resistance, resulting in a Pitting Resistance Equivalent number (PREN) of 26. Its copper content is recognised to improve resistance to sulphuric acid, which is replicated in Ferralium 255 with its 2% copper content. This balanced corrosion resistance across a number of aggressive environments is exploited in valve components, pump shafts, fasteners and marine applications.
The combination of corrosion resistance and strength makes Incoloy 925 a potential alternative to the more highly-alloyed and expensive Inconel 718. It is being increasingly specified by several large oil companies and OEMs in order to reduce project costs.