Jessop G81 is a British trade name, used historically by Jessop Steel, for the age-hardenable nickel-chromium-cobalt superalloy more widely known as Nimonic 90. It is a wrought alloy strengthened by a gamma-prime (Ni3(Al,Ti)) precipitate formed from titanium and aluminum additions, giving it meaningfully higher strength than the simpler solid-solution nickel-chromium-iron alloys (Haynes 600/601/690/75) covered elsewhere in this reference library, while still being hot-workable and weldable, unlike the fully cast, high-gamma-prime grades such as IN100 or MAR-M 247.

The alloy is designed for service up to roughly 1650°F (900°C) and is used primarily for gas turbine combustor and afterburner components, exhaust system hardware, and other structural parts that need creep resistance and oxidation resistance at high temperature without the extreme strength demands of rotating turbine blades and disks.

Jessop G81 / Nimonic 90 sits between the solid-solution Haynes 600-family alloys and the fully hardened cast superalloys in machining difficulty. Its gamma-prime content (roughly 4% combined Ti+Al) is meaningful but far below IN100 or MAR-M 247, so it presents a moderate abrasive-wear load on the tool rather than the severe wear seen on heavily precipitate-loaded cast grades — while still being noticeably tougher on tooling than plain solid-solution alloys like Haynes 600 or 75.

As with the rest of the nickel superalloy family, low thermal conductivity concentrates cutting heat at the tool edge instead of letting it escape into the chip, and the alloy work-hardens readily under light or interrupted cuts. Cobalt content adds to the tendency toward galling and built-up edge against common tool materials. Rigid setups, sharp positive-rake coated carbide suited to superalloys, consistent chip loads, and generous coolant delivery are the standard countermeasures, with cutting parameters generally falling between those used for solid-solution Ni-Cr-Fe alloys and fully hardened cast turbine grades.