Nimonic 90 takes the original Nimonic 80A composition and adds a substantial cobalt addition — around 18% — to raise strength further while keeping the same nickel-chromium-titanium-aluminum base that made 80A the founding age-hardenable grade of the family. That cobalt addition is the defining difference between the two alloys: it slows the coarsening of the gamma-prime strengthening precipitates at elevated temperature, which translates into better creep resistance and higher sustained strength than 80A can offer at similar operating temperatures.

The nominal chemistry runs roughly 58% nickel, 19.5% chromium, 18% cobalt, 2.4% titanium, and 1.4% aluminum, with iron and carbon held to low levels. This composition puts Nimonic 90 to work in more demanding turbine applications than 80A typically sees — blades, discs, rings, and other hot-section components operating at higher temperatures or under greater sustained stress, where the extra cobalt content earns its keep.

The tradeoff for that added strength is a more demanding machining profile than 80A: higher cutting forces, faster tool wear, and less forgiveness for light or hesitant cuts. Shops that already have 80A parameters dialed in should expect to back off cutting speeds and tighten up rigidity when they move to Nimonic 90.

Nimonic 90's added cobalt content pushes it noticeably further into "difficult" territory than the original 80A grade. Low thermal conductivity is still the underlying issue — heat stays concentrated at the cutting edge instead of leaving with the chip — but the higher cobalt level increases hot hardness and abrasiveness, which shows up directly as faster flank wear and shorter tool life at comparable cutting parameters.

The alloy work-hardens readily, so light cuts, dull edges, and interrupted engagement are all things to avoid: any of them can burnish the surface and leave a hardened layer that fights the next pass. Retained strength at temperature also promotes built-up edge and adhesive wear against carbide, particularly if cutting speeds run too high for the insert grade in use.

Sharp, positive cutting-edge geometries reduce the cutting forces this alloy generates, and a wear-resistant coated grade matched to the operation is essential rather than optional. Rigid workholding, minimal tool overhang, and consistent feed rates all pay off more on Nimonic 90 than they would on the softer 80A grade, since deflection and rubbing both accelerate tool wear and work hardening simultaneously.