Haynes 690 is Haynes International's trade name for the high-chromium nickel-chromium-iron alloy more commonly sold as Inconel 690. Like Haynes 600 and 601, it is a solid-solution alloy with no meaningful gamma-prime hardening, but it pushes chromium content much higher than either of those grades — roughly 29% nominal versus 15.5% for 600 and 23% for 601 — while cutting carbon content to a strict 0.05% maximum.

That combination of high chromium and low carbon was deliberately engineered to resist chloride- and caustic-induced stress-corrosion cracking, and it is what has made Haynes/Inconel 690 the standard material for steam generator tubing in pressurized water nuclear reactors, largely displacing Alloy 600 in that role. Outside nuclear service, its excellent resistance to oxidizing acids and general corrosion resistance also make it useful in chemical processing equipment exposed to nitric acid and similar aggressive media.

Haynes 690 is a solid-solution alloy like 600 and 601, so it does not carry the abrasive gamma-prime precipitate load of the cast turbine alloys — but its much higher chromium content (up to 31%) makes it noticeably tougher on tooling than either of those grades. Chromium contributes strongly to work hardening and to abrasive wear on cutting edges, so shops moving from 600/601 to 690 should expect faster edge wear at the same parameters.

Low thermal conductivity remains the fundamental constraint: heat stays concentrated at the tool-chip interface instead of dissipating into the chip, which is why nickel alloys as a family run at speeds well below comparable steels. The alloy also work-hardens rapidly under interrupted or light cuts, and its toughness produces long, continuous chips that need active chip-breaking geometry to avoid tangling and re-cutting. Rigid, low-deflection setups, sharp positive-rake coated carbide, steady feed rates, and generous coolant delivery are essential to controlling both tool life and surface finish.