Inconel 690 is a nickel-chromium-iron alloy distinguished by a notably higher chromium content than its predecessors in the Inconel 600 family — nominally around 29% chromium against roughly 61.5% nickel and 9% iron. That extra chromium isn't incidental; it's specifically what gives 690 its standout resistance to stress-corrosion cracking, particularly in the high-temperature, high-purity water environments found inside pressurized water reactors. This property made 690 the standard material for nuclear steam generator tubing, replacing earlier lower-chromium alloys precisely because the added chromium so effectively suppresses primary-water stress-corrosion cracking (PWSCC) over decades of reactor service.

Beyond the nuclear sector, 690's high-chromium chemistry also delivers strong general corrosion resistance in oxidizing acid environments — including nitric acid and nitric/hydrofluoric acid processing — and reasonable resistance in coal gasification atmospheres, so it shows up in tubesheets, baffles, and process hardware outside reactor applications as well. From a machining standpoint, 690 sits squarely in the solid-solution-strengthened nickel superalloy category: it doesn't respond to heat treatment for hardening, but its combination of high nickel and high chromium content produces the low thermal conductivity, rapid work hardening, and high hot strength that make this whole alloy family demanding to cut.

690's high chromium content that makes it so resistant to stress-corrosion cracking is also a big part of what makes it demanding to machine. Low thermal conductivity keeps cutting heat concentrated right at the tool edge instead of dissipating into the chip or the part, so tool-tip temperatures climb quickly even at moderate speeds. The alloy work-hardens fast under light or interrupted cuts, so a hesitant pass, a dull edge, or excessive dwell time can burnish the surface and make the next cut noticeably harder to start cleanly.

690 also retains a substantial fraction of its room-temperature strength at elevated temperatures, which keeps cutting forces high through the depth of cut rather than tapering off as heat builds up the way plain carbon steel would. Add in a strong tendency to gall and adhere to the tool face under pressure, and flank wear accelerates quickly on tooling that isn't matched to the job. The practical response is the same across this alloy family: sharp, positive-rake geometries to keep cutting forces down, steady feed rates that avoid rubbing, rigid workholding to control deflection and chatter, and ample coolant to manage heat at the tool-chip interface. A coated carbide grade with good hot hardness, run at a conservative and consistent speed, gives the most predictable tool life on 690.