Inconel 691 is a high-chromium nickel-chromium-iron alloy developed to push carburization resistance beyond what standard Ni-Cr-Fe grades in the 600/601 family deliver. Its typical chemistry runs around 59% nickel, 30% chromium, and 9% iron — a chromium level well above the base 600-series alloys and in the same elevated range as high-chromium grades like 690. That extra chromium is the working mechanism: at high temperature it promotes formation of a stable, continuous chromium-oxide surface layer that resists carbon pickup far better than lower-chromium alternatives, which is the core failure mode 691 is designed to fight in carburizing furnace and process atmospheres.

Because it's a less commonly published designation than mainstream grades like 600, 601, or 690, verified composition data for 691 is limited compared with those workhorse alloys, and we've kept the figures below to what's consistently supported rather than publishing a precise ASTM-style range we can't back up. Materially, though, 691 behaves like other high-chromium, solid-solution-strengthened nickel superalloys: no response to heat treatment for hardening, strong retained strength at elevated temperature, and machining characteristics driven by low thermal conductivity and rapid work hardening rather than by carbide precipitation or aging response.

691's high chromium content that drives its carburization resistance also puts it firmly in the difficult-to-machine category shared by every high-chromium nickel superalloy. Low thermal conductivity keeps cutting heat concentrated at the tool edge rather than carrying it off in the chip, so tool-tip temperatures build quickly even at conservative speeds. The alloy work-hardens fast under light or interrupted cuts — a hesitant pass or a dull edge burnishes the surface and makes the next pass harder to engage cleanly.

Like its higher-chromium relatives, 691 retains a large share of its strength at elevated temperature, so cutting forces stay high through the cut rather than tapering off as the workpiece heats up. A pronounced tendency to gall and adhere to the tool face accelerates flank wear on tooling that isn't matched to the job. The standard countermeasures apply: sharp, positive-rake geometries to hold cutting forces down, steady feed rates to keep the tool cutting rather than rubbing, rigid workholding to control deflection, and ample coolant to manage heat at the tool-chip interface. A coated carbide grade with good hot hardness and edge toughness, run at a conservative and consistent speed, gives the most predictable results.