420F is the free-machining version of standard Type 420 martensitic stainless, and the "F" tells the whole story: a deliberate sulfur addition (up to roughly 0.15%) that forms manganese sulfide inclusions throughout the microstructure. Those inclusions act as internal chip breakers and stress risers ahead of the cutting edge, and that's the entire reason this grade exists — 420F carries essentially the same 12-14% chromium and 0.15-0.35% carbon as plain 420, and hardens the same way, but it cuts dramatically easier.

That machinability boost comes with a real tradeoff: the sulfide inclusions that make chip control so much better also act as pitting initiation sites, so 420F has measurably lower corrosion resistance and lower transverse toughness than standard 420. It's the right choice specifically for parts that are machined complex, hardened, and don't need to survive an aggressive corrosive environment — cutlery, surgical and dental instruments, valve components, and other precision hardware where machinability and a good finish matter more than maximum corrosion resistance.

This is the whole reason 420F exists, so it's worth being direct about it: 420F machines meaningfully easier than standard 420, and the difference is obvious on the shop floor, not just on paper. The manganese sulfide inclusions break up chip flow, reduce built-up edge, and lower cutting forces across turning, milling, and drilling alike, letting shops run higher speeds and feeds than they could on the plain (non-"F") grade at the same hardness.

That said, 420F is still a 12%+ chromium martensitic stainless underneath the sulfur addition, and it's usually machined in the annealed condition before hardening. Standard carbide grades suited to the ISO M application range handle it well, and because chip control is already handled by the alloy itself, insert geometry can lean toward a sharper, more open chipbreaker than would be sensible on a gummier, non-free-machining stainless. Coolant still helps with heat management, though built-up edge is less of a persistent problem here than on 304 or 316.