Grade 420 is a higher-carbon martensitic stainless steel, built around roughly 12–14% chromium with enough carbon (commonly 0.15–0.40% depending on the specific subtype) to reach considerably higher hardness than 410 after heat treatment. Like the rest of the martensitic 4xx family, it carries essentially no nickel, is magnetic, and derives its strength and wear resistance from quenching and tempering rather than cold work. The extra carbon compared with 410 is what gives 420 its reputation as a "cutlery grade" — it can be hardened well into the 50s HRC, similar in principle to a tool steel.

420's combination of high achievable hardness, decent corrosion resistance, and good polishability makes it a common choice for knife blades, surgical and dental instruments, molds for plastics, and other parts needing a hard, wear-resistant, corrosion-resistant edge or surface. Published machinability ratings put it around 45% relative to a free-machining reference steel, noticeably lower than 410 — a direct consequence of its higher carbon content and the correspondingly higher forces and tool wear it generates at the machine, especially once hardened.

420's higher carbon content compared with 410 or 403 makes it noticeably more demanding on tooling. Even in the annealed condition it's harder and more abrasive than the lower-carbon martensitic grades, and once heat-treated to cutlery- or mold-grade hardness, machining largely shifts from conventional turning and milling to grinding and hard-milling with appropriate tooling.

In its annealed, pre-hardened machining condition, 420 still benefits from the same fundamentals as other martensitic stainless grades: a sharp, positive cutting edge that shears the material rather than rubbing it, which limits work hardening and keeps cutting forces predictable. Chip control is generally easier than with the lower-carbon grades since the higher carbon content promotes more natural chip segmentation. Rigidity and coolant matter more here than with 403 or 410, since cutting forces and heat generation both rise with the alloy's higher hardness and abrasiveness.

At stable cutting conditions, a semi-hard substrate with a thin CVD coating is a solid general-purpose choice for turning. Below roughly 390 SFM (120 m/min), a semi-hard substrate with a PVD coating tends to hold up better. For milling, a semi-hard substrate with a thin PVD coating is generally preferred.