Materials Glossary
Martensitic stainless steel is the 400-series family — 410, 420, 440C, and their relatives — that does something the common 300-series grades can't: it hardens by heat treatment, just like a carbon steel. That's the whole story of what it's for, and it's also the whole story of why it machines so differently depending on what state you catch it in.
Martensitic stainless steels typically carry roughly 11.5–18% chromium — enough for real corrosion resistance, though less than the austenitic grades — combined with carbon content that can run up to about 1%, plus small additions of elements like nickel, molybdenum, or sulfur depending on the grade. That carbon content is the key: it's what allows these steels to be heated into their austenite phase and then quenched into martensite, a hard, body-centered tetragonal structure, in essentially the same way a plain carbon steel hardens. The 300-series austenitic grades don't have enough carbon (or the right structure) to do this at all — martensitic is the stainless family built specifically to be heat-treatable.
410 is the general-purpose starting point — lower carbon, moderate hardness after treatment, and the best toughness of the common grades. 420 carries more carbon for higher achievable hardness, used in cutlery and surgical instruments. 440C sits at the top of the common range, with enough carbon and chromium to reach high hardness and strong wear resistance, which is why it shows up in bearings, valve parts, and premium cutlery. As carbon content rises across this family, hardness potential goes up and toughness comes down — the same tradeoff seen in plain high-carbon steel.
Martensitic stainless steel is normally supplied annealed — softened — specifically because that's when it machines and forms best. In that condition it cuts reasonably predictably, closer to a mild alloy steel than to a tool steel. Once it's quenched and tempered to a working hardness, machinability drops fast, and tool life on hardened 410 or 440C looks nothing like tool life on the same grade annealed. The practical takeaway: rough and shape the part before hardening whenever the process allows it, and treat any hardened-state machining as a finishing operation that calls for harder, more wear-resistant tooling and lighter cuts.