Grade 431 stands apart from the rest of the 4xx family covered on this page because it is a true martensitic stainless steel — it hardens through heat treatment rather than staying soft and magnetic like the ferritic grades. What sets 431 apart from simpler martensitic grades like 410 or 420 is a deliberate nickel addition of roughly 1.25–2.50%. Nickel doesn't contribute to hardenability the way chromium and carbon do, but it substantially improves toughness and impact resistance in the hardened and tempered condition, addressing the brittleness that plain high-chromium martensitic steels can show.

With 15–17% chromium and up to 0.20% carbon, 431 can be through-hardened to a useful working hardness while retaining good corrosion resistance for a martensitic grade — though still well below what an austenitic or ferritic stainless offers, since the higher carbon and lower chromium-to-carbon ratio needed for hardenability come at the expense of passive-layer stability. This combination of strength, toughness, and moderate corrosion resistance makes 431 a common choice for highly stressed components: pump and valve shafts, aircraft and marine fasteners, and other parts that need to survive both mechanical loading and a corrosive environment. It is typically supplied and machined in the hardened-and-tempered condition, which is the main practical difference from the annealed, easy-machining ferritic grades on this site.

431 machines very differently from the ferritic grades on this site, and the difference comes down to hardness and toughness rather than work hardening. Because 431 is almost always machined in the hardened-and-tempered condition to take advantage of its strength, the cutting tool is working against a genuinely harder, more abrasive material from the first pass — not a soft, annealed structure that hardens as you go. The nickel addition that gives 431 its toughness also makes it more resistant to chip separation than plain martensitic grades, so cutting forces and heat generation both run higher than on a ferritic stainless of similar chromium content.

A tougher carbide substrate with a wear-resistant coating generally outperforms a sharp, fragile edge on 431, since the priority shifts from minimizing work hardening (a non-issue here) to resisting abrasive flank wear and edge chipping under sustained cutting forces. Rigid setups, conservative feed rates, and adequate coolant for heat control all matter more on 431 than on the softer ferritic grades. Because hardness can vary with the specific heat treatment a part has received, it's worth confirming the actual hardness before committing to speeds and feeds — 431 supplied in the annealed condition machines considerably easier than the same alloy hardened and tempered to a structural spec.