This page covers 431 in its quenched-and-tempered (HT) condition — the state most 431 parts are actually machined in, since the alloy's whole purpose is to combine martensitic strength with a nickel-boosted toughness that plain chromium martensitic grades like 410 or 420 don't have. Heat treating 431 involves austenitizing and oil- or air-quenching to form martensite, then tempering to a target hardness that's normally somewhere in the high-20s to high-30s HRC, depending on the temper temperature the specification calls for. The roughly 1.25-2.50% nickel addition doesn't add hardenability on its own, but it substantially improves impact toughness at a given hardness, which is the entire reason engineers reach for 431 over a cheaper straight-chromium martensitic grade.

In the HT condition, 431 delivers a strength and toughness combination that few other stainless grades can match while still keeping usable corrosion resistance in mildly aggressive environments — not the equal of an austenitic grade, but respectable for a hardenable stainless. That profile is why 431 (HT) shows up in pump and valve shafts, aircraft undercarriage and fastener components, marine propeller shafts, and other highly loaded parts that also need to resist atmospheric or mild chemical attack. Because the part is already hardened by the time it reaches the machine in most shops, the cutting conditions on this page assume tempered martensite rather than the soft annealed structure some suppliers also offer.

Once 431 is quenched and tempered, the soft, easily sheared ferrite-carbide mix of the annealed condition is gone, replaced by a tempered martensitic structure that resists deformation far more aggressively. That structural change is what separates machining 431 (HT) from machining most other 4xx stainless grades on this site: cutting forces run consistently higher from the very first pass, and the material no longer has the annealed grade's tendency to smear or gum on the cutting edge. Instead, the dominant wear mechanism is abrasive flank wear driven by the hardness of the tempered matrix itself.

Because toughness rather than sharpness is the priority at this hardness, a robust carbide substrate with a wear-resistant coating and a slightly heavier edge preparation generally holds up better than a fine, fragile finishing geometry. Feeds should stay firm enough to keep the tool cutting under the hardened skin rather than rubbing across it, and depths of cut should be planned to avoid excessive tool deflection, since 431 (HT) transmits cutting forces into the workpiece and tool more directly than a softer alloy. Coolant helps manage the concentrated heat at the tool-workpiece interface, and because tempering temperature has a direct, predictable effect on final hardness, confirming the specific temper condition of the stock before setting speeds and feeds will save more tool life than any single geometry change.