440A is a hardenable martensitic stainless steel, registered as UNS S44002, and the lowest-carbon member of the AISI 440 family that also includes 440B and 440C. All three grades share the same basic chromium-molybdenum stainless chemistry — roughly 16-18% chromium with a small molybdenum addition for hardenability — and differ mainly in carbon content, which is what sets their maximum attainable hardness. 440A carries the least carbon of the three (0.60-0.75%), so it hardens to a lower ceiling than 440C, but that trade-off buys it noticeably better toughness and impact resistance for a given hardness level, along with slightly improved corrosion resistance since less chromium is tied up as chromium carbide.

Properly heat treated, 440A typically reaches the mid-50s HRC (around 54-56 HRC), which is still hard enough for cutlery, surgical and dental instruments, bearing races, valve components, and mold or die inserts that need a corrosion-resistant working surface without pushing to the extreme wear resistance — and extreme machining difficulty — of 440C. It is generally supplied and machined in the annealed condition (roughly 235 HB maximum) and hardened afterward, since carbide precipitation and hardening raise cutting forces substantially.

Note: the carbon figure above corrects an error found in this page's prior content, which listed carbon at 0.07% — a value that would not support the hardening response 440A is known for. 0.60-0.75% C is the figure specified under AISI/UNS S44002.

440A shares the core machining challenges common to hardenable chromium stainless steels: pronounced work hardening if the tool rubs instead of shears, low thermal conductivity that keeps generated heat concentrated at the cutting edge rather than carrying it off in the chip, and a tendency toward built-up edge at low cutting speeds. Because it carries less carbon than 440C, its annealed hardness and abrasive chromium-carbide content are both somewhat lower, which translates into less tool wear and slightly more forgiving cutting conditions — 440A is the more machinable of the two grades, though it is still meaningfully tougher to cut than a non-stainless alloy steel of similar hardness.

Sharp, positive-rake tooling with adequate feed to stay ahead of the work-hardened layer is the standard approach, along with generous coolant both for lubrication and to pull heat away from the edge. As with other hardenable stainless grades, the bulk of the material removal should happen in the annealed condition; once the part is quench-hardened and tempered, cutting forces rise and tool life drops off accordingly, so hardened-state machining should be limited to light finishing passes wherever possible.

Figures assume the annealed condition. Reduce speeds substantially once the part is hardened and tempered.

Turning

Parting / Grooving

Milling