440C is the highest-carbon member of the AISI 440 martensitic stainless family, registered as UNS S44004. Where 440A and 440B trade carbon content for toughness, 440C is built for maximum hardness and wear resistance: at 0.95-1.20% carbon against roughly 16-18% chromium, it hardens to 58-60 HRC, which puts it in the same performance territory as many through-hardening tool steels while retaining useful corrosion resistance. That combination makes 440C the standard choice for ball and roller bearings, precision balls, knife blades, injection nozzles, surgical and dental instruments, and valve seats — anywhere a hard, corrosion-resistant wear surface matters more than toughness.

The trade-off is machinability. 440C's high carbon content precipitates a heavy volume of hard chromium carbides throughout the martensitic matrix, and the same carbides that give the steel its wear resistance in service also abrade cutting edges aggressively in the shop. Compared to 440A, expect noticeably higher cutting forces, faster tool wear, and a stronger need for rigid setups and conservative feeds — this is one of the genuinely difficult-to-machine stainless grades, and it should be treated with the same respect given to hardened tool steel rather than a typical corrosion-resistant alloy.

440C combines every major stainless machining challenge with a high volume of hard carbides, and it is meaningfully harder to machine than 440A or the precipitation-hardening stainless grades. Work hardening is severe if the edge dwells or rubs; low thermal conductivity concentrates heat at the cutting zone instead of carrying it off in the chip; and the dense chromium-carbide network abrades flank faces and accelerates crater wear far faster than in lower-carbon stainless grades. In the annealed condition (typically around 97 HRB / 235-269 HB) 440C machines reasonably, but it work-hardens quickly, so consistent feed and a sharp, positive edge that shears cleanly rather than rubbing are essential.

Because 440C is usually specified precisely for its high hardened hardness, most of the roughing and semi-finish work should be done in the annealed condition, with final hardening and tempering (to the 58-60 HRC range) reserved until after the bulk of the material has been removed. Once hardened, 440C should be treated as a tool-steel-class material: rigid, vibration-free setups, reduced feeds and speeds, ceramic or CBN tooling for hard turning where volumes justify it, and abundant coolant to manage both heat and carbide-driven abrasive wear.

Figures assume the annealed condition. Once hardened to 58-60 HRC, treat 440C as a hardened tool steel: cut speeds fall substantially and CBN or ceramic tooling should be considered.

Turning

Parting / Grooving

Milling

A slightly heavier hone and lower rake than 440A are recommended here to protect the cutting edge from the abrasive carbide network in this higher-carbon grade.