S240 (UNS S11900), marketed by ATI as S240® Alloy, is a martensitic precipitation-hardening stainless steel engineered for a 240 ksi (1,655 MPa) tensile strength class with high fracture toughness and good corrosion and stress-corrosion-cracking resistance. It sits in the same high-strength PH stainless family as 13-8 PH and 15-5 PH, but is formulated to reach that strength level without cobalt and without the deep-freeze (sub-zero) conditioning step those established alloys require to fully transform to martensite — a meaningful simplification for shops running production heat-treat cycles.

S240 is double-vacuum melted (vacuum induction melting followed by vacuum arc remelting), a melt practice that delivers excellent macro- and micro-cleanliness and tight compositional control, supporting the good transverse toughness this alloy is known for even in heavy cross-sections. Aging temperature can be adjusted between roughly 950-1150°F (510-621°C) to tailor the final strength-to-ductility balance to the application. It's specified under AMS 5929 for bars and forgings, and is used in aerospace and other high-performance mechanical components where 240 ksi-class strength, good toughness, and better corrosion resistance than older high-strength PH grades are needed at a lower raw-material and processing cost than cobalt-bearing alternatives.

ATI does not publish a detailed elemental composition table for S240 in its public technical data sheet. What is documented and verified: S240 is a chromium-nickel-molybdenum martensitic precipitation-hardening stainless steel — the same general alloy family as 13-8 PH, 15-5 PH, and Custom 465 — strengthened by age-hardening a low-carbon martensitic matrix rather than through cobalt additions used in some competing high-strength grades. For heat-specific composition, consult a current ATI S240 mill certification.

S240 is machined in the annealed (solution-treated) condition wherever possible, the same practice used for 13-8 PH, 15-5 PH, and other precipitation-hardening stainless grades: features are cut close to final dimension before the aging heat treatment develops full strength, and only finish operations, if any, are performed on the hardened part. In the annealed condition, ATI's own guidance calls for machine speeds 20-30% lower than those used on Type 304 stainless, reflecting the higher alloy content and greater cutting resistance of a Cr-Ni-Mo martensitic stainless compared with a standard austenitic grade.

Unlike an austenitic stainless, S240 does not work-harden aggressively under the tool — its martensitic structure behaves more like a hardenable alloy steel than a gummy 300-series stainless, so chip control and edge wear are driven mainly by alloy hardness and abrasive carbide-forming elements rather than by a rapidly hardening surface skin. A tough, wear-resistant coated grade with adequate edge support handles the moderate-to-high cutting forces of the annealed condition well. If any machining is required after aging to the 240 ksi class, expect substantially higher cutting forces, reduced speeds, and a grade selected specifically for hardened tool steel and PH stainless work. Rigid setups and consistent coolant application help control heat buildup and dimensional stability, which matters given the tight tolerances these components typically require.