Swiss Machining Tool Steel: A2, D2, O1, H13 and S7

Soft, in the annealed condition, then hardened afterward. Tool steels are supplied annealed at roughly 200 to 250 HB, and essentially all Swiss machining happens at that hardness because the material cuts manageably, though slower and tougher than free-machining steel. After machining, the part goes out for harden-and-temper to reach its working hardness, commonly 56 to 62 HRC. Machining tool steel at that finished hardness is not done on a Swiss lathe; hardened tool steel is finished by grinding, EDM, or CBN hard turning on specialized equipment instead. So the Swiss shop's role is to produce an accurate soft part with the correct stock left on critical features for post-hardening grinding. This means the heat-treat behavior of the specific grade, how much it distorts and grows, directly drives the machining plan and how much grind allowance is left. Buyers should expect a tool-steel part to be a multi-step program involving the machine shop, an outside heat-treater, and often a grinder.

It depends heavily on the grade. Air-hardening grades like A2, D2, H13, and S7 are relatively dimensionally stable through heat treat, often moving only a few tenths of a thousandth per inch, which is why A2 in particular is favored when tight post-hardening tolerances must hold with minimal grinding. Oil-hardening O1 is less stable and distorts and changes size more, so it needs more grind allowance and careful fixturing through the quench. The standard way to handle distortion is to leave grind stock on critical features, commonly 0.005 to 0.015 inch per side depending on the grade's distortion tendency, machine the part soft, harden it, then grind those features to final size. Features that can tolerate the small as-hardened movement are machined to final dimension while soft. Good heat-treaters also use controlled heating, fixturing, and sometimes stress-relief steps to minimize movement. For the buyer this means tight-tolerance tool-steel parts almost always include a post-hardening grinding operation in the cost and lead time.

D2 is generally the most abrasive and demanding of the common tool steels to machine in the annealed state because of its high carbon and high chromium content, which forms hard chromium carbides distributed through the matrix. Those carbides wear cutting edges quickly even when the bulk material is soft, so D2 runs at lower speeds and consumes more tooling than the other grades. A2 and the other air-hardening grades like H13 and S7 are more cooperative, with H13 being moderately machinable and S7 reasonably tough but workable. O1 cuts acceptably soft but its poorer dimensional stability complicates the hardening side rather than the cutting. Across all of them, surface speeds in the annealed condition typically run 60 to 150 SFM with coated carbide and steady feeds to avoid work-hardening the surface. The practical implication is that D2 parts cost more in tooling and cycle time, so if the wear resistance of D2 is not strictly required, a more machinable grade like A2 may be a better overall choice.

Use tool steel when the part genuinely faces abrasive wear, edge retention demands, impact, or hot strength that ordinary alloy steel cannot meet. A2 and D2 earn their cost in punches, dies, and wear components that see real abrasion; S7 suits shock and impact tooling; H13 belongs in hot-work die-casting and forging applications and high-temperature parts. These grades are specified for those properties, and the multi-step soft-machine-then-harden-then-grind workflow is simply the price of getting them. If, however, a part only needs moderate strength and toughness and does not face severe wear or heat, a heat-treatable alloy steel like 4140 or a hardenable stainless will machine faster and cheaper, avoid the high-carbide abrasiveness of grades like D2, and often reach adequate hardness around 28 to 45 HRC with simpler processing. The decision should be driven by the actual service demands. When the part is a true tooling or wear component, tool steel is correct; when it is a general mechanical part, an alloy steel usually delivers the needed properties at lower machining cost.