Turning Tool Steel: Soft-Machine, Harden, Then Hard-Turn

Tool steels are supplied and almost always machined in the annealed condition, where they are soft enough to turn with standard carbide. Even annealed, they are tougher than mild steel because of their high carbon and alloy content (chromium, molybdenum, vanadium, tungsten), so expect lower surface speeds than 1018, roughly 250 to 500 SFM depending on grade, with coated carbide, rigid setups, and good coolant. The high-alloy, high-carbide grades like D2 are abrasive even annealed and wear tools faster. The standard workflow is: rough and semi-finish turn in the annealed state leaving finishing stock, send out for heat treatment (harden and temper to the target HRC), then finish to final dimension. Because heat treatment distorts parts and changes size, you almost never finish-machine to final tolerance before hardening on anything that matters. After hardening, you have two paths to final dimension: grinding, the traditional route for hardened tool steel, or hard turning with CBN tooling, which has become a practical and often faster alternative for many parts up to 60+ HRC. Which you choose depends on geometry, tolerance, finish, and volume, and a good shop will plan this before cutting the first chip, because the whole process, stock allowance, fixturing, and tolerances, is built around the heat-treat step in the middle.

O1 is an oil-hardening cold-work steel, the classic general-purpose tool steel: easy to heat treat, dimensionally stable enough for many tools, and the most forgiving to machine of this group in the annealed state. A2 is an air-hardening cold-work steel with better dimensional stability and toughness than O1, harden in air with less distortion, and good wear resistance; it machines reasonably annealed and is a workhorse for dies, punches, and gauges. D2 is the high-carbon, high-chromium cold-work steel prized for wear resistance, and it is the abrasive one. Its high volume of hard chromium carbides chews tooling even in the annealed condition and makes it the most demanding of this set to machine; hard-turning D2 after heat treat requires CBN and patience. H13 is the hot-work standard, a chromium-molybdenum-vanadium steel built to resist thermal fatigue and softening at high temperature, used for die-casting dies, extrusion tooling, and hot forming; it machines moderately well annealed. S7 is the shock-resisting grade, tough and impact-resistant for chisels, punches, and tooling that takes blows; it machines reasonably and is valued where toughness matters more than maximum hardness. The practical point is that machining difficulty in the annealed state tracks roughly with alloy and carbide content: O1 and S7 are friendlier, A2 and H13 moderate, D2 the toughest. After hardening, all of them require grinding or CBN hard turning, with the higher-hardness, higher-carbide grades being the slowest.

Hard turning, single-point turning of hardened steel typically from 45 to 65 HRC using CBN (cubic boron nitride) inserts, has changed tool-steel finishing. For many parts it replaces grinding, holding tolerances of ±0.0002 to ±0.0005 in and surface finishes of 8 to 16 Ra µin (sometimes better), at higher material-removal rates and with simpler setups than cylindrical grinding. It runs at low speeds with light depths of cut, rigid machines, and usually dry or with minimal coolant, because thermal shock can crack CBN. It is best for continuous cuts; heavily interrupted cuts chip CBN and may still favor grinding. The persistent challenge is heat-treat distortion. Quenching changes both size and shape, and slender or asymmetric parts can move several thousandths or warp, which is precisely why you leave finishing stock and finish after hardening rather than before. Air-hardening grades like A2 and H13 distort less than oil- or water-hardening grades, which is a real selection factor when dimensional stability matters, sometimes worth choosing A2 over O1 purely to reduce post-heat-treat correction. Planning for distortion means adequate and uniform finishing stock (so there is material everywhere to clean up the warp), stress-relieving rough-machined parts before final heat treat for tight-tolerance work, and designing geometry to minimize distortion, balanced sections, avoiding sharp transitions. The shops that hold tight tolerances on hardened tool steel are the ones that engineer the whole sequence around the quench, not the ones that simply machine harder.

Tool steel turned parts cost more than ordinary steel parts for several stacked reasons: the bar stock is more expensive (high alloy content), annealed machining is slower and harder on tooling (especially abrasive grades like D2), there is an outside heat-treatment step with its own cost and lead time, and final finishing by grinding or CBN hard turning is a precise, slower operation. A finished, hardened, ground tool-steel part is a multi-step job, not a one-op turning. Lead time is driven heavily by the heat-treat step, which is usually sent to a specialized heat-treater and adds days to the schedule, plus any stress-relief cycles for tight-tolerance work. Material availability in the specific grade and size also matters; common grades like A2, O1, and D2 in standard sizes are readily available, while specialty sizes or premium powder-metal tool steels take longer. The cost-control levers are choosing the grade no harder than the application needs (don't specify D2's wear resistance if A2 suffices, since D2 costs more to machine), minimizing material removal in the hardened state by roughing soft, and being realistic about tolerances and finishes so you don't drive unnecessary grinding. The honest framing: you choose tool steel because the part must be hard and wear-resistant, dies, punches, molds, cutting tools, gauges, and that hardness is the entire point, so the heat-treat and hard-finishing overhead is unavoidable. If a part does not need that hardness, an alloy steel like 4140 will cost far less to produce.