CNC Machined Tool Steel Parts

Karl Gillihan

🔨 TOOL STEEL

CNC Machining Tool Steel: A2, D2, O1, H13 and S7 Grades

Tool steel is bought to be hard, which is exactly what makes machining it a sequencing problem rather than a simple cutting job. The whole game is deciding what to cut soft, what to harden, and what to grind afterward, because the same metal that holds a cutting edge or a die surface in service will fight your end mill once it is heat treated. Get the order of operations right and the part is precise and durable; get it wrong and you are grinding scrap.

ISO 9001AS9100

The soft-then-hard workflow that defines tool-steel machining

Tool steels are almost always supplied annealed, in a soft, machinable state around 200-250 BHN, precisely so they can be cut. The standard workflow is to rough and largely finish-machine the part in the annealed condition, leave grinding stock on critical features, send it out for heat treatment to bring it to working hardness (often 55-62 HRC), then grind or hard-mill the critical surfaces to final tolerance. Trying to machine fully hardened tool steel with conventional cutters is impractical for most geometry. The reason this sequence matters is heat-treat distortion. Quenching and tempering cause dimensional change and warping that no amount of pre-machining precision survives, so tight features must be finished after hardening. The amount of expected movement depends heavily on the grade, which is one of the main reasons certain tool steels exist. For buyers, the practical consequences are that tool-steel parts carry multiple process stages, soft machining, heat treat at an outside facility, then grinding, that lead times are longer than for a single-op part, and that the heat-treat specification (target hardness, condition) must be on the drawing. A part quoted without a hardness callout is incomplete.

A2, D2, O1, H13 and S7: choosing by distortion, wear and toughness

These five cover most tooling needs, and they differ mainly in dimensional stability through heat treat, wear resistance, and toughness. O1 is an oil-hardening grade, inexpensive and easy to machine and heat treat, but with more distortion and less wear resistance, suited to short-run tooling, gauges and fixtures. A2 is an air-hardening grade with much better dimensional stability through hardening (air quench means less warp) and good wear resistance, making it a popular general-purpose die and punch steel that holds size well. D2 is the high-chromium, high-carbon wear champion: excellent abrasion resistance and edge retention for blanking dies, punches and forming tools, but its high carbide content makes it abrasive and harder to machine and grind, and it is less tough. H13 is the hot-work die steel, designed to resist thermal fatigue and softening at high temperature, the standard for die-casting dies, extrusion tooling and plastic-mold cores that run hot. S7 is the shock-resistant grade, prized for toughness and impact resistance in chisels, punches and dies that take impact, accepting lower wear resistance in exchange. The buyer choice is an engineering tradeoff: D2 for wear, S7 for impact, H13 for heat, A2 for dimensional stability in general tooling, O1 for cost on short runs. Specify by the dominant failure mode the tool faces.

Machining the soft material and finishing the hard part

Annealed tool steel machines like a medium-carbon alloy steel: moderate surface speeds (roughly 60-150 SFM with carbide depending on grade), rigid setups, and good coolant. D2's heavy carbide content makes it more abrasive on tooling even when annealed, so it cuts slower and wears tools faster than O1 or A2. Programmers leave appropriate grinding stock, typically 0.010-0.030 in on critical surfaces, knowing heat treat will move the part. After hardening, finishing shifts to grinding and, increasingly, hard milling with CBN or coated carbide on rigid machines. Surface grinding, jig grinding and wire EDM are standard for achieving precise dimensions and fine finishes on hardened tool steel, with ground finishes reaching well below 16 microinch Ra and tolerances to a few tenths of a thousandth on precision tooling. Wire and sinker EDM handle hardened features and sharp internal corners that cutters cannot. For buyers, this means tool-steel parts often involve a chain of specialized processes and the total cost reflects multiple operations and outside heat treat. It also means the right supplier is one equipped for the full chain, soft machining, heat treat coordination, and precision grinding or EDM, rather than a generalist who can only do the first step.

Frequently Asked Questions

Because hardened tool steel, typically 55-62 HRC, is impractical to cut with conventional tooling for most geometry, while the same steel in its supplied annealed state, around 200-250 BHN, machines readily like a medium-carbon alloy steel. The standard workflow is to rough and largely finish-machine in the annealed condition, leaving grinding stock on critical features, then send the part out for heat treatment to reach working hardness, then grind or hard-mill the critical surfaces to final tolerance. The reason critical features are finished after hardening is heat-treat distortion: quenching and tempering cause dimensional change and warping that pre-machining precision cannot survive, so any tight tolerance machined before heat treat would be lost. The amount of movement depends on the grade, which is part of why air-hardening steels like A2 (low distortion) and oil-hardening O1 (more distortion) exist as distinct choices. For buyers, this means a tool-steel part is inherently a multi-stage job, soft machining, outside heat treat, then precision grinding or EDM, with longer lead times and a mandatory hardness callout on the drawing, and it should go to a shop equipped for the full process chain.

Choose by the dominant condition the tool faces. O1 is an oil-hardening grade that is inexpensive and easy to machine and heat treat but distorts more and wears faster, ideal for short-run dies, gauges and fixtures where cost matters more than longevity. A2 is air-hardening with excellent dimensional stability through heat treat and good wear resistance, the popular general-purpose choice for dies and punches that must hold size precisely. D2 is the high-carbon, high-chromium wear champion with outstanding abrasion resistance and edge retention for blanking and forming dies and punches, but it is abrasive to machine and grind and less tough, so it is reserved for high-wear, low-impact tooling. H13 is the hot-work standard, engineered to resist thermal fatigue and softening at temperature, used for die-casting dies, extrusion tooling and hot-running mold components. S7 is the shock-resistant grade prized for toughness and impact resistance in chisels, punches and dies that take heavy blows, trading away some wear resistance. So: D2 for wear, S7 for impact, H13 for heat, A2 for stability, O1 for low-cost short runs. Specify by the failure mode you are designing against.

Enough that it dictates your entire process plan, and the amount depends strongly on the grade and the quench. Oil-hardening grades like O1 distort the most because the rapid oil quench induces uneven stresses, so parts can warp and change size noticeably; air-hardening grades like A2 distort far less because the slower, more uniform air quench reduces thermal gradients, which is a primary reason A2 is chosen for precision tooling that must hold geometry. In all cases, quenching and tempering cause some dimensional growth and warping, so the standard practice is to leave grinding stock, often 0.010-0.030 in, on critical surfaces during soft machining and then grind or hard-mill those surfaces to final tolerance after hardening. Large, thin or asymmetric parts move more than compact symmetric ones. For buyers, the implications are concrete: do not expect tight tolerances to survive heat treat on as-machined surfaces, budget for post-hardening grinding or EDM on critical features, choose a low-distortion grade like A2 when geometry stability matters, and always specify target hardness and condition on the drawing so the heat treater and grinder can plan correctly.

Both are possible, and modern shops increasingly hard-mill rather than relying solely on grinding. After hardening to 55-62 HRC, the traditional finishing methods are surface grinding, jig grinding and wire or sinker EDM, which reliably achieve precise dimensions, fine finishes below 16 microinch Ra, and sharp internal features that cutters cannot reach. Grinding remains the standard for flat and cylindrical precision surfaces, and EDM handles hardened sharp corners, intricate die details and through-hardened features. However, hard milling with CBN or specialized coated-carbide tooling on rigid, high-precision machines has become common, letting shops mill hardened tool steel directly for mold cavities and die surfaces, reducing electrode and grinding steps. The choice depends on geometry, tolerance and finish: simple precision flats and diameters go to grinding, intricate hardened detail and sharp corners go to EDM, and complex 3D hardened surfaces increasingly go to hard milling. For buyers, the practical point is that finishing hardened tool steel requires specialized capability beyond ordinary machining, so the right supplier is one with grinding, EDM and hard-milling resources, and the total cost reflects these precision post-hardening operations on top of the soft machining and heat treat.

Plan for longer lead times than a single-operation part, because tool-steel work is inherently multi-stage. A typical job involves soft machining, outside heat treatment, then precision grinding, hard milling or EDM, and each handoff adds time. Soft machining itself is comparable to other alloy steels, but heat treatment at an outside facility commonly adds several days to a week including queue and the controlled quench-and-temper cycle, and precision grinding or EDM finishing adds more depending on complexity and tolerance. For a straightforward die component, a realistic end-to-end lead time is often two to four weeks at low volume, longer for complex molds or tight-tolerance precision tooling requiring extensive grinding or EDM. Grades that machine and grind slower, notably abrasive D2, can extend the schedule further. To compress lead time, send a clean drawing with the hardness and condition specified up front so heat treat and grinding can be planned in parallel, choose a low-distortion grade like A2 when it suits the application to reduce grinding, and source to a shop that handles the full chain in-house or has tight heat-treat partnerships, since coordinating multiple vendors is usually the biggest schedule risk.

Last updated: July 2026

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