Tool and die steels split into six functional families — cold-work, hot-work, shock-resistant, high-speed, plastic-mold, and die-casting grades — with cold-work grades typically running 0.5–1.5% C, hot-work at 0.25–0.6% C with Cr-Mo-W, and high-speed steels (HSS) at ~1.0% C with W/Mo/V, all heat-treated to a working hardness of roughly HRC 58–64 [S9]. Daido Steel's product page explicitly states that tool steel "is used for machining tools and jigs of both metallic and non-metallic materials, as well as dies and molds for forging, die-casting, stamping and plastic injection" [S2], which is the cleanest working definition of the segment.
What this article is: a working reference for buyers, heat-treat shop owners, and process engineers who have to pick a grade off a mill test report and survive the consequences. What it is not: a metallurgical textbook. We will stay close to specifications, dimensional realities, and the failure modes that show up on a shop floor — and we will name the families AISI has used for the past century because that is the language every mill cert still ships in [S5].
How the Six Families Break Out on a Mill Test Report
AISI groups tool and die steel by the service condition, not the chemistry, and the working taxonomy is six buckets: W (water-hardening, plain carbon ~0.6–1.4% C), O (oil-hardening cold-work, ~0.9% C, 0.5–1.0% Cr), A (air-hardening cold-work, ~1.0% C, 5% Cr, 1% Mo), D (high-C high-Cr cold-work, 1.5–2.3% C, 12% Cr), H (hot-work, 0.25–0.6% C with Cr-W-Mo), and M/T (molybdenum/tungsten high-speed, ~1.0% C, up to 18% W or 9% Mo) [S5]. Plastic-mold and die-casting tooling is generally ordered as P20 (1.3Mn-1.7Cr-0.4Mo, pre-hardened to ~HRC 30–36) or H13 (0.4C-5Cr-1.5Mo-1V) for hot-chamber and cold-chamber die-casting dies, while H21 (0.3C-9W-3Cr) covers higher-temperature hot-work dies [S2].
The chemistry range is wide because the failure mode is wide. Cold-work dies fail by wear, chipping, and dimensional drift under compression at room temperature, so they need high carbon and high carbide volume. Hot-work dies fail by thermal-fatigue cracking, gross cracking, and plastic deformation at 400–600 °C working surface temperatures, so they need lower carbon, higher hot-strength elements (W, Mo, V) and toughness [S5]. M2 high-speed steel (1.0C-4Cr-5Mo-6W-2V) is the most-specified HSS in cutting-tool production and the chemistry is essentially fixed by AISI/SAE standardisation [S2].
Selection Criteria That Survive a Heat-Treat Lot
Three parameters decide the grade before chemistry: working temperature, abrasive wear budget, and impact load. Cold-work is the most abrasive / lowest temperature regime and steers you into D2 (1.5C-12Cr) or D3 (2.0C-12Cr) where abrasive wear and edge retention dominate. Hot-work routes you to H11 (0.35C-5Cr-1.5Mo-0.4V) or H13 (0.4C-5Cr-1.5Mo-1V) where surface temperatures hit 540–620 °C in die-casting and hot-forging dies [S5].
Hardness in service is the second gate. W-group water-hardening steels top out near HRC 65 at the surface, harden through only on thin sections, and lose hardness fast above 150 °C — they are cheap stamping die material, not die-casting die material. A2 and D2 air-harden to HRC 57–62 with low distortion, and that is the practical reason D2 dominates blanking and forming tooling [S5]. H13, the dominant die-casting die steel, is supplied pre-hardened to roughly HRC 40–48 for structural mould bases and re-hardened to HRC 48–52 for cavity inserts after machining [S2].
Heat-treat response is the third gate and the one most often ignored. W-series needs water quenching, which distorts and cracks large sections. A-series, D-series, H-series and M/T-series all air-harden or oil-harden with much lower distortion, and the difference between a die that cracks in quench and a die that ships is the choice of an air-hardening grade on a die-casting die block [S5]. Heat treatment "describes the practical aspects when performing heat treatment on tool steels" with austenitising, quenching, and tempering sequenced for secondary hardening, and that sequence is the same for every H-grade and M/T-grade [S5].
Comparison of the Six Families Across Four Decision Criteria
The following comparison lines the six families up against the four decision criteria that drive 90% of grade picks on a shop floor — typical working hardness, dominant failure mode, heat-treat distortion risk, and indicative application window. Cold-work grades (W, O, A, D) lead in hardness and wear resistance; hot-work H-grades lead in toughness and thermal-fatigue resistance; HSS M/T-grades lead in red-hardness above 600 °C. P20 plastic-mold steel is supplied pre-hardened to HRC 30–36, deliberately soft, because plastic-injection moulds need to be machined after hardening and then polished or textured [S5].
By working hardness, the order is W1/W2 (~HRC 64) > M2 HSS (~HRC 62–65) > D2 (~HRC 58–62) > A2 (~HRC 57–62) > H13 (~HRC 48–52 die-casting cavity) > P20 (~HRC 30–36 as supplied). By thermal limit, the order flips: H13 keeps hardness at 600 °C, M2 keeps cutting hardness to ~620 °C, D2 is rated to ~250 °C continuous, and W1 collapses above 200 °C [S5]. For die-casting work, the only AISI families that survive thermal fatigue at the die surface are H11, H13, and the higher-W H21/H26; cold-work D2 will thermal-crack inside a few thousand shots in an aluminium cold-chamber cell [S2].
The next step down the spec sheet is a heat-treat check: a grade that has to be water-quenched (W-series) cannot be used on any die block over ~25 mm section thickness without through-hardening distortion cracking. For sections above 75 mm, A2, D2, H13 or M2 are the standard picks because they air-harden with low distortion and a predictable hardness gradient [S5].
Real Use Cases by Industry Segment
Stamping and blanking dies for sheet metal, washer, and electrical lamination tooling default to D2 or D3 (AISI D-series) or DC53 (8% Cr Japanese cousin of D2) where wear life is the KPI. Cold-heading die inserts are typically M2 or M4 where higher impact and faster cycling push the spec toward HSS chemistry. Die-casting dies for aluminium and magnesium cold-chamber work default to H13, with H21 reserved for brass and higher-melting alloys [S2].
Plastic-injection moulds for automotive interior trim, appliance housings, and consumer electronics default to P20 (1.2738) for cavity-and-core, with H13 used for hot-runner manifolds and S7 (1.7Ni-1Cr-0.4Mo) for slides and wear strips. Tool-room jigs, fixtures, and gauges default to O1 (oil-hardening) or W1 (water-hardening) because the volume of material needed is small, the section is thin, and the cost-per-pound dominates [S2].
Forging dies (closed-die hammer and press forging) route to H11, H12, or H13, with H21 and H26 used where the die surface sees higher temperature and where tungsten's hot-strength pays for itself [S5]. Accu-Die & Mold in Stevensville, Michigan is one example of a tool-room that has specialised in "die cast tooling for a variety of markets … aluminum and magnesium die cast dies" for over 25 years, and the standard die-set material on that shop floor is H13 cavity-and-core [S7]. Forte Tool & Die, another foundry-tooling supplier, builds sand-casting patterns and permanent-mould tooling, where pattern wear drives the grade to AISI P20 or aluminium bronze insert pairs rather than hardened tool steel [S4].
Where Tool Steel Ends and Engineering Steel Begins
Tool steel is a sub-class of alloy steel, not the same as alloy steel in the AISI 41xx/43xx/86xx/87xx sense. The "alloy steel" bucket is a chemistry definition — about 5% total alloying elements of Mn, Cr, V, Ni, W — while tool steel is a function-plus-chemistry definition: the steel is bought, machined, heat-treated, and put into a service where it cuts, forms, or shapes another material [S9]. This matters at the spec sheet: an AISI 4140 bar is the same chemistry family as an H13 die block (both Cr-Mo-V), but only H13 is specified to tool-and-die tolerances, ultrasound-tested to AMS 2301 or equivalent, and supplied with a verified hardenability band [S2].
A related article on carbon steel types and classifications covers the AISI 10xx plain-carbon side of the same mill, and a carbon-steel TCO breakdown frames the same materials through 30-year cost lines — both worth cross-reading when you are deciding whether to pay the tool-steel premium. The casting-process connection also matters: tool steel is the upstream of nearly every die-casting die and tool-die steel on a die-casting machine's platens.
Limits, Failure Modes and Sourcing Reality
Three failure modes define whether a tool-and-die grade is fit for service. First, gross cracking in quench, which kills any W-series die on a thick section and is the #1 cause of new-die scrap on shop floors. Second, thermal-fatigue "heat-checking" on hot-work dies, a network of fine surface cracks that appears after thousands of heating-cooling cycles and limits die-casting die life to roughly 100,000–250,000 shots on aluminium cold-chamber work for H13 in production data. Third, abrasive wear on cold-work dies, which limits D2 blanking die life to a few hundred thousand hits before re-grinding or re-sharpening is needed [S5].
Supply-side reality: Chinese mills (Kunshan Benchi Special Steel, among others) now supply the full AISI spectrum — die steel, tool steel, alloy steel, carbon steel, bearing steel — at 30–50% of the Western mill list price, with mill test certs in EN 10108 / DIN 17350 format on request [S6]. Daido Steel continues to dominate the high-cleanliness, high-toughness end (DC53, DH2F, SKD61) and the plastic-mold P20 family, with the published product line covering "dies and molds for forging, die-casting, stamping and plastic injection" [S2]. US tool-and-die shops like Cedar Rapids Tool & Die, Midwest Tool and Machining, and Weiss-Aug remain the downstream that takes the bar stock, machines it, heat-treats it, and delivers a finished tool [S1][S3][S8].
Two trackable signals to watch between now and end-2026: (1) Chinese mill pricing on D2 and H13 has been creeping upward through 2025–2026 as scrap and ferro-chrome costs rose, narrowing the price gap to Western mills by several percentage points; (2) H13 ESR (electroslag-remelted) and H13 VAR (vacuum-arc-remelted) sub-grades are becoming a near-default for high-cavity-count automotive die-casting dies, driven by fatigue-life and inclusion-control spec sheets at the OEM tier [S5][S6].
For component-level specifications, see die casting machine.