Author: Site Editor Publish Time: 2026-10-07 Origin: Site
H13, H11 and H21 are the three hot work tool steels most specified for dies that run at 400 °C and above. All three are covered by ASTM A681, but they take three different metallurgical routes to the same goal — holding hardness and resisting thermal fatigue under cyclic heat and pressure. H13 is the 5% chromium Cr-Mo-V grade that dominates modern die casting; H11 is the lower-vanadium variant, tougher and slightly less wear-resistant; H21 is the 9% tungsten grade, the legacy choice for very high operating temperatures where tungsten's hot hardness beats chromium's thermal fatigue resistance. The selection rule in one line: die casting and most general hot work → H13; high-toughness hot work and aerospace forgings → H11; brass extrusion and high-temperature forging where tungsten is needed → H21.
| Property | H13 (T20813) | H11 (T20811) | H21 (T20821) |
|---|---|---|---|
| ASTM A681 class | H (Cr-Mo-V hot work) | H (Cr-Mo-V, lower V) | H (W-based hot work) |
| Equivalent grades | 1.2344, SKD61, 4Cr5MoSiV1 | 1.2343, SKD6, 4Cr5MoSiV | 1.2581, SKD5, 3Cr2W8V |
| Carbon, % | 0.32–0.45 | 0.33–0.43 | 0.26–0.36 |
| Chromium, % | 4.75–5.25 | 4.75–5.25 | 3.00–4.00 |
| Molybdenum, % | 1.10–1.75 | 1.10–1.60 | 0.40 max |
| Tungsten, % | — | — | 8.50–10.00 |
| Vanadium, % | 0.80–1.20 | 0.30–0.60 | 0.30–0.60 |
| Silicon, % | 0.80–1.20 | 0.20–0.50 | 0.50 max (often 0.20–0.40) |
| Austenitize, °C | 995–1040 | 980–1040 | 1100–1200 (typ. 1150) |
| Quench | Air (or oil for large) | Air | Oil (or air for thin) |
| Temper, °C | 595–650 (double) | 540–650 (double) | 595–675 (double) |
| Working hardness, HRC | 44–52 (typ. 46–48) | 40–50 (typ. 46–48) | 44–52 (typ. 46–50) |
| Hot hardness at 600 °C, HRC | ~36–40 | ~32–36 | ~42–46 (best) |
| Thermal fatigue resistance | High | High | Medium |
| Toughness (Charpy, room temp, working hardness) | Highest (~20–28 J at 46 HRC) | High (~22–30 J at 46 HRC) | Lowest (~10–18 J at 48 HRC) |
| Max service temperature, °C | ~540–595 | ~510–540 | ~600–650 |
| Relative cost | Medium | Medium | High (tungsten surcharge) |
Read the table as a trade-off curve: H13 and H11 are close cousins — both Cr-Mo-V steels that derive hot hardness from chromium and molybdenum carbides, with H13's higher vanadium giving better wear and thermal fatigue resistance. H21 is metallurgically a different animal: tungsten carbides give it better hot hardness above 600 °C, but the high tungsten content drops toughness and thermal fatigue resistance below H13 at every temperature point.
| Element (%) | H13 | H11 | H21 |
|---|---|---|---|
| Carbon | 0.32–0.45 | 0.33–0.43 | 0.26–0.36 |
| Manganese | 0.20–0.50 (often 0.60 max) | 0.20–0.50 | 0.10–0.40 |
| Silicon | 0.80–1.20 | 0.20–0.50 | 0.50 max |
| Chromium | 4.75–5.25 | 4.75–5.25 | 3.00–4.00 |
| Molybdenum | 1.10–1.75 | 1.10–1.60 | 0.40 max |
| Tungsten | — | — | 8.50–10.00 |
| Vanadium | 0.80–1.20 | 0.30–0.60 | 0.30–0.60 |
| Phosphorus | 0.030 max | 0.030 max | 0.030 max |
| Sulfur | 0.030 max | 0.030 max | 0.030 max |
Three chemistry differences drive everything else. First, H13's higher vanadium (0.80–1.20%) is the single most important difference from H11. Vanadium forms fine MC carbides that resist coarsening at high temperatures, which is why H13 holds hot hardness and resists thermal fatigue better than H11 despite the two having nearly identical chromium and molybdenum. Second, H11's silicon is capped lower (0.20–0.50%) than H13 (0.80–1.20%). H11 was developed for aerospace structural forgings, where high silicon would hurt impact properties; H13's higher silicon improves oxidation resistance at die-casting temperatures and is part of why it dominates that application. Third, H21's 9% tungsten is the legacy metallurgy — tungsten carbides are stable at temperatures where chromium carbides start to coarsen, so H21 holds hardness above 600 °C better than H13. The price is toughness: at room temperature, H21 at 48 HRC has Charpy values around 12 J, while H13 at the same hardness is 22–28 J.
| Step | H13 | H11 | H21 |
|---|---|---|---|
| Preheat | 650 °C, then 850 °C | 650 °C, then 850 °C | 650 °C, then 900 °C |
| Austenitize | 995–1040 °C (typ. 1020 °C) | 980–1040 °C (typ. 1010 °C) | 1100–1200 °C (typ. 1150 °C) |
| Soak time, min/mm | 0.5–1.0 | 0.5–1.0 | 0.8–1.2 (longer, high W diffusion is slow) |
| Quench | Air (preferred) or oil for large sections | Air | Oil (preferred) or air for thin sections |
| Quench hardness, HRC | 50–54 | 50–54 | 54–58 |
| First temper | 595 °C × 2 h, slow cool to room temp | 540 °C × 2 h | 595 °C × 2 h |
| Second temper | 595–650 °C × 2 h to target hardness | 540–650 °C × 2 h to target | 595–675 °C × 2 h to target |
| Working hardness range, HRC | 44–52 (typ. 46–48 die cast) | 40–50 (typ. 46–48) | 44–52 (typ. 46–50) |
| Sub-zero treatment | Optional, converts retained austenite, improves dimensional stability | Optional | Less common (lower retained austenite) |
Two heat-treat realities matter for selection. First, H13 and H11 air-harden — vacuum furnace with high-pressure gas quench (6–10 bar nitrogen) is the standard heat-treat route for premium die-casting inserts, and the slow quench preserves die geometry. H21 traditionally oil-quenches, though modern vacuum furnaces can gas-quench H21 in thin sections; the higher austenitizing temperature (1150 °C) means surface decarburization is a serious risk and protective atmospheres are mandatory. Second, double tempering is non-negotiable for all three. The first temper transforms retained austenite to fresh martensite; the second tempers that martensite. Skipping the second temper leaves untempered martensite in the structure, and the die fails by brittle cracking within the first few hundred cycles.
H13 is the most-used hot work tool steel in the world. The 5% chromium plus 1.4% molybdenum plus 1% vanadium combination gives the best balance of hot hardness, thermal fatigue resistance, toughness and wear at temperatures up to about 540 °C — which is exactly the operating range of aluminum die casting, hot forging of steel, and brass and copper extrusion. H13 dominates:
Aluminum and magnesium die casting dies, the single largest application. Die surface temperatures of 400–500 °C with cyclical thermal shock from molten metal at 670–720 °C make thermal fatigue the dominant failure mode, and H13's vanadium carbides resist the surface check-initiation that ends die life.
Hot forging dies and inserts for steel and alloy forgings. Forging temperatures are higher than die casting (1100 °C stock, die surface 300–500 °C), but the contact is intermittent — H13's toughness absorbs the impact and the Cr-Mo-V carbides hold the impression detail.
Hot extrusion dies, holders and liners for aluminum and copper alloys. H13 holds die bearing geometry under high-temperature sliding contact.
Plastic injection molds for high-temperature engineering polymers (PEEK, PPS, glass-filled nylon) where the mold runs at 150–200 °C. H13's polishability and thermal conductivity make it the standard upgrade from P20.
Pressure die casting cores and inserts for brass and copper, where die surface temperatures reach 600–700 °C — though above 600 °C, H21 tungsten hot work steel starts to outperform H13.
H13 tool steel is the metallurgical default for any new hot work tool unless the application specifically demands higher toughness (H11), higher hot hardness above 600 °C (H21), higher wear resistance (H13 modified, Amodie, or premium V-modified grades), or specialty grades like H11 for impact-loaded forging. The Premium grade of H13 — vacuum-arc-remelted (VAR) or electroslag-remelted (ESR), with very tight sulfur and phosphorus limits and microstructure control for thermal fatigue — is the specified grade for die-casting inserts in automotive structural castings.
H11 sits chemically between H13 (more vanadium, more wear) and the older H12 (which adds tungsten). The lower vanadium (0.30–0.60% versus H13's 0.80–1.20%) means fewer vanadium MC carbides, which means a softer, tougher matrix at the same hardness. H11 is the standard grade for:
Aerospace structural parts machined from forged bar. H11 (and its European variant 1.2343) was originally developed for aircraft landing gear and rotor components, where the combination of through-hardening in oil, high impact toughness, and good fatigue resistance made it the right tool steel for high-stress forged structural parts. The lower silicon (max 0.50% versus H13's 0.80–1.20%) is part of why H11 has slightly higher Charpy values than H13 at the same hardness.
Hot forging dies for high-impact applications where H13's thermal fatigue resistance is needed but impact loads are higher than die casting. Forging hammers and high-energy-rate forging (HERF) machines commonly specify H11.
Die casting dies for moderate cycle counts where H13 is over-specified. H11 is typically 10–15% cheaper than H13 in equivalent bar.
Plastic mold components for moderately abrasive filled polymers.
H11 tool steel is most often specified as a substitute for H13 when either (a) impact toughness is the dominant failure mode and H13's vanadium-content wear advantage is unused, or (b) cost matters and H13 is over-spec. For die casting, the modern trend is to skip H11 and go straight to H13 — the small cost premium is earned back by longer die life — but for impact-loaded forging applications, H11 remains the better metallurgical choice.
H21 is the metallurgy of an earlier era. The 9% tungsten forms tungsten carbides that resist coarsening at temperatures where chromium carbides start to soften, which gives H21 its single defining advantage: hot hardness above 600 °C. At 600 °C, H13 retains roughly 36–40 HRC; H21 holds 42–46 HRC. At 650 °C the gap widens further — H13 drops below 30 HRC, H21 still holds 38–42 HRC.
H21 is the choice for:
Brass and copper die casting dies, where die surface temperatures reach 700 °C and chromium-based steels soften and check rapidly. The tungsten carbide structure holds hardness at temperatures that defeat H13.
Hot extrusion dies and mandrels for brass, copper and titanium alloys — the bearing sections that see the highest combination of temperature and sliding pressure.
Hot forging dies for high-temperature alloys and stainless steels, where stock temperatures exceed 1100 °C and die surface temperatures climb past 600 °C.
Dies for glass manufacturing — glass container molds and feeder components that operate continuously at 700–900 °C.
H21 hot work tool steel pays two penalties for its hot hardness. First, toughness drops sharply. At room temperature and 48 HRC, H21's Charpy un-notched is 10–18 J; H13 at the same hardness runs 22–28 J. The tungsten carbides are large and act as crack-initiation sites under impact. Second, thermal fatigue resistance is worse than H13. Tungsten carbides have a different coefficient of thermal expansion from the matrix, and the cyclic thermal stresses of die casting cause surface checking to start earlier on H21 than on H13. The rule of thumb: H21 only earns its place when operating temperature exceeds 600 °C; below that, H13 wins on every other axis. The legacy tungsten-grade alternative H12 hot work steel sits between H13 and H21 — Cr-Mo-V plus 1.5% W — and is mostly a niche grade today.
| Property | H13 | H11 | H21 |
|---|---|---|---|
| Working hardness, HRC | 44–52 | 40–50 | 44–52 |
| Hot hardness at 540 °C, HRC | ~42 | ~40 | ~45 |
| Hot hardness at 600 °C, HRC | ~36–40 | ~32–36 | ~42–46 (best) |
| Hot hardness at 650 °C, HRC | ~28–32 | ~26–30 | ~38–42 |
| Thermal fatigue resistance (cyclic surface check) | High (best of three) | High | Medium |
| Toughness, Charpy un-notched, J at working hardness | 22–28 (best) | 22–30 (best) | 10–18 |
| Wear resistance, hot | High | Medium | High (at temperature) |
| Oxidation resistance at 600 °C | Good (Si helps) | Medium | Medium |
| Through-hardenability in section | ~150 mm (air), ~300 mm (oil) | ~150 mm (air), ~300 mm (oil) | ~75 mm (oil) |
| Machinability rating (annealed, % of B1112) | 55–65 | 60–70 | 45–55 |
| Thermal conductivity at 400 °C, W/m·K | ~28 | ~30 | ~25 |
| Cost (bar, premium ESR) | 1.0 baseline | 0.85–0.95× H13 | 1.3–1.6× H13 |
The decision matrix: H13 is the right answer when operating temperature stays below 540 °C and thermal fatigue is the dominant failure. H11 is the right answer when impact loads matter more than wear. H21 is the right answer only when operating temperature exceeds 600 °C and the tungsten hot hardness pays back the toughness penalty.
| Application | Operating temperature | Recommended grade | Reasoning |
|---|---|---|---|
| Aluminum die casting, die body | 400–500 °C | H13 (premium ESR) | Best thermal fatigue resistance; industry default |
| Aluminum die casting, core pins, slides | 500–580 °C | H13 (premium, nitrogen-cooled) | Higher local temperature; premium grade avoids checking |
| Magnesium die casting | 450–550 °C | H13 (premium) | Same thermal fatigue logic, similar to Al |
| Brass die casting | 650–750 °C | H21 (or H13 with intensive cooling) | H21 holds hot hardness above 600 °C |
| Hot forging, steel stock | 300–500 °C die surface | H13 or H11 | H13 for wear; H11 for high-impact hammers |
| Hot forging, high-temperature alloys | 500–700 °C | H21 | Above H13's thermal fatigue range |
| Hot extrusion, aluminum | 450–550 °C | H13 | Standard for Al extrusion dies |
| Hot extrusion, copper/brass | 600–800 °C | H21 | Tungsten hot hardness essential |
| Hot extrusion, titanium | 700–900 °C | H21 or specialty grades | Very high temperatures exceed H13 |
| Plastic injection, glass-filled polymers | 150–200 °C mold | H13 (or P20 for low stress) | Polishability + thermal conductivity |
| Plastic injection, high-temperature polymers (PEEK) | 200 °C mold | H13 (or H11) | Holds mold geometry at elevated temperature |
| Press hardening (22MnB5 sheet) | 600–700 °C die surface | H11 or H13 (premium) | High impact + high temperature; H11 toughness wins |
| Copper tube piercing plug | 700–900 °C | H21 | Tungsten hot hardness critical |
| Aerospace forged landing gear | room temp | H11 (1.2343) | Standard aerospace forging grade |
The pattern: H13 covers the bottom 80% of hot work applications. H11 covers the subset where impact toughness is the differentiator. H21 covers the high-temperature niche above 600 °C. The single most common selection mistake is using H21 below 600 °C — the tungsten premium is wasted and the toughness penalty is real.
The most common mistake is specifying H21 for die casting because "tungsten is better at high temperature." Brass die casting is one of the few applications where this is true; for aluminum and magnesium, the operating temperature sits inside H13's sweet spot and H21's worse thermal fatigue resistance will shorten die life. A second mistake is specifying H11 instead of H13 to save 10% on material cost — the die-life difference for a thermal-fatigue-dominated application is typically 15–25% in favor of H13, so the cost saving is illusory past the first re-grind.
The third mistake is under-specifying H13 quality for premium die casting. Conventional electric-furnace H13 has higher sulfur and phosphorus, larger primary carbide clusters, and shorter thermal fatigue life than ESR or VAR premium H13. For a die running 100,000+ shots in aluminum structural casting, premium H13 earns its 20–30% price premium; for a prototype die running 5,000 shots, conventional H13 is fine.
The fourth mistake is skipping the second temper. The first temper on H13 leaves 5–10% retained austenite that the second temper converts to fresh martensite. Skipping the second temper leaves untempered martensite in the structure, and the die fails by brittle cracking in the first few hundred cycles — a completely avoidable failure.
Q: Is H13 the same as SKD61?
Yes, essentially. SKD61 is the JIS G4404 designation for the same composition as ASTM A681 H13. DIN 1.2344 and GB 4Cr5MoSiV1 are also equivalent. Cross-grade substitution is normal practice in the industry.
Q: Can I use H13 for a brass die casting die instead of H21?
Yes, but with intensive internal cooling and acceptance of shorter die life. H13 surface checking starts earlier than H21 above 600 °C. If the brass casting runs high-volume production, H21 (or H21-modified with niobium) is the standard answer.
Q: Why does H21 need higher austenitizing temperature than H13?
Tungsten carbides dissolve more slowly and at higher temperatures than chromium carbides. To get enough carbon and alloy into solution for full hardening response, H21 needs 1100–1200 °C austenitize versus H13's 995–1040 °C. The trade-off is surface decarburization risk, which is why H21 heat treatment needs protective atmospheres.
Q: What is "Premium H13" and when do I need it?
Premium H13 is ESR (electroslag remelted) or VAR (vacuum arc remelted) material with very low sulfur (typically <0.005%), low phosphorus (<0.015%), tight microstructure (no primary carbide clustering, ASTM E45 Method A ≤ 2 for inclusions), and a controlled temper curve. For aluminum die casting inserts running 100,000 shots or more, premium H13 extends thermal fatigue life by 30–50% over conventional electric furnace H13 and is the standard for automotive structural casting dies. For prototype dies or low-volume work, conventional H13 is fine.
Q: H11 vs H13 for forging — which is right?
For forging hammers and high-impact presses, H11 (or the European 1.2343) tends to outperform H13 because its slightly lower hardness at the same temper temperature is offset by better impact toughness. For press forgings with high stock temperatures but moderate impact, H13's wear and thermal fatigue resistance give longer die life. Many shops stock both and switch by forging type.
Q: Can H21 be air-hardened?
In thin sections (under 25 mm) and with high-pressure gas quench in a vacuum furnace, yes. In larger sections, oil quench is the standard because H21's hardenability is shallower than H13's. The risk of oil quench is cracking in sections with sharp corners or section changes — preheat and post-temper immediately.
If you are specifying a hot work tool and want a second opinion on H13, H11 or H21 for your die casting, forging or extrusion application — or whether you should be looking at premium H13 (ESR/VAR), H13-modified grades, or specialty tungsten grades — send the drawing, the operating temperature range and the cycle count. Qilu Metal supplies H13, H11 and H21 in flat bar, round bar and forged block to ASTM A681, DIN EN ISO 4957 (1.2344 / 1.2343 / 1.2581) and JIS G4404 (SKD61 / SKD6 / SKD5), with conventional, ESR and VAR quality routes, full mill test reports and traceability. Email enquiry@qilumetal.com and our metallurgy team will quote the size, the quality level and the heat-treatment recommendation matched to your operating temperature and die geometry.
