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H11 / AMS 6487 / AMS 6437

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H11 is a medium-alloy chromium steel that comes from the hot-work tool steel class but is used for aerospace structural parts. Within the alloy steel group it stands out for retaining its strength at high temperature; its UNS designation is T20811.

The defining property of this material is that it retains most of its strength of approximately 1900 MPa up to 538 °C. The 5% chromium provides hot hardness and scaling resistance, while the 1.3% molybdenum and 0.5% vanadium provide temper resistance and fine grain. Limiting the carbon content to 0.40% helps preserve toughness at this strength level.​‌​​‌​

Heat treatment: austenitise at 1010 °C for 15-45 minutes and air cool, then temper in three stages of 2-3 hours each at 538 °C and above, air cooling between stages. Tempering three times completes the transformation of retained austenite and so gives dimensional stability.

It is used in aircraft structural parts, particularly landing gear components. It is supplied as round bar.​‌​​‌​

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What AISI H11 Is — and Why It Has Two Separate Lives

AISI H11 (UNS T20811 / W.Nr. 1.2343 / EN X37CrMoV5-1) is a 5 % chromium, air-hardening hot-work tool steel: nominally 0.38 C – 5.0 Cr – 1.3 Mo – 0.4 V – 1.0 Si. Medium carbon plus high chromium plus molybdenum delivers three things at once: deep air hardening, secondary hardening (hardness rising again during tempering) and strength retention at elevated temperature.​‌​​‌​

The one number that defines H11 is vanadium: 0.30–0.60 %. In its sibling H13 the same band is 0.80–1.20 %. Everything else — carbon, chromium, molybdenum, silicon — is effectively identical. That half-point of vanadium turns two steels into two different commercial products, and that is the most important section on this page.

H11’s little-known second life is this: the same steel, bought to AMS 6487 premium aircraft quality, is used not as a tool steel but as an ultra-high-strength STRUCTURAL steel — landing gear, rocket motor cases, critical fasteners. Carpenter’s own wording is explicit: strength in excess of 260,000 psi and high creep and rupture strength between 427 and 649 °C (800–1200 °F). Same chemistry, same heat-treatment logic, entirely different purchasing specification and entirely different inspection regime. It is opened up in its own section below.​‌​​‌​

H11 versus H13 — Vanadium at 0.4 % against 1.0 %​‌​​‌​

This is the most important section on an H11 page, and almost no distributor page gets it right. The difference is concentrated in one element and one band, and once the mechanism is understood the commercial consequence follows on its own.

Standards by Product Form

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H11’s standards map is split in two and the split matters on the order line: the tooling side (ASTM A681, EN ISO 4957, NADCA) and the aerospace structural side (AMS). Same steel, two acceptance regimes. Each row below states which world it belongs to.

Code Acceptance and Temperature Ceilings — the Honest Answer

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The expected ASME table is EMPTY here too, and for the same reason: H11 is not a pressure-equipment material. But H11 does have a real and very important temperature discussion — on the metallurgical side rather than the code side.​‌​​‌​

Product Forms With NO Standard — the Commercially Most Valuable Section​‌​​‌​

This is the section your sales engineers should memorise. H11’s coverage is limited to tool-steel product forms on the ASTM A681 side, and to bar, forgings and forging stock only on the AMS side.

Chemical Composition

Three different composition bands circulate under the name H11, and all three are called “H11”. The differences look small, but the width of the carbon band is hardness scatter in disguise — and the aerospace band has been narrowed deliberately.​‌​​‌​

Heat Treatment — All of H11 Is Here

Buying H11 is the easy part. This steel’s entire character — hardness, toughness, dimensional stability and heat-checking resistance — is decided in the chain of preheat, austenitize, quench rate and DOUBLE (or even triple) tempering. And the most frequently skipped link in that chain is the number of tempers.​‌​​‌​

Double — or triple — tempering: why it is not negotiable​‌​​‌​

H11 carries appreciable RETAINED AUSTENITE after quenching. 5 % chromium, 1.3 % molybdenum and 0.4 % carbon push M_s/M_f down far enough that part of the structure is still austenite when the quench ends.
The first temper does two things: it tempers the martensite already present and it destabilises the retained austenite. On cooling from that temper, the austenite transforms to fresh, untempered martensite. So at the end of the first temper, your part contains brittle, hard, completely untempered martensite.
The second temper is what tempers that fresh martensite. This is why “temper twice” is not advice but a metallurgical necessity, and why the part must be cooled to room temperature between the two cycles — otherwise the transformation does not complete.
When is a third temper needed? It is standard in aerospace structural use: the published route is austenitize at 1010 °C for 15–45 minutes, air cool, then temper THREE times for 2–3 hours at a temperature not lower than 538 °C (1000 °F), air cooling after each. In heavy sections, at high working hardness and on critical parts, the third cycle is a margin against any remaining retained austenite.
Each temper cycle is held for at least 2 hours, extended with section thickness — and it is the PART reaching temperature that counts, not the furnace.

The secondary hardening peak — and where the sources diverge​‌​​‌​

H11 does not soften monotonically as it is tempered. At low temperature the martensite tempers and hardness falls a little; then fine molybdenum and vanadium carbides begin to precipitate and hardness RISES AGAIN. This is secondary hardening, and it is why H11 is a hot-work steel at all.
The published data diverge on where the peak sits [D]:
One European publisher’s table (oil from 1020 °C, ø20 mm specimen): 200 °C → 52 HRC (1880 MPa), 300 °C → 52.5 HRC (1915 MPa), 400 °C → 54.5 HRC (2040 MPa), 550 °C → 46 HRC (1520 MPa), 650 °C → 30 HRC (950 MPa) — on that table the peak is around 400 °C.
A producer’s tempering graph, by contrast, shows roughly 53 HRC at 200 °C, 51 at 400 °C, 47 at 600 °C and 45 at 650 °C — that is a far flatter curve and markedly higher hardness at 600 °C.
Do not ignore the discrepancy; its explanation is the austenitizing temperature. A higher austenitizing temperature dissolves more chromium, molybdenum and vanadium into the matrix, and the secondary hardening peak both rises and shifts to a higher tempering temperature. There is therefore no such thing as “the tempering curve of H11”. Whenever you publish one, state the austenitizing temperature, the specimen diameter and the number of cycles — without them the table is unusable.

Premium Aircraft Quality H11 (AMS 6487) — H11’s Little-Known Second Life​‌​​‌​

Filing H11 away as a tool steel misses half of it. The same chemistry, vacuum melted and with the carbon band narrowed, is used in aerospace and defence as an ultra-high-strength structural steel — on the same shelf as 4340, 300M and maraging 250.

Mechanical Properties

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Publishing a single table of mechanical properties for H11 is wrong, because the same steel can sit anywhere between 30 HRC and 55 HRC depending on the tempering temperature chosen. Every row below states which heat-treated condition it belongs to.​‌​​‌​

Physical Properties

Two numbers among H11’s physical properties are commercially decisive: its low thermal conductivity (about 25 W/m·K — less than half that of plain carbon steel) and its relatively high thermal expansion. Together they explain why heat checking is the number one failure mode of a hot-work die.​‌​​‌​

Thermal Fatigue (Heat Checking) Resistance​‌​​‌​

The number one cause of death of a hot-work die is not wear but heat checking. A fine crack network appears on the surface, invisible at first; it deepens a little each cycle; eventually it prints onto the casting surface and the die is scrapped.

Welding — Repair Welding Above All

H11 is weldable, but welding here is not a manufacturing method — it is a REPAIR method. Filling a worn area of a hardened die, restoring a broken corner, recovering a pocket that has gone oversize. And a repair weld done wrong finishes the die completely.​‌​​‌​

Machining and EDM​‌​​‌​

H11 machines well in the annealed condition — published machinability is 75–80 % of 1 % carbon steel. In the hardened condition (46–52 HRC) the work moves to hard turning/milling, grinding and EDM. And EDM is the finishing method that does the most damage in H11.

Corrosion and WHERE IT FAILS​‌​​‌​

H11’s 5 % chromium is the number people get wrong most often. The stainless threshold is around 10.5 % Cr; H11 is at half of it. H11 is not stainless, is not corrosion resistant and does rust. The chromium is there for hardenability and hot strength, not for corrosion.​‌​​‌​

Frequently Asked Questions​‌​​‌​

Our die is dying from a heat-check network. Should we move from H13 to H11?

Check four things before you move; in most cases the problem is not the material.
1. Is the die being preheated? Shooting hot metal into a cold die produces the most severe thermal shock seen in the field and burns a substantial share of the die’s life in the first hundred shots. 2. Was the white layer removed from the EDM surfaces and was the part re-tempered? If that step was skipped, your die surface carries an untempered, brittle martensite layer under tensile stress, and the first cracks will start there. 3. Was the quench fast enough? A slowly cooled block passes the hardness test but is low in toughness — and toughness is heat-checking resistance. 4. Is there decarburization? Heat treating without a protective atmosphere leaves a soft surface layer, and thermal fatigue starts exactly there.
If all four are clean, the material discussion becomes meaningful. Classic tool-steel practice points to H11: halve the vanadium and the carbide volume fraction drops, fracture toughness rises, and the crack both starts later and runs slower. But let us be honest: some current commercial sources argue the exact opposite — that H13’s higher hot hardness makes it superior — and we could not find a peer-reviewed dataset measuring the two on the same rig.
Our suggestion points elsewhere: instead of going from H13 to classic H11, look at a MODIFIED H11. A silicon-reduced, VAR- or ESR-melted, premium-class 1.2343 derivative can deliver 80–100 J Charpy V at room temperature — against the 13.6–33.9 J published for classic H11. Three to seven times the toughness is a far bigger prize than half a point of vanadium. And the producer reports that this toughness lets the tool run at 2 HRC higher working hardness without loss of toughness, which limits the formation of thermal fatigue cracks. Upgrade the class, not the grade.​‌​​‌​

The heat treater’s report says 52 HRC and the die broke anyway. How?

Because hardness is NOT an acceptance criterion in H11. Toughness is, and hardness does not measure it.
There are at least three ways of arriving at the right hardness by the wrong route, and all three pass a hardness test:
1. Slow quenching. If cooling is too slow, carbides precipitate on the grain boundaries on the way down from austenite. The matrix still hardens — the hardness holds — but the grain boundaries have been weakened. Fracture then comes along the grain boundaries, without warning. That is exactly why the die-casting world imposes a minimum cooling rate.
2. A single temper. H11 carries appreciable retained austenite after quenching. The first temper destabilises it and, on cooling, converts it to fresh, untempered martensite. The part leaves the furnace carrying brittle, never-tempered martensite inside it — and its hardness reads correctly for exactly that reason. The second temper exists to temper that fresh martensite and cannot be skipped.
3. Austenitizing too high. At 1030–1040 °C hardness and hot strength rise, but the austenite grain coarsens and toughness falls. None of that appears on a hardness report.
And there is a fourth, very insidious possibility: the part may have been left standing at room temperature before tempering. As-quenched H11 must go to tempering immediately once it reaches 50–70 °C; left standing, it cracks on its own, and that crack can be too fine to see.
What to ask for: not a hardness report but (a) the cooling curve or a record of the minimum cooling rate, (b) the number of tempering cycles with the time and temperature of each, (c) confirmation that intermediate cooling went all the way to room temperature, (d) the austenitizing temperature and (e) a Charpy specimen where possible. Do not have a critical die treated by a heat treater who will not supply these.​‌​​‌​

Our aerospace customer wants AMS 6487 and we hold ASTM A681 H11. Can we use it?

No — and the reason comes in three layers.
First layer: the carbon band. In ASTM A681 H11 carbon is 0.33–0.43 %. In AMS 6487 it is 0.38–0.43 % — the upper half of that band only. If your material came in at 0.34 % carbon it is fully compliant with ASTM and outside AMS 6487. And that is not an academic detail: structural use targets strength above 260 ksi (1793 MPa), which cannot be reliably reached with carbon at the bottom of the band.
Second layer: the melt route. AMS 6487 says Consumable Electrode Vacuum Re-Melted — VAR is mandatory. ASTM A681 imposes no melt route at all. That condition governs more than the chemistry does: in a structural steel at the 1793 MPa level, an inclusion is a direct fatigue crack initiator and heat treatment cannot remove it.
Third layer: the inspection and traceability regime. Aerospace material arrives with heat number traceability, an inspection level and release documentation. A block on a die shop’s rack does not carry that chain, however identical its chemistry.
Watch the reverse direction too: AMS 6487 material can be used in place of ASTM A681 H11 (it is narrower and cleaner) — but doing so is almost always money wasted. An aluminium die casting die gains nothing from VAR melting; what it needs is NADCA-class cleanliness and verified toughness, not aerospace traceability. Keep the two worlds apart: AMS is for aerospace, NADCA is for tooling, and ASTM A681 is the common floor beneath both.​‌​​‌​

The price gap between H11 and H13 is small. Why not just always buy H13?

For most work you genuinely can — and most of the sector already does. H13 is the world standard for aluminium die casting, it has the widest availability, it sits at the centre of the NADCA acceptance criteria and its heat-treatment window is the best understood of any grade. For a job you do not know, the default should be H13.
There are four situations where you should specifically look for H11:
1. Very heavy section. As section grows, cooling rate falls and toughness gives way. Low-vanadium H11 is tougher than H13 at the same section. On large forging dies and heavy blocks H11 is still preferred.
2. Severe thermal shock and/or water cooling. Classic tool-steel practice points to H11 here, and one US source states explicitly for H11 that it “permits water cooling in service”.
3. The die is dying by breaking in one go. A large crack, a corner breaking out, a full separation — these are toughness failures, not wear failures, and the answer is to reduce H13’s vanadium, not to increase it.
4. Aerospace structural use. That sits outside this argument altogether: there the choice is not H13 but AMS 6487 H11, and it has no alternative.
Conversely, the case where you should NOT reach for H11 is just as clear: if the die is dying by wear (dimensional loss, rounded edges, erosive washout), H11’s lower vanadium takes you further in the wrong direction. The decision criterion is not the price gap but the way the die dies — and you read that off the surface of the die you just scrapped.​‌​​‌​

Common datasheet errors — check before you order

1. AMS 6485 and AMS 6488 listed as if still current. AMS 6485H was CANCELLED in October 2006; AMS 6488H in January 2008. The current H11 aerospace specification is AMS 6487 (rev. M, 2021). A datasheet listing those two has not been updated in at least fifteen years — treat everything else on it with the same suspicion.
2. The sentence “H11 and H13 are effectively the same steel”. WRONG. Vanadium is 0.30–0.60 % against 0.80–1.20 % — roughly double. That is the only difference and it changes the whole balance of wear, toughness and heat checking.
3. Thermal conductivity given as 42.2 W/m·K. It is an outlier. Two independent European producers publish 24.9–25 W/m·K at 20 °C. Do not use 42.2 — and it is a large enough error to double your die cooling calculation.
4. Publishing a single “tempering curve”. H11’s hardness–tempering curve shifts with the austenitizing temperature. One source puts the secondary hardening peak at about 400 °C; another shows a far flatter curve still at about 47 HRC at 600 °C. A tempering table that does not state the austenitizing temperature, the specimen diameter and the number of cycles is unusable.
5. Writing “double tempering is recommended”. It is not recommended, it is REQUIRED. The first temper converts retained austenite into fresh, untempered martensite; the second temper exists to temper it. On the aerospace route the number of cycles is THREE.
6. Conflating the stress-relief temperatures. Stress relief before hardening (350 °C up to 600–670 °C depending on the source) and stress relief after hardening (28–42 °C below the final tempering temperature) are not the same operation. Confusing them softens a hardened part.
7. Giving a single soft-annealing temperature. Published values scatter across 750–800, 800–810, 850 and 871 °C. Do not average them; follow the sheet of the producer you are actually buying from. The common acceptance criterion is the outcome: annealed hardness ≤229 HB.
8. Publishing a Charpy value without qualification. The band published for classic H11 is 13.6–33.9 J; for a modified/premium H11 it is about 80–100 J. Three to seven times. Without the hardness, the tempering temperature, the specimen orientation and the quality class, a toughness number is meaningless.
9. The sentence “it contains 5 % chromium, so it is corrosion resistant”. WRONG. The stainless threshold is around 10.5 % Cr. H11 rusts. The chromium is there for hardenability and hot strength.
10. Skipping the post-EDM step. The white (recast) layer must both be removed mechanically AND be followed by re-tempering. Doing one and skipping the other leaves half the damage in place.
11. Reading “air hardening” as “can be cooled slowly”. A dangerous misreading. H11 hardens in air, but slow cooling precipitates grain-boundary carbides and destroys toughness — and the hardness test will not show it.
12. Assuming it is harmless to let a quenched part stand. An H11 part that has reached 50–70 °C must go straight to tempering; left standing, it cracks on its own.
13. Treating ASTM A681 and AMS 6487 as equivalent. AMS 6487 narrows carbon to 0.38–0.43 % and mandates VAR. Every AMS 6487 is an A681 H11; not every A681 H11 is an AMS 6487.
14. Confusing the EN designation. The current EN name is X37CrMoV5-1. X38CrMoV5-1 circulates as an older or alternative designation; X40CrMoV5-1 is 1.2344, that is H13 — a completely different grade.
15. Mistaking a producer nominal for the standard band. Five producers, all selling “H11”, publish carbon between 0.36 and 0.40, silicon between 0.3 and 1.1 and vanadium between 0.40 and 0.50. In a die-life comparison, a claim of “the same material” without a producer name is meaningless.
16. Leaving the temperature claims unlabelled. 538 °C is a resistance-to-softening figure; 427–649 °C is a creep and rupture strength range; and the service temperature limit is whatever tempering temperature you chose. Three different numbers, three different meanings — label each one.​‌​​‌​

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