H13 Tool Steel (1.2344 / SKD61): Properties, Heat Treatment and Applications

10 Oct 2026

Meta description: H13 tool steel explained: 1.2344 and SKD61 equivalents, chemical composition, hot hardness, heat treatment steps, ESR grades and die casting applications.

H13 tool steel sits at the center of hot work tooling. It is the default material for aluminum die casting dies, a mainstay of extrusion tooling and hot forging dies, and a common choice for mold inserts that must survive abrasive or high-temperature service. The reason is a balance that is difficult to match: good hot hardness, strong resistance to thermal fatigue cracking, useful toughness at working hardness levels of 44–52 HRC, and moderate cost.

The grade is known by several designations that describe essentially the same steel. AISI H13 in the United States, DIN 1.2344 in Europe, JIS SKD61 in Japan and GB 4Cr5MoSiV1 in China all refer to a 5% chromium hot work tool steel with molybdenum and vanadium additions. The equivalents are close but not always identical: chemistry tolerances and melting practice differ between producers, and those differences can matter in demanding dies.

This article covers what H13 is, its composition and properties, how to heat treat it correctly (with typical temperatures and hardness targets), when ESR and premium versions are worth the extra cost, where the steel is used, the machining and polishing practices that protect its performance, and the mistakes that most often shorten die life.

What Is H13 Tool Steel?

H13 is a chromium-molybdenum-vanadium hot work tool steel containing approximately 5% chromium. It belongs to the same family as H11 (DIN 1.2343), which carries less vanadium and molybdenum, and H10 (1.2365), a lower-chromium grade with higher molybdenum. Among the three, H13 offers the best combination of wear resistance and temper resistance, while H11 types are generally tougher and slightly more thermally conductive.

The principal cross-references are:

AISI H13 — United States
DIN 1.2344 / X40CrMoV5-1 — Europe (EN ISO 4957)
JIS SKD61 — Japan (JIS G4404)
GB 4Cr5MoSiV1 — China (GB/T 1299)
AFNOR Z40CDV5 — France

The specifications overlap closely, but permissible ranges for carbon, vanadium and molybdenum differ slightly, so a mill certificate should always be read against the standard named on the purchase order. Grades marketed as "H13 modified" — with tightened composition ranges and cleaner melting practice — are common in die casting, where consistency from die to die matters as much as absolute properties.

Chemical Composition of H13 Tool Steel

Element	Typical range (wt.%)	Function
Carbon (C)	0.32–0.45 (commonly 0.37–0.42)	Hardness, temper resistance, carbide formation
Silicon (Si)	0.80–1.20	Deoxidation; contributes to temper resistance
Manganese (Mn)	0.20–0.50	Hardenability and deoxidation
Chromium (Cr)	4.75–5.50	Hardenability, oxidation and hot corrosion resistance
Molybdenum (Mo)	1.10–1.75	Secondary hardening, hot strength, temper resistance
Vanadium (V)	0.80–1.20	Wear resistance, grain refinement, carbide stability
Phosphorus (P)	≤ 0.030 max	Residual element; kept low for toughness
Sulfur (S)	≤ 0.030 max	Residual element; reduced further by ESR for cleanliness

The exact limits depend on the standard used — AISI, DIN, JIS and GB ranges overlap but do not coincide. Every additional alloying element is balanced against the others: chromium provides hardenability and the oxidation resistance that protects die surfaces, molybdenum and vanadium form the stable carbides responsible for hot hardness and temper resistance, and carbon ties them together. In ESR-remelted grades, sulfur is typically reduced well below the standard limit and oxygen content is lower, which is a cleanliness difference rather than a chemistry one — but it is visible on the certificate and in the polishing room.

Properties of H13 Tool Steel

The properties below describe H13 tool steel in the correctly heat treated condition; the heat treatment that produces them is covered in the next section.

Hot Hardness and Temper Resistance

H13 resists softening at elevated temperature better than most tool steels because molybdenum and vanadium form stable alloy carbides that resist coarsening. After correct heat treatment it reaches roughly 54–56 HRC as quenched, and when tempered to a working hardness of 46–48 HRC it typically retains on the order of 80% of its room-temperature hardness at 500 °C. It continues to carry useful load to approximately 550–600 °C, beyond which softening accelerates. This is the property that lets aluminum die casting dies cycle between water cooling and 450–500 °C metal contact without collapsing.

Thermal Fatigue Resistance

Heat checking — the fine network of cracks that develops on a die surface — is the result of cyclic thermal stress. The surface expands and contracts with every shot while the bulk of the die stays relatively cool, and the repeated strain eventually exceeds the steel's fatigue limit. H13 delays this process through high temper resistance and good ductility at working hardness, which is why it dominates die casting. Within the 5% chromium family, grades with higher molybdenum and lower chromium (H10 and H11 types) can offer slightly better heat-checking resistance and higher thermal conductivity, while H13 compensates with better wear and erosion resistance.

Toughness and Crack Resistance

At 44–48 HRC, H13 retains good toughness and notch ductility, which matters for forging dies that see impact loading and for large dies that must tolerate thermal gradients during start-up. Toughness is influenced by melting practice as much as by hardness: ESR grades have fewer inclusions and more isotropic properties, particularly in the transverse direction of large blocks, where conventionally melted steel is weakest. Most gross cracking failures in service trace back to heat treatment errors or mechanical overload rather than to the steel itself.

Oxidation and Corrosion Resistance

The chromium content gives H13 useful resistance to scaling and oxidation in hot air, but it is not a stainless steel. At room temperature the material rusts like any alloy tool steel, and dies must be protected during storage and shipping with rust-preventive treatment. H13 is not the correct choice for molding PVC or other chemically aggressive plastics, where stainless mold steels such as 1.2083 or 1.2316 belong.

Physical Properties

Density: approximately 7.8 g/cm³
Thermal conductivity: approximately 24–26 W/(m·K) at room temperature
Modulus of elasticity: approximately 210 GPa
Typical working hardness: 44–52 HRC, depending on application

Heat Treatment of H13 Tool Steel

Heat treatment is the single largest variable in H13 tool steel performance. Two dies made from the same heat of steel can differ substantially in life depending on preheating, austenitizing temperature, quench rate and tempering practice. The sequence below reflects standard industry practice; exact parameters should follow the steel producer's data sheet and be verified by hardness testing.

1. Preheating

Large dies should be preheated in two stages, typically 550–650 °C followed by 800–850 °C, with equalization at each stage before proceeding. Slow, controlled heating avoids the thermal gradients that crack heavy sections. Small tools can be charged directly into a furnace at the first preheat temperature, but the second stage remains worthwhile for complex geometry.

2. Austenitizing

The usual austenitizing range is approximately 1000–1030 °C. The lower end of the range favors toughness and finer grain; the upper end produces slightly higher as-quenched hardness at some cost in ductility. Soak time is typically 30–60 minutes once the section is uniformly at temperature, with longer holds for heavy blocks; furnace thermocouples or a load probe are the reliable way to judge this. Temperatures above roughly 1050 °C promote grain growth and are avoided outside special cases.

3. Quenching

Vacuum furnaces with high-pressure nitrogen gas quenching — typically at pressures of 5–10 bar — are the standard method for die blocks, producing clean surfaces and controlled cooling. Oil quenching is used where faster cooling is needed or where older equipment dictates it, and salt bath or interrupted quenching serves special cases. The important practical point is section size: heavy blocks cool slowly at the core, and if the quench is too slow the core will not reach full hardness. Cooling should be stopped at approximately 50–80 °C, not at room temperature, and tempering should follow immediately.

4. Tempering

H13 must be double tempered: two cycles at temperature, cooling to room temperature between them. Each cycle is typically two hours at temperature for average sections, with longer times for large blocks. The double temper transforms retained austenite left from quenching and completes the secondary hardening reaction, producing a stable structure and consistent dimensions.

Tempering temperature is chosen to hit the required working hardness. The table below shows typical results for standard H13 austenitized at approximately 1020–1030 °C and double tempered:

Tempering temperature (two cycles)	Typical hardness
400 °C	54–56 HRC
450 °C	54–56 HRC (secondary hardening peak)
500 °C	52–55 HRC
550 °C	50–52 HRC
600 °C	46–49 HRC
620 °C	44–46 HRC
650 °C	40–43 HRC

These values are approximate and shift with composition, austenitizing temperature and section size — always confirm with hardness testing on samples or test coupons. One rule from hot work practice is worth repeating: the final tempering temperature should be at or above the maximum temperature the die will see in service, so that the steel does not continue to soften on the job.

5. Recommended Working Hardness

Aluminum die casting dies (cores, inserts, slides): 44–48 HRC
Extrusion tooling (dies, mandrels, container components): 46–50 HRC
Hot forging dies (hammer and press): 42–48 HRC
Wear-critical mold inserts: 48–52 HRC

ESR H13 and Premium Grades

Standard H13 tool steel is produced by electric furnace melting with vacuum degassing, and for many tools that is entirely sufficient. When die life and surface quality are critical, premium H13 tool steel grades refined by electro-slag remelting (ESR) are specified instead. The base heat is remelted through a molten slag layer, which absorbs non-metallic inclusions and produces an ingot with a cleaner, more uniform structure and reduced segregation in large sections.

The practical benefits are consistent with that cleanliness: better and faster polishability, more uniform texture etching, improved transverse toughness in heavy blocks, and more predictable thermal fatigue behavior in die casting. Grades sold under names such as 8407 represent this premium class — an ESR-remelted, tightly controlled H13 with very low sulfur and oxygen content. They carry a price premium over conventional H13, but the additional cost is modest relative to the value of extended die life, fewer polishing hours and reduced risk of a premature failure in a die that costs far more than the steel it contains.

ESR is easiest to justify for: large aluminum die casting dies with long production runs, dies that will be polished or textured, tooling where a single heat check network forces an early weld repair, and heavy forging dies where transverse toughness governs cracking risk.

Applications of H13 Tool Steel

Aluminum Die Casting Dies

This is H13's home application. Cores, inserts, slides and other die components run at 44–48 HRC and face molten aluminum at 650–700 °C, thermal cycling with every shot, and erosion from metal flow at the gates. The dominant failure mode is heat checking, followed by washout and, less often, gross cracking. Premium ESR grades are commonly specified for large dies and for components that govern the interval between maintenance shutdowns. Zinc and magnesium die casting use the same grade at similar or slightly higher hardness.

Extrusion Tooling

Aluminum extrusion dies, mandrels, dummy blocks and container components are typically made from H13 or H11 types hardened to 46–50 HRC. The tooling sees sustained high temperature and abrasive metal flow rather than rapid thermal cycling, so wear resistance and hot strength dominate, and nitriding of critical surfaces is common practice to extend die life between corrective polishes.

Hot Forging Dies

H13 is widely used for hammer and press forging dies, usually at 42–48 HRC, where a combination of impact toughness, hot hardness and thermal fatigue resistance is required. Large die blocks benefit from ESR steel and from careful preheating before production; most forging die cracking failures in service begin with inadequate preheat.

Plastic Mold Inserts and Other Tooling

H13 also appears in plastic molds as hardened inserts: gate blocks, cores and cavity details exposed to glass-fiber filled compounds, and areas where high wear resistance is needed locally. It is also used for hot shear blades, hot punches and similar tooling where a combination of toughness and hot hardness is required. Because H13 is not corrosion resistant, it is not the right grade for PVC or other corrosive resins.

Machining, Polishing and Surface Treatment

In the annealed condition, H13 tool steel is typically around 190–230 HB and machines with conventional carbide tooling at moderate speeds. Standard practice is to rough machine with an allowance appropriate to the section size, stress relieve at approximately 600–650 °C, then finish machine or grind. Stress relief at this stage removes the machining stresses that would otherwise combine with hardening stresses and increase distortion.

After hardening, finishing is done by grinding, hard milling or EDM. EDM leaves a recast "white layer" on the cut surface that is brittle, cracked and in residual tension; it should be removed by a light grind or polish — typically a few hundredths of a millimeter — before the tool goes into service, particularly on die surfaces and cavity details that will see thermal or mechanical cycling.

Polishing follows the usual sequence of stones, abrasives and diamond paste. Clean ESR material takes a finer finish faster, which is why premium grades are specified for dies that must be polished repeatedly during their life. For wear protection, gas nitriding is widely used on extrusion and forging tools, producing surface hardness in the range of approximately 900–1100 HV with case depths typically from 0.05 to 0.30 mm. On die casting dies, nitriding is applied selectively and with care, because a hard but brittle case can accelerate heat checking if the case is too thick. PVD coatings appear in some applications but do not replace correct base hardness.

Common Mistakes in H13 Heat Treatment and Use

Most H13 tool steel failures are process failures, not steel failures. The recurring ones:

Single tempering. A single temper leaves retained austenite that transforms in service, causing dimensional movement, hardness loss and cracking risk. Double tempering is not optional for H13.
Tempering below the service temperature. A die tempered at 550 °C that runs at 600 °C will soften in production and heat check early. Temper at or above the expected maximum service temperature.
Insufficient preheating. Charging a cold die block into a hot furnace, or starting production with a cold die, introduces thermal gradients that can crack the tool. Use two-stage preheat in heat treatment and preheat dies before production.
Overheating during austenitizing. Temperatures above approximately 1050 °C coarsen the grain and reduce toughness for minimal hardness gain.
Wrong working hardness. Dies run too hard for forging chip and crack; dies run too soft for die casting wash out and heat check. Match the working hardness to the application.
Quenching to room temperature before tempering. Cool only to approximately 50–80 °C, then temper immediately. Allowing a quenched die to sit cold invites cracking.
Inadequate quench rate in heavy sections. Low gas pressure in a vacuum furnace can leave the core below the required hardness; verify hardness after quenching rather than assuming.
Ignoring the EDM recast layer. Left in place, the white layer becomes the initiation site for cracking.
Using H13 where more hot strength is needed. Copper alloy die casting and some high-temperature processes require more highly alloyed hot work grades; H13 is not a universal answer.

Selecting an H13 Tool Steel Supplier

Because the grade is produced worldwide, the meaningful differences between suppliers lie in melting practice, forging quality, testing and documentation. Useful questions include:

What is the melting route — EAF with vacuum degassing, ESR, or both — and is it stated on the certificate?
Is the chemical analysis per heat supplied, and does it fall within the standard ordered?
Has the material been ultrasonic tested, to which standard (for example SEP 1921 or ASTM A388) and acceptance class?
What is the delivery hardness range, and how uniform is it across large blocks?
What forging reduction was applied to heavy sections?
Does every shipment include an EN 10204 3.1 mill test certificate?

A producer that controls melting, remelting, forging, heat treatment and machining in a single plant can answer these questions from its own process records. Hubei Keyspark Special Steel Co., Ltd. (Keyspark Steel), based in Huangshi City, Hubei Province, China, is an example: the company produces hot work tool steel including H13 grades through in-house steelmaking (EAF, LF, VD and ESR), forging on a 5,000-ton press and hammers, precision heat treatment and machining, and includes a mill test certificate with every shipment covering chemical composition, hardness, ultrasonic and surface inspection, dimensions and mechanical properties.

Conclusion

H13 tool steel remains the reference grade for hot work tooling because its balance of hot hardness, thermal fatigue resistance and toughness covers more applications than any single competing steel. But the grade name on a certificate guarantees far less than buyers often assume: performance depends on where the steel was melted, how it was forged and how carefully it was heat treated. Specify cleanliness where it matters, temper to the right working hardness with two cycles at or above the service temperature, preheat before service, and buy from a producer that can document all of it. Producers such as Keyspark Steel, which melt, remelt, forge and heat treat in-house, can document exactly that. Done properly, H13 dies deliver the long, predictable life the grade is known for.

Frequently Asked Questions

What is the difference between H13, 1.2344 and SKD61?

They are essentially the same 5% chromium hot work steel under different standards — AISI H13 in the United States, DIN 1.2344 in Europe and JIS SKD61 in Japan (GB 4Cr5MoSiV1 in China). The composition ranges overlap closely, but tolerances differ, so the mill certificate should always be checked against the standard that was ordered.

What hardness should H13 be used at?

Typical working hardness is 44–52 HRC. Aluminum die casting dies commonly run at 44–48 HRC, extrusion tooling at 46–50 HRC, hot forging dies at 42–48 HRC and wear-critical inserts up to approximately 52 HRC. Tempering temperature — usually 540–620 °C — is selected to achieve the target hardness with double tempering.

Why does H13 need double tempering?

Double tempering transforms retained austenite left after quenching and completes the secondary hardening reaction. A single temper leaves an unstable microstructure that can cause dimensional movement, hardness loss and cracking in service. Two cycles of approximately two hours each, with cooling to room temperature between them, are standard practice.

Is ESR H13 worth the extra cost?

For dies where heat checking, polish quality or cracking risk is critical — large aluminum die casting dies, polished or textured inserts — ESR H13 typically offers better cleanliness and homogeneity, which translates into more predictable die life and faster polishing. For less demanding tooling, conventional vacuum-degassed H13 tool steel is often adequate.

What causes heat checking in H13 dies?

Heat checking comes from cyclic thermal stress: the die surface expands and contracts with each shot while the bulk of the die remains cooler, and a network of fine cracks eventually forms. Tempering above the service temperature, the correct working hardness, proper die preheating and controlled cooling all delay it, and cleaner, more temper-resistant steel extends life further.

Can H13 be polished to a mirror finish?

Yes. H13 is routinely polished, and ESR grades with low inclusion content can reach the fine finishes required for many plastic molding inserts and textured dies. Its corrosion resistance is limited, however, so for transparent parts or corrosive resins, stainless mold steels such as 1.2083 or 1.2316 are usually the better choice.