H13 Hot Work Die Steel: Chemical Composition by Standard

Why the Composition of H13 Is a Purchasing Document, Not a Grade Name

H13 is the most widely consumed hot work die steel in the world, and it is also one of the most loosely specified. Two mills can both deliver "H13" and hand over material whose carbon differs by 0.10 percentage points, whose vanadium differs by 0.30 points, and whose sulphur is ten times higher in one heat than the other. Those differences do not show up in a visual inspection, in a hardness test on delivery, or in a machined trial part. They show up months later as a die that softens at 600 °C, a cavity that washes out after 20,000 shots, or a block that cracks after the third temper.

The grade is defined by a family of designations rather than by a single document: AISI H13 and UNS T20813 in the American system, the 4Cr5MoSiV1 designation in GB/T 1299, SKD61 in JIS G 4404, and 1.2344 / X40CrMoV5-1 in the German DIN and EN systems. Each carries its own composition window and its own permitted tolerances, and each window was written for a different industrial purpose. Reading the composition against the designation the mill actually worked to is the only reliable way to know what was bought.

Nominal Chemistry of the H13 Family

The grade is a 5% chromium hot work die steel with molybdenum and vanadium additions and a medium carbon content. The nominal windows are: carbon 0.32-0.45%, silicon 0.80-1.20%, manganese 0.20-0.50%, chromium 4.75-5.50%, molybdenum 1.10-1.75%, vanadium 0.80-1.20%, phosphorus 0.030% max, sulphur 0.030% max, with copper normally limited to 0.25% max and nickel to 0.30% max as residual elements.

The apparent agreement is misleading. The carbon ranges differ by up to 0.13 points between the widest and narrowest designation, and the die casting industry's own specification, NADCA 207, tightens carbon further to 0.37-0.42% for large dies. A mill certificate that states "H13" without quoting the applied standard has told the buyer very little.

Carbon: The Variable That Sets Hardness and Secondary Hardening

Carbon governs two things at once. In the as-quenched condition it sets the hardness of the martensite, and the relationship is steep in the 0.3-0.5% range: at 0.32% the quenched hardness sits near the lower end of the 52-56 HRC band, and at 0.45% it approaches the upper end, with a corresponding loss of toughness. In the tempered condition carbon does something more important for a hot work die: it combines with the carbide formers and re-precipitates as fine alloy carbides during tempering, producing the secondary hardening peak that lets the die hold hardness at 500-600 °C.

This is why a die steel with carbon at the bottom of the window tempers to a lower peak hardness and softens earlier, and why carbon at the top of the window gives better hot hardness at the cost of notch toughness and resistance to heat checking. For aluminium die casting, where thermal fatigue cracking is the dominant failure mode, the middle of the range is normally preferred. For hot forging dies that see mechanical loading at high temperature, a slightly higher carbon and a tempering schedule that stops just short of the peak hardness is the conventional compromise.

What Each Alloying Element Is Doing

Chromium at 4.75-5.50% provides the hardenability needed to through-harden sections up to a few hundred millimetres, and it supplies the oxidation and scaling resistance that keeps a die from losing its surface at 600 °C. It also forms the M7C3 and M23C6 carbides that contribute to wear resistance. Chromium alone is not sufficient for hot hardness; a 5% Cr steel without molybdenum and vanadium would soften rapidly above 500 °C.

Molybdenum at 1.10-1.75% raises hardenability, resists tempering, and is the element most responsible for the resistance to softening that distinguishes H13 from plain 5% Cr steels. It also suppresses temper embrittlement, which is why the grade can be tempered in a single step at 550-600 °C without the embrittling hold that older 5% Cr grades required. Vanadium at 0.80-1.20% is the grain refiner: it forms stable MC carbides that pin austenite grain boundaries during austenitising, and it dissolves and re-precipitates during tempering to sharpen the secondary hardening peak. Vanadium below 0.60% leaves the steel prone to grain coarsening if the austenitising temperature overshoots.

Silicon at 0.80-1.20% raises temper resistance and improves fluidity in the melt, but it is also a ferrite former and, above roughly 1.3%, it degrades toughness and weldability without adding useful hot hardness. A certificate showing silicon at 1.5-1.9% is a warning that the heat was made to a loose internal window rather than to a die steel specification. Manganese at 0.20-0.50% is a deoxidation residual; higher manganese raises the risk of banding and of retained austenite. Phosphorus and sulphur at 0.030% max are the toughness limits, and for premium die blocks made by electroslag remelting the specification is often tightened to 0.015% P and 0.005% S to improve transverse toughness and polishability.

Melting Route Changes the Effective Composition

Two heats can meet the same ladle analysis and behave differently because of segregation. In a conventionally electric-furnace melted and vacuum degassed ingot, chromium and molybdenum segregate toward the centreline and the top of the ingot, producing banded carbide rows that appear as streaks after polishing and that act as crack paths under thermal cycling. Electroslag remelting or vacuum arc remelting reduces segregation markedly, and the resulting block machines and polishes more uniformly.

For large die blocks the practical consequences of segregation are dimensional: a 500 mm thick block cut from an ingot centre can show composition drift across the section that exceeds the product analysis tolerance, even though the ladle certificate is perfectly in specification. Buyers of heavy blocks should therefore ask for the analysis position to be stated and for ultrasonic examination to an agreed level, since internal porosity and shrinkage are the other classic defects of the ingot route.

Composition Versus Heat Treatment Response

The composition window only makes sense alongside the heat treatment it was designed for. Conventional practice austenitises at 1000-1030 °C, holds at temperature long enough to dissolve the chromium and vanadium carbides without coarsening the grain, quenches in oil or in a vacuum furnace with high-pressure gas, and tempers twice at 540-600 °C with an intermediate cool to room temperature. Double tempering is not optional on this grade: the first temper converts retained austenite and the second tempers the freshly formed martensite, and a single temper leaves a structure that will move in service.

A heat at 0.34% C and 0.85% V will not reach 52 HRC at a 600 °C temper no matter how the schedule is adjusted, and a heat at 0.44% C with 1.15% V may crack in a large section if it is austenitised at the top of the range and tempered too low. The composition and the schedule have to be matched, and the mill certificate is the input to that decision.

Verification and Acceptance on Delivery

Chemical verification starts with a spectrometric analysis to a recognised method such as ASTM A751, performed on the sample position agreed at the order stage. A check analysis on the delivered block is not redundant: it confirms that the block matches the certificate, which is the only defence against a mixed stock yard. Carbon, chromium, molybdenum and vanadium should all be reported individually, since a grade statement alone cannot be reconciled with the standard quoted on the purchase order.

Hardness is the second gate. A delivery hardness survey across the block face detects both inadequate annealing of the supplied condition and gross composition error. For pre-hardened die blocks supplied at 28-32 HRC or at 40-45 HRC, the survey also reveals decarburisation and soft spots from insufficient section size capability.

Microstructure and cleanliness belong in the specification for critical dies. Macroscopic examination for banding and carbide segregation, grain size determination by comparison methods, and non-metallic inclusion ratings give a far better prediction of die life than the ladle analysis alone. Ultrasonic examination of heavy blocks is standard practice and should be agreed as an acceptance level, not left as an optional extra.

Procurement Pitfalls That Recur Most Often

The commonest problem is an off-specification substitution presented as an equivalent. A "1.2343 type" or a modified 5% Cr steel with lower molybdenum and vanadium will machine and harden acceptably in a small trial die and then fail early in production, because the hot hardness that molybdenum and vanadium provide is exactly what the unverified substitution removes. The second problem is silicon above the die steel window, which reduces toughness without any compensating benefit. The third is sulphur at the 0.030% ceiling rather than at a controlled low level, which is acceptable for general work but unsuitable for polished cavity work and for large blocks where transverse toughness matters.

The fourth problem is documentation. A certificate that lists a single composition figure per element, with no standard quoted, no heat number tied to the block, and no test method, cannot be used to resolve a subsequent dispute. Buyers should require the standard, the heat number, the product analysis, the hardness survey and the ultrasonic report as one package, and should state in the order which designation's window governs where several are quoted.

FAQ

Q: What is the standard chemical composition of H13 tool steel?
Roughly 0.32-0.45% C, 0.80-1.20% Si, 0.20-0.50% Mn, 4.75-5.50% Cr, 1.10-1.75% Mo, 0.80-1.20% V, with phosphorus and sulphur each at 0.030% max. The exact carbon window depends on the designation applied, ranging from 0.32-0.42% under GB/T 1299 and JIS G 4404 to 0.37-0.43% under DIN 1.2344.

Q: Why does NADCA 207 specify a narrower carbon range?
Die casting dies fail mainly by thermal fatigue, and carbon at the top of the AISI window raises hardness at the expense of toughness. Narrowing carbon to 0.37-0.42% keeps the secondary hardening response consistent from heat to heat so that a die programme validated on one block behaves the same way on the next.

Q: What hardness does H13 reach after quenching and tempering?
As-quenched hardness is normally 52-56 HRC depending on carbon and section size. After double tempering at 540-600 °C the usual working range is 48-52 HRC for die casting and hot forging, with lower hardness selected where thermal fatigue resistance dominates.

Q: Is a higher vanadium content always better in H13?
No. Vanadium above the 1.20% window improves wear resistance and grain refinement but consumes carbon in stable MC carbides that do not dissolve at the austenitising temperature, reducing the carbon available for secondary hardening. Excess vanadium also makes the steel harder to anneal and to machine.

Q: How can a buyer check that a delivered block is genuine H13?
Check the spectrometric analysis against the quoted designation, survey hardness across the block face, and confirm heat number traceability from certificate to block. For critical dies add macroscopic examination for segregation, inclusion rating and ultrasonic examination of the section.

Q: Does the melting route change the composition requirement?
It changes the acceptable limits rather than the nominal window. Electroslag remelted die blocks are typically ordered with phosphorus and sulphur tightened to about 0.015% and 0.005% in order to reduce segregation and improve transverse toughness and polishability.