H13 vs SKD61 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
H13 (AISI/ASTM designation) and SKD61 (JIS designation) are two of the most commonly specified hot-work tool steels worldwide. Engineers, procurement managers, and manufacturing planners frequently face the choice between them when specifying dies, molds, and hot-forming tooling — considerations typically include heat-resistance, wear life, weldability, and supply-chain conformance. The selection dilemma often reduces to whether to prioritize specification and standards alignment (which influences inspection certificates, dimensional tolerances, and acceptance criteria) versus material chemistry and heat-treatment practice that govern in-service performance.
Both grades are chemically and metallurgically very similar; the primary practical distinction lies in standards, allowable tolerances, and delivery-condition requirements. Because they converge on the same alloy strategy (Cr–Mo–V hot-work steel), they are often interchangeable in function, but not always interchangeable on paper in regulated contracts or where traceability and certification to a particular standard are required.
1. Standards and Designations
- H13: Common designations and standards include AISI H13, SAE J431 (historical), ASTM A681 (tool steel bars), AMS standards (for aerospace substrates), and various EN/ISO equivalents (often referenced as X40CrMoV5-1 / 1.2344 depending on the exact spec and delivery condition).
- SKD61: JIS G4404 (SKD61) is the Japanese Industrial Standard designation. Equivalent materials are commonly cross-referenced to AISI H13 or DIN/EN tool-steel grades, but details may vary by mill and spec.
- Classification: Both H13 and SKD61 are hot-work tool steels (alloy/tool steels), not stainless, not HSLA.
2. Chemical Composition and Alloying Strategy
| Element | Typical Range H13 (wt%) | Typical Range SKD61 (wt%) |
|---|---|---|
| C | 0.32–0.45 | 0.32–0.45 |
| Mn | 0.20–0.60 | 0.20–0.50 |
| Si | 0.80–1.20 | 0.70–1.20 |
| P | ≤0.03 (trace) | ≤0.03 (trace) |
| S | ≤0.03 (trace) | ≤0.03 (trace) |
| Cr | 4.75–5.50 | 4.75–5.50 |
| Ni | ≤0.30 | ≤0.30 |
| Mo | 1.10–1.75 | 1.10–1.50 |
| V | 0.80–1.20 | 0.80–1.20 |
| Nb, Ti, B, N | ≤trace | ≤trace |
Notes: - Ranges are typical commercial ranges; exact composition varies by standard and mill. SKD61 and H13 use the same alloying strategy: mid-range carbon for hardness and wear, significant chromium for temper resistance and oxidation resistance, molybdenum and vanadium for secondary hardening, strength at elevated temperature, and carbide dispersion for wear resistance. - Minor differences between standards often appear in impurity limits, trace elements, and allowable composition windows rather than in fundamental alloying approach.
How the alloying affects properties: - Carbon controls achievable hardness and wear resistance but increases hardenability and cracking risk. - Chromium provides hardenability, elevated-temperature strength, and oxidation resistance. - Molybdenum improves temper resistance and secondary hardening. - Vanadium forms stable carbides that improve wear resistance and refine grain size, enhancing toughness. - Silicon and manganese are deoxidizers and contribute to strength; excessive Mn can reduce toughness.
3. Microstructure and Heat Treatment Response
Typical microstructures: - In the annealed or normalized condition: tempered martensite or bainitic/pearlitic structures depending on cooling; soft annealed microstructures contain carbides dispersed in ferrite/pearlite for machinability. - After standard quench-and-temper cycles: tempered martensite matrix with dispersed chromium/molybdenum/vanadium carbides that provide high strength and hot hardness. Secondary hardening from Mo and V produces increased hardness after tempering at elevated temperatures typical for hot-work steels. - SKD61 and H13 develop essentially the same microstructural constituents under equivalent heat treatment cycles.
Heat treatment routes and their effects: - Normalizing: refines prior austenite grain size and homogenizes microstructure; recommended before rough machining for large sections. - Hardening (austenitize and quench): austenitize typically in the 1000–1050 °C range (exact temp per spec), followed by oil or air quench depending on section size and required toughness. Produces martensite and retained austenite. - Tempering: multiple tempering cycles (often 2–3) are used to reduce residual stresses and develop desired hardness while promoting secondary hardening (due to Mo and V carbides). Higher tempering temperatures lower hardness but improve toughness. - Thermo-mechanical processing (for forgings and large dies): controlled forging temperatures and subcritical anneals reduce segregation and refine carbides, improving toughness and fatigue life.
4. Mechanical Properties
| Property | Typical H13 (quenched & tempered) | Typical SKD61 (quenched & tempered) |
|---|---|---|
| Tensile strength | ~1000–1800 MPa (depending on tempering) | ~1000–1800 MPa (similar range) |
| Yield strength | ~800–1500 MPa | ~800–1500 MPa |
| Elongation (A%) | ~5–12% | ~5–12% |
| Impact toughness (Charpy V) | ~15–45 J (depends on hardness/tempering) | ~15–45 J (comparable) |
| Hardness (HRC) | ~40–52 HRC (typical operational range) | ~40–52 HRC |
Interpretation: - Both grades display similar strength and toughness profiles because they share the same primary alloying elements. Strength and toughness trade-offs are principally controlled by carbon content and tempering practice rather than the H13 vs SKD61 label. - Toughness is enhanced by careful normalization and tempering, as well as reduced section thickness and proper heat-treatment practice. - The practical difference is seldom in intrinsic mechanical properties but in guaranteed delivery conditions, heat-treatment instructions, and acceptance criteria specified by the selected standard.
5. Weldability
Weldability considerations for both grades: - Carbon equivalent and equivalent-composition indices are used to evaluate cold-cracking risk and preheat/postheat requirements. Common empirical formulas include:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
and
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
- Interpretation: Both H13 and SKD61 have moderate carbon and significant alloying (Cr, Mo, V) that raise CE and $P_{cm}$. This means:
- Preheating and controlled interpass temperatures are usually required to avoid hydrogen-induced cold cracking.
- Post-weld heat treatment (PWHT) or tempering may be necessary to restore toughness and reduce residual stresses in service-critical parts.
- Weld filler selection should match chemistry and accommodate carbides/secondary hardening tendencies.
- Practical guidance: Welding is feasible with appropriate procedures (low-hydrogen consumables, preheat, slow cooling), but repair welding of highly stressed hot-work tooling requires metallurgical control and subsequent tempering cycles.
6. Corrosion and Surface Protection
- Both H13 and SKD61 are non-stainless tool steels; they do not offer notable corrosion resistance compared with stainless grades. They resist oxidation and scaling at elevated temperatures better than simple carbon steels due to Cr content, but are not corrosion-resistant in wet or aggressive chemical environments.
- Common surface protection strategies:
- Protective coatings (PVD/CVD) for wear and mild corrosion resistance.
- Surface nitriding or carbonitriding to increase surface hardness and improve fatigue/wear resistance (must consider residual stress and distortion).
- Paints, epoxy coatings, or galvanizing for non-high-temperature service (galvanizing not appropriate for high-temperature die surfaces).
- PREN formula for stainless alloys is not applicable to H13/SKD61, but for completeness the PREN expression is:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
Use PREN only for evaluating pitting resistance of stainless steels; it does not apply to hot-work tool steels.
7. Fabrication, Machinability, and Formability
- Machinability: In the annealed condition both steels machine reasonably well; hardness and carbide content increase tool wear in hardened condition. Carbide-forming elements (V, Mo) increase abrasion, reducing tool life for high-precision milling/drilling unless carbide tooling is used.
- Formability: Cold forming is limited on hardened material. Soft-annealed billets can be formed; hot forging/forging followed by controlled heat treatment is common to produce large die blocks and forgings.
- Grinding and EDM are standard finishing operations for hardened tools; EDM is commonly used for complex cavities and repair work.
- Surface treatments such as nitriding can reduce machinability and require adjustment of finishing operations.
8. Typical Applications
| H13 (AISI) Typical Uses | SKD61 (JIS) Typical Uses |
|---|---|
| Hot-work die casting dies | Hot-work die casting dies |
| Hot forging dies and inserts | Hot extrusion dies and plungers |
| Hot shear blades and punches | Forging dies and die-casting tooling |
| Plastic injection mold cores (high thermal cycles) | High-temperature forming tooling |
| Heat-resistant tooling for aluminum and brass | Press and die components for hot forming |
Selection rationale: - Both grades are chosen for hot-work applications where resistance to thermal fatigue, thermal softening, and abrasive wear are primary requirements. - Choose based on load type, cycle frequency, operating temperature, and required surface life; metallurgical performance is similar, so logistical and contractual considerations (specification, certification) often drive the choice.
9. Cost and Availability
- Relative cost: Generally similar on a per-kg basis because chemistry and processing are equivalent. Regional market factors can make SKD61 more economical in Asia, whereas H13 (specified to ASTM/AMS) may be preferred and more readily stocked in North America and Europe.
- Product forms and availability: Both grades are widely available in bar, plate, forgings, and pre-hardened blocks. Availability of specific product forms (large forged blocks, precision-ground plates) depends on regional mill capabilities and inventory.
- Lead times: Specifying a particular standard (JIS vs ASTM/AMS) can affect lead times and inspection paperwork; for critical components, confirm mill test certificates and permitted deviations in advance.
10. Summary and Recommendation
| Attribute | H13 | SKD61 |
|---|---|---|
| Weldability | Moderate; requires preheat/PWHT | Moderate; identical welding precautions |
| Strength–Toughness balance | High strength with good toughness after proper HT | Equivalent balance under equivalent HT |
| Cost & availability | Widely available globally; preferred in ASTM/AMS contexts | Widely available, often more economical in Asia; JIS-certified |
Recommendations: - Choose H13 if: your supply chain, contracts, or certification requirements call for AISI/ASTM/AMS specifications, or if your sourcing and inspection processes are configured around those standards. It is the pragmatic choice for North American and European procurement where paperwork and acceptance criteria favor H13/ASTM. - Choose SKD61 if: your procurement is regionally centered in Japan or Asia, contract documents reference JIS standards, or you require compatibility with JIS-certified mill test documentation. SKD61 can offer cost or lead-time advantages in those markets. - In functional terms, choose either grade when the primary needs are hot hardness, thermal fatigue resistance, and wear resistance — but always specify heat-treatment instructions, hardness acceptance ranges, and required inspection/certification to ensure the delivered product meets engineering intent.
Final note: From a metallurgical-performance viewpoint H13 and SKD61 are effectively equivalent alloys. The decisive factor for many industrial users is standards conformance and the associated supply-chain documentation and tolerances, not a substantial difference in in-service behavior. When specifying tooling, include explicit heat-treatment parameters, acceptance hardness ranges, and repair-welding procedures to ensure predictable performance regardless of whether the material is labeled H13 or SKD61.