Understanding H13 Steel Hardness

H13 steel’s hardness is a key property in its applications. H13 steel is a widely used 5% chromium (Cr) hot-work tool steel, also known as 4Cr5MoSiV1, 1.2344, SKD61, with a carbon content of approximately 0.4%. It belongs to the H group of tool steels, which exhibit resistance to softening at high temperatures.

Its characteristics include:

1. Hot Hardness & Temper Resistance: H13 exhibits good “red hardness” and maintains high hardness and strength at elevated temperatures, making it resistant to thermal softening. Its high tempering resistance allows it to withstand high-temperature surface treatments without significant loss of core hardness.
2. Good Wear Resistance: Due to its vanadium content, H13 is very resistant to abrasion and erosive wear, especially at high temperatures. 
3. Toughness & Impact Strength: H13 possesses excellent impact strength and good ductility.
4. Deep Hardenability: It is a deep-hardening, air-hardening steel, allowing large sections to be hardened by air cooling with minimal residual stresses.
5. Good Machinability: When properly annealed, H13 has a machinability rating of 70 compared to a 1% carbon steel rated at 100. Please refer to H13 Tool Steel Machinability.

Hardness of H13 Tool Steel

For H13, increasing the hardening temperature can increase tempered hardness, but attempts to obtain excessively high hardness may lead to little or no additional hardness gain while causing a strong loss in toughness. In actual practice for dies, a compromise is made where dies are tempered to near-maximum hardness levels at which they still possess sufficient toughness to withstand loading.

The tempering process is crucial to adjust hardness and, more importantly, to increase toughness and relieve internal stresses in hardened tools. 

1. After air quenching, the hardness range of H13 steel is 52–54 HRC.
2. After tempering at different temperatures, the longitudinal mechanical properties of H13 steel bars at room temperature are as follows:

Hardness (HRC)	Temperature (°C)	Temperature (°F)
52	527	980
50	555	1030
48	575	1065
46	593	1100
44	605	1120

3. For die-casting tooling, the optimum working hardness for H13 is typically 44–48 HRC. For tooling requiring shock resistance, it is 40–44 HRC. H13 combines good red hardness, abrasion resistance, and resistance to heat checking at hardness levels in the range of 45–50 HRC.
4. The typical core hardness for H13 after nitriding is ~45 HRC, with surface hardness exceeding 1000 HV (>70 HRC).
5. When used in extrusion processes, H13 has a hardness of 48–52 HRC for punches and dies. In hammer forging and mechanical press forging, H13 dies have a hardness range of 47–56 HRC.

Chemical Composition¹

Element	Carbon (C)	Chromium (Cr)	Molybdenum (Mo)	Vanadium (V)	Silicon (Si)	Manganese (Mn)	Phosphorus (P)	Sulfur (S)
Content (%)	0.32 – 0.45	4.75 – 5.50	1.10 – 1.75	0.80 – 1.20	0.80 – 1.25	0.20 – 0.60	≤ 0.030	≤ 0.030

Key Alloying Elements and Their Roles:

• Chromium (Cr): Chromium significantly improves hardenability, wear, corrosion, and oxidation resistance, as well as high-temperature properties.
• Molybdenum (Mo): Molybdenum is a strong carbide former, creating hard, stable carbides like Mo2C and double carbides. It increases wear resistance and improves machinability and mechanical properties in chromium steels. Molybdenum helps maintain hardness at higher tempering temperatures and increases hot hardness and high-temperature properties. 
• Vanadium (V): Vanadium is added to increase wear resistance, especially abrasion and erosive wear at high temperatures. It forms V-rich MC carbides, which are extremely hard (up to 84 HRC equivalent) and significantly contribute to tempering resistance and secondary hardening. Vanadium is also a potent hardenability element and strengthens the steel by forming vanadium carbonitride precipitates. It increases strength while retaining ductility and promotes a fine-grained structure.
• Carbon (C): Carbon is the most important element, enabling the steel to harden through martensite formation. Increasing carbon content leads to higher working hardness and elevated temperature hardness, and it increases the number of hard, stable, complex carbides, which greatly contribute to wear resistance.
• Silicon (Si): Silicon is commonly added as a deoxidizer in molten steel. It can slightly increase maximum attainable tempered hardness and alter carbide morphology when combined with higher nitrogen content.
• Manganese (Mn): Manganese acts as a deoxidizer and enhances hardenability. It strengthens and toughens steel by dissolving in ferrite and stabilizes carbides. 

Variation in hardness with tempering temperature for H13 steel²

* All specimens air cooled from 1025 °C (1875 °F) and tempered 2 h at temperature. AQ, as-quenched. Please refer to H13 steel heat treatment.

Applications of H13 Tool Steel

• Hot Work Tooling:
  • Die-casting dies (aluminum, magnesium, zinc, white metal).
  • Forging dies.
  • Extrusion dies, mandrels, cores, dummy blocks, stems.
  • Hot shear blades, gripper dies, hot-nut tools, casings.
• Plastic Molding: Injection molds, plastic mold cavities, and components.
• Cold Work Tooling: Found in applications where extra toughness is required at the sacrifice of some wear resistance, offering better hardenability and wear resistance than common alloy steels like 4140. It can be used for severe cold punching and scrap shears.
• Other Applications: Shrink rings (e.g., for cemented carbide dies), wear-resisting parts.

1. Bringas, J. E. (Ed.). (2002). Handbook of Comparative World Steel Standards (2nd ed., p. 434). ASTM International. ↩︎
2. ASM International. (1991). ASM Handbook, Volume 4: Heat Treating (p. 520). ASM International. ↩︎