Selection of Tool Steel for Die-Casting Dies
Die casting is a high-volume process where molten metal is injected under high pressure into a closed die cavity. In each cycle, the die continuously absorbs and releases heat, so it functions not only as a forming tool but also as a component subjected to repeated thermal loading.
In production, die damage is not random. It typically develops under three interacting conditions. The surface is repeatedly heated by molten metal and then rapidly cooled, creating cyclic stress that eventually leads to heat checking. At the same time, high injection pressure and rapid metal flow lead to erosion and localized deformation. In addition, continuous contact with molten alloys promotes soldering and surface degradation, especially in aluminum casting.
What determines die life is not the presence of these conditions, but which one becomes dominant in a specific application.
Selection Factors for Die-Casting Tool Steels
Material selection should begin with identifying how the die actually fails in service, rather than comparing general material properties.
Resistance to Thermal Fatigue (Heat Checking)
In most aluminum and magnesium die-casting operations, dies fail due to surface cracking caused by repeated thermal cycling. When cracks initiate early and propagate quickly, increasing hardness does not solve the problem. What matters more is the material’s ability to resist cyclic stress and slow crack growth. In such cases, steels with good toughness and stable microstructure under repeated heating are required.
Hot Strength and Tempering Resistance
When casting temperature increases, especially with brass or copper alloys, the failure mode often shifts. Instead of cracking, the die may begin to soften, deform, or lose dimensional stability. If the surface hardness cannot be maintained during service, the die will fail even if cracking is controlled. In these conditions, resistance to softening becomes more critical than toughness.
Balance Between Hardness and Toughness
Higher hardness improves resistance to wear and erosion, but it also increases the risk of crack propagation under thermal stress. This is why most die-casting dies are operated at 42–50 HRC. Below this range, deformation becomes more likely. Above it, cracking tends to accelerate. When dies fail due to early cracking, reducing hardness or selecting a tougher grade is usually more effective than further increasing hardness.
Recommended Tool Steels
H13 Tool Steel Supplier | 1.2344 | SKD61
H13 is used as the baseline material for most die-casting applications because it provides a stable balance between thermal fatigue resistance, hot strength, and toughness.
It is suitable when the die primarily fails due to heat checking and no extreme condition dominates. In aluminum and magnesium die casting, where thermal cycling is unavoidable but not excessively severe, H13 delivers consistent and predictable performance.
Cleaner variants, such as ESR grades, further improve resistance to crack initiation by reducing internal defects, which is important in long production runs.
AISI H11 Tool Steel | 1.2343 | SKD6
H11 becomes relevant when H13 does not last as expected, and failure is dominated by cracking rather than wear or deformation.
This typically happens in dies exposed to aggressive cooling, large temperature gradients, or high-frequency thermal cycling. Under these conditions, the higher toughness of H11 helps slow crack propagation and extend die life.
In practice, switching from H13 to H11 is a direct response to premature fracture problems.
AISI H21 Tool Steel | 1.2581 | SKD5
H21 is selected when temperature becomes the limiting factor instead of cracking.
If the die surface softens during operation, shows plastic deformation, or cannot maintain hardness at elevated temperatures, then conventional chromium-based steels are no longer sufficient. In these cases, a tungsten-based steel such as H21 is used because it retains strength and hardness at higher temperatures.
However, its lower toughness makes it unsuitable for applications subject to severe thermal shock. It is therefore used selectively, only when high-temperature stability is the primary requirement.
AISI P20 Tool Steel | 1.2311 | 3Cr2Mo
P20 is chosen when production requirements are driven more by cost and delivery speed than by maximum die life.
Because it is supplied in a pre-hardened condition, it allows direct machining without heat treatment, reducing lead time and avoiding distortion. This makes it suitable for short-run production or low-temperature casting, such as zinc.
However, under higher thermal loads, especially in aluminum die casting, P20 lacks sufficient tempering resistance and will fail quickly. Its use is therefore limited to applications where shorter tool life is acceptable.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Applications | Selection Logic |
| AISI H13 | 42–52 HRC | Al, Mg, Zn (General Use) | Used when heat checking is the main failure mode |
| AISI H11 | 44–50 HRC | Al, Zn (Severe Thermal Cycling) | Selected when cracking occurs earlier than expected |
| AISI H21 | 38–52 HRC | Brass, Copper | Used when high-temperature softening becomes critical |
| AISI P20 | 28–32 HRC | Zinc (Short Run) | Used when cost and fast machining are prioritized |