Selection of Tool Steel for Die Casting Cores and Inserts
Cores and inserts are typically the first components to fail in die casting tooling. Their lifespan directly determines maintenance frequency, tooling cost, and production downtime.
During operation, molten metal enters the cavity under pressures of 20–150 MPa, with extreme cases reaching 700 MPa. At the same time, inserts are exposed to rapid surface heating followed by forced cooling. This creates steep thermal gradients between the surface and the core, leading to repeated expansion and contraction in each cycle.
Under these conditions, failure is driven by identifiable mechanisms rather than random damage. In production, inserts typically fail in one of the following ways:
- Surface material loss caused by high-velocity melt flow (washing)
- Adhesion between molten alloy and tool surface (soldering)
- Surface crack networks from cyclic thermal stress (heat checking)
- Sudden fracture due to stress concentration or thermal shock
Tool steel selection must therefore be based on which failure mode dominates your process, not on general material properties.
Core Selection Logic
For die-casting cores and inserts, material selection is determined by whether failure is dominated by thermal softening or by cracking.
If the material loses strength at temperature, erosion and deformation accelerate.
If the material lacks toughness, cracking will occur before wear becomes relevant.
The correct grade is chosen by identifying the primary failure mechanism.
Thermal Fatigue (Heat Checking)
When inserts fail by surface cracking, the root cause is cyclic thermal stress. The surface expands under heat while the interior constrains it, creating repeated tensile and compressive stress.
Materials with stable microstructure at elevated temperature delay crack initiation and reduce the crack propagation rate.
Hot Strength and Softening Resistance
If inserts show plastic deformation or rapid surface wear, the limiting factor is high-temperature strength. The material must retain hardness under operating temperature to resist erosion and maintain dimensional stability.
This becomes critical in high-speed or high-temperature casting conditions.
Fracture Resistance (Toughness)
When failure appears as sudden cracking, especially in thin sections or sharp geometries, toughness becomes the controlling factor.
Increasing hardness in this case usually accelerates failure rather than solving it. Material selection must shift toward grades with higher fracture resistance.
Resistance to Soldering
In aluminum die casting, molten aluminum reacts with iron, forming intermetallic layers. This leads to sticking, tearing, and surface damage during ejection.
This behavior is primarily influenced by the base material composition and becomes more severe under high-temperature and long-cycle exposure.
Recommended Tool Steels
H13 (1.2344 / SKD61)
H13 is the standard choice for most die-casting inserts because it offers a balanced combination of thermal fatigue resistance and toughness.
It performs reliably under cyclic heating and cooling and can operate under water-cooled conditions without immediate cracking. This makes it suitable for high-cycle die casting of aluminum, magnesium, and zinc.
The typical hardness range is 46–52 HRC, adjusted to balance wear and fracture resistance.
H13 should be considered the baseline material when failure is distributed between surface degradation and moderate cracking.
H11 (1.2343 / SKD6)
H11 is selected when failure is dominated by cracking rather than wear.
Compared with H13, it offers higher toughness and better fracture resistance, making it suitable for small cross-section cores, sharp transitions, and complex geometries with high stress concentrations.
In practice, when H13 inserts fail prematurely due to cracking, switching to H11 is often more effective than adjusting hardness or heat treatment.
Typical hardness range is 42–47 HRC, often adjusted downward to further improve fracture resistance.
H21 (1.2581)
H21 is used in high-temperature casting environments such as brass and copper alloys, where thermal softening becomes the primary failure mode.
Its high tungsten content allows it to maintain hardness at elevated temperatures where H13 would begin to lose strength. This significantly reduces deformation and washing under extreme heat.
However, H21 has low toughness and is sensitive to thermal shock. It is not suitable for water-cooled or rapidly cycled conditions.
Typical hardness range is 40–44 HRC.
P20 (1.2311 / 1.2738)
P20 is not typically used for high-load die casting inserts. Its role is mainly structural or cost-driven.
It is commonly applied in holder blocks, large low-stress inserts, or zinc alloy applications where thermal load is limited. Its advantage lies in machinability, dimensional stability, and lower cost.
Typical hardness is 28–32 HRC (≈300 HB), with higher hardness variants used selectively in low-demand insert applications.
Practical Selection Summary
| Tool Steel | Key Strength | Limitation | When to Use |
| H13 | Balanced thermal fatigue resistance and toughness | Not optimized for extreme temperature or severe stress concentration | General Al/Mg/Zn die casting inserts |
| H11 | Higher toughness, better resistance to cracking | Lower hot strength than H13 | Small, complex, or high-stress cores |
| H21 | Superior high-temperature hardness | Low toughness, sensitive to thermal shock | Brass/copper casting with high thermal load |
| P20 | Cost-effective and easy to machine | Limited high-temperature performance | Holder blocks or low-load zinc applications |
Final Decision Guidance
Material selection should start from how your inserts fail in production.
- If failure is dominated by heat checking and surface degradation, H13 provides the most stable overall performance.
- If inserts fail by cracking, especially in thin or complex sections, switching from H13 to H11 is usually more effective than increasing hardness.
- If deformation or erosion occurs under high temperature, upgrading to H21 becomes necessary, provided thermal shock can be controlled.
- If thermal load is low and cost reduction is the priority, P20 is sufficient for non-critical components.
At Aobo Steel, these grades are supplied in annealed or pre-hardened condition for bulk procurement, allowing distributors and manufacturers to perform machining and final heat treatment according to their own tooling requirements.