Selection of Tool Steel for Copper and Brass Extrusion
In copper and brass extrusion, a heated billet is forced through a die under high pressure to form continuous profiles. Typical process temperatures range from 650–1100°C, with unit pressures exceeding 690 MPa and reaching up to 1035 MPa in heavy-duty operations.
Under these conditions, tooling failure is primarily driven by three mechanisms:
plastic deformation at elevated temperature, surface wear from metal flow, and thermal fatigue caused by repeated heating and cooling. These mechanisms directly determine tool life and dimensional stability, and therefore define the requirements for tool steel selection.
Selection Factors
Tool steel selection must address the dominant failure modes in copper and brass extrusion rather than general material properties.
Hot Hardness and Tempering Resistance
At the extrusion temperature, insufficient hot strength results in plastic deformation of the die bearings and loss of dimensional accuracy. The steel must retain hardness and compressive strength under sustained thermal exposure.
Wear and Erosion Resistance
Continuous metal flow causes surface material loss, especially in high-velocity regions. Tool steels must contain stable carbides to resist abrasion and prevent rapid profile degradation.
Impact Toughness
Load fluctuations during billet entry and unstable flow can generate localized stress. Insufficient toughness leads to cracking or edge failure, particularly in high-stress areas.
Thermal Fatigue Resistance (Heat Checking)
Repeated heating and cooling cycles generate surface cracks that propagate over time. Resistance to thermal fatigue is critical for maintaining die surface integrity and service life.
Recommended Tool Steels
Based on the balance between hot strength, wear resistance, and thermal fatigue resistance, the following grades are used for different tooling roles:
AISI H10 Tool Steel | 1.2365 | SKD7
H10 is selected when thermal fatigue is the dominant failure mode rather than wear. It offers better resistance to heat checking than tungsten-based steels, making it suitable for mandrels and dummy blocks in brass extrusion, where temperature cycling is frequent. Typical hardness is 42–48 HRC.
AISI H21 Tool Steel | 1.2581 | SKD5
H21 is selected when resistance to thermal softening is the primary requirement. It maintains strength under sustained high temperature and compressive load, making it suitable for extrusion dies and dummy blocks. Typical working hardness is 42–48 HRC to reduce the risk of cracking under load.
AISI H12
H12 is selected for components subjected to high mechanical load but lower thermal exposure. Its higher toughness reduces the risk of cracking under pressure, making it suitable for support components such as liners, rams, and bolsters. Typical hardness is 40–46 HRC.
AISI H19
H19 is used in the most severe conditions where both high temperature and high pressure must be sustained without softening. The cobalt addition improves resistance to thermal softening, allowing it to maintain dimensional stability. It is typically used for die inserts and cores at 45–52 HRC.
AISI H26
H26 is used in more severe conditions where higher temperatures and stronger metal flow accelerate wear. Compared with H21, it offers better resistance to high-temperature wear and erosion but lower toughness. It is typically applied in high-load dies operating at 45–50 HRC.
Summary Table
| Tool Steel Grade | Alloy Base | Working Hardness | Primary Advantage | Typical Application |
| AISI H21 | Tungsten | 42–48 HRC | Resistance to thermal softening under load | Extrusion dies, dummy blocks |
| AISI H26 | Tungsten | 45–50 HRC | High-temperature wear and erosion resistance | Heavy-load dies |
| AISI H10 | Chromium | 42–48 HRC | Resistance to thermal fatigue (heat checking) | Mandrels, dummy blocks |
| AISI H19 | Cr-W-Co | 45–52 HRC | Stability under combined high temperature and pressure | Die inserts, cores |
| AISI H12 | Chromium | 40–46 HRC | High toughness under compressive load | Liners, rams, bolsters |