Selection of Tool Steel for Hot Extrusion Dies
Hot extrusion is a forming process in which heated billets are forced through a die under high pressure to produce continuous profiles. During operation, the die is subjected to sustained compressive stress, sliding contact with flowing metal, and repeated exposure to elevated temperatures. Typical billet temperatures range from 400–500°C for aluminum alloys and can exceed 1100°C for steel, which results in fundamentally different thermal loads on the die surface.
Although dies are preheated and cooled to control bulk temperature, the working surface still experiences cyclic heating and localized thermal gradients. These conditions lead to two dominant damage mechanisms. One is progressive thermal fatigue, which initiates surface cracking due to repeated expansion and contraction. The other is continuous wear along the bearing surface caused by metal flow and friction. In higher-temperature applications, local softening can also occur, reducing load-bearing capacity and accelerating deformation.
Tool steel selection must therefore be based on which mechanism dominates under specific operating conditions, rather than treating all extrusion environments as equivalent.
Critical Selection Factors
The performance of hot extrusion dies depends on how the material maintains strength, resists cracking, and manages heat under cyclic loading.
Hot strength determines whether the die can maintain its shape under load at elevated temperature. When hot strength is insufficient, the bearing surface deforms plastically, which leads to dimensional instability and loss of profile accuracy. This becomes critical in copper alloy and steel extrusion, where surface temperatures are significantly higher than in aluminum processing.
Toughness governs the material’s resistance to crack initiation and propagation under repeated thermal and mechanical stress. In extrusion, cracks typically form at the surface and grow over time rather than resulting from a single overload event. Materials with insufficient toughness will develop heat checking that propagates rapidly into structural failure.
Thermal fatigue resistance is governed by both microstructural stability and heat-transfer capability. Steels with higher thermal conductivity reduce surface temperature gradients, which directly slows crack initiation. Conversely, materials that retain hardness at elevated temperatures reduce wear but may accumulate higher thermal stress if heat is not dissipated efficiently.
Because these factors interact, selection must be aligned with the dominant failure mode. Applications with severe thermal cycling prioritize crack resistance, while high-temperature extrusion requires retention of hardness. In high-speed conditions, heat dissipation becomes the controlling factor.
Recommended Tool Steels
H13 Tool Steel Supplier | 1.2344 | SKD61
H13 is the standard choice for aluminum extrusion because it provides a stable balance of thermal fatigue resistance, wear resistance, and toughness at moderate temperatures. Its alloy system supports good resistance to softening while maintaining sufficient ductility to delay crack propagation.
In aluminum extrusion, where die surface temperatures typically remain below 550°C, H13 maintains adequate hardness to resist wear at the bearing while limiting the rate of thermal cracking. However, when surface temperatures exceed this range, hardness decreases more rapidly, leading to accelerated wear and loss of dimensional stability.
AISI H11 Tool Steel | 1.2343 | SKD6
H11 contains lower vanadium content than H13, resulting in reduced carbide volume and improved toughness. This makes it more resistant to crack initiation under conditions involving rapid temperature change or aggressive cooling.
In applications where dies are exposed to frequent thermal cycling or external cooling, H11 reduces the risk of early crack formation compared to H13. The reduction in carbide content also means lower resistance to abrasive wear, so it is more suitable where cracking, rather than wear, is the primary cause of failure.
AISI H21 Tool Steel | 1.2581 | SKD5
H21 is a tungsten-alloyed hot-work steel designed for elevated-temperature applications. Tungsten improves resistance to thermal softening, allowing the material to retain hardness at temperatures where chromium-based steels lose load-bearing capacity.
This makes H21 suitable for extrusion of brass, bronze, and steel, where higher surface temperatures require sustained hot strength. However, the increased resistance to softening is accompanied by lower toughness. Under cyclic loading, this increases sensitivity to crack propagation, particularly when thermal gradients are uncontrolled.
AISI H10 Tool Steel | 1.2365 | SKD7
H10 is characterized by lower chromium content and higher thermal conductivity compared to H13. This allows faster heat transfer away from the die surface, reducing thermal gradients and slowing the initiation of heat checking.
This characteristic is particularly important in high-speed extrusion and in components such as mandrels, where heat accumulation rather than wear is the primary limitation. Compared to H13, H10 sacrifices some wear resistance but provides more stable performance in conditions where temperature control governs tool life.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Advantage | Ideal Application |
| AISI H13 | 44–50 HRC | Balanced performance under moderate temperature | Aluminum extrusion |
| AISI H11 | 38–52 HRC | Higher toughness and crack resistance | Thermal cycling conditions |
| AISI H21 | 36–54 HRC | Retains hardness at high temperature | Copper alloys and steel extrusion |
| AISI H10 | 39–54 HRC | High thermal conductivity and heat dissipation | High-speed extrusion, mandrels |