Selection of Tool Steel for Hot Trimming Dies
Hot trimming is a secondary operation in forging and die casting used to remove flash, gates, and overflows immediately after forming, while the workpiece remains hot. This improves cutting efficiency but exposes the die to simultaneous mechanical loading and thermal cycling.
Trimming temperature depends on the material. Nonferrous alloys may be processed near 150°C, while steels and ferrous alloys often exceed 980°C. Under these conditions, three damage mechanisms interact: abrasive wear from the oxide scale, thermal fatigue from repeated heating and cooling, and edge deformation due to thermal softening. In most cases, tool life is limited by the mechanism that develops fastest under a given cycle.
Selection Factors
Tool steel selection is determined by the dominant failure mode under specific trimming conditions.
Wear resistance and toughness form the primary trade-off. High carbide content improves resistance to abrasive scale and slows edge rounding, but reduces impact resistance and increases the risk of chipping. In operations with heavy flash, misalignment, or unstable loading, insufficient toughness leads to early edge failure before wear becomes critical.
Thermal fatigue resistance controls crack initiation and propagation under cyclic heating. When trimming involves repeated short cycles with rapid temperature changes, surface cracks form and gradually link into networks. Once established, these cracks accelerate material removal and reduce edge stability. Steels with higher toughness and tempering stability delay this process.
Hot hardness becomes decisive when contact time or heat input increases. If the steel softens during service, the cutting edge plastically deforms even if wear resistance is adequate. This is typical in slower cycles, thicker sections, or conditions where heat accumulates rather than dissipates between strokes.
Recommended Tool Steels
AISI D2 Tool Steel |1.2379 | SKD11
D2 is used in hot trimming when abrasive wear is the dominant failure mode and thermal exposure is short. Its high carbon and chromium content produce a large volume of hard carbides, which slows edge wear and maintains cutting sharpness.
Its limitation is low toughness and moderate resistance to thermal softening. It is therefore limited to thin-flash trimming, high-speed operations, or cases where the tool–workpiece contact time is brief. Typical hardness is 58–60 HRC, chosen to maintain wear resistance while limiting the risk of brittle fracture.
AISI H11 Tool Steel | 1.2343 | SKD6
H11 is selected when failure is driven by impact loading or thermal fatigue. Compared with D2, it provides lower wear resistance but significantly higher toughness and resistance to crack propagation.
It is suitable for general hot-trimming conditions where the load varies or impacts are significant, such as upset forging. Under these conditions, edge failure is more often caused by cracking than by wear. Typical hardness is 46–50 HRC, balancing strength with resistance to fracture and heat checking.
AISI H21 Tool Steel | 1.2581 | SKD5
H21 is used when thermal softening limits tool life. The tungsten content improves high-temperature strength, allowing the edge to retain its geometry under sustained heat.
It is well-suited for heavy flash trimming, thick sections, or slower cycles in which heat accumulates during operation. Compared with H11, it provides better resistance to softening but lower toughness. Typical hardness is 50–52 HRC, chosen to maintain edge stability under thermal load.
AISI S7 Tool Steel | DIN 1.2355
S7 is selected when impact is the primary failure driver and edge chipping is the main risk. Its high toughness allows it to absorb repeated shocks without cracking.
Its wear resistance and hot hardness are limited, so it is not suitable for conditions dominated by abrasion or high temperature. It performs best in trimming operations with unstable loading or severe mechanical shock. Tempering above the expected service temperature is required to reduce the risk of in-service softening.
6G or 6F2 Low-Alloy Steels
These steels are used for punches and support components where failure is driven by bulk fracture rather than edge wear. Their Ni-Cr-Mo composition provides high core toughness and resistance to heavy compressive and impact loads.
They are not intended for cutting edges. Instead, they are applied where structural reliability and resistance to cracking are more important than hardness. Typical hardness is 41.8–45.7 HRC, providing a balance between strength and ductility.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Advantage for Hot Trimming |
| D2 | 58–60 HRC | Resists abrasive wear in short-contact trimming |
| H11 | 46–50 HRC | Resists cracking under impact and thermal cycling |
| H21 | 50–52 HRC | Maintains edge strength under sustained heat |
| S7 | — | Absorbs shock and prevents edge chipping |
| 6G / 6F2 | 41.8–45.7 HRC | Provides core toughness for heavy-duty components |