Selection of Tool Steel for Hot Shear Blades
Hot shearing is used to cut heated bars, billets, and heavy sections during forging or extrusion. Although the material is softened, deformation is non-uniform, so the blade must simultaneously withstand high shear and compressive stresses to complete separation. During operation, the cutting edge is exposed to sliding contact against the oxide scale, while repeated heating and intermittent cooling generate strong thermal gradients.
In service, failure is governed by identifiable mechanisms. If the steel cannot maintain its strength at the temperature, the edge plastically deforms, increasing the cutting force and accelerating wear. Oxide scale promotes continuous abrasive wear along the edge. Thermal cycling produces surface stress that initiates cracks, which can propagate and lead to edge failure. Tool steel selection, therefore, focuses on maintaining edge stability under temperature, load, and cyclic thermal stress.
Selection Factors
The performance of hot shear blades depends on whether the material can resist the dominant failure modes under specific operating conditions. These requirements are interdependent and must be evaluated together.
Hot hardness indicates the edge’s ability to resist plastic deformation at elevated temperatures. If hot strength is insufficient, the edge rounds rather than shears, leading to a rapid loss of cutting efficiency.
Toughness controls resistance to edge chipping under dynamic loading. In practice, misalignment, scale buildup, or uneven sections introduce localized impact, making insufficient toughness a primary cause of premature edge failure.
Resistance to thermal fatigue governs crack initiation under repeated heating and cooling. Materials with sufficient strength at temperature delay crack formation and slow crack growth, extending service life under cyclic conditions.
Abrasion resistance determines how quickly the edge wears down due to oxide scale and sliding contact. Increasing wear resistance improves edge retention, but it must be balanced against toughness to avoid brittle fracture.
In practical selection, temperature level defines the required hot hardness, while cooling intensity and impact severity determine the necessary toughness and thermal fatigue resistance.
Recommended Tool Steels
H13 Tool Steel Supplier | 1.2344 | SKD61
H13 is the standard choice when both thermal fatigue resistance and toughness are required. At 45–50 HRC, it maintains sufficient hot strength to resist deformation while retaining impact resistance. Its alloy system provides stable performance under cyclic heating, making it suitable for conditions involving intermittent water or air cooling. It is typically selected when thermal shock and mechanical loading are both present.
AISI H21 Tool Steel | 1.2581 | SKD5
H21 is used when resistance to high-temperature softening is the controlling requirement. Its tungsten-rich carbide structure maintains hardness at temperatures up to about 620°C, allowing the edge to resist deformation under sustained heat. However, its lower resistance to thermal shock limits its use to stable thermal conditions without aggressive cooling. It is best suited for continuous high-temperature cutting where deformation, rather than cracking, is the primary risk.
AISI S7 Tool Steel Supplier | DIN 1.2355
S7 is selected when impact loading is the dominant failure factor. Its high toughness allows it to resist chipping under intermittent or unstable cutting conditions. Although its hot hardness is lower than that of hot-work steels, it can be used in applications where temperatures are moderate and mechanical shock governs tool life. It is typically used where edge failure occurs by chipping rather than softening or wear.
AISI A8 (High-Toughness Cold/Hot-Work Steel)
A8 is applied where temperature remains moderate, but abrasive wear and mechanical load are significant. Below approximately 480°C, it provides better wear resistance than H13 while maintaining adequate toughness. This makes it suitable for heavy-section cutting where load distribution is uneven and edge wear, rather than thermal softening, limits tool life.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Advantage | Best Use Case |
| AISI H13 | 45–50 HRC | Balanced resistance to thermal fatigue and impact | General hot shearing with thermal cycling and cooling |
| AISI H21 | 45–50 HRC | High resistance to softening at elevated temperature | Continuous high-temperature cutting without rapid cooling |
| AISI S7 | 54–58 HRC | High impact toughness | Intermittent cutting where chipping is dominant |
| AISI A8 | 52–56 HRC | Wear resistance under moderate temperature | Heavy sections with controlled temperature (<480°C) |