Selection of Tool Steel for Bending and Forming Dies
Bending and forming dies plastically deform sheet metal along a defined axis. During forming, the inner radius is compressed while the outer radius is in tension. Under repeated operation, the combined effects of sliding contact and lateral forces lead to three dominant failure modes: abrasive wear, galling caused by material adhesion, and fatigue-related cracking or edge damage.
Selection Factors
Selecting tool steel for bending and forming dies is essentially a trade-off between wear resistance and toughness, with additional attention to galling behavior and distortion during heat treatment.
Wear resistance determines how well the die maintains its edge geometry under continuous sliding contact. When wear resistance is insufficient, edges round quickly, dimensional accuracy is lost, and tool life drops sharply in high-volume production.
Toughness controls the die’s ability to resist cracking and edge chipping under cyclic loading and side thrust. This becomes critical in thicker sections, unstable forming conditions, or setups where alignment cannot be perfectly controlled.
Surface compatibility is directly related to galling behavior. In low-lubrication environments, adhesion between the workpiece and die surface can lead to material pickup, unstable forming, and surface defects on the final part.
Manufacturing constraints are mainly linked to heat treatment response. Air-hardening steels are generally preferred because they reduce distortion and improve dimensional stability, whereas oil- or water-hardening steels are usually limited to simpler geometries where the risk of distortion is manageable.
Recommended Tool Steels
AISI A2 Tool Steel | 1.2363 | SKD12
A2 is typically used as the baseline material when no single failure mode dominates. At 58–62 HRC, it maintains a practical balance between wear resistance and toughness, enabling reliable performance across a wide range of forming conditions. Its air-hardening characteristic reduces distortion during heat treatment, making it suitable for dies that require stable dimensions. In most medium-volume applications with relatively stable loading conditions, A2 provides consistent and predictable performance.
AISI D2 Tool Steel | 1.2379 | SKD11
D2 is chosen when wear becomes the primary limiting factor in tool life. Its high carbide content significantly improves resistance to abrasive wear, making it suitable for long production runs and materials that accelerate surface wear. However, this same carbide structure reduces toughness, increasing the risk of chipping or cracking if the die is exposed to impact, side loading, or unstable forming conditions. As a result, D2 performs best in controlled processes where wear dominates and mechanical shock is limited.
AISI O1 Tool Steel | 1.2510 | SKS3
O1 is generally applied where cost control and machinability are more important than maximum durability. With a working hardness of 58–60 HRC, it provides sufficient performance for less demanding applications but cannot match the wear resistance of higher alloy steels. It is more practical for short- to medium-length production runs, especially when the tooling geometry is simple and the forming conditions are not severe.
AISI S7 Tool Steel Supplier | DIN 1.2355
S7 becomes necessary when tool failure is driven by cracking rather than wear. It maintains high toughness even at working hardness levels of 54–58 HRC, allowing it to absorb impact and resist fracture under heavy forming loads. In operations involving thick sections, high force, or unavoidable misalignment, S7 provides a level of reliability that standard cold-work steels cannot achieve, although this comes at the cost of lower wear resistance.
Grade W1 (Water-Hardening Tool Steel)
W1 is limited to simple, low-cost applications where dimensional precision is not critical. Although it can achieve high surface hardness, the risk of distortion during water quenching restricts its use to uncomplicated geometries and short production runs. It is typically selected only when budget constraints outweigh performance requirements.
Summary Table
| Tool Steel Grade | Typical Hardness (HRC) | Primary Advantage | Best For |
| A2 | 58–62 | Balanced wear resistance and toughness; low distortion | General-purpose forming, medium runs |
| D2 | 58–62 | High wear resistance | High-volume production with stable conditions |
| O1 | 58–60 | Low cost and good machinability | Short runs, simple tooling |
| S7 | 54–58 | High resistance to cracking and impact | Heavy-duty forming, unstable conditions |
| W1 | 58–64 (Case) | Low cost, high surface hardness | Simple shapes, low-precision applications |