Selection of Tool Steel for Cold-Heading Dies
Cold heading is a bulk-forming process used to produce high-volume components, such as fasteners, by plastic deformation at room temperature. The dies are subjected to cyclic impact loading and compressive stresses up to approximately 2413 MPa (350 ksi).
Although classified as a cold process, localized temperatures at the die-workpiece interface can rise by several hundred degrees Celsius due to deformation energy and friction. This affects surface hardness and accelerates wear.
Under these conditions, die failure is primarily governed by abrasive wear, adhesive galling, and fatigue cracking. In high-load applications, insufficient toughness leads to edge chipping or sudden brittle fracture.
Selection Factors
Tool steel selection for cold-heading dies is determined by the interplay among wear resistance, toughness, and compressive strength.
High surface hardness is required to resist abrasion, adhesion, and plastic deformation. In most applications, the working hardness ranges from 56–62 HRC. However, increasing hardness reduces toughness, which increases the risk of cracking under cyclic impact.
As a result, cold-heading dies are not designed as components with uniform hardness. Instead, performance depends on a hardness gradient: a hard, wear-resistant surface supported by a tougher core that absorbs impact stress.
This requirement is achieved through two approaches:
Shallow-hardening steels develop a hard surface with a relatively tough core during heat treatment. They are suitable for solid dies under high impact.
Deep-hardening, high-alloy steels provide uniform high hardness and are therefore used as inserts, where a tougher outer casing carries the mechanical load.
The selection is therefore not only material-based, but also depends on whether the die is designed as a solid tool or an insert.
Recommended Tool Steels
AISI D2 Tool Steel |1.2379 | SKD11
D2 is used in applications where abrasive wear is the dominant failure mechanism and impact loading is moderate.
Its high carbon and chromium content produces a large volume of hard carbides, providing strong wear resistance at 58–60 HRC. However, its relatively low toughness makes it unsuitable for solid dies under heavy impact.
For this reason, D2 is typically used as an insert, where the surrounding die structure absorbs the load.
S1 and S7 (Shock-Resisting Tool Steels)
S1 and S7 are selected when impact-induced fracture is the dominant failure mode.
S7 is widely used due to its air-hardening behavior and stable toughness. It is typically used at 56–58 HRC to reduce the risk of impact-induced cracking. Compared with higher-hardness steels, it provides lower wear resistance but significantly higher resistance to fatigue fracture.
S1 is preferred in dies with stress concentration, where crack initiation must be controlled.
These steels are suitable for solid dies operating under severe impact conditions, where wear is not the primary limiting factor.
AISI M2 Tool Steel | 1.3343 | SKH51
M2 is used in high-load applications involving severe wear, high contact stress, or work-hardening materials.
It offers higher compressive strength and wear resistance than D2 and is typically used at 62–64 HRC. Unlike conventional cold-work steels, M2 maintains hardness under elevated temperatures, which helps prevent surface softening and plastic deformation during high-speed forming.
Due to cost, M2 is primarily used as an insert in critical wear zones.
W1 and W2 (Water-Hardening Tool Steels)
W1 and W2 are shallow-hardening steels used for punches and solid dies under high impact loading.
They can achieve a surface hardness above 60 HRC while maintaining a tougher core, owing to their limited hardenability. This structure allows the die to absorb impact without cracking.
However, water quenching introduces a high risk of distortion and cracking, which limits its use to simple geometries and smaller sections.
Summary Table
| Tool Steel Grade | Typical Hardness | Primary Selection Role |
| W1 / W2 | Surface: 60+ HRC / Core: 40–50 HRC | Solid dies under high impact with simple geometry |
| S1 / S7 | 56–58 HRC | Solid dies require maximum resistance to impact and fatigue |
| D2 | 58–60 HRC | Inserts for wear-dominated applications |
| M2 | 62–64 HRC | Inserts for high load, high wear, and thermal softening conditions |