Selection of Tool Steel for Cold Forming Hexagon Nuts
The cold forming of hexagon nuts is a multi-stage process performed under high compressive loads and repeated impact, typically below 200 °C. Tool failure is not driven by thermal softening, but by mechanical overload and surface damage.
In practice, tooling fails primarily due to abrasive wear, adhesive wear (galling), and plastic deformation under excessive contact stress. Secondary failure modes include edge chipping and fatigue cracking under cyclic loading.
Selection Factors
Tool steel selection for hexagon nut forming is fundamentally a balance between wear resistance and toughness under high compressive stress.
High hardness improves resistance to wear and deformation, which is critical for maintaining die geometry over long production runs. However, increasing hardness and carbide content reduces toughness, raising the risk of edge chipping or brittle fracture under repeated impact.
Because forming temperatures are relatively low, thermal properties such as red hardness and resistance to heat checking are not decisive. Material selection should focus on controlling wear, maintaining dimensional stability, and preventing fracture under cyclic loading.
Recommended Tool Steels
AISI D2 Tool Steel |1.2379 | SKD11
D2 is used in applications where abrasive wear and dimensional stability are the primary requirements, and impact loading is limited.
At a working hardness of 58–60 HRC, D2 provides strong resistance to surface wear while maintaining precision during heat treatment. However, its lower toughness makes it less suitable for conditions involving repeated shock or unstable loading.
D2 is best suited for precision dies where wear resistance is critical, and loading conditions are well controlled.
M4 High-Speed Steel (HSS)
M4 is used when wear is the dominant failure mechanism and production volume is high. It performs best under stable loading conditions where edge chipping is not the limiting factor.
Performance: In production, M4 tooling can exceed 1 million parts when wear controls tool life.
Enhancement: With optimized heat treatment or surface hardening to 63–65 HRC, service life can reach approximately 2.4 million parts.
M4 is the preferred choice for ultra-high-volume production where maximum wear resistance directly determines tool life.
T1 Tungsten High-Speed Steel
T1 is selected when both wear and impact are present, but neither is extreme. Compared with higher-alloy HSS grades, it provides a more balanced response under mixed loading conditions.
With standard heat treatment (oil quench at 1270 °C and double temper at 595 °C), T1 typically delivers around 350,000 parts.
T1 is suitable for medium- to high-volume production where improved toughness is required to reduce the risk of chipping.
Carbon Tool Steel (W-Type Drill Rod)
For non-critical components such as guide pins or die holders—where direct wear and impact are minimal—carbon tool steel provides a cost-effective solution.
At 58–60 HRC, it offers sufficient compressive strength and rigidity without the cost of high-alloy steels.
Summary Table
| Tool Steel Grade | Key Properties / Hardness | Primary Advantage in Nut Forming |
| M4 | 63–65 HRC | Maximum Longevity: Best for wear-dominant, ultra-high volume production |
| T1 | Quenched 1270 °C / Tempered 595 °C | Balanced Performance: Suitable for combined wear and impact conditions |
| D2 | 58–60 HRC | Stability & Wear Resistance: Ideal for precision dies under controlled loading |
| Carbon Steel | 58–60 HRC | Economy: Used for low-wear structural or support components |