Selection of Tool Steels for Forming & Drawing

In metal-forming processes, dies are frequently subjected to extremely high contact pressures, high sliding speeds, and high temperatures. The rationale for selecting tool and die steel is to prevent plastic deformation under high stresses and to effectively suppress adhesive wear, which is commonly encountered when processing stainless steel or advanced high-strength steel (AHSS).

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Bending and Forming Dies

During bending operations, the die fillet endures localized compressive loads while the workpiece outer layer experiences tensile stresses. Primary failure modes include abrasive wear at the fillet and cracking induced by insufficient toughness. Such operating conditions demand tool steel that combines high toughness with high hardness: high toughness to withstand impact loads; high hardness to delay edge wear and maintain precise bending angles, thereby effectively controlling springback.

Deep Drawing Dies

Under deep-drawing conditions, materials experience combined circumferential compressive and radial tensile stresses, which can easily cause wall thinning or flange wrinkling. The dies must withstand adhesive wear under high-pressure sliding contact. Selecting tool steel with a low coefficient of friction and high compressive yield strength effectively prevents adhesive wear and surface scratches on the drawn parts.

Roll Forming Dies

Roll forming is a progressive bending process in which strip steel is continuously passed through a series of rollers. The rollers endure both axial and radial loads, requiring tool steel with exceptional dimensional stability to maintain precise tolerance control throughout prolonged production. The tool steel’s high wear resistance prevents dimensional deviations in the profile, while its excellent toughness delays fatigue failure caused by cyclic contact stresses.

Hydroforming Dies

During hydroforming of sheet metal and tubing, the die cavity is subjected to high internal pressure, forcing the workpiece to conform precisely to the die walls. High-cycle fatigue and elastic deflection are the primary mechanical characteristics of hydroforming. The die steel must possess high yield strength to suppress plastic deformation, coupled with high fatigue strength to reduce the risk of cracking induced by pulsating hydraulic loads.

Surface Treatment and Substrate Stability

In AHSS or stainless steel forming applications, PVD or TD surface treatments are commonly employed to suppress adhesive wear and reduce friction. Coating performance depends on the substrate’s ability to support the hard thin layer, necessitating avoidance of the “eggshell effect” induced by substrate deformation. Compared to D2, DC53 (8% Cr) serves as a superior coating substrate. It exhibits significant secondary hardening properties during high-temperature tempering at 500–550°C, ensuring the substrate does not soften during coating. It consistently maintains a hardness of >60 HRC, providing stable support for the coating under high loads.

Forming High-Strength Steel (HSS)

Compared to low-carbon steel, forming HSS and AHSS involves extremely high contact pressures and temperatures, significantly increasing the risk of adhesive wear and die deformation. Conventional D2 steel struggles to meet these demanding conditions due to its low toughness and coarse eutectic carbides in the microstructure, which readily induce crack initiation. For high-strength steel applications, powder metallurgy grades or DC53 (8% Cr modified steel) should be selected. Their combination of high compressive strength and high toughness reduces plastic deformation in dies and reduces the risk of chipping.

Comparison Table

Grade	Galling Resistance	Compressive Strength	Toughness	Best Application
D2 (1.2379)	Moderate	High	Low	Heavy-duty forming of AHSS; substrate for PVD/TD coatings; complex dies requiring toughness.
DC53 (8% Cr)	High	Very High	High	General-purpose forming, short-run dies, and less demanding tolerances.
Cr12MoV	Low/Moderate	Moderate	Moderate	General purpose forming; short-run dies; less demanding tolerances.

FAQ

How do I select tool steel for bending dies?

You need steel combining high toughness to withstand impact loads and high hardness to delay edge wear. This combination helps maintain precise bending angles and effectively control springback.

What properties prevent failure in deep drawing dies?

Select tool steel with a low coefficient of friction and high compressive yield strength. These properties prevent adhesive wear and surface scratches caused by high-pressure sliding contact during the drawing process.

Which tool steel is best for forming high-strength steel (AHSS)?

Powder metallurgy grades, such as DC53, are recommended due to their high compressive strength and toughness. Conventional D2 often fails in these applications because its low toughness can induce cracking.

Why is DC53 preferred over D2 for PVD coated dies?

DC53 exhibits significant secondary hardening, maintaining >60 HRC after high-temperature tempering. This stability prevents substrate softening and the “eggshell effect,” providing superior support for the coating compared to D2.

What are the requirements for roll forming die steel?

Roll forming rollers require exceptional dimensional stability to maintain precise tolerances. They also need high wear resistance to prevent deviations and excellent toughness to delay fatigue failure from cyclic contact stresses.

How to choose die steel for hydroforming applications?

Dies require high yield strength to suppress plastic deformation and high fatigue strength to reduce the risk of cracking. The steel must withstand high-pressure internal loads and pulsating hydraulic forces.

When should I use D2 tool steel for forming?

D2 is best suited for abrasive wear applications and simple forming dies where toughness is a secondary concern. It offers high compressive strength but has low toughness and moderate galling resistance.