Selection of Tool Steels for Cold Forming & Extrusion

Cold forming and extrusion dies endure extreme tribological and mechanical stresses. The process achieves metal displacement through plastic flow under non-uniform transient pressures, with die internal pressures typically exceeding 1895 MPa (275 ksi) and punch pressures reaching up to 2370 MPa (344 ksi).

Under these operating conditions, the primary failure modes include abrasive wear, fracture, upsetting or bulging, and high-cycle fatigue. To withstand high unit pressures and prevent workpiece-to-tool interlocking, cold-work tool steels exceeding standard grades—such as DC53 and M2 high-speed steel—should be selected. These grades exhibit high compressive yield and fatigue strengths, effectively resisting permanent deformation and delaying crack initiation.

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Cold Extrusion Dies and Punches

In both forward and reverse extrusion processes, the punch serves as the load-bearing member, subject to high stresses. The punch must possess high compressive yield strength to prevent buckling or upsetting; punches made from M2 high-speed steel can withstand compressive stresses of 2200–2500 MPa. Die inserts endure radial and circumferential stresses during operation and require high fatigue strength. Typically, inserts are prestressed using a shrink ring to suppress cracking caused by internal pressure cycles.

Cold Heading Dies

During cold-heading processes for upsetting wire or bar stock, dies endure impact loads. Primary failure modes include fatigue failure of the inner wall or cracking due to insufficient toughness. The surface hardness of the die must exceed 60 HRC to resist scoring, while the core requires high impact toughness to inhibit crack propagation. To achieve this, impact-resistant grades or high-toughness high-speed steels must be selected, exhibiting superior performance compared to standard high-carbon, high-chromium steels.

Thread Rolling Dies

Thread-rolling die failure manifests as flaking and chipping due to fatigue at the thread crest. The steel must possess resistance to subsurface shear stresses to suppress crack initiation and propagation. For processing work-hardened alloys or high-strength fasteners, the die must be hard enough to exceed 64 HRC and highly rigid to maintain thread profile tolerances and prevent edge chipping.

Material Selection Strategy

Materials for heavy-duty cold-forming tools are typically selected from M2 high-speed steel (HSS) and high-performance cold-work tool steels, such as DC53.

M2 (HSS): When compressive loads exceed the yield strength of D2, select M2 for punches and dies. This steel grade offers high compressive yield strength, making it suitable for applications requiring a tool pressure exceeding 2100 MPa. Additionally, M2 exhibits red hardness, counteracting the effects of adiabatic heating on tools during rapid extrusion.

DC53 (8% Cr): For applications where D2 fails due to chipping or cracking, select DC53 (8% Cr modified). Its finely distributed carbides provide superior toughness over D2 while maintaining hardness at 62–64 HRC, enhancing resistance to adhesive wear and scoring on metal contact surfaces.

Heat Treatment and Compressive Strength

The properties of tool steel are determined by heat treatment. To prevent plastic deformation, the tool steel’s hardness must ensure that its yield strength exceeds the applied process stress. High-speed steel and DC53 require high-temperature tempering to promote the complete transformation of retained austenite. If transformation is incomplete, retained austenite may undergo strain-induced phase transformation under cyclic stress, converting to untempered martensite. This leads to dimensional instability and tool embrittlement.

Comparison Table

Steel Grade	Compressive Strength	Toughness	Fatigue Resistance	Typical Application
M2 (SKH51)	Very High (>2200 MPa)	Moderate/Good	Excellent (High elastic limit)	Heavy-duty extrusion punches, Thread rolling dies,
DC53	High (Superior to D2)	High (Finer carbides than D2)	High (Resists micro-chipping)	Cold heading dies, High-load stamping, Trimming dies
D2 (1.2379)	Moderate/High (up to 2000 MPa)	Low (Prone to chipping)	Moderate (limited by coarse carbides)	General purpose forming, blanking dies

FAQ

What causes tool failure in cold forming?

Primary failure modes include abrasive wear, fracture, upsetting, and high-cycle fatigue failure due to non-uniform transient pressures. Thread rolling dies may also experience flaking and chipping at the thread crest.

Which tool steel is best for high pressure extrusion?

M2 high-speed steel is suitable for applications with tool pressures exceeding 2100 MPa. It offers high compressive yield strength and exhibits red hardness to counteract adiabatic heating effects during rapid extrusion.

When should I use DC53 instead of D2 steel?

Select DC53 when D2 fails due to chipping or cracking. DC53 features finely distributed carbides that provide superior toughness compared to D2 while maintaining a hardness of 62–64 HRC.

What hardness is required for cold heading dies?

The surface hardness must exceed 60 HRC to resist scoring. Additionally, the core requires high-impact toughness to inhibit crack propagation caused by impact stresses during upsetting of wire or bar stock.

How to prevent failure in thread rolling dies?

The die steel must resist subsurface shear stresses to suppress crack initiation. For work-hardened alloys, ensure die hardness exceeds 64 HRC to maintain thread profile tolerances and prevent edge chipping.

Why do M2 and DC53 require high-temperature tempering?

High-temperature tempering promotes the complete transformation of retained austenite. If incomplete, retained austenite may convert to untempered martensite under stress, leading to dimensional instability and tool embrittlement.

How to prevent cracking in cold extrusion die inserts?

Die inserts are typically prestressed using a shrink ring to suppress cracking caused by internal pressure cycles. Inserts require high fatigue strength to endure the radial and circumferential stresses generated during operation.