In modern manufacturing, cold extrusion is a core process for mass-producing precision components across the automotive, aerospace, and industrial machinery sectors. The service life of cold extrusion dies often directly determines a company’s profitability. Selecting the most suitable tool steel for punches and dies represents a strategic decision critical to cost-effectiveness.
Cold extrusion, also known as cold forging or cold impact extrusion, is typically performed at temperatures below 200°C (390°F). The general process involves placing a metal blank into a fixed die cavity, where a high-speed punch forces the metal to undergo plastic flow, ultimately achieving the desired shape.
This process imposes extremely demanding requirements on mold materials. Mold steel must withstand extreme compressive stresses, severe abrasive wear, and dynamic fatigue loads within an extremely short timeframe.
General-purpose steel can no longer meet the requirements above; heat-treated tool and die steel must be used. The fundamental rationale for selecting tool and die steel is to strike a balance between high hardness and high toughness.
The Critical Demands on Cold Extrusion Tooling
Cold extrusion dies face three core challenges.
1. Extreme Pressure and Compressive Stress
Cold extrusion operates by inducing plastic flow in metal at room temperature. To force the metal through a specific die cavity, the die must withstand immense counterforce. Qualified punches and dies must endure peak pressures reaching 2415 MPa (350 ksi). Under such pressure, ordinary steel deforms like clay. Compressive strength is the primary consideration for tool and die steel, ensuring the die does not undergo plastic deformation or collapse under extreme pressure.
2. Wear Resistance
Under high pressure, the metal blank and mold surface undergo intense relative sliding, leading to abrasive wear and adhesive wear. Wear resistance directly determines mold lifespan and the dimensional accuracy of the finished product. This is why high-carbon, high-chromium tool steels, such as the D series, are widely used in this field.
3. Toughness and Fatigue
To withstand high pressure and wear, we need to increase hardness; however, excessive hardness renders steel brittle. Cold extrusion is a dynamic cyclic process. Particularly for slender punches with high length-to-diameter ratios, insufficient toughness under repeated impacts makes them highly susceptible to fatigue fracture or premature chipping.
Key Tool Steel Categories for Cold Extrusion
In summary, the material selection requirements for cold extrusion processes are high hardness, high wear resistance, and sufficient toughness.
1. High-Speed Steels (HSS)
High-speed steel is not only used for cutting tools; it also excels in cold extrusion dies, particularly for punches and die inserts.
HSS exhibits exceptional hot hardness, maintaining high hardness up to 600°C. During high-speed extrusion, where instantaneous high temperatures occur, the cutting edges of HSS dies do not soften.
Example: Aço M2 is the standard choice for cold extrusion punches. It has a wear resistance rating of 7 and a toughness rating of 3.
M4 steel is suitable for cold extrusion applications demanding higher wear resistance. It has a wear resistance rating of 9 and a toughness rating of 3.
T15 steel is suitable for cold extrusion, even under higher wear-resistance requirements. It has a wear resistance rating of 9 and a toughness rating of 1. T15 offers top-tier wear resistance, but this comes at the expense of toughness. It is suitable for precision dies operating under extremely stable conditions with minimal lateral forces but experiencing severe wear.
2. Cold Work Tool Steels (D- and A-Series)
For buyers and engineers in the cold forming field, the most common choices typically revolve around three materials: AISI D2 (1.2379), AISI A2(1.2363), e AISI O1(1.2510).
AISI D2 (1.2379) is a high-carbon, high-chromium aço para ferramentas de trabalho a frio distinguished by its exceptional wear resistance and resistance to softening. It scores 8 for wear resistance and 2 for toughness. D2 is the optimal choice for long-life, high-volume production dies. It is widely used in cold extrusion dies, punching dies, and various shearing blades.
When D2 dies exhibit chipping or fracturing during use, AISI A2 often serves as the optimal alternative. It scores 6 for wear resistance and 5 for toughness, offering a balanced combination of these properties. A2 is a versatile cold-forming steel that maintains good wear resistance while providing superior impact resistance compared to D2, making it suitable for slightly more demanding applications.
AISI O1, a low-cost oil-hardening tool steel, still holds a place in the market. It scores 4 for wear resistance and 8 for toughness. While O1’s wear resistance falls short of D2, its toughness is exceptional. If your molds primarily fail due to fracture rather than wear, or if you require a low-cost tool material, O1 offers a highly cost-effective choice.
3. Hot Work Tool Steels (H-Series)
In cold extrusion die applications, hardness is crucial but not the sole factor. When dies face extremely complex stress environments or extrude high-strength steels, excessively high hardness can instead cause instantaneous brittle fracture. In such cases, a different material selection strategy is required. To achieve necessary fracture resistance while sacrificing some compressive strength, we strongly recommend using H11 ou H13. These steels absorb impact energy like springs rather than shattering like glass. Medium-carbon aços para ferramentas de trabalho a quente like H11/H13 achieve a toughness rating of 9.
H11/H13 also serves another purpose. To prevent the shattering of expensive carbide or high-hardness tool steel inserts within the mold, one or more “Shrink Rings” made of H11 or H13 (1.2344) can be fitted around the exterior of the inserts. This configuration is termed a prestressed composite mold. Leveraging thermal expansion and contraction, rings made from H11/H13 are heated and fitted over the insert. Upon cooling, this creates significant inward pre-stress. This force counteracts outward tensile stress during cold extrusion, preventing insert cracking. For this application, H11/H13 does not require extremely high hardness; heat treatment to 46-48 HRC is typically sufficient. This hardness range ensures excellent toughness and structural stability.
Advanced Tooling Materials: Carbides and Powder Metallurgy
For applications requiring ultra-high performance, production rates, and maximum mold life, conventional tool steels may be substituted with advanced materials.
Cemented Carbides
Cemented carbides are not ordinary steels; they are manufactured through powder metallurgy using hard compounds of refractory metals and binding metals. The compressive strength of cemented carbides far exceeds that of traditional tool steels. According to data, cemented carbide dies can withstand peak pressures up to 3100 MPa (450 ksi). Cemented carbide punches can withstand working pressures up to 2760 MPa (400 ksi). In contrast, ordinary tool steel may approach its yield strength at 2400 MPa. Carbide is most commonly used for mold cavity inserts and counter-extruding punches. In these applications, it minimizes wear under high pressure, ensuring dimensional accuracy persists after producing tens of thousands of parts.
However, cemented carbide is too brittle and highly susceptible to tensile stress and impact. Therefore, cemented carbide dies are always used in conjunction with tool steel. They must be encased within an exceptionally robust, highly ductile tool steel die case—specifically, shrink rings—to ensure the expensive cemented carbide inserts do not fracture.
Powder Metallurgy (P/M) Tool Steels
Traditional tool steel involves casting molten steel into large ingots, which are then forged. Powder metallurgy (P/M) tool steel is entirely different. First, molten steel is atomized into extremely fine powder particles. These powders are then compressed into dense ingots using hot isostatic pressing (HIP) technology under high temperature and pressure. Finally, the ingots undergo forging or rolling. Powder metallurgy (P/M) tool steel resolves the issue of carbide segregation. This microstructure achieves a perfect balance of high wear resistance and high toughness. Molds become more resistant to wear while also being less prone to fracturing.
Design and Fabrication for Longevity
Selecting mold materials is only half the battle. The manufacturing precision, heat treatment processes, and especially the geometric design and surface finish of the mold often determine its ultimate service life. Even with the right material selection, poor punch design or rough surface machining will prevent the mold from performing as intended.
When designing the end shape of cold extrusion punches, many designers intuitively assume that sharper points facilitate easier die entry. However, this is precisely incorrect. For steel cold extrusion, the most durable design employs a flat end face combined with an extremely small transition radius (R). This radius should not exceed 0.51 mm (0.020 inches). Cold extrusion relies heavily on lubricants like phosphating and saponification. A pointed tip design displaces the lubricant layer on the metal surface upon contact, causing direct metal-to-metal contact that instantly triggers severe galling and wear. In contrast, a flat-bottom design effectively traps the lubricant.
Cold extrusion dies require surface finish not merely for aesthetic purposes. All grinding marks must be eliminated from punch and die surfaces to achieve an ultra-fine finish of 0.10 µm (4 µin). Polishing and grinding of cold extrusion dies must follow the direction of metal flow. If polished transversely, the resulting microscopic texture increases frictional resistance, elevates extrusion pressure, and ultimately leads to premature die failure.
Heat Treatment and Surface Enhancement
Our customers have encountered this situation: molds pass hardness tests during factory inspection, yet suddenly crack shortly after machine operation begins. This is most likely due to retained austenita. During the quenching process, if the austenite within the steel is not fully transformed into martensita, the remaining portion is termed retained austenite. This phase is unstable and spontaneously transforms into brittle martensite during mold service, especially under high stress or high-speed impact. This transformation involves volume expansion, generating immense internal stresses within the mold that ultimately lead to fatigue fracture.
For high-stress applications such as cold extrusion dies, never perform only one or two têmpera cycles to save costs. The industry standard practice is triple tempering. This ensures the majority of retained austenite is successfully transformed, completely eliminating internal instability factors within the die.
Once the base material achieves sufficient strength and toughness, we must also apply a protective layer to the mold to withstand extreme wear. Nitriding is the most common surface hardening method. Through this process, an extremely hard compound layer forms on the mold surface, further enhancing its wear resistance.
| Tool Component | Recommended Steel Grades (Selection Examples) | Dureza de Trabalho (HRC) | Key Property Emphasis | Supporting Components |
| Punções | T15, M4, D4, M2 | 60–66 | High Compressive Strength/Wear Resistance | Punch shank: A2, O1, S7 (56–58 HRC) |
| Die Inserts | T15, M4, D4, M2, D2, Carbide | 58–66+ | High Hardness/Wear Resistance | Die holder/Retainer Rings: H11, H13 (46–48 HRC) |
| Extremely Difficult Extrusion | H11/H13 or Tungsten Carbide | 46–62+ | Toughness to resist cracking |