Unit
- High-speed steels (HSS): These tool steels contain tungsten, molybdenum, cobalt, and vanadium to retain hardness at elevated cutting temperatures. Typical grades like M2 and T1 are used for cutting tools and punches where edge retention is critical. HSS can be heat treated to 62-65 HRC.
- Cold work tool steels: Grades such as D2, A2, and O1 provide high wear resistance and dimensional stability for cold forming dies and blanking punches. D2 is a high-carbon, high-chromium steel that offers excellent abrasion resistance but lower toughness in comparison to A2.
- Hot work tool steels: H13 and H11 are common grades that maintain strength at temperatures up to 600 degrees Celsius. They are used for hot forging dies, extrusion dies, and die casting cores. Their resistance to thermal fatigue is achieved through careful alloying with chromium and molybdenum.
- Powder metallurgy tool steels: Produced by atomizing molten steel and then consolidating via hot isostatic pressing, these steels offer a fine, uniform carbide distribution. Grades like ASP23 provide superior wear resistance and grindability compared to conventional HSS, making them suitable for high-performance dies.
- Carbon tool steels: Water-hardening steels like W1 are inexpensive and used for tools subject to low stress. They have low hardenability and require quench in water or brine, leading to a risk of distortion. These are typically chosen for simple tools with short production runs.
- Alloying elements: Chromium improves hardenability and corrosion resistance. Vanadium forms hard carbides that enhance wear resistance. Molybdenum and tungsten contribute to red hardness. Cobalt increases hot hardness but reduces toughness. Each element must be balanced to achieve the desired property profile.
- Hardness and toughness trade-off: As hardness increases to improve wear resistance, toughness generally decreases. Die designers must select a steel that provides adequate toughness to resist chipping and cracking under the specific loading conditions of the operation. For high-impact applications, a tougher grade like S7 may be chosen over a high-wear grade like D2.
- Standard specifications: Tool steels are classified by AISI (American Iron and Steel Institute) designations. International standards such as DIN (Germany) and JIS (Japan) use different numbering systems but cover equivalent grades. Understanding cross-references is important for global sourcing and material substitution.
- Surface coatings and treatments: Physical vapor deposition (PVD) and chemical vapor deposition (CVD) apply thin coatings like titanium nitride (TiN) or titanium carbonitride (TiCN) to tool surfaces. These coatings reduce friction, improve wear resistance, and allow higher operating speeds. For dies, treatments like nitriding or carburizing produce a hard case while retaining a tough core, extending service life significantly.
- Heat treatment considerations: Proper annealing, austenitizing, quenching, and tempering cycles are essential to achieve the desired microstructure and mechanical properties. For example, high-speed steels require controlled preheating and multiple tempering steps to transform retained austenite. Distortion and cracking risks are minimized by using appropriate heating rates, protective atmospheres, and stress-relief practices during die manufacture.
- Selection criteria for die materials: Key factors include production volume (short-run vs. high-volume), work material (abrasiveness, hardness), operating temperature, and loading type (impact, compression, shear). For high-volume blanking of abrasive sheet metal, a wear-resistant D2 or powder metallurgy steel is chosen. For warm forming with moderate temperatures, hot work H13 balances strength and toughness. Cost and machinability also influence final grade selection.
- Non-metallic tool materials in dies: When extreme wear resistance or chemical inertness is required, carbide (WC-Co) or ceramic inserts may be used in die components. Carbide offers hardness up to 90 HRA and high compressive strength, suitable for wire drawing dies and deep draw punches. However, such materials are brittle and must be supported by a tough steel substrate or used in modular inserts to prevent catastrophic failure.
Equipment Specification
Equipment: AISI D2 Cold Work Tool Steel
- Hardness (as-delivered/annealed)
- 200-240 HB
- Hardness (heat treated)
- 58-62 HRC
- Typical Applications
- Blanking dies, forming rolls, gauges, shear blades
- Standard
- AISI D2, DIN 1.2379, JIS SKD11
Quick Revision
- Tool steels are categorized by application: high-speed, cold work, hot work, powder metallurgy, and carbon steels.
- Alloying elements such as Cr, V, Mo, W, and Co impart specific properties like wear resistance, hardenability, and red hardness.
- The trade-off between hardness and toughness guides material selection for different die types.
- Standard AISI designations and cross-references to DIN/JIS allow consistent sourcing and substitution.