2: Tool Steels

Overview of Tool Steels

• Tool steels: High alloy steels designed for specialty purposes, primarily used to shape other alloys or materials.

• Distinguished by high cleanliness and precise alloy controls, often produced in small batches with extensive quality inspections.

Compositions and Classes of Tool Steels

• Over 70 commercial types of tool steels, grouped into five major classes:

  • Cold Work Steels: Used for shaping at room temperature. Different grades include:

    • Water Hardening (W): Cheapest, essentially carbon steels with extra additions.

    • Oil Hardening (O): Improved hardenability and lower distortion upon hardening.

    • Air Hardening (A): Hardenable to six-inch section thicknesses.

    • D Series (D2): Most preferred cold work steels, excellent wear resistance, a mix of oil and air hardening.

  • Shock Resisting Steels (S): Lower carbon, lower hardness, better toughness for impact resistance, used for chisels and similar tools.

  • Hot Work Steels (H): Designed for high-temperature applications (up to 620°C) using refractory elements to stabilize properties. Main types include tungsten and molybdenum additions.

  • High Speed Steels (M & T): Premium steels with high alloy content, used for cutting metal alloys, hardness up to 67 Rockwell C, and the ability to maintain hardness at high temperatures (up to 540°C).

  • Mold Tool Steels: Used for forming injection mold cavities, relatively lower hardness than other tool steels.

Special Purpose Tool Steels

• Special Purpose Tool Steels: A catchall category for tools used in structural applications such as wrenches or riveting tools.

Specific Grades Within Tool Steel Classes

• Key grades include:

  • Cold work: O1, A2, D2, A11.

  • Shock resisting: S7.

  • Hot work: H13.

  • High speed: M2.

• Reference for common wear rates and substitute materials:

  • Martensitic stainless steels (420, 440C) and high-strength, high-hardenability alloy steels (4140, 4340).

  • Cemented carbide (C2): Exceptional abrasion resistance but classified as a ceramic rather than a metal alloy.

Hardenability and Wear Rates

• Tool steels are classified by their hardenability:

  • Cold work steels show increasing hardenability with grade.

  • D-series (D2) has the highest hardenability.

• Hot work tool steel (H13) demonstrated extreme hardenability, reaching up to 50 Rockwell C at 12-inch thicknesses.

• Wear Resistance: Chart analyzed wear rates of tool steels against an ASTM standard (60 mesh silica), showing:

  • D-series cold work and high-speed tool steels (M-series) exhibit the lowest wear rates.

  • Reference materials: 1020 steel (low wear), cemented carbide (high wear).

Conclusion

• Understanding the characteristics and classifications of tool steels is crucial for selecting the appropriate type for specific applications, ensuring optimal performance and durability.