In the precision-driven world of industrial manufacturing, tool steel plays a pivotal role. Just as a skilled craftsman must understand each tool's characteristics, engineers and designers must thoroughly comprehend different tool steel properties to select the most suitable material for specific applications. Tool steel, a special type of alloy steel, is widely used in tool manufacturing due to its exceptional hardness, wear resistance, and ability to maintain shape under high temperatures. This article systematically introduces tool steel classification, characteristics, and common grades, providing a comprehensive and practical material selection guide.

Tool Steel Overview

Tool steel is a high-carbon alloy steel specifically designed for manufacturing various tools. Its primary characteristics include exceptional hardness, wear resistance, and dimensional stability under high temperatures. Tool steel typically contains 0.5% to 1.5% carbon along with carbide-forming elements such as tungsten, chromium, vanadium, and molybdenum. These alloying elements significantly enhance the steel's properties, enabling it to meet various demanding application requirements.

Based on different application needs, tool steel can be categorized into six main types:

• Water-hardening steel: This type hardens through water quenching and primarily consists of high-carbon steel. Water-hardening steel is relatively inexpensive and suitable for applications with modest performance requirements.
• Cold-work steel: Cold-work steel offers high hardenability and wear resistance, commonly used for manufacturing large components. Based on hardening methods, it can be further divided into oil-hardening, air-hardening, and high-carbon high-chromium steels.
• Shock-resistant steel: This type provides excellent impact toughness but relatively lower wear resistance, making it suitable for tools subjected to high impact loads.
• High-speed steel: High-speed steel maintains high strength and hardness at elevated temperatures, making it ideal for cutting tools and machining implements.
• Hot-work steel: Hot-work steel maintains good strength and hardness at high temperatures, suitable for tools performing cutting or forming operations in heated conditions.
• Special-purpose steel: Also called plastic mold steel, this category is specifically designed for zinc die casting and plastic injection molding applications.

The selection of tool steel type typically depends on factors such as working temperature, required surface hardness, strength, impact resistance, toughness, and cost considerations.

Tool Steel Characteristics and Performance

The most prominent properties of tool steel include hardness, wear resistance, and deformation resistance. Additionally, many tool steels can maintain cutting edges at high temperatures, making them suitable for forming other materials through cutting, machining, stamping, or forging operations.

• Hardness: A key indicator of tool steel's resistance to plastic deformation and wear. High hardness enables greater load-bearing capacity and extended service life.
• Wear resistance: The ability to resist abrasion. High wear resistance helps maintain dimensional accuracy and shape precision.
• Toughness: The capacity to absorb energy and resist fracture. High toughness allows tools to withstand impact loads without brittle failure.
• Heat treatment properties: Directly affecting final hardness, strength, and toughness. Different tool steels require specific heat treatment processes to achieve optimal performance.
• Machinability: The ease with which steel can be cut, formed, and finished. Good machinability reduces manufacturing costs and shortens lead times.

Common Tool Steel Grades

Below are detailed descriptions of several common tool steel grades, including their properties, applications, heat treatment methods, and hardness levels.

D2 Tool Steel

• Applications: Ideal for long-term tooling applications requiring precise tolerances, such as blanking, punching, and forming dies.
• Heat treatment: Hardened at 1800°F to 1875°F (982°C to 1024°C), typically tempered between 900°F to 960°F (482°C to 516°C).
• Hardness: Achieves 62-64 HRC after hardening and tempering.

O1 Tool Steel

O1 is an oil-hardening tool steel with relatively easy machinability. It offers a good balance between toughness and wear resistance, making it a popular choice for general-purpose tools.

• Applications: Commonly used for knives, shear blades, and other tools requiring sharp, durable edges.
• Heat treatment: Hardened by heating to 1475°F to 1500°F (802°C to 816°C) followed by oil quenching.
• Hardness: Reaches approximately 57-62 HRC after tempering.

S7 Tool Steel

S7 is a shock-resistant, air-hardening tool steel offering high toughness and impact resistance, making it excellent for tools experiencing significant mechanical shock.

• Applications: Used for chisels, punches, and rivet sets requiring high impact resistance.
• Heat treatment: Hardened at 1725°F to 1850°F (941°C to 1010°C), tempered at 400°F (204°C) for cold-work applications or up to 1000°F (538°C) for hot-work applications.
• Hardness: Achieves 60-62 HRC after hardening.

H13 Tool Steel

H13 is a 5% chromium hot-work tool steel known for maintaining hardness at elevated temperatures, making it ideal for hot-work applications.

• Applications: Widely used for aluminum and zinc die casting, as well as aluminum extrusion and press dies.
• Heat treatment: Hardened by heating to 1825°F to 1900°F (996°C to 1038°C) and tempered between 950°F to 1150°F (510°C to 621°C).
• Hardness: Typically reaches 56-57 HRC after hardening and tempering.

M2 Tool Steel

M2 is a high-speed tool steel offering excellent toughness and wear resistance, particularly effective for tools that must maintain sharpness at high temperatures.

• Applications: Commonly used for cutting tools like drills, taps, and end mills.
• Heat treatment: Hardened at approximately 2200°F to 2250°F (1204°C to 1232°C) and tempered at 1000°F to 1050°F (538°C to 566°C).
• Hardness: Achieves about 62-64 HRC after tempering.

A2 Tool Steel

A2 is an air-hardening tool steel known for its balanced toughness and wear resistance. With 5% chromium content, it provides high hardness and excellent dimensional stability after heat treatment.

• Applications: Typically used for blanking/forming punches, die trim steels, and injection molds.
• Heat treatment: Air quenched from hardening temperature, usually tempered at 350°F to 400°F (177°C to 204°C) to achieve 63-65 HRC.
• Hardness: Reaches 63-65 HRC after hardening.

Tool Steel Selection

Selecting the appropriate tool steel grade depends on specific application requirements, such as needs for wear resistance, toughness, or high-temperature hardness retention.

Key considerations: When selecting tool steel, evaluate these factors:

• Workpiece material: Different materials impose varying requirements on tool steel properties. Harder materials demand more wear-resistant tool steels.
• Processing method: Various operations (stamping, forging, injection molding, etc.) require different combinations of strength, toughness, and heat resistance.
• Production volume: Higher production quantities necessitate more wear-resistant tool steels to extend tool life.
• Cost: Significant price variations exist between grades, requiring careful balance between performance and budget.

Selection process: Follow these general steps:

1. Define application requirements: Determine expected loads, temperatures, impacts, and other operating conditions.
2. Evaluate workpiece material: Understand material hardness, strength, and machinability.
3. Select appropriate tool steel type: Choose between cold-work, hot-work, or other categories based on application needs.
4. Choose specific grade: Select from available grades (D2, O1, H13, etc.) within the chosen category.
5. Perform heat treatment: Apply appropriate heat treatment per grade specifications to achieve optimal performance.

Conclusion

Understanding each grade's unique properties enables informed decisions that ensure optimal tool performance and extended service life. Whether using A2, D2, O1, S7, H13, or M2, each tool steel grade offers distinct advantages. By aligning these properties with application requirements, manufacturers can achieve superior results.

This guide provides a comprehensive overview of various tool steel grades, helping navigate material selection complexities and make optimal choices for specific needs.