First of all, there is no precise definition. In most people’s understanding, the differences between tool, ordinary, and carbon steels are:
1. It contains alloy elements.
2. It must undergo heat treatment before use.
Tool steel contains several alloy elements and improves its properties through heat treatment processes such as normalizing, quenching, and tempering.
In a book titled Steel Products Manual of Iron and Steel Society, the author defines tool steel as follows:
“Tool steels are carbon-, alloy-, or high-speed steels, capable of being hardened and tempered. They are usually melted in electric furnaces and produced under tool steel grade practices to meet special requirements. They may be used in certain hand tools or mechanical fixtures for cutting, shaping, forming, and blanking materials at ordinary or high temperatures. Tool steels are used in many different applications. They are chosen for their wear resistance, strength, toughness, and other important properties. These features help ensure the best performance.”
In this definition, tool steels have three conditions.
- Hardenable by heat treatment
- Special manufacturing processing
- Application in cutting or forming operations
In addition to nationally standardized grades, there are also enterprise-specific grades. Although the steel composition is the same, manufacturers consequently give the material special traits through their own design and production methods. They establish a good reputation through long-term practical testing and gradually form their grades.
One of the important naming standards is the ASTM (American Society for Testing and Materials) designation. This naming system consists of a letter and number combination. The letter signifies a specific characteristic, while the number, typically in sequential (often historical) order, represents a particular developed steel.
For example, common types of tool steels include: A2 tool steel, D2 tool steel, D3 tool steel, H11 tool steel, H13 tool steel
ASTM Classification of Tool Steels
| Classification ASTM | Symbol |
|---|---|
| Water-hardening tool steels | W |
| Shock-resisting tool steels | S |
| Oil-hardening cold work tool steels | O |
| Air-hardening, medium-alloy cold work tool steels | A |
| High-carbon, high-chromium cold work tool steels for Dies | D |
| Plastic mold steels | P |
| Hot work tool steels, chromium, tungsten | H |
| Tungsten high-speed tool steels | T |
| Molybdenum high-speed tool steels | M |
In addition to this classification, steels are also identified by designations in the Unified Numbering System (UNS) for Metals and Alloys, established in 1975 by ASTM and the Society of Automotive Engineers (SAE).
An additional notable classification for tool steels is derived from the former DIN 17350 Standard, now recognized as ISO Standard 4957. This system assigns designations based on chemical composition and provides a numerical identifier for each grade.
Another common classification is Japanese grades. The designation of Japanese steel is specified according to JIS standards, which stand for Japanese Industrial Standard. In the text below, × represents numbers.
- Carbon tool steel: Designated by SK××, where ×× represents the average carbon content. The JIS 4401-2000 standard includes 11 grades, such as SK140.
- Alloy tool steel: JIS G4404-2000 standard for alloy tool steel, with SKS×(×) representing cutting tool steels and high-impact tool steels. SKD× and SKT× represent hot work tool steel, while SKS×(×) and SKD×(×) represent cold work tool steel.
- High-speed tool steel: Designated by SKH×(×) in the JIS G4403-2000 standard.
We have another article about different standards. To learn more, please click on An Overview of JIS, ASTM, ISO, and EN Standards.
KEY ALLOYING ELEMENTS AND THEIR EFFECTS IN TOOL STEELS
Tool steels typically contain medium to high carbon levels, contributing to their hardness and wear resistance. However, the addition of various alloying elements can significantly enhance their performance:
- Chromium (Cr): Provides deeper hardness penetration, improves wear resistance, and increases toughness.
- Cobalt (Co): Used in high-speed steels to enhance red hardness, allowing them to operate at higher temperatures.
- Manganese (Mn): Improves soundness, promotes deeper and faster hardening, and reduces quenching temperatures.
- Molybdenum (Mo): Increases hardness penetration, reduces quenching temperatures, and enhances red hardness and wear resistance.
- Nickel (Ni): Adds toughness and wear resistance, often used in conjunction with hardening elements.
- Tungsten (W): Increases wear resistance and provides red hardness characteristics. Higher tungsten content improves wear resistance, especially in combination with high carbon.
- Vanadium (V): Refines grain size, increases toughness, and provides exceptional wear resistance, particularly in high-speed steels.
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