Introduction
Tool steel properties play a pivotal role in determining the suitability of this versatile material for a wide range of applications across industries.Tool steels derive their name from their primary application – the manufacturing of tools. Cutting tools, dies, machine tool parts and other implements that require high strength, toughness and wear resistance are often made from various grades of tool steel. At a basic level, all tool steels share the common property of being able to take and hold a cutting edge through heat treatment. Beyond this, their material compositions and heat treatment responses can vary widely depending on the intended use.
Understanding tool steel properties is important for manufacturers, machinists, die makers and others who work with these specialized alloys on a regular basis. Choosing the right grade for a given tooling application can make the difference between a tool that lasts for years versus one that fails prematurely. In this guide, we will explore the key factors that differentiate tool steel types, with a focus on composition, heat treatment effects and common applications. Frequently asked questions will also be addressed. By the end, readers should have a solid working knowledge of tool steel properties.
Composition
The composition of tool steels is carefully controlled to impart certain desirable characteristics. Common alloying elements include:
- Carbon (C) – The primary hardening element in steel. Higher carbon contents increase hardness but reduce toughness. Levels typically range from 0.5-1.5% in tool steels.
- Tungsten (W) – Adds hardness and wear resistance without significantly reducing toughness. Often used to substitute for some carbon in high-speed steels.
- Molybdenum (Mo) – Increases hardness, strength, and wear resistance. Also improves temperature stability.
- Vanadium (V) – Enhances hardness, strength, and wear resistance when tempered. Also improves grindability.
- Chromium (Cr) – Provides temperature stability and resistance to softening at elevated temperatures. Improves toughness.
- Cobalt (Co) – Acts similarly to tungsten by adding hardness and wear resistance. Used in some high-speed and hot-work tool steels.
- Nickel (Ni) – Increases toughness without lowering hardness too much. Improves temperature stability.
The exact percentages and combinations of these alloying elements determine a tool steel’s final properties. Grades intended for heavy duty cutting or hot work applications contain more alloying elements, while those for general tooling use fewer.
Heat Treatment
Heat treatment is critical to realizing the full potential of a tool steel’s composition. All tool steels are hardened by heating to an austenitizing temperature followed by quenching, usually in oil. This transforms the steel’s microstructure to martensite – a very hard but brittle phase.
To relieve brittleness, quenched tool steels must then be tempered. Tempering involves reheating to temperatures ranging from 350-700°F for a specified time. This changes the martensite into a tempered martenedite that is harder and tougher than plain carbon steel but less brittle than as-quenched martensite.
The tempering temperature and time determines the final properties. Lower temperatures around 350-450°F produce maximum hardness, while higher 500-700°F ones maximize toughness at the expense of some hardness. Most tool steels are tempered to achieve an optimized balance of the two.
Heat treatment response varies greatly between grades. High-speed steels are designed to retain hardness even when tempered at higher temperatures. Hot work grades stabilize at very high temperatures. Proper heat treatment unlocks each tool steel’s full potential.
Applications
Different tool steel grades are optimized for specific applications based on expected stresses, temperatures, and wear conditions:
- Carbon/low-alloy steels – General tooling, taps, reamers, broaches. Good strength and toughness.
- High-speed steels (HSS) – Cutting tools for ferrous metals. Hold a keen edge at high speeds and temperatures.
- Cold work steels – Punches, dies, extrusion dies. Resist abrasive wear at room temperature.
- Hot work steels – Forming and forging dies. Stable at temperatures up to 2000°F.
- Plastic mold steels – Injection molding dies. Heat resistant with good surface hardness.
- Die steels – Blank and forming dies. High strength, toughness, and wear resistance.
- Stamping steels – Progressive and compound dies. Tough with good surface hardness.
Proper selection based on factors like material being cut/formed, temperatures, pressures and more is key to optimal tool performance.
Common Grades
Some of the most widely used tool steel grades include:
| Grade | Composition | Key Properties | Common Uses |
|---|---|---|---|
| O1 | 0.95% C | Good all-around properties for general tooling | Cutting, forming, and gauging tools |
| A2 | 0.95% C, 5% Cr | Higher temperature stability than O1 | Cold work tools, dies |
| D2 | 1.5% C, 12% Cr | Extreme wear resistance | Plastic molding and forming dies |
| M2 | 6% Cr, 5% Mo, 1% V | High hardness and strength | HSS drills, reamers, taps |
| M42 | 2.9% C, 5.5% Cr, 1% Mo | Excellent wear resistance | Metal cutting tools |
| S7 | 1.5% C, 5% Cr, 1.5% Mo | Stays hard at high tempering temps | Plastic molding and hot work dies |
Proper heat treating is still required to achieve the grades’ full potential wear resistance and hardness properties.
Conclusion: Tool Steel Properties
With so many available grades, tool steels may seem complex. However, understanding their key properties like composition, heat treatment responses and common applications provides a solid foundation for selection. Proper care must also be taken in heat treating to fully unlock each grade’s potential. This guide aimed to comprehensively cover the fundamentals of tool steel metallurgy in an approachable manner. Armed with this knowledge, users can now confidently choose the right tool steel to withstand demanding industrial environments.
FAQs
What is the difference between tool steel and high-speed steel?
While all high-speed steel is a type of tool steel, not all tool steel is high-speed. HSS contains alloying elements like tungsten and molybdenum that allow it to retain hardness and cutting ability at higher temperatures than regular tool steels. HSS grades are optimized for metal cutting applications.
How do I heat treat tool steel properly?
Careful control of austenitizing temperature, oil quenching, and double tempering is required. Specific temperatures and times will vary by grade – always follow manufacturer’s recommendations. Using a heat-treating facility is best to achieve consistent, defect-free results.
Which tool steel is best for plastic molding dies?
Plastic mold steel grades like D2, S7, and T1 are commonly used as they combine heat resistance with high hardness and strength. D2 offers the highest wear resistance, while S7 provides good strength retention at elevated tempering temperatures.
What causes tool steel to crack or chip during use?
Common causes are using a grade not suitable for the application/stresses, improper heat treating leading to defects, excessive pressures or temperatures, or inadequate clamping that allows vibration/shock loading. Maintaining optimal cutting parameters also prevents premature failure.