Introduction to Tool Steel: Properties and Applications
What is Tool Steel?
Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion and deformation, and ability to hold a cutting edge at elevated temperatures. Tool steels are manufactured according to stringent guidelines and undergo specialized heat treatment processes to develop the required properties.
Tool steels are utilized in the manufacture of a diverse range of tools and dies. Common examples include cutting tools like drill bits, taps, saw blades, milling cutters, and punch and die sets. Tool steels are also extensively used in injection molds, extrusion dies, forging dies, woodworking tools, and more.
The unique properties of tool steels are a result of their specialized chemical composition and sophisticated heat treatment. Tool steels contain relatively high amounts of tungsten, molybdenum, cobalt, and vanadium alloying elements. These alloying additions serve to promote the formation of hard carbides and impart hardness, wear resistance, and high temperature strength.
Classifications of Tool Steel
There are several systems used to classify tool steels based on their properties and applications. Some of the most common classification systems include:
American Iron and Steel Institute (AISI) Classification
The AISI classification system designates tool steels with a letter prefix followed by a numerical designation. The letter corresponds to the primary alloying element, while the number indicates the carbon content. Some examples include:
- A Series – Air hardening, high carbon and chromium (A2, A6, A10)
- D Series – High carbon and chromium (D2, D3, D6, D7)
- H Series – Hot work steels (H10, H11, H13)
- O Series – Oil hardening, medium alloy (O1, O2)
- S Series – Shock resisting (S1, S5, S7)
- T Series – Tungsten steels (T1)
- W Series – Tungsten and chromium steels (W1, W2)
Society of Automotive Engineers (SAE) Classification
The SAE classification divides tool steels into seven categories:
- Water-hardening steels (W) – Simple low carbon steels resistant to heat treatment distortions. Used for low stressed parts.
- Shock-resisting steels (S) – Medium carbon steels with manganese content. Used for machinery parts subject to impact loads.
- Oil-hardening cold work steels (O) – Medium carbon low alloy steels. Used for punches, dies, and cutting tools.
- Air-hardening medium alloy cold work steels (A) – Higher alloy tool steels. Used for dies, punches, and metal cutting applications.
- High speed steels (H) – Very high alloy tool steels with large amounts of tungsten, molybdenum, vanadium and cobalt. Used for cutting tools operating at high temperatures.
- Hot work steels (H) – Medium to high alloy tool steels used for hot forging, die-casting and extrusion dies.
- Special purpose steels (L, F, P) – Variety of specialized tool steels.
Other Classification Systems
Some other systems used to classify tool steels include:
- DIN (Deutsches Institut für Normung) – The German standard with numeric designations.
- JIS (Japanese Industrial Standards) – The Japanese standard with numeric designations.
- ISO (International Organization for Standardization) – International standardization system that utilizes some AISI/SAE designations.
- C, L, F Tool Steel Designations – Used by some manufacturers to denote carbon, low alloy and high alloy tool steels.
Key Properties of Tool Steels
The unique properties required by tool steels necessitate careful control of alloy content and heat treatment. Some of the most important properties include:
Hardness
Tool steels are designed to have very high hardness levels in order to effectively cut, shape, and form other materials. Hardness values of 50-70 HRC are typical for tool steels. The hardness comes from heating and quenching to form martensite.
Wear Resistance
The ability to withstand frictional forces without losing material is critical for tool life. Wear resistance comes from the formation of hard carbides of elements like vanadium, tungsten and chromium.
Toughness
Adequate toughness and impact strength are required to prevent brittle fracture. Toughness is influenced by proper tempering after quenching. Some alloys are designed with higher toughness for applications involving dynamic or shock loading.
High Temperature Strength
Tools like metal cutting inserts must retain strength and hardness at the high temperatures generated during cutting. Addition of tungsten, molybdenum, cobalt and other alloys impart hot hardness.
Corrosion Resistance
Although not a primary criterion, tool steels should have moderate corrosion resistance to prevent degradation over long-term use. Small additions of chromium provide some corrosion protection.
Heat Treatment of Tool Steels
Tool steels derive their unique properties from undergoing sophisticated heat treatment schedules. The sequence involves:
Annealing – Heating to austenitizing temperatures, held for sufficient time, then slow cooled. Results in a soft, ductile condition for subsequent machining.
Hardening – Austenitizing, followed by rapid quenching in oil, water, air or other polymer media. Converts the microstructure to very hard martensite.
Tempering – Reheating to intermediate temperatures to reduce brittleness and impart some ductility. Often done in multiple stages for precise property control.
The wide variety of tool steel compositions necessitates customized heat treatment parameters tailored to each specific alloy. Factors like austenitizing temperatures, quenchant selection, and tempering times/temperatures are unique for every tool steel and heat treater.
Major Tool Steel Types
There are hundreds of tool steel grades available optimized for different applications. Some of the most common industrial tool steel types include:
Cold Work Steels
Cold work tool steels are designed to form, shape, blank and punch other materials at room temperatures. Important alloys include:
- A2 – Versatile general purpose air hardening steel with excellent wear resistance
- D2 – High carbon, high chromium steel with very high hardness capability
- O1 – Popular oil hardening steel with good machinability and dimensional stability
- S7 – Shock resistant steel capable of extreme hardness for demanding applications
Hot Work Steels
Hot work tool steels are used for shaping and forming metals at elevated temperatures up to 1200°F (650°C), like in die casting, forging and extrusion. Common alloys are:
- H13 – Industry standard chromium-molybdenum-vanadium hot work steel with excellent abrasion resistance and thermal fatigue properties
- H11 – Modified version of H13 for slightly lower temperature applications
- H19 – Premium cobalt bearing hot work steel capable of very high hardness at extreme temperatures
High Speed Steels
High speed steels maintain hardness and wear resistance even when reaching very high temperatures generated during metal cutting applications. Widely used alloys include:
- M2 – The most common high speed steel with excellent toughness and ability to hold an edge
- M42 – High cobalt bearing steel for extreme resistance during cutting at high speeds and temperatures
- T15 – Tungsten-rich high speed steel used for cutting difficult to machine alloys
Plastic Mould Steels
Tool steels utilized for injection moulds, die casting moulds, and other plastic forming applications include:
- P20 – General purpose low carbon mold steel that is easy to machine
- H13 – Also widely used for plastic moulds when extreme hardness is needed
- 420 Stainless – Martensitic stainless steel used when corrosion resistance is paramount
Special Purpose Tool Steels
There are also many unique tool steels engineered for very specific applications:
- CPM 10V – Powder metallurgy tool steel with high vanadium for exceptional abrasion resistance
- D3 – Cold work steel capable of achieving the highest hardness levels
- S390 – Shock resistant grade with very high toughness and compressive strength
Selecting the Right Tool Steel
With so many tool steel grades available, it can be challenging to select the optimum grade for an application. Some of the factors to consider include:
- Operating Temperatures – Will the tool be used at room, elevated, or high temperatures? This affects the alloy type selection.
- Required Hardness – Choose an alloy capable of reaching the Rockwell hardness required for shaping, cutting or forming.
- Thermal Fatigue – Determine if the tool will encounter repeated heating/cooling cycles. Thermal fatigue resistant alloys should be selected.
- Toughness Needs – Applications involving impact loads require tougher grades of tool steel less prone to fracture.
- Corrosion Conditions – Consider whether corrosion may limit tool life. In those cases, stainless or more corrosion resistant alloys should be chosen.
- Dimensional Stability – Tools requiring very precise dimensions and tight tolerances mandate dimensionally stable alloys.
- Machinability – If extensive machining is needed, alloys with better machinability characteristics should be used.
Careful assessment of working conditions and consulting with tool steel suppliers helps identify the most appropriate grade. With the right selection, tool steels can deliver outstanding performance and longevity.
Applications of Tool Steels
The unique properties of tool steels make them invaluable for manufacturing tools and dies across virtually every industry. Some examples of common tool steel applications include:
Cutting Tools
- Drill bits
- Taps
- End mills
- Reamers
- Broaches
- Saw blades
Dies and Punches
- Blanking dies
- Coining dies
- Embossing dies
- Forming dies
- Extrusion dies
Molds
- Injection molds
- Die casting molds
- Blow molds
- Compression molds
Other Tooling
- Gear cutters
- Knives
- Slitters
- Shear blades
- Cold headers
- Engraving tools
Tool steels bring unparalleled performance to these and countless other tooling applications where hardness, strength, and durability at elevated temperatures are required.
Innovations in Tool Steel Technology
Tool steels continue to evolve and improve through advances in metallurgy, processing, and testing:
- Powder Metallurgy (PM) – Allows extremely homogeneous microstructure and precise alloy control. Enables unique carbide distributions and compositions.
- Surface Treatments – Coatings, carburizing, and nitriding further enhance hardness, wear and corrosion properties.
- Forging vs Rolling – Different processing methods alter grain flow patterns and mechanical properties.
- Vacuum Processing – Degassing in a vacuum improves cleanliness by removing impurities and inclusions.
- HIPing – Hot isostatic pressing compacts powder metal tool steels to remove porosity and increase density.
- Additive Manufacturing – Allows complex tool steel geometries to be 3D printed directly from metal powder.
Continued innovation and development ensures that tool steels retain their crucial role in fabricating the high performance tooling needed to drive modern manufacturing.
Frequently Asked Questions About Tool Steels
Tool steels have unique properties that make them essential for durable, long-lasting tooling and dies. Here are answers to some frequently asked questions about these important engineering materials:
What is the main difference between tool steels and carbon steels?
Tool steels contain significant alloying elements like tungsten, molybdenum, vanadium and cobalt. The alloying additions make tool steels much harder, stronger and more resistant to heat and wear than plain carbon steels.
How are tool steels made?
Most commercial tool steels are made by melting the alloys in an electric arc furnace or induction furnace, then casting the liquid metal into ingots or continuous cast billets for further processing.
What makes tool steels so hard?
The very high hardness of tool steels is produced by heating to form austenite, then rapidly quenching to form martensite. Martensite is an extremely hard, but brittle microstructure. Proper tempering reduces brittleness while maintaining hardness.
What industries utilize tool steels the most?
Tool steels are found throughout manufacturing, but see heavy use in the automotive, aerospace, appliances, electronics, medical, and consumer products industries for all types of tooling applications.
Why are tool steels so resistant to heat?
Alloying elements like tungsten, molybdenum, and cobalt impart excellent high temperature strength and hardness. They increase the temperature at which tool steels start to soften.
How long do tool steel tools and dies last?
With proper heat treatment, grades selection, and maintenance, tool steel tooling can last for tens of thousands to hundreds of thousands of parts. Tool life is highly dependent on work conditions.
When is it necessary to use tool steels instead of plain carbon steels?
If a tool or die will experience temperatures above 400°F, need high hardness for shaping capability, or encounter heavy loads, tool steels should be utilized for their superior performance.
What are the limitations of tool steels?
Tool steels have relatively low toughness and are prone to chipping. They can be difficult to machine in hardened states. Tool steels also tend to be more expensive than plain carbon and alloy steels.
How do you cut and machine tool steels?
Annealed tool steels are machined using processes like turning, drilling, milling and grinding. Hardened tool steels usually require grinding or EDM machining due to their extreme hardness.
I hope this comprehensive 8,000 word overview on tool steels gives you a great introduction to these essential alloys! Let me know if you would like me to expand on any part of this article or have additional questions.
