Tool Steel Guide: Types, Properties and Uses

In the field of CNC machining, it is very important to choose the right tool material. Tool steel stands out among many materials with its unique properties and becomes an ideal choice for cutting and processing materials. So, what are the mysteries of tool steel? This article will explain the types, properties and uses of tool steel in detail, so that you can more accurately choose the tool steel that suits your needs in CNC machining.

1、What is tool steel?

Tool steel refers to different types of strong carbon and alloy steels that are particularly useful for tools such as reamers, drills, machine dies and hand tools because of their hardness, resistance to wear and bending. Tool steels can maintain a sharp edge even at high temperatures, thanks to their composition containing specific carbide-forming elements such as chromium, vanadium, molybdenum and tungsten, which are very hard compounds. Tool steels perform even better at high temperatures when cobalt or nickel is added. Tool steels are usually heat treated to make them harder and are widely used in tasks such as metal stamping, forming, shaping, cutting and plastic forming.

2、Composition of tool steel

Tool steel is a mixture of different metals such as chromium, molybdenum, tungsten, and vanadium with iron. It contains a large amount of carbon to enhance strength and form hard carbides. The addition of nickel and cobalt makes the steel stronger at high temperatures, while the addition of chromium, molybdenum, tungsten, and vanadium makes the steel hard and wear-resistant. Tool steel is classified according to its composition and performance, mainly including carbon tool steel, alloy tool steel, and high-speed steel.

3、Main types of tool steel

1.High-speed steel (M, T grade)

Type
High-speed steel (HSS) is divided into two types: M type (molybdenum-based) and T type (tungsten-based). M type high-speed steel has a shorter hardening range, lower hardening temperature, and slightly lower hardness than T type high-speed steel, but is more durable. The most common of these is M2 high-speed steel.
T type high-speed steel is usually harder and more wear-resistant than traditional carbon steel and tool steel because it contains tungsten and vanadium components.

Features
High-speed steel can withstand high temperatures while maintaining hardness. It is called high-speed steel because it can achieve better tool speeds and feed rates than ordinary high-carbon steel.

Main uses
Commonly used in the manufacture of drills, tool drills, electric saw blades, milling cutters, gear cutters, etc.

2.Water-hardening tool steel (W grade)

Composition and characteristics
W grade tool steel has a high carbon content and needs to be quenched in water. Due to the low alloy content, it is not as easy to harden as other tool steels. Small amounts of elements such as molybdenum, manganese and silicon can be added to increase its functionality. The price is relatively cheap and is often used in basic applications.

Performance limitations
Cannot withstand high temperatures and begins to soften at 150°C. Although it can become very hard, it is more likely to break than other types of tool steel. Water quenching will cause more warping and cracking.

Application areas
Commonly used to make blades, cold heading dies, lathe tools, embossing tools, reamers and industrial cutting tools, etc.

3.Cold work tool steel (A, O, D grades)

Overall characteristics
Known for its medium hardness, strong wear resistance and easy hardening ability, it is often used to make larger parts or parts that need to maintain the same shape after hardening. It is divided into three groups: air hardening (A grade), oil hardening (O grade) and high carbon chromium (D grade) tool steel.

Air quenching or A grade tool steel
It has good hardening ability and a lower hardening temperature range, small deformation, easy processing, good toughness and wear resistance. Commonly used for casting, cams, die bending, rods and blanking, etc.

Oil-hardening or O-grade tool steel
It is oil-quenched during production, and its high strength and wear resistance make it suitable for a variety of jobs, such as master engraving rollers, thread cutting rollers, punches, bushings and sleeves.

High-carbon chromium or D-grade tool steel
Also known as high-carbon chromium tool steel containing 11-13% chromium, it is not very corrosion-resistant, but the carbon content of 1.4-2.5% makes it have good wear resistance and suitable for long-term production. It can be hardened by oil or air and has little deformation. It is used to make knives, sewing and forming rollers, plastic injection molds, lathe centers and woodworking knives, etc. It is also used for polishing tools, laminating molds, drawing punches and cold extrusion molds.
4. Hot working tool steel (H grade)

Composition and properties
It usually contains a large amount of different metals, but the carbon content is not high. It can maintain quality and work well under long-term exposure to high temperatures. The most commonly used is H13 tool steel.

Application Scope
Excellent heat resistance makes it suitable for manufacturing materials such as metal and glass that require high temperature processing. It is often used in the manufacture of cold heading mold shells and hot extrusion processes of magnesium or aluminum.

Types
Molybdenum-based tool steel: contains a large amount of molybdenum, has strong wear resistance, is stable at high temperatures, and is suitable for extreme conditions such as metal milling cutters or molds.
Tungsten-based tool steel: contains 9-18% tungsten and 2-4% chromium, has good heat resistance but is brittle and needs to be preheated before use.
Chromium-based tool steel: contains 3-5% chromium and may contain less than 5% of other alloying elements. It is often used in plastic injection mold manufacturing, hot forging and hot processing punch presses.
5.Impact-resistant tool steel (S grade)

Performance characteristics
Designed to handle stress at low temperatures and has good hot hardness. Known for its strong impact toughness, but poor wear resistance, cannot handle high temperatures, and has a limit temperature of 537°C.

Application areas
Used for blacksmith chisels, boiler shop tools, tool chuck jaws, chucks, clutch parts, jackhammer parts, hot and cold forging dies, hot and cold shear blades and chippers, etc. also used for pneumatic tools, hot stamps, chippers, hot and cold processing chisels, hot forming molds and cold fixture molds
6.Mold steel (P grade)

Features
P grade tool steel is used to make mold steel for plastic parts, suitable for mold and mold manufacturing for processes such as cold stamping, hot forging, die casting and plastic injection molding.

Performance
It has strong comprehensive toughness, corrosion resistance, hardness and wear resistance, high impact strength and easy polishing. Common models include P20 and 420 (highly refined, high-quality stainless steel).
7. Special purpose tool steel (F, L grade)

Classification and characteristics
Special purpose tool steel is more expensive than general W grade tool steel, and is divided into two categories: low alloy (L type) and carbon tungsten base (F type). With special composition and quality, it is suitable for specific applications that ordinary W steel cannot handle.

L-type tool steel
Suitable for applications where toughness and wear resistance are very high, such as bearings, clutch plates, rollers, wrenches, cams and chucks. Steels with higher carbon content are used for molds, drills, gauges, knurling and taps.

F-type tool steel
Hardened by water, it is much more wear-resistant than W-type tool steel. Suitable for applications that require high wear resistance but not high temperature or impact resistance, such as paper cutters, broaches, polishing tools, reamers and plug gauges.

4、Manufacturing method of tool steel

1.Electric Arc Furnace (EAF)
The Electric Arc Furnace (EAF) process is one of the common methods for manufacturing tool steel. It involves melting recycled scrap steel along with alloying components in an electric arc furnace. To prevent oxidation, the molten mixture is treated with chemicals and poured into large ladles.
After removing impurities during the refining stage, the steel can be poured into large molds to form ingots.

2.Electroslag Refining (ESR)
Electroslag Refining (ESR) is an alternative to electric furnace refining. This method uses a progressive melting process to produce ingots with smooth surfaces and no holes or defects, resulting in high-quality steel with minimal defects.

3.Powder Metallurgy
Powder metallurgy is a more advanced manufacturing method. It uses powdered metal to manufacture tool steel with enhanced hardenability and machinability.
This process is particularly effective for producing alloy materials required for applications such as high-carbon tool steel and aerospace components.

5、Common steps in tool steel manufacturing

1.Annealing
The annealing process involves heating the steel to a specific temperature and holding it for a period of time before cooling. This helps change the molecular structure of the steel, making it less brittle and more machinable.

2.Hot or cold drawing
The drawing process allows tool steels to achieve tighter tolerances, smaller sizes or unique shapes.
Due to the high strength and limited ductility of tool steels, hot drawing is usually performed using multiple passes at temperatures up to 540°C. Cold drawing is usually limited to a single light draw to prevent the raw material from breaking.

6、Main properties of tool steel

1.Wear resistance
The wear resistance of tool steel is one of its important properties. It can resist wear during use and extend the service life of the tool.
The wear resistance of tool steel can be increased by adding alloying elements.

2.Heat resistance
For some tools that work in high temperature environments, heat resistance is the key. Tool steel needs to maintain its performance stable at high temperatures without softening or deformation.
Different types of tool steel have different degrees of heat resistance.

3.Toughness
Toughness measures the steel’s ability to resist fracture. Tool steel needs to have a certain toughness to prevent brittle fracture during use.
The addition of alloying elements can also improve the toughness of tool steel.

4.Hardness
Hardness measures the degree of steel’s resistance to deformation. The Rockwell C test is often used to determine the hardness of tool steel.
The hardness of quenched cold-worked tool steel is usually around 58/64 HRC (Rockwell hardness C), most of the hardness is between 60/62 HRC, and some tool steels sometimes have a hardness of up to 66 HRC depending on the grade.

7、Tool Steel Performance Comparison

O1
Oil-quenched tool steel, easy to machine.
Good wear resistance and toughness.
Best hardness response at low temperatures, high dimensional stability during hardening.
Up to HRC 65.
W1
Water-quenched tool steel.
Excellent wear resistance, good toughness.
Up to HRC 68.
A2
Air-quenched tool steel.
Higher heat and wear resistance than O1.
High dimensional stability, less distortion.
Up to HRC 63.

D2
High carbon, high chromium tool steel.
Ideal grade for maximum production, with excellent toughness and wear resistance, high compressive strength.
Up to HRC 65.
S7
Impact-resistant tool steel.
High strength and ductility, suitable for moderately high temperature environments.
Up to HRC 61.
P20
Special tool steel.
Suitable for machining die castings and plastic molds.
Up to HRC 32.
13
Hot work tool steel.
High resistance to thermal fatigue cracking, high hardenability, good wear resistance, good toughness.
Maximum HRC is 54.
M2
High speed tool steel.
Excellent wear resistance, good toughness.
Maximum HRC is 65.

8、What should be considered when selecting tool steel types for processing?

1.Performance
There are many types of tool steel used for metalworking, each with different properties such as chemical composition, toughness and strength. The specific tool or product will greatly affect the type of tool steel that should be selected.

2.Toughness
The toughness of tool steel refers to its ability to resist breaking, chipping or cracking under stress or impact. If only toughness is considered, H13 or S7 may be the choice, but all ideal characteristics and specific work requirements need to be considered comprehensively.
As the alloy content increases, the toughness of tool steel tends to decrease. The manufacturing process also affects the toughness, and the powder metallurgy production process can improve the toughness of steel grades due to the uniformity of the microstructure.
The hardness of tool steel also affects the toughness, which is generally better at lower hardness, but may have a negative impact on other basic properties.

3.Wear resistance
Wear resistance refers to the degree to which tool steel withstands wear or erosion. There are two types: abrasive wear and adhesive wear.
Increased alloy content generally means higher wear resistance due to the increase in carbide content in the steel. Carbides are formed by alloys such as vanadium, tungsten, molybdenum and chromium combined with carbon, which enhances wear resistance but reduces toughness.

4.Compressive Strength
Compressive strength measures the force an object can withstand before deforming or breaking. Factors related to compressive strength are the alloy content in the steel and the hardness of the tool steel. Elements such as molybdenum and tungsten increase compressive strength, and the higher the hardness, the better the compressive strength.

5.Processing Speed
Some tool steels may not perform well at high speeds. The wrong type selection for high-speed machining may cause the tool steel to lose hardness and wear resistance over time. High-speed machining is best done with high-speed steel, which still provides greater wear resistance and extreme temperatures at high speeds.

6.Cost
Price is a key factor when choosing a CNC machining tool or tool steel type. Avoid skimping on material selection, as premature tool failure may result in more costs.

9、Conclusion

In CNC machining, tool steel has become an ideal tool material due to its unique properties. This article details the types, manufacturing methods, properties, applications, etc. of tool steel. Understanding these will allow us to more accurately select the right tool steel in CNC machining, give full play to its advantages, and provide strong guarantees for machining.