As the name suggests, tool steel is used for the fabrication of tools.
The properties of tool steels make them the ideal materials for the fabrication of cutters, dies, punches, bits, and a variety of other hand and machine tools.
What is instantly noticeable is that these steels have to be able to stand up forces such as sheering, torsion, tension, compression, as well as temperature abuse without damage.
But what makes tool steel so special? What properties make tool steels ideal for their purposes? How do these properties come about?
Let’s take a closer look at this type of steel and find out what makes steel qualify as tool steel.
What is tool steel?
Tool steels are a variety of steels belonging to the carbon and alloy steel family.
Tool steels contain between 0.5 to 1.5% carbon as well as other alloying metals. Their various properties make them the ideal materials for the fabrication of various types of tools. These tools have to be able to withstand the forces associated with their specific uses without breaking, deforming, shaving off, or losing their structural properties due to heat.
What are the properties of tool steel?
The main properties of tool steel are:
The toughness of a material is its ability to resist fracture by absorbing the energy of a strike by plastically deforming.
Hammers and other tools that apply or receive blunt forces have to be tough so that they don’t crack or shatter.
Lever tools such as crowbars and other pry tools also need to have enough toughness so that they do not break due to high bending forces.
Hardness is the ability of a material to withstand forces without becoming permanently deformed. Hardness also takes into consideration the material’s ability to resist fracture due to frictional forces from a sharp object. In other words, its ability to resist being scratched.
Cutting tools such as sheers, and lathe turning tools, as well as files, scribing tools, and drill bits have to be hard to maintain sharpness for a prolonged period.
Hard tools have to find the right balance between being hard but not too brittle where they break easily.
The wear resistance property of a material is somewhat related to its hardness.
Wear resistance is a material’s ability to resist abrasion. This means that particles of the material are not easily sheared off due to frictional forces between other materials.
This property is necessary for lathe turning tools and drill bits because their mode of operation depends on them cutting away other material via shear forces.
This is a very important property for high-speed tools.
Heat resistance is the ability of a metal to withstand elevated temperatures without change to its physical properties, especially hardness.
This is necessary for tools that produce high temperatures while being used such as drill bits. It is also important for tools used in the metal forging process.
It would be unwanted for your drill bit, after heating up, to become malleable after it is cooled.
Tool Steel Grades
There are seven grades, groups, or classifications that tool steels are categorized by. All these grades can be further subdivided into different types for specific groups. Some of these will be further discussed below.
1. Water-Hardening Tool Steel (W)
Water hardening tool steels are so-called because they are cooled by water quenching.
It is a low-cost tool steel compared to the other groups, has a high level of hardness, and are comparatively brittle. They are not recommended for use in temperatures above 150°C as they begin to soften noticeably at this temperature.
W-group tool steels are alloyed with manganese, silicon, and molybdenum to increase toughness.
This group of steel is less frequently used today than previous centuries but is still popular for the production of springs.
2. Cold -Work Tool Steel (O, A, and D)
Cold work tool steels are grouped into three categories, namely, oil-hardening (O series), air-hardening (A series), and high carbon-chromium (D series).
a. O Series (Oil-Hardening)
O series tool steels are typically hardened at about 800°C, oil quenched for cooling, and then tempered below 200°C.
These steels are less likely to distort or crack during heat treatment compared to water hardened steel.
O series can be further subdivided into O1, O2, O6, and O7 groups. This grouping is determined by the types and percentages of different alloying materials used in their fabrication.
b. A Series (Air-Hardening)
A series tool steel is further subdivided into A2, A3, A4, A6, A7, A8, A9, and A10.
They have low distortion during heat treatment mainly due to their high chromium content (except for A10 which doesn’t contain chromium).
Air-hardening tool steels have a good balance of wear resistance and toughness and also good machinability.
c. D Series (High Carbon-Chromium)
This grade of steel can either be air or oil hardened.
It contains between 10 and 13% chromium and can retain their hardness up to 425°C.
D series tool steels are used to make different types of dies such as forging dies.
3. Hot Work Tool Steel (H)
Hot work tool steels are developed for tools used in high-temperature fabrication. They maintain good toughness and hardness and fair wear resistance even after prolonged use in high-temperature applications.
These steels are known for their low carbon content and high alloy content. They are grouped in three main categories ranging from H1 to H59.
H1 to H19 contains about 5% chromium.
H20 to H39 contains about 9 to 18% tungsten and 3 to 4% chromium.
H40-H59 are molybdenum based.
4. Shock Resisting Tool Steel (S)
Shock resisting tool steel has been developed to be used in situations where the tool is subjected to repeated impact and shock. Impact drill bits and chisels are examples of tools that are made from shock resisting tool steel.
These materials have high toughness and hardness and relatively low wear resistance.
The toughness is achieved with a low carbon content of about 0.5%.
Good shock resistance and high hardenability are achieved by alloying with chromium-tungsten, silicon-molybdenum, or silicon-manganese.
5. Mould Steels (P)
The P represents plastic mold steels.
Mold steels are used to fabricate plastic injection molding dies and zinc die casts.
They are subdivided into P2, P3, P4, P5, P6, P20 and P21.
Mold steels are low carbon steel and their main alloying elements are chromium and nickel. They have good machinability so that they can be easily shaped into various mold designs. The faces of the molds have to be hard to withstand abrasive friction while the core has to be tough to absorb shock during compression.
6. High-Speed Tool Steel (T and M)
High-speed tool steels are developed to fabricate tools that do high-speed metal cutting.
These steels need to have good hardness and wear resistance and like the other classifications of tool steels, can be further subdivided to vary their properties for different specific uses.
Tungsten High-Speed Steel (T) contains a high percentage of tungsten along with chromium and vanadium. This version of high-speed steel is no longer frequently used.
Molybdenum High-Speed Steel (M1, M2, M7, and M50) contains molybdenum, tungsten, and chromium and is used for a range of things from drill bits to high-temperature ball bearings.
Cobalt High-Speed Steel (M35 and M42) contains cobalt as an additional alloy which increases heat resistance and hardness.
7. Special Purpose Tool Steel (L and F)
These tool steels are fabricated for special uses.
The specific features of these steels are not found in the other groups but they are far too expensive to be produced for regular use.
They are subdivided into two categories:
a. Low Alloy types (L)
These have similar characteristics to water hardening tool steels.
The main alloying element is chromium with small percentages of molybdenum, vanadium, and nickel.
They are used where high wear resistance and toughness are of utmost importance such as cams and clutch plates.
b. Carbon-Tungsten type (F)
These contain carbon (above 1%) and tungsten as their main alloying element. They have high wear resistance but are brittle. They are therefore used for low impact applications such as to make taps and broaches.