Steels are generally divided into structural and tool steels. Tool steels are used, among other purposes, for cutting hand and machine tools. All tool steels are stainless steels.
Heat treatment gives tool steels their working hardness (hot hardness and strength). This treatment influences the many properties (e.g. hot hardness, application temperature, tensile strength, wear resistance) of the tool steel and adapts it for specific applications. When increasing hardness, for example, the toughness decreases. Here it must be considered which property significantly influences the tool life depending on the application.
Tool steels are characterized according to
- their composition (unalloyed and alloyed)
- the cooling agents (water, oil and air hardeners) and
- the application temperature (cold work, hot work and high speed steel).
The application temperature is differentiated between cold work steel (unalloyed) and hot work steel (alloyed). Cold work steels are usually used when the application temperature at the surface does not exceed 200 °C during use. On the other hand, application temperatures of more than 200 °C occur with hot work steels.
A special class of these hot work steels are high speed steels (HSS), in which the application temperature can rise to over 600 °C.
High Speed Steels (HSS)
(High speed steel (HSS)), is primarily used as a cutting material (for cutting tools) and is a high-alloy tool steel. HSS is also used for the manufacturing tools because it is very good for grinding (which also permits regrinding of blunt tools, for example).
Compared to cold work steels, cutting speeds three to four times higher and thus high application temperatures can be achieved. This is due to the heat treatment in which the steel is annealed at over 1,200 °C and then cooled.
HSS obtains its hardness from its basic structure, which consists mainly of iron and carbon. In addition, alloying additions of more than 5 % are contained, making HSS a high-alloy steel.
Advantages of HSS in general
- Application temperature over 600°C
- High cutting speeds
- High strength (high breaking strength)
- Good grindability during production
- Good regrindability of blunt tools
- Relatively low price
The name additions HSSE-Co 5 or HSSE-Co 8 indicate the percent of cobalt content. The higher the cobalt content, the harder the tool steel. The cobalt content increases the hot hardness resistance and you can better cut materials that are difficult to cut. Co 5 contains, 4.8 - 5 % cobalt and Co 8, 7.8 - 8 % cobalt. With increasing hardness, however, the toughness decreases.
| Designation | Cobalt content |
|---|---|
| HSS | 0,5 - 1,5 % |
| HSSE-Co 5 | 4,8 - 5 % |
| HSSE-Co 8 | 7,8 - 8 % |
Wear resistance due to coating
In addition to the cutting speed and the resulting application temperature, wear also influences the service life of the tool. This can be remedied by a coating that allows higher cutting speeds.
HSS tools can be finished with various coatings that increase the wear resistance and, above all, the service life of the tool.
Coatings we are using:
- TiN
- TiAlN
- AlTiN
- TiCN
- RUnaTEC
An overview and a comparison of the coatings can be found in our article TiN, TiAlN, AlTiN... a comparison of the coatings.
Applications
High speed steel, with its various degrees of hardness and coatings, is suitable for various applications.
Which high speed steel you need for your application depends on your cutting process, whether you are drilling, threading or countersinking.
In the following graphic we give you a guide for which material you can use which high speed steel. It still depends on the respective cutting process. Nevertheless, you can see that with increasing cobalt content, the hardness of the material to be cut increases (e.g. HSS cuts steel up to 900 N/mm2, with the HSSE-Co 8 you drill in steel up to 1300 N/mm2).
Conclusion and summary
Drills are made of alloyed high speed steel (HSS). With this tool steel, application temperatures of up to 600 °C can be reached, which can occur when cutting e.g. steel or metals.
As the hardness of the material increases, you can use high speed steel with a higher cobalt content (5% or more). How high the cobalt content should be depends on your application. For example, if you want to drill stainless steel, you usually use an uncoated HSSE-Co 5 twist drill. In some cases tool steel HSS with a TiAlN coating is sufficient.
Now you can classify high speed steel and choose the right tool for your application.
If you have any questions plaese do not hesitate to contact us.
