Tool steels are yet another classification or type of metal you can find on the market.
It is easy to derive from the name that tool steels are part of the steels group. This is the most common group of metals used in engineering today.
Tool steels, along with stainless steels, are a sub-group of alloy steels. Both of them contain alloying elements that give them necessary properties for certain applications. And we are about to find out what these properties are respective to the tool steel grade.
What Is a Tool Steel?
As mentioned before, manufacturing industries use different type of steels for various purposes. Either as part of the manufacturing process or as part of the final product.
Judging by the name, you may have an idea on the main purpose of a tool steel. Whether you are manufacturing hand tools or machine tools, like dies or drill bits, tool steel is the perfect choice. Some of the main properties that make this alloy steel the best option for tool manufacturing include:
- Fairly high material hardness that makes it resistant to deformation and flattening.
- Great toughness which makes it resistant to breakage and chipping.
- Wear resistance which includes resistance to abrasion and erosion.
- Good thermal properties that provide it with the ability to retain its shape and sharp cutting edges even at very high temperatures.
Other uses for tool steels that should be highlighted are modifying and repairing machine tools and dies, something which could be very relevant for some metal manufacturing industries. Also, the production of injection moulds finds this type of steel very useful thanks to its resistance to abrasion.
Tool steels come in different grades. Each grade has more specific properties depending on how it is treated. For example, some grades receive extra chemical properties from components like vanadium. Vanadium increases the resistance to corrosive wear of the tool steel.
Others include a smaller amount of manganese to reduce the possibility of cracks during quenching.
Now, if you are asking how is this possible, keep reading and you’ll find out.
How Are Tool Steels Made?
There are different processes that can be used to produce tool steels, but all of them have something in common. The production of tool steel has to be carried out in environments with controlled conditions to ensure high quality.
Regarding the contents, tool steels usually have in the range of 0.5% to 1.5% of carbon. Other components, such as tungsten, vanadium, chromium and molybdenum, are added in different proportions to achieve more specific properties.
Some of the most used processes in tool steel production include:
- Electric arc furnace melting
- Electroslag refining
- Primary breakdown
- Hot and cold drawing
- Continuous casting
Electric Arc Furnace (EAF) Melting
This process is also known as primary melting. It is based on the melting of metal chips which are obtained from milling processes and suppliers. Basically, EAF melting uses leftovers of different metal processing methods.
Electric arc furnace melting is widely used because the production costs are low. Still, some extra treatment may be needed to achieve the highest possible quality and properties. An example of this is annealing to prevent cracking.
The process consists of two steps:
- Melting the scrap quickly in the furnace.
- Refining the melted metal in a separate vessel which allows the possibility to process great amounts of metal.
It is necessary to avoid the contamination of the melt during the process. That is why controlled conditions are so important.
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Electroslag Refining (ESR)
Electroslag refining is also known as electroslag remelting. Throughout the process, the metal is melted progressively. The resulting ingots have good surface quality without notable imperfections.
Moreover, the tool steel produced through this process provides other great qualities unnoticeable to the naked eye:
- Better hot workability
- Higher cleanliness
- Improved ductility
- Increased fatigue resistance
Although a rather expensive process, electroslag refining is a good choice for the most specialised applications.
This tool steel manufacturing process requires certain machinery. Such as open-die hydraulic presses or rotary forging machines.
However, the variety of cross sections achievable with the primary breakdown process makes up for the cost of the machines. It is possible to produce square, rectangular, hollow and stepped profiles. The maximum lengths range somewhere between 6…12 metres.
Most importantly, tool steel manufactured by primary breakdown provides increased quality, few to no imperfections and great straightness.
Nowadays, rolling mills are used in a row. Those rows can include more than 20 mills. The process starts with an induction furnace where the steel is heated.
There are also other options for heating the metal, such as walking-beam furnaces. The heating process is carried out in a quick fashion in order to avoid the loss of carbon, which is known as decarburisation.
Hot rolling comes next to give the metal its initial shape. Each roll presses the sheets a little thinner until the required thickness is achieved.
Depending on the dimensional requirements, cold rolling may come next in line. This rolling method allows very precise tolerances to the final product.
With the technological developments that have taken place in the steel industry, the process is automated. Rolling produces sheets of steel in coils. The time it takes to make one coil is about 12 minutes.
Hot and Cold Drawing
Using drawing to produce tool steel makes it possible to obtain great tolerances, smaller sizes and certain profiles.
Because of tool steel’s high strength and reduced ductility, cold drawing is limited to a single light pass. This helps to prevent the cracking of the material.
Hot drawing with temperatures up to 540° C, allows multiple passes. This also increases the strength of the tool steel.
This is a process that helps to reduce costs when producing tool steels. After casting, other treatments and processes are used to obtain better properties. Some of the most common processes used after casting include annealing, hammer forging and rolling.
It is important to highlight that most of the processes that have been used traditionally to produce tool steels have a common flaw – the cooling periods are quite long. This sometimes results in coarse structures that provide low quality and limited properties.
In order to eliminate this problem, many developments have been made. The most current processes, such as powder metallurgy and osprey processes, are capable to produce tool steels with high contents of carbon, chromium and improved properties. These include:
- Better machinability
- Better response to heat treatment
- Increased grindability
The only problem with these new processes is the increase of costs due to a need for special machinery and expertise. However, we can expect a decrease in the prices as these processes become more common in the future.
Most Common Tool Steel Grades with Applications
Tool steels are divided into 5 groups. Each of them has specific features regarding aspects like surface hardness, strength or toughness, working temperature, shock resistance and cost.
These five groups are:
The cold-working group consists of three grade types: oil-hardening, air-hardening and D-grades.
Water Hardening or W-Grades
This group contains low-cost high carbon steels with high hardness. The price factor makes it the most widely used amongst tool steels.
However, fragility is a side-effect of the W-grade’s hardness. Also, they are not suitable for working at elevated temperatures.
The name derives from the fact that all steels in this group are water quenched. Water quenching may result in cracks and warping more often than oil quenching or air hardening. This is also why the sales, although still leading, have been decreasing compared to other grades.
The most common applications of W-grade tool steels include:
- Cutters and knifes
- Razor blades
- Lathe tools
Air Hardening or A-Grades (Cold-Working)
A-grade tool steels have a higher content of chromium which results in a better response to heat treatment. The machinability of A-grade tool steels is quite good. In addition, they have great wear resistance and toughness properties.
The most common applications of A-grade tool steels include but are not limited to:
- Bending dies
- Blanking dies
- Coining dies
- Embossing dies
- Lamination dies
- Chipper knives
- Lathe centres
- Plastic injection moulds
- Cold extrusion punches
In this group we find the tool steels that combine W-grade and A-grade characteristics. On one hand, they contain a higher amount of carbon compared to the water hardening type. On the other hand, they have the properties described above which are typical of the air hardening type.
Because of their high chromium content, D series tool steels are often also categorised as stainless. But the corrosion protection is actually pretty limited.
The most common applications include:
- Burnishing tools
- Cold extrusion dies
- Lamination dies
- Woodworking knives
- Lathe centres
- Drawing punches
- Plastic injection moulds
- Seaming rolls
- Forming rolls
Oil Hardening or O-Grades (Cold-Working)
This tool steels group has great resistance to abrasion and high toughness properties. It is considered to be a general-purpose steel, making it very versatile.
Most of the applications are similar to those of A-grade and D-grade tool steels, but also include:
- Chasers for thread cutting
- Master engraving rolls
Shock-Resisting or S-grades
This group contains low carbon tool steels and have very high toughness values. That allows them to be very resistant to shock at both low and high temperatures.
However, they are not very resistant to abrasion because of the same low carbon content.
The most relevant applications of S-grade tool steels are:
- Jackhammer parts
- Blacksmith chisels
- Cold working chisels
- Hot working chisels
- Clutch parts
- Hot forming dies
- Cold gripper dies
- Chipper knives
- Pneumatic tools
- Hot stamps
High-Speed Tool Steel
These tool steels are especially common in cutting tools.
Mechanical cutting methods result in a lot of heat generation. High-speed steels do not lose their hardness at high temperatures, though, making this a perfect use-case for them.
Common applications for high-speed steels:
- Power-saw blades
- Drill bits
- Milling cutters
- Gear cutters
- Router bits
Hot-Working or H-Grades
When cutting material at very high temperatures, you may want to use a tool steel from this group. They high toughness and hardness values and keep their characteristics while working at high temperatures for long periods.
This is achieved by having a low carbon content, but a high content of other alloying elements.
The most common applications of H-grade tool steels include:
- Hot forging
- Dummy blocks for hot extrusion
- Plastic injection moulds
- Hot working punches
The choice of the tool steel you really need depends on the properties your specific application requires. The most common mechanical properties to consider are surface hardness, toughness, working temperature and shock resistance.
At the same time, it is important to include the cost of each material into the assessment matrix.
Also, it is very useful to answer questions about the requirements of sharp edges or cutting, how important abrasion resistance is, and the heat treatment method required.
With all this information you should be ready to go and make your choice!