What is Tool Steel?
Tool steel is a type of carbon engineering steel. As you can guess from the name, it is often used to make, modify or repair hand tools or machine dies. Tool steels are notable for their hardness, resistance to abrasion and deformation. They can retain a cutting edge at very high temperatures which is why they are often used in the shaping of other materials through cutting, pressing, coining or extruding. Their resistance to abrasion lends to their use in the production of injection molds.
Groups of Tool Steel
Tool steels are categorized in six groups. The choice of group depends on factors of strength toughness, surface hardness, shock resistance, working temperature, and cost. The six groups are:
- Special purpose.
How is Tool Steel Made?
The manufacture of tool steels takes place under carefully controlled conditions to produce the required quality. Tool steel has a carbon content of between 0.5% and 1.5%. The manufacturing process introduces alloying elements that form carbides, commonly tungsten, chromium, vanadium and molybdenum.
The most important manufacturing processes for tool steel are as follows:
Tool steel is often made from around 75% scrap – a mixture of mill scrap and purchased scrap. It’s very important to avoid contamination of the scrap, especially from metals which cannot be oxidized like nickel, cobalt and copper.
The majority of tool steel production is done through Electric Arc Furnace (EAF) melting.
There are two stages:
- The scrap is melted rapidly in the furnace.
- The hot metal is transferred to a separate ladle or converter vessel to be refined. This process is known as secondary refining, and it allows for great efficiency and the processing of large volumes.
The refined metal is then transferred into the casting station and poured into ingots. The resulting ingots are usually annealed (heated and cooled slowly) to prevent cracking.
Electroslag remelting or refining (ESR) is a progressive melting process used to produce ingots with smooth surfaces and no pipe (holes) or porosity (imperfections). ESR ingots give improved hot workability, better processing yields, increased cleanliness, better transverse tensile ductility and fatigue properties.
ESR is an expensive process, and the costs saved through the increase in yield are not always sufficient to offset the costs of ESR processing. However for some specialized tool steel applications ESR is worth it.
Vacuum arc remelting (VAR) is a process sometimes used alongside ESR. However its use in tool steels is limited to specialized applications with specific bearing requirements. In the VAR process, heat is supplied via an arc in a high-vacuum environment. The resulting steel has a refined macrostructure and microstructure and excellent chemical uniformity.
The breakdown method used for tool steels employs either an open-die hydraulic press or rotary forging machine. These processes are extremely versatile and can produce lengths of 6 to 13 m (20 to 43 ft) in squares, rectangles, hollows or stepped cross sections. The final product is very high quality, having few cracks, laps or seams, and a high degree of straightness can be achieved.
In modern steel manufacture, up to 26 rolling mills are used in a row. The metal is heated via a gas-fired pusher, walking-beam furnace, or high powered induction furnace. Rapid heating is used to prevent decarburization (loss of carbon content). The process is automated by computers and measuring devices are used to monitor the diameter tolerance and surface quality of the metal. Through this process, a coil of steel sheet can be produced in less than 12 minutes.
Hot and Cold Drawing
Drawing operations are usually used on tool steels to produce better tolerances, smaller sizes, or special shapes. As tool steels are of high strength and limited ductility, cold drawings are limited to a single light pass in order to prevent breakage. Warm drawing at temperatures up to 540 °C (1000 °F) is used in multiple passes to strengthen the metal.
Continuous casting of tool steel is sometimes done for economic reasons. Following casting, the billets are annealed and sometimes ground, then forged by hammer or rotary, after which they can be rolled. Electroslag rapid remelting (ESRR) is a modern process which runs at higher temperatures than ESR.
Powder metallurgy (P/M) is used to produce highly alloyed steels such as high-carbon, high-chromium and high-speed. This process has become increasingly popular in recent years. Using traditional methods, the production of high-carbon, high-alloy tool steels are particularly challenging. The relatively slow cooling times for these methods results in the formation of undesirable coarse structures of eutectic carbide, which results in non-uniform heat-treat response, poor transverse qualities and low toughness.
In P/M, the problems of traditional methods are overcome. A fine, uniform distribution of carbides can be produced using P/M which results in improved machinability in the annealed condition, a faster response to hardening heat treatment, and improved grindability. However, there is a downside — a reduction in wear resistance.
The Osprey Process remains a very specialized activity limited to sites in Japan and the UK, However, it has tremendous technical and commercial potential. The molten alloy is poured from an induction furnace through a nozzle and blasted with high-pressure gas atomization jets, causing the formation of small droplets. The droplets are collected and used to form billets, hollows and sheets.
The advantages of the Osprey process are similar to P/M. Tool steel produced from Osprey material have a uniform distribution of fine carbides. However, the Osprey process is currently not as economically competitive as P/M.
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