Tool steel is a type of carbon alloy 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 abrasion-resistance lends to their use in the production of injection moulds.
Groups of Tool Steel
Tool steels are categorized into six groups. The choice of the group depends on factors of strength toughness, surface hardness, shock resistance, working temperature, and cost. The six groups are: water-hardening, cold-work, shock-resisting, high-speed, hot-work and 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.
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.
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.