Tool steel is basically a certain type of carbon alloy steel. As you can easily tell from its name, it’s used to make, modify and repair hand tools or machine dies.
How Tool Steel is made
Tool steels are manufactured under properly controlled conditions in order to produce the required quality. They contain 0.5%-1.5% carbon content. The manufacturing process introduces alloy elements which from carbides, most commonly chromium, tungsten, molybdenum and vanadium. Here’s a breakdown of the most important processes involved in the manufacturing of tool steel:
Often, tool steel is made from almost 75% scrap, which is essentially a mixture of mill and purchased scrap. It’s highly important to ensure that the scrap isn’t contaminated, especially from metals that can’t be oxidized like copper, nickel and cobalt. Most of tool steel production involves Electric Arc Furnace melting. Here, we have two stages:
- The scrap is rapidly melted in the furnace
- The hot metal is then transferred to a convertor vessel for refining. This is referred to as secondary refining. It allows for greater efficiency and processing of larger volumes.
Afterwards, the refined metal is transferred into the casting station before being poured into ingots. The ingots are heated and cooled slowly in order to prevent cracking.
Electro-Slag Re-melting (ESR)
Electro-slag refining is a progressive melting process that’s used to produce smooth-surfaced ingots that have no holes or porosity. ESR ingots have improved hot workability, increased cleanliness, better processing yields, better transverse tensile ductility, and fatigue properties.
Electro-slag refining is an expensive process. The costs saved by increasing the yield aren’t always enough to offset the costs incurred by the process. ESR is, however, worth it for various specialized tool steel applications.
Vacuum Arc Re-melting (VAR)
This process is sometimes used alongside electro-slag refining. However, its use in manufacturing tool steels is limited to applications with specific bearing requirements. VAR involves heat supply through an arc in high-vacuum environments. The resulting steel features a refined macro- and micro-structure with excellent chemical uniformity.
The breakdown method that’s used for tool steel makes use of either a rotary forging machine or an open-die hydraulic press. Both of these processes are extremely versatile. They can produce lengths of 20 to 43 feet in hollows, squares,rectangles or stepped cross-sections. The final product is of high quality, having very few cracks, seams or laps. Quite a high degree of straightness can be obtained.
In modern manufacturing of steel, up to 26 rolling mills are used in a single row. The metal gets heated via a gas-fired pusher, high-powered induction furnace or walking-beam furnace. Rapid heating is applied to prevent loss of carbon content (decarburization). This process is automated by computing devices. Measuring tools are used to help monitor the metal’s diameter tolerance and surface quality. Through rolling, a steel sheet coil can be produced in even less than 12 minutes.
Drawing operations are used on tool steels to help produce better tolerances, special shapes or smaller sizes. Tool steels are of limited ductility and high strength. As such, cold drawings are limited to only one light pass to prevent breakage. Warm drawing (temperatures of up to 1,000 degrees Fahrenheit) is used in more than one pass to strengthen the metal.
At times, continuous casting of tool steel is done for economic reasons. After casting, the billets are heated, cooled slowly and sometimes ground. They’re then forged by rotary or hammer, after which they can now be rolled.
Powder Metallurgy (P/M)
This is done to produce highly alloyed steels like high-carbon, high-speed and high-chromium. Powder metallurgy is increasingly growing popular nowadays. Production of high-alloy tool steels is particularly challenging when using traditional methods as they have relatively slow cooling times. The slow cooling times result in formation of undesirable coarse structures which in turn result in poor transverse qualities; low toughness and non-uniform heat-treat response.
Powder metallurgy overcomes the challenges faced when using traditional methods. A fine and uniform distribution of carbides is produced, which results in improved machinability in their annealed condition. Powder metallurgy provides for fast response to hardening heat treatment and improved grind-ability.
This remains a very specialized activity, limited to sites in the U.K. and Japan. However, it has great technical and commercial potential. Molten alloy is poured from the induction furnace through a nozzle and then blasted with high pressure gas-atomization jets. This causes the formation of small droplets which are then collected and used to form sheets, billets and hollows. Tool steel that’s produced from the Osprey process has a fine, uniform distribution of carbides. The Osprey process may not be as economically competitive as powder metallurgy.
Tool Steel Groups
Tool steels are categorized into six different groups. The classification is dependent on factors like strength toughness, shock resistance, surface hardness, the working temperature and cost. The groups are as listed below:
Tool steel is manufactured in many different grades. It’s used to make tools like stamping dies, axes and construction equipment. Also, it’s used in other applications which require materials with mechanical properties like that of tool steel.
Tool steel is notable for its hardness and resistance to deformation. It’s able to retain a cutting edge even at extremely high temperatures. That explains why it’s often used to shape other materials through pressing, cutting, extruding or coining. Tool steels’ resistance to abrasion explains their use in production of injection molds.