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Tool steel, as the name suggests, is often used to produce and repair machine tools and hand tools (among other applications). Known for its extreme hardness, tool steel has good abrasion resistance and can hold a cutting edge at high temperatures.
Different grades of tool steel are made by adding different amounts of carbon, as well as other elements such as chromium, tungsten, and molybdenum. Each type and form of steel is produced with different methods, depending on the characteristics desired.
The most popular forms of tool steel are O-1 and A-2 tool steels, which are both part of the cold-work group of tool steels.
- O-1 is a general purpose oil-hardening steel with good hardness, strength, and wear resistance. It is mainly used for items like knives and forks.
- A-2 is an air hardening steel with good machinability along with a balance of wear resistance and toughness. A-2 is the most common variety of air-hardening steel and is often used for blanking and forming punches, trimming dies, and injection mold dies.
How to Make Tool Steel
The basic process of making tool steel starts with recycled steel scrap which is melted in an electric arc furnace, along with any alloying elements. The molten mixture is poured into a giant ladle and mixed with chemicals to prevent oxidation. After impurities are removed through this refining stage, the steel is allowed to flow down into large molds to make ingots.
After un-molding, the red-hot ingots are forged with huge mechanical dies to press them into the desired size and shape. The finished forgings are annealed by re-heating to reduce internal stresses formed during the forging process. Then they are allowed to cool slowly either in water, air, or an oil bath, allowing the metal crystals to re-form. This annealing process keeps the steel soft enough to work without cracking. It can then be cold- or hot-rolled into the desired shape.
An alternative method of making tool steel is to position the ladle of hot molten steel over a vertical open-ended mold. The molten steel runs down through the cooled mold, and begins to harden near the mold’s inner surface. As more steel passes through the mold, the partially hardened metal continues to move down and then out onto water-cooled rollers. The result is a long, continuous bar or rod of steel.
For a tool steel with better surface quality and fewer imperfections, manufacturers use a process called electro-slag re-melting (ESR). Giant water-cooled molds are filled with a pool of heated slag containing reactive chemicals. A consumable steel ingot, called an electrode, is lowered down into the slag. An electric current passed through the electrode causes it to melt, its liquid drops of steel pulled by the current towards the bottom of the mold. As the drops flow downward through the hot slag, impurities react with chemicals in the pool and float to the top. Only pure steel droplets reach the bottom, where they solidify and eventually build up a homogeneous steel ingot inside the mold.
Another, more advanced process uses powdered metal to form tool steel with improved hardening and machinability. This process has better results for tool steel with higher percentages of carbon and alloying material, required for applications such as aerospace components.
Industrial Metal Supply offers tool steel bar in O-1 and A-2 grades. Note that not all sizes and alloys are stocked in every IMS location, but we can get it to you in any case.