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How to Harden Steel

Steel is an alloy of iron used in construction, the manufacture of metal tools and other applications. In uses exposing it to a lot of wear and tear, the steel must be hardened and then tempered so it stands up to the level of usage it's intended for. With the right tools and a little practice, you'll be able to harden steel in no time.


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    Start with steel with sufficient carbon content. Hardening steel causes the structure of carbon to crystallize, similar to the way coal or graphite changes to diamond under the heat and pressure within the earth. Without other metals alloyed with it, steel to be hardened needs to have a carbon content of about 0.6 percent or higher. This kind of steel is called high-carbon steel or tool steel. (Steel alloys such as silver steel and gauge plate are also high-carbon steels.)
    • Medium carbon steels, with a carbon content of 0.4 to 0.55 percent, can also be hardened, but these steels need to have other metals alloyed with them so that the steel can be hardened more thoroughly.
    • Mild steels, with a carbon content of 0.4 percent or less, cannot be hardened directly. They can, however, be coated with other materials to make them harder in a process called case hardening.
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    Heat the steel. Heat the entire piece of steel slowly at first. Then, concentrate the heat on the area that is to be hardened, such as a chisel point or screwdriver blade tip, until that area glows red hot.
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    Quench the steel in a fluid. Dipping the hot steel into a liquid or gas rapidly cools it, hardening the metal. Several fluids may be used as a quench.
    • Oil is the most commonly used quench liquid especially if you are unsure of the exact chemical composition of your steel. Common oils used are vegetable, mineral, cottonseed or whale oil. Oil cools the steel more slowly than water which makes it less prone to cracking but hardens just as much as water. Complex steels such as most modern steels with several alloys tend to crack if quenched to quickly such as with water. If bubbles are allowed to form, they can slow the cooling process and create soft spots in the metal, so the quenching liquid is usually agitated to prevent bubbles from forming. Quenching oils may cause fumes, spatters and spills and do constitute a possible fire hazard if not monitored properly.
    • Salt water quenches hot steel faster than plain water, because the salt makes bubbles pop faster. However, it must be rinsed off immediately, because it will corrode steel faster than fresh water.
    • Glycol polymers mixed with water provide an intermediate rate of quenching between water and oil, with the rate of cooling dependent on the amount of glycol. They corrode the steel less than water and are less likely to catch fire than oil, but the ratio of polymer to water must be monitored constantly to ensure consistent results.
    • Cryogenic quenches are used to prevent soft and brittle spots from forming and forcing the steel to become more thoroughly hardened. These quenches are more likely to be used with high-carbon than medium-carbon steels.
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    Clean the steel. This removes the quenching fluid and prepares the steel for tempering. If a liquid other than water was used to quench the steel, water may be used to rinse the steel off. If not, a light abrasive, such as emery cloth, may be used.
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    Heat the steel again, but not as much. This second heating tempers the steel to take away the brittleness created by the hardening process. The steel may be heated in a furnace or a bath of oil, sodium and potassium nitrate mixture or lead. The temperature to which the steel is heated during tempering determines how hard the tempered steel will be; the higher the temperature, the softer but tougher it will be.
    • During the tempering process, the heated steel shows surface oxide colors according to how hot it is. Colors start with light straw at about 200 °C (392 °F) through to purple at about 300 °C (572 °F). These colors are used as a guide for the tempering temperature according to how the steel will be used. Alloyed steels require higher temperatures to reach a particular color than carbon steels, however.
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    Let the steel cool. This time, unlike the rapid cooling during quenching, the steel can be cooled either quickly or slowly, depending on the desired properties of the finished steel. Steel alloys subject to becoming brittle again after tempering should only be cooled slowly, however.


  • Holes drilled in steel before heating and tempering should always be drilled through completely and then plugged with asbestos wool to prevent cracking.

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Categories: Metalwork and Wire Projects