by Jenny Knodell, IQS Editor
Many of the strongest metal products available today like knives, swords, train parts, die casting molds and musical instruments are made of extremely resistant, durable and tough materials. Although metals like steel and iron are the best metals for the job, sometimes they need a little help to obtain the desired hardness and strength properties for the tough jobs that the automotive, weapons, transportation and military industries have in mind. In order to make these metals even stronger than they already are, the products and parts are altered physically, mechanically and even chemically to obtain certain properties.
Because heat treating requires such high temperatures which use extreme amounts of energy, it is a very costly process that uses huge ovens and baths. Standard heat treating strengthens the metal’s internal structure which is a crystalline lattice pattern, bringing out finery in grains that have increased tensile strength. Aluminum, titanium and magnesium are all commonly heat treated, but the most common metal by far is steel, as it has a large amount of carbon. Carbon content is necessary for the hardening process to take place—the more carbon in the steel, the harder it may become.
There are a couple different methods of heat treatments. Most harden the metal, increasing their strength, wear and resistance properties, which is done by heating and rapid cooling. Case hardening is used to harden the surface of the metal by infusing nitride or cyanide, forming a thin layer of a harder alloy on the exterior of the metal. Only low carbon metals are used during this process. Tempering reduces the brittleness of the metal, which in turn toughens and sometimes increases flexibility. When heat followed by rapid cooling of the metal occurs, the martensite and bainite are transformed into ferrite and cementite, which are much less brittle. Glass may also be tempered to increase its strength and eliminate sharp shards if shattered. The rapid cooling process is done by rapidly reducing the air temperature, or by quenching, which involves bathing the metal part in a low temperature water, oil or salt bath.
Case Hardening Quenching
Heat treatments may also soften the metal, which increases both flexibility and ductility. The process is similar to hardening, but the rapid cooling via quenching is left out. When metals are slowly heated and then allowed to cool on their own back to room temperature, their properties are softened and they become much easier to shape. This is called annealing, and it is used when a part must undergo further secondary processing like stamping and shaping.