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In Metalworking, what is Critical Temperature?

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  • Written By: Patrick Roland
  • Edited By: A. Joseph
  • Last Modified Date: 18 November 2016
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Metalworking often focuses on the benefits of heating material to extreme temperatures for pliability, but rarely are the advantages of extreme cooling considered. By cooling a metal to a very low temperature called its critical temperature, an electrical phenomenon called superconductivity can be observed. This method is an important advance in electrical work and has been utilized with a variety of metals, but aluminum and steel generally are the most common.

A metal's critical temperature differs from substance to substance and, for purposes of conductivity, might not be possible to reach. Generally, metals must be cooled to temperatures around 0 degrees Kelvin (minus-459 Fahrenheit, minus -273 Celsius) using liquid nitrogen until a noticeable phase change occurs. The change involves a nonexistent electrical resistance, also referred to as becoming a superconductor. This allows for energy to pass more easily than through traditional wiring.

Superconductivity usually is the purpose of the critical temperature process. When a metal is cooled to this critical temperature, research has shown it to be a better conductor than wires at room temperature. There is no electrical resistance, so electrons can pass freely through this metal, resulting in nearly no energy lost through heat. Superconductor loops using metals cooled to a critical temperature can last several years with practically no deterioration, compared to traditional systems that must be replaced frequently because of heat.

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Aluminum is considered an excellent metal to be used with critical temperature superconductivity. Its light weight and malleability make it a prime choice for wires and other materials used in conducting electricity. Aluminum often is used in industries needing to pass large amounts of energy, such as a power plant or a large factory.

Steel and its many alloys have been found to be another type of metal that handles this treatment well. The critical temperature of steel is useful in more ways than simply conducting electricity. Isothermal annealing is a process created to control the metal's rate of temperature changes, also called a temperature gradient, that has a particular piece of steel cooled to just above the critical temperature, then lowered below that point and brought back up. Quenching is another steel critical temperature process that does not involve superconductivity or liquid nitrogen, but the metal instead is cooled to that point in water, oil or brine in order to increase its carbon content.

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