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How does heating metal affect its properties?

Posted August 14th 2017

Applying heat to different metals can have a dramatic effect on them and can completely alter their structural, magnetic and electrical properties. There are several methods that can be used to change metals through heat, in order to enhance more favourable qualities, and the varying methods that are used will depend upon the metal and the desired result.

Thermal Expansion

Heating metal can increase its volume, length and surface area, as the heat displaces atoms from their usual position which alters the structure. This is known as thermal expansion and the amount of growth depends on the metal. Examples of this can be seen in everyday life when things such as pipework in bathrooms and the plumbing of houses expand and contract in hotter and cooler months. A common side effect of this is burst pipes.

Magnetism

Iron, cobalt and nickel are all naturally magnetic materials, or ferromagnetic materials. When heat is applied to them it can reduce their natural magnetic properties to a point so low that it is completely gone. This point, which is different for every metal, is known as the Curie temperature. For cobalt this is 1110 degrees Celsius whereas Nickel is much lower at only 330 degrees Celsius.

Resistance

Some metals are able to effectively reduce, or halt, the flow of an electric current. This is known as resistance and how resistant a metal is depends on how quickly electrons are able to pass through it. When metal is heated, electrons can gather energy more quickly which allows them to move faster and thus increases the level of resistance as they are more likely to scatter and collide. Similarly, a drop in temperature can result in a drop in resistance as the electrons move more slowly.

Different heat treatments include the processes of annealing, normalising, hardening and tempering. These are used to alter the properties of various metals and gain an end result better suited to the intended use of the metal. The aim may be to strengthen, soften, increase ductility or provide uniformity to name a few.

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