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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.

CHANGES IN MECHANICAL PROPERTIES

Heating metals can also affect their mechanical properties, including hardness, strength, and ductility. These changes are primarily attributed to the alteration of the microstructure within the metal. The microstructure of a metal consists of grains, grain boundaries, and defects. When a metal is heated, several important processes occur:

1. ANNEALING

Annealing is a heat treatment process that involves heating a metal to a specific temperature and then slowly cooling it. This process relieves internal stresses, improves ductility, and reduces hardness. It also allows for the formation of new, more equiaxed grains, which can enhance the metal's mechanical properties.

2. TEMPERING

Tempering is another heat treatment technique that follows processes like hardening or quenching. It involves reheating the metal to a lower temperature and holding it for a specific time. Tempering reduces the hardness and brittleness of hardened metals, making them tougher and more ductile. The temperature and duration of tempering can be adjusted to achieve the desired balance of strength and ductility.

3. HARDENING

Contrary to annealing and tempering, hardening involves heating the metal to a high temperature followed by rapid cooling, known as quenching. This process results in the formation of a hard and brittle microstructure. Hardened metals are strong but lack ductility. However, they can be tempered to regain some ductility while maintaining high strength.

4. ELECTRICAL CONDUCTIVITY

The electrical conductivity of a metal can be significantly affected by heating. In general, heating metals increases their electrical resistance. This phenomenon can be explained by considering the movement of electrons within the metal lattice. As temperature rises, the increased thermal vibrations of atoms impede the flow of electrons, leading to higher resistance.

In certain applications, such as in electrical circuits and conductors, it is crucial to consider the effect of temperature on a metal's conductivity. Engineers must account for changes in resistance to ensure the proper functioning of electrical components over a range of operating temperatures.

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.

THERMAL CONDUCTIVITY

Thermal conductivity is the ability of a material to conduct heat. Metals are known for their high thermal conductivity, making them excellent conductors of heat. However, heating metals can affect their thermal conductivity to some extent. As metals are heated, lattice vibrations increase, which can hinder the flow of heat energy. This results in a slight decrease in thermal conductivity at elevated temperatures.

The effect of temperature on thermal conductivity is relatively small for most metals, but it is a crucial consideration in applications where heat dissipation is critical, such as in heat sinks for electronic devices.

CORROSION RESISTANCE

Corrosion is a natural process that can deteriorate the surface of metals over time. Heating metals can influence their corrosion resistance in several ways. For example, when a metal is heated, its microstructure may change, affecting the exposure of its surface to corrosive environments. Additionally, the formation of oxide layers on the metal's surface can be influenced by temperature.

In some cases, heat treatment processes like annealing can improve a metal's resistance to corrosion by promoting the formation of a more protective oxide layer. Understanding how heating affects corrosion can be crucial in selecting the right materials for applications exposed to aggressive environments.

GRAIN GROWTH

During the heating of metals, particularly during annealing processes, grain growth can occur. This is a phenomenon where individual grains in the metal structure grow larger over time. Grain growth can lead to changes in the mechanical properties of the metal. For example, larger grains may result in reduced strength and increased ductility.

Engineers and metallurgists carefully control annealing processes to achieve specific grain sizes that optimise the desired mechanical properties for a given application.

PHASE TRANSFORMATIONS

Heating metals can induce phase transformations, which involve changes in the arrangement of atoms within the crystal lattice. Phase transformations can result in the formation of new crystalline structures with distinct properties. One common phase transformation is from the alpha phase to the beta phase, as seen in some alloys.

Understanding these phase transformations is crucial in materials science and metallurgy for tailoring the properties of metals to meet specific requirements.

CONCLUSION

Heating metals can have a profound impact on their properties, ranging from changes in mechanical strength and electrical conductivity to alterations in magnetic behaviour and corrosion resistance. These effects are a result of complex interactions at the atomic and microstructural levels.

 

Metallurgists and materials scientists leverage their understanding of these phenomena to design and engineer materials that can withstand extreme conditions, provide optimal performance, and meet the demands of various industries. Whether it's for constructing durable buildings, manufacturing advanced electronics, or producing high-performance alloys, the ability to control and manipulate metal properties through heating processes is a fundamental aspect of modern materials science and engineering.