Heat is a form of energy that, when absorbed by a metal, directly increases the kinetic energy of its constituent atoms. Within the orderly, repeating structure of a solid metal, known as a crystalline lattice, this energy manifests as more vigorous atomic vibration. The atoms begin to oscillate more forcefully around their fixed positions. This heightened motion is the underlying cause for the significant physical and chemical transformations metals undergo when exposed to increasing temperatures. Understanding this fundamental atomic response helps comprehend how thermal energy alters the metal’s dimensions, state, and physical properties.
Changes in Dimensions and State
The immediate consequence of increased atomic vibration is a change in the metal’s physical size, a phenomenon called thermal expansion. As the atoms oscillate more violently, the average distance between them increases, which forces the entire body of the metal to expand in length, width, and volume. Every metal possesses a unique material characteristic, known as the coefficient of thermal expansion, which dictates the degree of this dimensional change for a given temperature increase. For example, aluminum expands at roughly twice the rate of steel under the same conditions, a difference that engineers must account for when designing structures that combine different metals.
If heating continues past the solid state, the metal will eventually transition from a solid to a liquid at its melting point. This defined temperature is where the thermal energy supplied overcomes the interatomic forces holding the crystalline structure rigid. The solid and liquid phases can coexist in equilibrium, and further heat input breaks the remaining bonds rather than raising the temperature. The temperature remains constant until the entire mass is converted into a molten liquid.
A final, much higher energy state is reached if the liquid metal is heated further to its boiling point, leading to vaporization. The boiling point is the temperature at which the liquid metal’s vapor pressure equals the pressure of the surrounding atmosphere, causing the liquid to convert into a gaseous state. Because of the immense energy required to force the atoms completely apart and into a vapor, the boiling points of most structural metals are exceptionally high; for instance, iron boils at around 2,870°C.
Alterations to Electrical and Mechanical Properties
While a metal remains in its solid state, the increased thermal agitation profoundly impacts its ability to conduct electricity. Electrical current relies on the smooth flow of free-moving electrons through the metal’s crystal lattice. When the metal is heated, the vibrating atoms act as obstacles, scattering these flowing electrons. This scattering effect increases the metal’s electrical resistance, consequently lowering its overall electrical conductivity.
Metals exhibit a positive temperature coefficient of resistance, where resistance rises proportionally with temperature. Thermal energy reduces the mean free path of the electrons, making it harder for them to move through the material without collision. This change in resistance is so predictable that it is used as the foundational principle for devices like resistance thermometers.
In terms of structural integrity, heating a metal causes a significant reduction in its mechanical strength. The thermal energy weakens the bonds within the crystalline structure, which manifests as a noticeable reduction in hardness and yield strength; the metal effectively softens. This softening makes the metal easier to permanently deform under stress than at room temperature. For components operating under continuous load at high temperatures, “creep” becomes a major concern.
Creep is defined as the slow, permanent, time-dependent deformation of a solid material under a stress level that is below its yield strength. This process involves the thermally activated movement and rearrangement of atoms and crystal defects, such as dislocations, which permits the metal to gradually stretch or distort over time. Creep is typically most pronounced at temperatures exceeding about 40 to 50 percent of the metal’s absolute melting temperature, representing a significant limitation in the design of high-temperature machinery.
Chemical Reactions with the Environment
The introduction of heat dramatically accelerates the chemical reactivity of a metal, particularly with atmospheric gases. Heat provides the activation energy necessary to initiate and sustain reactions that would occur very slowly at room temperature. The most common reaction is oxidation, which involves the metal combining with oxygen to form a metal oxide layer on the surface.
At lower temperatures, this surface reaction is often referred to as tarnishing, which typically results in a thin, self-limiting film that can protect the underlying metal from further corrosion. However, at much higher temperatures, the rate of oxidation increases substantially, leading to the formation of a much thicker, non-protective layer called scaling.
Scaling is a common issue in industrial processes like hot rolling, where the metal is heated to over 1,000°C. The high temperature causes the oxide layer to grow rapidly, often creating a thick, flaking, and undesirable surface layer composed of various oxides. The presence of other elements in the environment, such as sulfur, also leads to accelerated corrosion through the formation of metal sulfides. This sulfurization reaction commonly occurs alongside oxidation at high temperatures, further degrading the metal’s surface properties.