What happens when metals are heated?

Metal heating leads to several consequences at once, here are the main ones.

1.The metal expands thermally in all directions, i.e. its length, width and surface area increase. When a metal is heated, its atoms and molecules begin to move faster, interatomic bonds weaken, which leads to an increase in the distance between them and an increase in the volume of the metal. When cooled, the dimensions are restored.

2.The vast majority of metals and alloys increase their ductility with increasing temperature, including iron, steel, copper, aluminum and its alloys, magnesium, brass, and others. At the same time, these metals acquire the ability to be forged, i.e., to change shape without breaking under the influence of an external force. For example, steel heated to 700°C (1292°F) requires 4.5 times the forging force as steel heated to 1200°C (2192°F). Other metals and alloys, such as gray cast iron, tin bronze, and zinc alloys, do not deform when heated; they are brittle and fracture on impact.
Metal crystals usually have a regular structure, with atoms arranged in a certain order. However, when a metal is heated, the atoms become more mobile and the metal becomes more ductile. Heating a metal can also cause a change in the type of its crystal structure. Changing the structure will lead to a decrease or increase in ductility, because the type of structure has a decisive influence on the properties of metals. This explains why the effect of plasticity changes during heating is observed differently for different metals.

3.The thermal radiation of metals when heated causes the dark cherry glow of steel, which is noticeable already when heated to 550°C (1022°F), and at 850°C (1562°F) it turns into bright red, and then into orange (950°C, 1742°F), yellow (1000°C, 1832°F), and white (1300°C, 2372°F and above).
As you can see, the spectrum of thermal radiation depends on temperature, so observing the colors of quenching can be used to estimate the temperature of the metal, which is often used in heat treatment and forging, especially before the invention of non-contact thermometers. Names of glow colors: "red heat", "white heat", are often still used by metallurgists instead of determining the exact temperature.
The change in radiation color is due to an increase in the energies of internal interactions, excitation and relaxation of metal atoms, with increasing temperature. The higher the temperature, the more intense this radiation. Its spectrum is gradually enriched with short-wave radiation resulting from interactions with increased energy. Therefore, the main contribution of infrared radiation at low temperatures changes with increasing temperature to the visible light range and to ultraviolet radiation at very high temperatures.

4.An increase in temperature can cause oxidation of metal surface and the formation of an oxide layer on it. In the case of steel, such a layer can form a thin transparent metal film that remains when the temperature drops to room temperature. In this case, the surface of the metal acquires a rainbow colors. This is due to the fact that the surface is covered with a thin transparent layer and works like a mirror. When this layer is very thin, it reflects only certain colors from daylight, which is the result of interference.
Before the advent of pyrometers, this effect was also used as an indicator of the heating temperature of iron and steel. The colors were used to judge the heating temperature of steel chips and, consequently, the cutter during drilling and cutting operations. In modern times, it is used to create markings on the surfaces of ferrous metals and titanium by localized heating, including laser annealing.

5.If the steel is heated further, above 1300 °C (2372 °F, the specific temperature depends on the steel grade), melting of the metal can begin. Melting occurs because atoms acquire high energy and interatomic bonds are destroyed, and atoms lose their static positions in crystals and can travel around the material volume. This leads to the loss of the original shape and should be prevented during heat treatment of metals.
So we have briefly reviewed the obvious processes that occur with a metal when it is heated. However, changes in the properties of metals caused by heat treatment are due to a number of not so obvious reasons. These changes are due to changes in the metal structure, and we will discuss this in more detail.