Often when we speak of heat, it concerns scenarios we’d rather avoid: getting sweaty and sunburnt from the summer sun, overheating laptops or car engines, maybe singeing your hair after leaning too close to a candle. And rightly so! In all these cases heat can be damaging, whether to your possessions or your own self.
Yet despite some risks, heat is greatly beneficial. The sun’s beams feel warm and pleasant after a cold winter. The flames of a stove burner or grill cook your food. And when it comes to metal, heat is no different. Too much heat can damage metal to the point of weakening or destroying it. But heat is also a transformative force, from forging the metal to hardening it and modifying its mechanical properties.
There are four ways the application of heat affects metal:
- Electrical Resistance
- Thermal Expansion
Heat is a crucial part of the metal making process, with furnaces heated to such high temperatures that metal ore turns molten. However, it’s not always necessary to go to such extremes to affect the properties of the metal. Any time metal is heated, the atoms making up the metal or alloy begin to move. Given enough heat and then carefully cooled, the crystal structure of the metal can be reshaped into a lattice that is much stronger than prior to heat treatment. In the case of high carbon steels, heat application allows the iron to absorb additional carbon, which produces an exceptionally hard and strong steel.
As metal is heated, the electrons within it will absorb energy and move faster. The faster the electrons move, the more likely they are to collide with the metal’s atoms and scatter them. Electrical resistance measures how strongly the metal works against this potential scattering as the temperature rises. That means the higher the electrical resistance of a metal, the lower its conductivity.
There are three elements with naturally magnetic properties, known as ferromagnetic metals: iron, cobalt, and nickel. But once heat is applied – and especially as the temperature goes up – the metal’s natural magnetism is reduced. If heated to a certain point known as the metal’s Curie temperature, its magnetic property will be reduced completely.
As metal is heated, it begins to expand; its volume, length and overall surface area will grow as it continues to absorb more heat. This is due to the atoms making up the metal, which increase their movement as the temperature rises. This movement creates vibrations within the metal’s structure to the point that it swells, an occurrence which is known as thermal expansion. For safety reasons, any metal structure such as bridges or buildings must be designed to accommodate a certain degree of thermal expansion and contraction. Otherwise, you run the risk of damaging the metal as it warms and cools.