Heating metal affects its strength, but the outcome depends on the metal’s composition, the temperature reached, and the duration of heating. While metal generally loses strength when hot, controlled heating and cooling processes are frequently used in manufacturing to deliberately change a metal’s permanent properties. Understanding how heat interacts with the metal’s internal atomic structure is key to predicting how its strength will be affected.
How High Temperature Affects Immediate Strength
The most immediate and temporary effect of applying high heat is a rapid reduction in strength, known as thermal softening. As the temperature rises, atoms gain kinetic energy and vibrate more intensely. This increased atomic vibration makes it easier for internal defects in the crystal structure, called dislocations, to move through the material.
The movement of dislocations is the mechanism of plastic deformation, making the metal significantly more pliable and less resistant to being shaped. This temporary weakening allows materials to be forged or hot-rolled, as yield strength and tensile strength drop dramatically while the metal is hot. Once the metal cools, atomic vibrations decrease, and dislocation movement becomes restricted again, restoring the metal’s strength properties.
Structural Changes from Controlled Heating
Controlled heating, often called annealing, causes permanent changes to the metal’s internal structure, typically resulting in a softer, weaker material even after cooling. This heat treatment relieves internal stresses that accumulate during previous manufacturing steps, such as cold-rolling or welding. When heated, the internal energy allows atoms to rearrange into more stable configurations.
A key change is recrystallization, where deformed grains are replaced by new, strain-free grains. If heating continues, the new grains grow in size, a process known as grain growth. Since grain boundaries act as barriers to dislocation movement, the formation of fewer, larger grains reduces the material’s hardness and yield strength.
When Heat Treatments Increase Strength
Despite the general softening effect of heat, specific heating and cooling cycles are employed to make metal stronger and harder. This strengthening is often achieved through a two-step process, most commonly seen in the heat treatment of steel. The first step involves rapidly cooling the metal from a high temperature, a process called quenching. Quenching traps carbon atoms within the iron crystal structure, creating a hard but brittle state known as martensite.
Tempering and Precipitation Hardening
The quenched metal is then reheated to a lower temperature in a process called tempering. Tempering relieves some internal stress and reduces brittleness while retaining high strength. This process allows a controlled amount of carbon to precipitate out, balancing the material’s hardness and toughness for practical applications. A different mechanism, precipitation hardening, is used for non-ferrous alloys like certain aluminum and nickel alloys. Here, heating and controlled aging cause tiny, uniformly scattered particles to form, which effectively block the movement of dislocations and increase the metal’s strength.