There are many ways to change the way a metal behaves and responds to precision machining. One of these methods is heat treatment. Heat treatment can be done before the part to make the material easier to process, or before the final hardening and heating stage. Heat treatments can affect many different aspects of metals, including strength, hardness, toughness, workability, formability, ductility and elasticity. It can also affect the physical and mechanical properties of a metal to change its use or alter its future work. Here, we take a closer look at the different types of heat treatments and how they affect parts during precision machining.

Quenching
In a heat treatment to harden metal, the metal is heated to a temperature that causes the elements in the metal to become a solution. Prior to this, defects in the metal lattice structure were the main source of defects. Heat treatment addresses these imperfections, placing the metal in a solution and strengthening it with tiny particles. Once the metal is heated thoroughly to the right temperature to create a solid solution, it is quickly quenched to trap particles in the solution.

In precipitation hardening, impurity particles are added to a metal alloy to further increase strength.

annealing
Annealing is a heat treatment method in which a metal such as aluminum, copper, steel, silver or brass is heated to a specific temperature, held at that temperature for a period of time to allow it to transform, and then air cooled. This process increases the ductility of the metal and reduces its hardness, making it easier to work. Copper, silver and brass can be cooled quickly or slowly, whereas ferrous metals such as steel must always be cooled gradually for annealing to occur. Annealing can be used before working on a metal to improve its stability, making harder materials less likely to crack or break.

Normalizing
Normalizing is an annealing process for steel in which the steel is heated to a temperature 150-200°F higher than annealing and held at the critical temperature long enough for transformation to occur. Steel treated in this way must be air cooled. The heat treatment of normalizing results in smaller austenite grains, while air cooling produces finer ferrite grains. This process improves the steel’s workability, ductility and strength. Standardization also helps remove columnar grains and dendrite segregations that can occur during the casting process of the part.

Tempering
Tempering is a heat treatment method used to increase the elasticity of iron-based alloys such as steel. Iron-based metals are very hard, but they are usually too brittle to be useful for most purposes. Tempering can be used to change the hardness, ductility, and strength of a metal, which often makes it easier to work. The metal will be heated to a temperature below the critical point because lower temperatures reduce brittleness while maintaining hardness. If you want to increase plasticity and decrease hardness and strength, higher temperatures are required.