Heat Treating

There are many ways to alter the ways metals perform and react to precision machining. One of these methods is heat treating. Heat treating can be applied to the part before to make the material more machinable, or the components may be machined before the final hardening and heating stages. Heat treating can affect a number of different aspects of the metal including strength, hardness, toughness, machinability, formability, ductility, and elasticity. It can also affect the physical and mechanical properties of metal to change the use of the metal or alter future work on the metal. Here we take a closer look at the different types of heat treating and how these affect the parts during precision machining.

5 Types of Heat Treating & Their Purpose in Precision Machining

Hardening
 

In heat treatment to harden a metal, the metal is heated to a temperature at which the elements of the metal go into solution. By bringing metal into a reliable fine particle solution to strengthen the metal, heat treting addresses these deficiencies. When the metal is heated to a high temperature, it is rapidly cooled.Previously, defects in the metal's crystal lattice structure are the primary source of "give" or plasticity.  Precipitation hardening adds impurity particles to the metal alloy to further increase strength.  

  Hardening 

With case hardening, the outer metal layer becomes hard while the inner metal remains soft. Iron and steel have low carbon content, they need additional carbon to the surface. Case hardening is a process that is often used as the last step after the part has already been machined. Chemicals, high temperatures and other elements are used to create a hardened outer layer. Hardening can be useful for metal applications that need a flexible metal with a durable wear layer. 
 

 Annealing 

Annealing is a heat treatment method in which metals such as aluminium, copper, steel, silver or brass are heated to a specified temperature, held at that temperature for a period of time to allow them to transform, and then annealed with air chilled This process increases the ductility of the metal and reduces the hardness to make it more machinable. Copper, silver and brass can be cooled quickly or slowly, while ferrous metals such as steel must always be cooled gradually for annealing to take place. Annealing can be used prior to machining metal to improve its stability, making harder materials less likely to crack or break.

Normalizing

Normalizing is an annealing process for steel where it is heated 150-200°F higher than in annealing and held at the critical temperature long enough for the transformation to occur. Steel treated in this way must be air cooled. The heat treating in normalization causes smaller austenitic grains, while air cooling produces more refined ferritic grains. This process improves machinability, ductility, and strength of the steel. Standardization is also useful to remove columnar grains and dendritic segregation that can occur during the casting of a part.

Tempering

Tempering is a method of heat treating used to increase the resilience of iron-based alloys like steel. Iron-based metals are very hard, but they are often too brittle to be useful for most purposes. Tempering can be used to change the hardness, ductility, and strength of metal, which usually makes it easier to machine. The metal will be heated to a temperature below the critical point as lower temperatures reduce brittleness while maintaining hardness. For increased plasticity with less hardness and strength, higher temperatures are required.

Another option is to purchase material that has been hardened or to harden the material before machining. While this makes it more difficult to machine, it eliminates the risk for the part sizes changing, unlike a post-machining heat treatment process. It can also eliminate the need for a grinding shop to get the tight finishes or tolerances.

The heat treatment can be an essential part of the precision machining process to transform metals and ensure your pieces and parts perform as you need them to.

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