Heat treatment can be applied to many metal alloys to significantly improve key physical properties such as hardness, strength, or machinability. These changes are due to changes in the microstructure and sometimes due to changes in the chemical composition of the material.
These treatments include heating the metal alloy to (usually) extreme temperatures followed by cooling under controlled conditions. The temperature to which the material is heated, the time to maintain the temperature and the cooling rate will greatly affect the final physical properties of the metal alloy.
In this paper, we review the heat treatment related to the most commonly used metal alloys in CNC machining. By describing the impact of these processes on the final part properties, this article will help you choose the right material for your application.
When will the heat treatment be carried out
Heat treatment can be applied to metal alloys throughout the manufacturing process. For CNC machined parts, heat treatment is generally applicable to:
Before CNC machining: when it is required to provide ready-made standard grade metal alloys, CNC service providers will directly process parts from inventory materials. This is usually the best choice to shorten the lead time.
After CNC machining: some heat treatments significantly increase the hardness of the material, or are used as finishing steps after forming. In these cases, the heat treatment is performed after CNC machining, because high hardness reduces the machinability of the material. For example, this is the standard practice when CNC machining tool steel parts.
Common heat treatment of CNC materials: annealing, stress relief and tempering
Annealing, tempering and stress relief all involve heating the metal alloy to a high temperature and then slowly cooling the material, usually in air or in an oven. They differ in the temperature at which the material is heated and in the order of the manufacturing process.
During annealing, the metal is heated to a very high temperature and then slowly cooled to obtain the desired microstructure. Annealing is usually applied to all metal alloys after forming and before any further processing to soften them and improve their workability. If no other heat treatment is specified, most CNC machined parts will have material properties in the annealed state.
Stress relief includes heating the parts to a high temperature (but lower than annealing), which is usually used after CNC machining to eliminate the residual stress generated in the manufacturing process. This can produce parts with more consistent mechanical properties.
Tempering also heats parts at a temperature lower than the annealing temperature. It is usually used after quenching of low carbon steel (1045 and A36) and alloy steel (4140 and 4240) to reduce its brittleness and improve its mechanical properties.
Quenching involves heating the metal to a very high temperature, followed by rapid cooling, usually by immersing the material in oil or water or exposing it to a cold air stream. Rapid cooling "locks" the microstructure changes that occur when the material is heated, resulting in extremely high hardness of the parts.
Parts are usually quenched after CNC machining as the last step of the manufacturing process (think of blacksmith immersing the blade in oil), because the increase in hardness makes the material more difficult to process.
Tool steels are quenched after CNC machining to obtain extremely high surface hardness characteristics. The resulting hardness can then be controlled using a tempering process. For example, the hardness of tool steel A2 after quenching is 63-65 Rockwell C, but it can be tempered to a hardness between 42-62 HRC. Tempering can prolong the service life of parts because tempering can reduce brittleness (the best results can be obtained when the hardness is 56-58 HRC).
Precipitation hardening (aging)
Precipitation hardening or aging are two terms commonly used to describe the same process. Precipitation hardening is a three-step process: first, the material is heated to a high temperature, then quenched, and finally heated to a low temperature (aging) for a long time. This leads to the dissolution and uniform distribution of alloying elements initially in the form of discrete particles of different compositions in the metal matrix, just as sugar crystals dissolve in water when the solution is heated.
After precipitation hardening, the strength and hardness of the metal alloy increase sharply. For example, 7075 is an aluminum alloy, which is usually used in the aerospace industry to manufacture parts with tensile strength equivalent to that of stainless steel, and its weight is less than 3 times. The following table illustrates the effect of precipitation hardening in aluminum 7075:
Not all metals can be heat treated in this way, but compatible materials are considered as superalloys and are suitable for very high performance applications. The most common precipitation hardening alloys used in CNC are summarized as follows:
Case hardening and carburizing
Case hardening is a series of heat treatment, which can make the surface of parts have high hardness while the underlining material remains soft. This is generally better than increasing the hardness of the part over the entire volume (e.g., by quenching) because the harder part is also more brittle.
Carburizing is the most common case hardening heat treatment. It involves heating low carbon steel in a carbon rich environment and then quenching the parts to lock the carbon in the metal matrix. This increases the surface hardness of steel, just as anodizing increases the surface hardness of aluminum alloy.