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, sometimes due to changes in the chemical composition of the material.
These treatments include heating metal alloys to (usually) extreme temperatures and then cooling them 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 metal alloys most commonly used in CNC machining. By describing the impact of these processes on the final part properties, this article will help you choose the right materials for your application.
When to conduct heat treatment
Heat treatment can be applied to metal alloys throughout the manufacturing process. For CNC machined parts, heat treatment is generally applicable to:
Before CNC processing: When ready standard metal alloys are required, CNC service providers will directly process parts from stock 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, heat treatment is carried out after CNC processing, because high hardness will reduce the machinability of materials. For example, this is the standard practice for CNC machining tool steel parts.
Common heat treatment of CNC materials: annealing, stress relief and tempering
Annealing, tempering, and stress relieving all involve heating a 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 in which it is manufactured.
During the annealing process, 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 machinability. If no other heat treatment is specified, most CNC machined parts will have material properties in the annealed state.
Stress relief includes heating parts to high temperature (but lower than annealing), which is usually used after CNC machining to eliminate residual stress generated during manufacturing. In this way, parts with more consistent mechanical properties can be produced.
Tempering also heats parts at a temperature lower than annealing temperature, usually used after quenching of low carbon steel (1045 and A36) and alloy steel (4140 and 4240) to reduce their brittleness and improve their mechanical properties.
quench
Quenching involves heating the metal to a very high temperature and then rapidly cooling it, 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 materials are heated, resulting in extremely high hardness of parts.
Parts are usually quenched as the last step of the manufacturing process after CNC processing (think of the blacksmith immersing the blade in oil), because the increase in hardness makes the material more difficult to process.
Tool steel is quenched after CNC machining to obtain extremely high surface hardness characteristics. The resulting hardness can then be controlled using the tempering process. For example, tool steel A2 has a hardness of 63-65 Rockwell C after quenching, but 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 result can be obtained when the hardness is 56-58 HRC).
Precipitation hardening
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 for a long time (aging). This leads to the dissolution of alloy elements in the form of discrete particles of different components and their uniform distribution in the metal matrix, just as sugar crystals dissolve in water when heating the solution.
After precipitation hardening, the strength and hardness of metal alloys increase rapidly. For example, 7075 is an aluminum alloy, which is usually used in aerospace industry to manufacture parts with tensile strength equivalent to stainless steel, and its weight is less than 3 times. The following table illustrates the effect of precipitation hardening in aluminium 7075:
Not all metals can be heat treated in this way, but compatible materials are considered 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 underlined material remains soft. This is usually preferable to increasing part hardness throughout the volume (e.g. by quenching), as harder parts are also more brittle.
Carburizing is the most common case hardening heat treatment. It includes 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 alloys.
How to specify heat treatment in your order:
When you place a CNC order, you can request heat treatment in three ways:
Refer to manufacturing standards: many heat treatments are standardized and widely used. For example, T6 indicators in aluminum alloys (6061-T6, 7075-T6, etc.) indicate that the material has precipitation hardened.
Specify the required hardness: This is a common method for specifying the heat treatment and case hardening of tool steels. This will explain to the manufacturer the heat treatment required after CNC machining. For example, for D2 tool steel, a hardness of 56-58 HRC is usually required.
Specify the heat treatment cycle: when the details of the required heat treatment are known, these details can be communicated to the supplier when placing an order. This allows you to modify the application's material properties specifically. Of course, this requires advanced metallurgical knowledge.
Rule of thumb
1. You can specify the heat treatment in the CNC processing order by referring to specific materials, providing hardness requirements or describing the treatment cycle.
2. Select precipitation hardening alloys (such as Al 6061-T6, Al 7075-T6 and SS 17-4) for the most demanding applications because they have very high strength and hardness.
3. When it is necessary to improve the hardness within the whole part volume, quenching is preferred, and only surface hardening (carburizing) is carried out on the part surface to increase the hardness.
