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Heat treatments for CNC machined parts

Written by Alkaios Bournias Varotsis



Heat treatments for CNC machined parts


Heat treatments can be applied to many metal alloys to drastically improve key physical properties (for example hardness, strength or machinability). These changes happen due to modifications to the microstructure and, sometimes, the chemical composition of the material.

Those treatments involve the heating of the metal alloys to (usually) extreme temperatures, followed by a cooling step under controlled conditions. The temperature the material is heated to, the time it is kept at that temperature and the cooling rate all greatly affect the final physical properties of the metal alloy.

In this article, we reviewed the heat treatments that are relevant to the most commonly used metal alloys in CNC machining. By describing the effect of these processes to properties of the final part, this article will help you choose the right material for your applications.

For guidelines on designing parts for CNC machining and for useful tips to help you stay on budget, you can refer to our previous articles here and here.

When are heat treatments applied

Heat treatments can be applied to metal alloys throughout the manufacturing process. For CNC machined parts, heat treatments are typically applied either:

Before CNC machining: When a standardized grade of a metal alloy is requested that is readily available, the CNC service provider will machine the parts directly from that stock material. This is often the best option for reducing lead times.

After CNC machining: Some heat treatments significantly increase the hardness of the material or are used as a finishing step after forming. In these cases, the heat treatment is applied after CNC machining, as high hardness reduces the machinability of a material. For example, this is standard practice when CNC machining tool steel parts.

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Common heat treatments for CNC materials

Annealing, stress relieving & tempering

Annealing, tempering and stress relieving all involve the heating of the metal alloy to a high temperature and the subsequent cooling of the material at a slow rate, usually in air or in the oven. They differ in the temperature that the material is heated to and in the order in the manufacturing process.

In annealing, the metal is heated to a very high temperature and then slowly cooled to achieve the desired microstructure. Annealing is usually applied to all metal alloys after forming and prior to any further processing to soften them and improve their machinability. If another heat treatment is not specified, most CNC machined parts will have the material properties of the annealed state.

Stress relieving involves the heating of the part to a high temperature (but lower than annealing) and is usually employed after CNC machining, to eliminate the residual stresses created from the manufacturing process. This way parts with more consistent mechanical properties are produced.

Tempering also heats the part at a temperature lower than annealing, and it usually employed after quenching (see next section) of mild steels (1045 and A36) and alloy steels (4140 and 4240) to reduce their brittleness and improve their mechanical performance.

Heat treatment Purpose Compatible materials
Annealing To improve the machinability of the metal alloy All metal alloys
Stress relieving To release residual stresses from mechanical deformation or temperature increase during processing All metal alloys
Tempering To reduce brittleness after quenching Mild steels (1045, A36)
Alloy steels (4140, 4240)
Tool steels (A2)


Quenching involves the heating of the metal to a very high temperature, followed by a rapid cooling step, usually by dipping the material in oil or water or exposing to a stream of cool air. Rapid cooling "locks-in" the changes in the microstructure that the material undergoes when heated up, resulting in parts with very high hardness.

Parts are usually quenched as a final step in the manufacturing process after CNC machining (think of blacksmiths dipping their blades in oil), as increased hardness makes the material more difficult to machine.

Tool steels are quenched after CNC machining to achieve their very high surface hardness properties. A tempering process may then used to control the resulting hardness. For example, Tool steel A2 has a hardness of 63-65 Rockwell C after quenching but can be tempered to a hardness ranging between 42 to 62 HRC. Tempering prolongs the service life of the part, as it reduces brittleness (best results are achieved for a hardness of 56-58 HRC).

Heat treatment Purpose Compatible materials
Quenching To increase the hardness of the metal alloy Mild steels (1045, A36)
Alloy steels (4140, 4240)
Tool steels (D2, A2, O1)

Precipitation hardening (aging)

Precipitation hardening or aging are two terms that commonly used to describe the same process. Precipitation hardening is a three step process: the material is first heated a high temperature, then quenched and finally heated to a lower temperature for a long period of time (aged). This causes the alloy elements that initially appear as discrete particles of different composition to disolve and distribute uniformly in the metal matrix, in a simillar way that sugar crystal disolve in water when the solution is heated.

After precipitation hardening, the strength and hardness of the metal alloys increase drastically. For example, 7075 is an aluminum alloy, commonly used in the aerospace industry, to manufacture parts of tensile strength comparable to stainless steel, while having less than 3 times the weight. The effects of precipitation hardening in aluminum 7075 are illustrated in the next table:

Aluminum 7075-0(tempered) Aluminum 7075-T6(precipitation hardened)
Ultimate Tensile Strength 280 MPa 510–540 MPa
Yield Strength 140 MPa 430–480 MPa
Elongation at Break 9-10% 5–11%
Hardness (HV) 68 175
Source: Matweb Not all metals can be heat treated with this method, but the compatible materials are considered superalloys and are suitable for very high-performance applications. The most common precipitation hardening alloys used in CNC are summarized below:
Heat treatment Purpose Compatible materials
Precipitation hardening (aging) To increase the hardness & strength of the metal alloy Aluminum alloys (6061-T6, 6068-T6, 7075-T6)
Stainless steel (17-4)

Case Hardening & carburizing

Case hardening is a family of heat treatments that result in parts with high hardness on their surface, while the underline materials remain soft. This is often preferred over increasing the hardness of the part throughout its volume (for example, by quenching), as harder parts are also more brittle.

Carburizing is the most common case-hardening heat treatment. It involves the heating of mild steels in a carbon-rich environment and the subsequent quenching of the part to lock the carbon in the metal matrix. This increases the surface hardness of steels in a similar way that anodizing increases the surface hardness of aluminum alloys.

Heat treatment Purpose Compatible materials
Case hardening & carburizing To increase the surface hardness of the metal part, while keeping the core soft Mild steels (1018, A36)

How to specify a heat treatment in your order

When you place a CNC order, there are three ways to request a heat treatment:

Give a reference to a manufacturing standard: Many heat treatments are standardized and widely used. For example, the T6 indicator in aluminum alloys (6061-T6, 7075-T6 and so on) shows that the material has been precipitation hardened.

Specify the required hardness: This is a common method of specifying a heat treatment for tool steels and case hardening. This will indicate to the manufacturer the heat treatment he needs to perform after CNC machining. For example, for D2 tool steel, it is common to request a hardness of 56-58 HRC.

Specify the heat treatment cycle: When the specifics of the required heat treatment are known, then these can be communicated to the supplier when placing the order. This way you can specifically modify the material properties for your application. Of course, this requires advanced metallurgical knowledge.

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Rules of Thumb

  • You can specify a heat treatment in your CNC machining order by referencing a specific material, by providing a hardness requirement, or by describing a treatment cycle.
  • Select a precipitations hardening alloy (for example Al 6061-T6, Al 7075-T6 and SS 17-4) for the most demanding applications, as they have a very high strength and hardness.
  • Prefer quenching when an increased hardness throughout the volume of the part is required, and case hardening (carburizing) to increase the hardness only on the surface of the part.

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