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Secondary processing after CNC processing: heat treatment

August 8, 2022

When you finish the CNC machining of the parts, your work is not finished. These original components may have unsightly surfaces, may not be strong enough, or may only be part of one component, which must be connected with other components to form a complete product. After all, how often do you use equipment made up of individual parts?
The key point is that the post-processing process is necessary for a series of applications. Here we introduce some precautions to you so that you can choose the correct secondary operation for your project.

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In this three part series, we will introduce options and considerations for heat treatment process, surface treatment and hardware installation. Any or all of these may be required to transition your part from a machined state to a customer ready state. This article discusses heat treatment, while the second and third parts examine surface treatment and hardware installation.
In this three part series, we will introduce the heat treatment process, finishing and hardware installation options and considerations. Any or all of these may be necessary to change your part from a machined state to a customer ready state. This paper discusses heat treatment.
Heat treatment before or after processing?

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Heat treatment is the first operation to be considered after processing, and it can even be considered to process preheating materials. Why use one method instead of the other? The order in which heat treatment and machining metals are selected may affect the material characteristics, machining process and tolerances of the parts.
When you use materials that have been heat treated, this will affect your processing - the harder materials have a longer processing time and faster tool wear, which will increase the processing cost. Depending on the type of heat treatment applied and the depth below the affected surface of the material, it is also possible to cut off the hardened layer of the material and first destroy the purpose of using the hardened metal. The machining process may also generate enough heat to increase the hardness of the workpiece. Some materials, such as stainless steel, are more susceptible to work hardening during machining, and extra care is required to prevent this.

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However, there are some advantages in choosing metals that have been preheated. For hardened metals, your parts can maintain tighter tolerances, and it is easier to purchase materials because pre heat treated metals are readily available. Moreover, if the processing is completed, the heat treatment will add another time-consuming step in the production process.
On the other hand, heat treatment after machining enables you to better control the machining process. There are many types of heat treatment, and you can choose which type to use to obtain the required material properties. The heat treatment after machining can also ensure that the heat treatment effect of the part surface is consistent. For the materials that have been preheated, the heat treatment may only have a certain depth of influence on the materials, so the machining may remove the hardened materials in some places and not in other places.

As mentioned earlier, the post-processing heat treatment increases the cost and lead time because this process requires additional outsourcing steps. Heat treatment may also lead to warpage or deformation of parts, thus affecting the tight tolerance obtained during machining.

heat treatment
Generally, heat treatment will change the material properties of metals. In general, this means increasing the strength and hardness of the metal so that it can withstand more extreme applications. However, some heat treatment processes, such as annealing, actually reduce the hardness of the metal. Let's look at different heat treatment methods.

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Hardening is used to make metal harder. The higher hardness means that the metal is less likely to be dented or marked upon impact. Heat treatment also increases the tensile strength of the metal, which is the force of material failure and fracture. The higher strength makes the material more suitable for certain applications.

In order to harden the metal, the workpiece is heated to a specific temperature higher than the critical temperature of the metal, or a point at which its crystal structure and physical properties change. The metal is maintained at this temperature and then quenched and cooled in water, brine or oil. The quenching fluid depends on the specific alloy of the metal. Each quenchant has a unique cooling rate, so it is selected according to the cooling rate of the metal.

Surface hardening
Case hardening is a type of hardening that affects only the outer surface of a material. This process is usually completed after processing to form a durable outer layer.
The hardening depth can be changed by modifying the process parameters
Precipitation hardening
Precipitation hardening is a process for specific metals with specific alloying elements. These elements include copper, aluminum, phosphorus and titanium. When the material is heated for a long time, these elements precipitate in the solid metal or form solid particles. This will affect the grain structure and increase the strength of the material.

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As mentioned earlier, annealing is used to soften the metal, as well as to release stress and increase the ductility of the material. This process makes the metal easier to process.

To anneal the metal, the metal is slowly heated to a certain temperature (higher than the critical temperature of the material), then maintained at that temperature, and finally cooled very slowly. This slow cooling process is accomplished by burying the metal in the insulating material or holding it in the furnace as the furnace and the metal cool down.

Stress relief of large plate processing
Stress relief is similar to annealing, that is, the material is heated to a certain temperature and cooled slowly. However, in the case of stress relief, the temperature is lower than the critical temperature. The material is then air cooled.
This process can eliminate the stress caused by cold working or shearing, but does not significantly change the physical properties of the metal. Although the physical properties do not change, eliminating this stress during further processing or part use helps to avoid dimensional changes (or warpage or other deformation).

When the metal is tempered, it needs to be heated to a point below the critical temperature and then cooled in air. This is almost the same as stress relief, but the final temperature is not as high as stress relief. Tempering increases toughness while maintaining most of the hardness of the material added by the hardening process.

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Last thought
Heat treatment of metals is often necessary to achieve the physical properties required for a particular application. Although heat treatment of materials before milling can save the overall production time, it will increase the processing time and cost. At the same time, the processed heat-treated parts make it easier to process materials, but add additional steps to the production process.