Secondary processing after CNC processing: heat treatment

When you finish CNC machining of parts, your work is not finished. These original components may have an ugly surface, may not be strong enough, or may only be part of a component, which must be connected with other components to form a complete product. After all, how often do you use a device made up of a single component?

The key point is that the post-processing process is necessary for a series of applications. Here we introduce some precautions so that you can choose the correct secondary operation for your project.

In this three part series, we will introduce the heat treatment process, surface treatment and hardware installation options and considerations. 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 check 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?

Heat treatment is the first operation to be considered after processing, and even processing preheating materials can be considered. Why use one method instead of another? Selecting the sequence of heat treatment and metal processing may affect the material characteristics, processing process and tolerance of the part.

When you use materials that have been heat treated, this will affect your processing – harder materials take longer to process, and tool wear is faster, which will increase processing costs. Depending on the type of heat treatment applied and the depth below the affected surface of the material, the hardened layer of the material can also be cut off and the purpose of using hardened metal can be destroyed first. The processing 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 require extra care to prevent this.

However, there are some advantages to choosing metals that have been preheated. For hardened metals, your parts can maintain tighter tolerances, and because the pre heat treated metal is readily available, it is easier to purchase materials. Moreover, if the processing is completed, another time-consuming step will be added in the production process.

On the other hand, heat treatment after machining enables you to better control the processing 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 materials that have been preheated, heat treatment may only have a certain depth of influence on the material, so machining may remove the hardened material in some places and not in others.

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

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.

sclerosis

Hardening is used to make metal harder. Higher hardness means that the metal is less likely to show dents or marks when impacted. Heat treatment will also increase the tensile strength of the metal, which is the force of material failure and fracture. 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 the point where its crystal structure and physical properties change. The metal is kept at this temperature and then quenched and cooled in water, salt water or oil. The quenching solution depends on the specific alloy of the metal. Each quenching liquid has a unique cooling speed, so it is selected according to the speed of cooling metal.

Case hardening

Case hardening is a type of hardening that affects only the outer surface of the material. This process is usually completed after processing to form a durable outer layer.

Precipitation hardening

Precipitation hardening is a process for specific metals with specific alloy elements. These elements include copper, aluminum, phosphorus, and titanium. When the material is heated for a long time, these elements precipitate or form solid particles in the solid metal. This will affect the grain structure and increase the strength of the material.

annealing

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

To anneal the metal, the metal is slowly heated to a specific temperature (above the critical temperature of the material), maintained at that temperature, and finally cooled very slowly. This slow cooling process is accomplished by burying the metal in the insulating material or keeping it in the furnace while the furnace and metal are cooling.

Stress relief of large plate processing

Stress relief is similar to annealing, in that the material is heated to a certain temperature and slowly cooled. However, in the case of stress relief, this temperature is below the critical temperature. The material is then air cooled.

This process can eliminate the stress caused by cold working or shearing without significantly changing the physical properties of the metal. Although the physical properties will not change, the elimination of this stress will help to avoid dimensional changes (or warping or other deformation) during further processing or part use.

tempering

When tempering the metal, 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.

Final thoughts

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