Time to read: 5 min
Once you’ve finished CNC machining your parts, your work isn’t done. Those raw components might have ugly finishes, may not be strong enough, or be just one part of an assembly that’s got to be connected with others to make a complete product. After all, how often do you use devices composed of a single part?
The point is, post-machining processes are necessary for a range of applications, and we’re here to walk you through some considerations so you can choose the right secondary operations for your project.
In this three-part series, we’ll cover options and considerations for the heat treatment process, finishes, and hardware installation. Any or all of these may be necessary to take your part from its machined state to customer-ready. This article discusses heat treating, while Parts II and III examine finishes and hardware installation.
Heat Treating Before or After Machining?
Heat treatment is the first operation to consider after machining, and you might even consider machining pre-heat treated material. Why use one method over the other? The order in which you choose to heat treat and machine your metal can affect the material properties, the machining process, and the tolerances of your part.
When you use a material that has already been heat-treated, this affects your machining — harder materials take longer to machine and wear down tools faster, which adds to machining costs. Depending on the type of heat treating applied and how deep below the surface the material was affected, you can also cut away the hardened layer of the material and defeat the purpose of using hardened metal in the first place. There’s also a chance that the machining process will generate enough heat to increase the hardness of the workpiece. Certain materials, like stainless steel, are more prone to being work hardened during machining, and extra care is necessary to prevent this.
Choosing a pre-heat treated metal does have some advantages, however. With hardened metal, your part can hold tighter tolerances, and sourcing your material is easier because pre-heat treated metals are readily available. And, if you wait until after machining, heat-treating adds another time-consuming step in the production process.
On the other hand, heat treating after machining gives you more control over the process. There are multiple types of heat treating, and you can choose which to use to get the material properties you need. Heat treating after machining also ensures that the heat treating effects are consistent across the surface of your part. With pre-heat treated material, the heat treating may have only affected the material to a certain depth, so machining may remove the hardened material in some places and not others.
As mentioned, heat treating after machining adds cost and lead time, due to the extra step of outsourcing this process. Heat treating can also cause parts to warp or otherwise deform, which can affect the tight tolerances achieved during machining.
Heat Treatments
In general, heat treating will change the material properties of your metal. Usually, this means increasing the strength and hardness of the metal so that it can stand up to more extreme applications. However, certain heat treating processes, like annealing, actually reduce the hardness of metals. Let’s take a look at the different heat treating methods.
Hardening
Hardening is used to — you guessed it — make metal harder. A higher hardness means that the metal is less likely to dent or be marked when impacted. Heat treating also increases the tensile strength of the metal, which is the force at which the material fails and breaks. Higher strength makes the material more suitable for certain applications.
To harden a metal, the workpiece is heated to a specific temperature above the metal’s critical temperature, or the point at which its crystal structure and physical properties change. The metal is held at that temperature, then cooled by quenching it in water, brine, or oil. The quenching liquid depends on the specific alloy of the metal. Each quenching liquid has a unique rate of cooling, so it’s chosen based on how quickly you need to cool the metal.
Case Hardening
Case hardening is a type of hardening that only affects the outer surface of the material. This process is often done after machining to create a durable outer layer.
Precipitation Hardening
Precipitation hardening is a process used on specific metals that have certain alloying elements. These elements include copper, aluminum, phosphorus, and titanium. When the material is heated over an extended period of time, these elements precipitate or form solid particles within the solid metal. This affects the grain structure, increasing the strength of the material.
Annealing
As mentioned previously, 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 significantly more machinable.
To anneal a metal, the metal is heated slowly to a specific temperature (above the material’s critical temperature), then held at that temperature, and finally cooled extremely slowly. This slow cooling process is accomplished by burying the metal in an insulating material or keeping it in the furnace while both the furnace and metal cool.
Stress Relieving for Machining Large Plates
Stress relieving is similar to annealing in that the material is heated to a certain temperature and cooled slowly. However, in the case of stress relieving, that temperature is lower than the critical temperature. The material is then air-cooled.
This process relieves stresses from cold working or shearing but does not significantly change the physical properties of the metal. Although the physical properties don’t change, relieving this stress is beneficial to avoid dimensional changes (or warping or other deformation) during further machining processes or during the use of the part.
Tempering
When tempering a metal, it’s heated to a point lower than its critical temperature, then cooled in air. This is almost the same as stress relieving, but the final temperature is not as high as stress relieving. Tempering increases the toughness while maintaining most of the hardness of a material that may have been added through a hardening process.
Final Thoughts
Heat treating metal is often necessary to achieve the physical properties needed for a given application. And while heat treating material before milling can save overall production time, it adds time and cost to the machining process. Meanwhile, heat-treating parts after machining makes it easier to machine the material but adds an extra step to the production process.
Depending on your application, you’ll have to weigh the benefits of added hardness and tighter tolerances with the disadvantages of longer machining time associated with pre-heat treated material. And the good news is, China Manufacturing parts, Inc. has a wide range of both pre-heat treated and standard metals that can be heat treated after machining — plus our experts can help you choose the right material for your next CNC project.
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