Heat treatment is a critical finishing step that can fully transform how a part works. It’s a process that makes parts harder, stronger, and more resistant. In this guide, we’ll explain how you can use heat treatment to improve a CNC-machined part.
What Is CNC Machining?
CNC machining is a topic that we discuss a lot on our site. It’s the process of using Computer Numerical Controlled (CNC) machines that cut, shape, and create parts for fabrication. A common example we have is a CNC milling machine. Our machinist “tells” the mill what to do, and it can create cuts and features on its own without any additional input. The computer does the thinking, measuring, and cutting after being programmed correctly. This guide is specifically about CNC-machined parts because heat treatment is very versatile and broad for this topic. With 3D printing, injection molding, and casting, heat treatment either is impossible or much narrower.
What Does Heat Treatment Do?
The simple goal of a heat treatment is to change the material properties of a part and increase the hardness. It is also used to make parts stronger, more durable, and more ductile.
Heat treatment works because of a few principles in physics and material science. For a machine shop to heat treat a part, there are a few different options. The general process entails getting a part really hot, then controlling how it cools down.
When Heat Treatment Is Done
Heat treatment can be done either before or after machining the part. If it’s done before, then it’s up to the material vendor to do the treatment, not the shop.
By hardening the raw material, the machine shop will need to be more deliberate with how they run their CNC machines. Harder bits might need to be used, more cutting fluid should be utilized, and lower speeds might be set up to reduce chatter and heat generation.
If you want a general heat treatment on your part, it makes sense to do it before the machining. The operation takes a few days, but that time is negligible when the raw material is bought with an already-applied heat treatment.
For a modern machine shop, in-house heat treatment has to be done after the part is made. This is especially true if the part needs to hit very high hardness levels. If it’s too hard before machining, bits can break, and tooling can get jammed.
Think about it — if you fully heat treat the raw material, then the CNC machine has to cut through stronger, harder, more durable materials. CNC machines prefer softer materials in order to get better results and spend less time during the process.
As a result, most shops will heat treat the final part after all the machining and fabrication work is done. This might still occur before the final finishing steps are performed (like painting or sandblasting).
For a short version of this topic, remember this: Pre-fabrication heat treatment can save time, but there’s a limit to how hard the part can be for the machine’s sake. Post-fabrication heat treatment can create incredibly hard parts, since they’re already cut.
The Different Heat Treatment Options
Different shops can offer different forms of heat treatment. For all of them, the goal is the same but the process is different.
Annealing is the most common form of pre-fabrication heat treatment. It involves heating a part and then slowly cooling it at a controlled pace. The heated part is typically white-hot metal, kept hot for about an hour per inch of material, and then the cooling process typically takes a full day.
For steels, annealing takes the metal up to around 1500 degrees F.
It can also be done again after the part is machined, but it’s not necessary.
Purpose: Make a part harder while still optimizing its ability to be machined.
Another treatment option is quenching. This works similarly to annealing, with a few key differences. When a metal is quenched, it is heated up to its white-hot state, then left to cool at whatever pace the medium allows — it typically cools very quickly.
This process “shocks” the molecules of the metal, causing them to line up and become stiffer.
Quenching can be done in the open air of a shop, in a bucket of water, or in a container of oil or brine. The operation is a lot faster, and the part isn’t heated as hot as an annealed metal would be.
If you’ve seen a blacksmith work, you’ll notice that they dip their swords in a bucket of water or oil. This is quenching.
Purpose: Make a part harder and more brittle.
Shops will usually reserve tempering for use after quenching. It can be thought of as a low-scale version of quenching. The part is heated up to a much lower temperature — somewhere around 500 degrees F for steels — and left to cool slowly in the open air.
This process might be repeated a few times. Each time, it makes the part less brittle.
The combination of quenching and a few cycles of tempering is still much faster than the full annealing process.
Purpose: Make a part less brittle after certain heat treatments.
After machining, a part might go through a stress relief operation. No, this doesn’t entail massaging the piece of metal. Instead, the part gets heated up to around 1000 degrees F for steel, and held at a constant temperature before allowing the part to slowly cool.
This process will remove some of the damage done from impacts and cuts during fabrication. It will harden the part as a whole, and eliminate weak spots.
Purpose: Remove mechanical deformation, weak spots, and micro-mechanical damage.
Be Sure to Specify the Required Hardness
When you want your part to be hardened, you don’t need to know which operation to use, just what hardness you’d like. When you send a part to get quoted, it helps to include the required hardness. From there, the machine shop will pick which option to use.
As you just learned, heat treatment is the process of heating up and then cooling metal. It makes the final part a lot harder and stronger, whether it’s done before or after machining the part.
If you’re in the market for hardened and heat-treated parts, you can get started at Rapid Axis. We have a full range of finishing steps that will harden and strengthen your parts. We are a full-scale machine shop that offers dozens of different services. Reach out today for a free quote so our experts can get started on your project.