Why Include Heat Treatment of Metal in Cut-Off Parts Specifications?
We’ve talked elsewhere about how heat expansion may be a factor when deciding how precise the dimensions of a metal part need to be.
Namely, if even a slight difference in tolerance caused by heat expansion would affect how the part performs, then it is wise to avoid specifying a very tight tolerance whenever there may be a risk of variation in environmental temperatures. And that means variation anywhere in the process, from part cut-off and inspection to assembly and end use.
But what about when you deliberately heat treat metals as a normal part of your manufacturing process? What, then, do you need to keep in mind when creating specifications for small, cut-off metal parts, such as those we produce here at Metal Cutting Corporation?
Why Heat Treat Metals?
Why are metals heat treated? There are many important reasons. But basically, the process for heat treating of metals involves the controlled application of heat to alter the physical — and sometimes, chemical — properties of the material.
While we do not heat treat metals as part of our services at Metal Cutting, we are mindful of its potential dimension-altering effects. But before we explore the unintentional consequences to metal part dimensions, let’s discuss some benefits of heat treating and annealing.
How to Heat Treat Metal to Produce Different Results
Depending on the method used, heat treated metals become harder or softer, more or less brittle, or stronger or weaker. Based on the desired end results, the method may involve:
- Using several treatments
- Altering the temperature at which the metal is heat treated
- Varying the length of time heat is applied
- Controlling how quickly or slowly the material is cooled
The way in which you apply and remove heat as you heat treat metal has an impact on both the yield strength and the hardness of the end product.
Why Heat Treat Metals Such as Stainless Steel?
For example, stainless steel is commonly heat treated to increase its hardness; on the downside, it may also become more brittle.
Conversely, stainless steel may be heat treated to increase its ductility, which helps to minimize cracking and increase workability. However, that process may also decrease the steel’s hardness.
Therefore, achieving a metal with the desired characteristics — such as a stainless steel with high tensile strength but relatively low brittleness — may require multiple treatments at different temperatures and different lengths of time.
Annealed vs. Heat Treated Metal: Not the Same Thing
Often used interchangeably with the term heat treating, annealing is a specific method used to soften metals, with the goals of increasing their ductility and decreasing brittleness.
Annealing can also be used to increase the homogeneous nature of metals, as well as to restore their ductility prior to further handling. For example, as 316 stainless steel is worked, it can pick up undesirable magnetic properties; however, annealing the stainless steel can restore it to (or very close to) its original nonmagnetic condition.
Annealing is performed using specialized furnaces in which conditions are tightly controlled. The metal is heated to a high temperature — generally, slightly above the recrystallization temperature.
The material is held at high temperature for anywhere from several hours to several days and then allowed to cool (in the case of steels and other ferrous metals, very slowly).
Where You Heat Treat Metals Is Another Factor
When you heat treat metal, the atmosphere in which the process occurs is also important, because it has an impact on the material surface and strength.
For instance, if you heat treat tungsten in a normal atmosphere, it will produce oxidation that makes the surface finish porous. However, working in a controlled environment — such as a vacuum or a sealed nitrogen, argon, or hydrogen atmosphere — allows you to heat treat the metal without oxidation.
Low carbon steel can be annealed in a carbon-rich environment to case-harden the steel with a high carbon surface layer that has good fatigue and wear resistance. This technique is used to improve the hardness and durability of products such as carbon steel wire springs and forgings.
However, if “carburization” is not a desired trait, annealing should be performed in an environment that is low in or free of carbon.
How Alloy Composition Can Affect Metal Heat Treating
Sometimes, finding the right “formula” for both an alloy’s composition and the method for heat treating of metal is essential to achieving the desired end properties. A great example is the shape memory property of nickel-titanium alloy — otherwise known as NiTi or nitinol.
This “springy” material was first popularly used to make eyeglass frames that can bend and seemingly magically return to their original shape. Today, NiTi is widely used for medical device tubing applications such as catheter guidewires, stents, and microsurgery needles.
(You can learn more about NiTi and other tubing materials in our white paper, Medical Device Tubing in the 21st Century: Who Needs It?)
The shape memory property of NiTi depends on heat treating. It is what enables NiTi to undergo deformation at one temperature and then recover its original, undeformed shape when heated above its so-called transformation temperature.
However, NiTi is notoriously difficult to make, because of both the high reactivity of titanium and the fact that even slight variations in composition can affect the transformation temperature.
For instance, if the titanium atoms combine with oxygen or carbon, the NiTi crystal structure can lose titanium, causing the transformation temperature to be lowered. If there is too little nickel and the material is aged for too long, the transformation temperature is increased.
Specify Heat Treated Metal Up Front
Whatever method will be used to heat treat metals, it is vitally important to identify the process in the specifications for your cut-off metal parts. It is remarkable how metal heat treating can change certain dimensions of carefully machined parts and have no effect on other parameters.
For example, if you have a straightness specification, you must consider whether heat treatment and any resulting expansion of the metal will have an impact on the dimensions of your cut parts and, ultimately, how well they will perform — and if so, you may need to adjust your tolerances accordingly.
Heat Treated Metal Can Even Impact Packaging
In addition, it is important to make sure heat treated metal parts will be packaged properly to avoid distorting or damaging the previously cut parts while they are in transit.
For instance, at Metal Cutting, we always want to know if the parts we cut will then be sent to a third party for heat treating or annealing. Since parts are made more pliable by annealing, inadequate packaging could cause annealed parts to become warped when they are repackaged and sent to you (or sent back to us for additional processing).
Parts such as rods can become bowed if they expand and are not properly packaged after metal heat treating. Other parts can start rubbing together and get scratches in their surface finish if the packaging no longer holds the heat treated metal parts securely.
Yet Another Reason for Thorough Specs
The bottom line is that with heat treated metal, the methods and effects must be considered and shared with your manufacturing partners when you create the specifications for your small parts requirements. It will help to optimize manufacturability and get you the results you want.
To learn more about maximizing the accuracy and effectiveness of your specs, download our free guide, How to Fine-Tune Your Quote Request to Your Maximum Advantage: Frequently Asked Questions in Small Parts Sourcing.