Why Include Heat Treatment by a Third Party in Your Cut Off Parts Specifications?
In our last blog, we talked about the importance of considering heat expansion when deciding how precise a metal part’s dimensions need to be. 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. That means anywhere in the process from part cut off and inspection to assembly and end use.
But what about when you are deliberately heat treating metal as a normal part of your manufacturing process? What are some of the considerations to keep in mind when creating specifications for small, cut off metal parts, such as those we produce here at Metal Cutting Corporation?
Different Methods of Heat Treating Metal Produce Different Results
The heat treatment process involves the controlled application of heat to alter the physical (and sometimes, chemical) properties of a metal. While we do not perform heat treating as part of our services at Metal Cutting, we are mindful of the dimension-altering effects of heat treating metal. Before we explore the unintentional negative consequences to metal part dimensions, let’s discuss some of the benefits of heat treating and annealing.
There are many important reasons to heat treat. Depending on the method used, heat treating metal causes the material to become harder or softer, more or less brittle, or stronger or weaker. Based on the desired end results, a method may involve using several treatments, altering the temperature at which the material is treated, varying the length of time heat is applied, and controlling how long the material is cooled. The way in which you apply and remove heat has an impact on both the yield strength and the hardness of the end product.
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. So, achieving a stainless steel with the desired characteristics — such as high tensile strength but relatively low brittleness — may require multiple treatments at different temperatures and different lengths of time.
Annealing and Heat Treating Are Not the Same
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 non-magnetic 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 — held there 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 Is Another Factor
When heat treating metal, the atmosphere in which the process occurs is also important, because it has an impact on the material surface and strength. For instance, heat treating tungsten in a normal atmosphere will produce oxidation, which 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 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.
Alloy Composition Can Affect Heat Treating — and Vice Versa
Sometimes, finding the right “formula” for both an alloy’s composition and the method for heat treating 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 — a springy material 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, microsurgery needles, and long tubing that needs to be tightly coiled for efficient packaging. (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, which 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.
It’s Important to Mention Heat Treatment Up Front
As we hinted above, whatever method will be used for heat treating metal, it is vitally important to identify the process in the specifications for your cut off metal parts. For example, if you have a straightness specification, you must consider whether heat treating 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. It is remarkable how heat treating can change certain dimensions of carefully machined parts and have no effect on other parameters. 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, here 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 heat treating; other parts can start rubbing together and get scratches in their surface finish if the packaging no longer holds the treated parts securely.
The bottom line is that, when heat treating metal, the methods and effects must be taken into account and shared with your manufacturing partners when you create the specifications for your small parts requirements. To learn more about maximizing the accuracy and effectiveness of your specs — and your budget — download our free guide, How to Fine-Tune Your Quote Request to Your Maximum Advantage: Frequently Asked Questions in Small Parts Sourcing.