When you start a new project that involves metals, it’s important to know which materials you are going to use to get the outcome you want. You wouldn’t build a house with cardboard or an airplane with cement.
When it comes to heat-treating outcomes, understanding the specific material is even more important. The chosen heat-treating process or specification is linked directly to the type of material in order to get the outcome that is required for your project.
Why does knowing the specific material matter?
Heat treating is the process of heating up a metal to change its molecular structure and properties. Every material has its own specific tolerance to heat-treating temperatures and methods of processing. The results can be catastrophic if a material goes through the wrong process. The following issues can result from incorrect processing:
- Low hardness
- Warping and cracking
The type of material chosen specifically limits the capability of the heat to change its molecular structure and properties. For example, 410 stainless steel cannot get as hard as 440 stainless steel regardless of the heat-treat process. At our facility, we have been able to trace 80% of all heat-treat failures to materials being specified incorrectly.
Correct Material Identification Leads to Accurate Results
Ideally, everyone will work with a certified material supplier that provides a material certification for the purchased material. If there is no certification or if the certification is in doubt, however, it may be necessary to use an alternate method of identification.
One method of accurately identifying materials and avoiding these issues is called spectrometry, which is a method of measuring energy waves on something called the electromagnetic spectrum. X-rays are one form of this energy wave on this spectrum that can be measured through spectrometry.
By bombarding a material with high-energy X-rays and measuring the energy emitted, we can determine the specific material. The emitted energy levels, known as characteristic X-rays, indicate the types of elements present as well as their respective concentrations (Fig. 1)
This idea is very similar to the concept of fluorescence seen on glow-in-the-dark material, such as those seen in biology with fish and other animals as well as children’s toys. This material is able to glow in the dark because it absorbs the energy from the light and will appear to someone to glow in contrast to darker lighting (Fig. 2).
For example, a high concentration of characteristic X-ray levels associated with iron and chromium indicates some type of stainless steel.
At Byington, we offer material identification using X-ray fluorescence and provide certification including:
- Which material best matches the AISI (American Iron and Steel Institute) designation
- The alloying elements making up the material as well as their concentration
- The material’s error tolerance
- The spectrometric analysis results
- A magnified image of the surface tested
This use of spectrometry has helped us identify all types of materials for our customers, from basic-grade steel to rare-earth super-metal alloys (Fig. 3). This test helps ensure that the correct process or specification has been identified in order to get the desired heat-treating outcome.
Some images generated through our material analysis are provided on our material analysis certificates (Fig. 4).
- Top left – All the detected elements as well as their concentration from the material analysis. This also informs us what AISI designation the material falls under (important for heat treatment).
- Top right – An image of the material surface analyzed.
- Bottom – A generated graph of the energy readings (characteristic X-rays) converted into the elements and their concentrations for the material.
In an effort to improve your heat-treating outcomes, it is important to know for certain the type of material you are working with. It is best to work with a metal supplier that provides you with certification of the material you purchased. If you do not know for certain what material you are working with, you cannot choose the correct heat-treating process or specification. In this case, it is advised that you have a spectrometry test completed in order to optimize your heat-treating outcome and prevent expensive errors as a result of using the incorrect material.
For more information: Contact Dylan Porter, director of engineering, Byington Steel Treating (Commercial Heat Treater of the Year 2014), 1225 Memorex Dr., Santa Clara, CA 95050; tel: 408-727-6630; e-mail: firstname.lastname@example.org
Why Material Choice Matters for Aerospace
Identifying the correct material for proper heat treatment is especially important for parts intended to service the aerospace industry. Designs to be used in planes and rockets usually require a very high strength-to-weight ratio. This design aspect is critical to a safe and successfully functioning product. The key to achieving a high strength-to-weight ratio is defining the material and what state it is in. If either the material or the state is the incorrect choice or not adequate for the purpose, a critical failure can be the result.
Take, for example, an aircraft wing (Fig. 5). An aircraft wing is designed to provide enough surface area to generate the required lift and to be lightweight and strong enough to hold itself in place and handle the stresses and pressures exerted on it.
If an aircraft wing were to fail, that would be catastrophic! That is why every aspect of the wing needs to be ideally optimized to serve the design purpose while remaining structurally sound. Material choice and the material state are no exception. When designers create these aircraft wings to meet all of the necessary requirements, they are counting on the material making up the wing to satisfy the design as intended. The most common aircraft wing designs use specific aluminum alloys as the structural support, most of which is heat treated to give it the strength needed to endure the elements. Material identification and heat treatment are keys to help make those design goals a reality.
The Landing Gear
Let’s take a look at another critical design piece that is dependent on material choice and how it is heat treated – the landing gear. In 2005, Flight LAX 050921 needed to make an emergency landing due to a misaligned front landing gear (Fig. 6). Due to its alignment, it fell to a very skilled pilot and the landing-gear structure to hold up a 70-ton aircraft moving at 200 mph. Luckily for all the passengers, the landing gear is made up of a specially designed alloy that goes through a custom heat-treatment process. Due to this alloy/heat-treatment combination, the structure was able to stay upright and not break under the stress (Fig. 7).