At Semco Carbon, we are constantly thinking of new, relevant, and useful topics involving graphite materials and graphite applications to share with the readers of this blog. If something has to do with the manufacture of graphite products, we’ve probably got a lot to say, and a story to tell, about it.
During a recent meeting, our application engineers were working on a specific issue that one of our customers was having. This customer had been experiencing premature component breakage in a new high temperature process they were testing. After thinking about the specifics of their application, we realized that this experience could highlight the importance of the coefficient of thermal expansion.
Ok, so here’s the part where we get technical. By definition, thermal expansion is the tendency of matter (in this case, graphite) to change in volume in response to change in temperature. The coefficient of thermal expansion is the value that allows us to calculate the amount of volume change that will occur along with the change in temperature over a specific range.
Why is the coefficient of thermal expansion important in the graphite application industry? Well, graphite materials are often exposed to very high temperatures. Because of these extreme conditions, graphite is definitely susceptible to physical changes when doing its job in these graphite applications. That being said, one graphite grade will perform differently than another under the same variations in temperature. Understanding this attribute of graphite, and being able to anticipate how a specific graphite grade will perform under field or process conditions, is important. It allows us to customize solutions, and produce graphite product, based on a customer's set parameters.
Let’s go back to the application engineers meeting for a moment and consider the problem our customer was confronted with. We will then explain our thinking process and present the solution we offered. By the end, you should have a clear understanding of why we spend our time brushing up on concepts like the coefficient of thermal expansion.
Our customer was in the process of setting up a new line of furnaces that were designed to run at higher temperatures than their regular furnaces. For the graphite components that would be part of these furnaces, the customer specified the graphite grade which had been used successfully in their regular furnace line for years. Yet, during test runs of the new furnaces, the graphite components or connectors, manufactured by our customer from various other materials, would exhibit cracks and fail.
As you can probably guess, our analysis focused on the higher temperatures of the new furnaces since none of the components experienced the same degree of failure in our customer’s past applications. The graphite materials utilized in these new furnaces were capable of withstanding the operating temperature and process environment. By itself, the graphite material was not the problem. Since the parts that experienced failures were interconnected, we assumed that the issue had to be related to how these various materials worked together within the system. We went back over the materials specification sheets, which revealed vastly different coefficients of thermal expansion between the graphite and the other materials used. We concluded that the difference in coefficients caused materials to expand in volume at different rates. That in turn caused significant stress on all connected parts, which resulted in the premature failures.
After this analysis, a big question remained: why did the same materials not exhibit the same failures in the customer’s standard furnace line? The answer was both the higher operating temperature and the ramp rate or speed at which those temperatures were reached in the new furnaces. While stress was experienced within the components in the standard line of furnaces, it was not enough to damage the components. Raising the upper operating temperature to a new threshold level was causing excessive stress between parts.
Most graphite material specification sheets will list the value for the coefficient of thermal expansion, but sometimes the coefficient is extrapolated based upon limited data. Also, the value is set over a known temperature range. The catch is that all graphite manufacturers do not use the same range of temperatures to set their published coefficient of thermal expansion values. This creates problems because most of the commercially available graphite materials will have different values over different temperature ranges. As an example, the coefficient of thermal expansion from 100 to 500 degrees is different from the coefficient of thermal expansion over 3,100 to 3,500 degrees.
After we completed our analysis, we recommended a different graphite grade to our customer. This new grade of graphite material had a lower coefficient of thermal expansion value. We also suggested that our customer change out various non-graphite materials within the system. Tests done after the incorporation of the new materials were successful. Little or no damage appeared on the internal components, the same ones that had previously experienced premature wear.
This case study reveals how changes perceived as relatively minor can in fact dramatically change the operability of heat treat systems. It also reveals that materials considered proven and reliable can still fail if not integrated properly into new systems.
We thought this specific case study and other relevant experiences could be used to shed light on the importance and proper evaluation of the attributes found on a typical graphite material specification sheet. Look for other blogs over the next few months that will detail other attributes listed on the standard material specification sheet.