Fig. 1 – Thermal Expansion Curves for several metals
In this article I will explore the effect that thermal expansion has on joint clearance, and thus, on brazed joint strength and quality. It’s an important concept, and although it is well known in the brazing world, many folks today still do not take this topic seriously enough when designing brazed assemblies. This article is based on one I wrote many years ago for an in-house brazing publication at a brazing filler metal supplier, and will be written in two parts. Next month’s segment will look more closely at polymorphic metals, such as carbon steels, and will attempt to explain why they exhibit their very strange thermal expansion curves.
Please note that ALL metals expand (grow) when they are heated, and contract (shrink) when they are cooled. This fact has been thoroughly explored over the years, and data-tables have been published showing how fast each metal expands as temperature increases. This important information about the expansion characteristics of each metal should always be used in developing braze procedures when different kinds of base metals are to be brazed to each other. The success or failure of a braze procedure may very well depend on it!
Fig.1 shows typical thermal expansion data for some common base metals. It is obvious from these curves that different metals expand at different rates when heated. As an example, look at the two curves representing 302-stainless steel, and the curve representing tungsten carbide (WC). Notice from this chart that the stainless grows at a much faster rate, when heated, than does tungsten carbide. If these two metals were being brazed together, this difference in their expansion rates will be very important to take into consideration. Let’s look at this further.
Let’s assume that we were asked to braze a 302-stainless steel bar into a tungsten carbide ring, as shown in Fig. 2. As these two metals are heated up to brazing temperature, the brazing joint clearance between them will change as the stainless steel expands at a faster rate than the carbide. Thus the gap between them will get smaller (perhaps even close completely as they are heated).