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).

Room temperature radial clearance

Figure 2.

Always remember that the optimal braze joint clearance that is needed for a successful braze occurs AT BRAZING TEMPERATURE, since only at brazing temperature is the brazing filler metal (BFM) liquid and therefore able to flow through the joint by capillary action. Thus, it is critical that you use the metal expansion data from the charts to “back-calculate” from the needed clearance at brazing temperature down to the room temperature clearance needed to give you that desired clearance at brazing temperature when the two metals are heated (for most brazing we typically want a 0.000″-0.005″ {0.000-0.125mm} clearance between the faying surfaces of the joint at brazing temperature). For certain BFMs, it may need to be kept even tighter.

Back to our example. Look at the assembly-diagram in Fig. 2.

What if the I.D. of the thick-walled tungsten carbide ring exactly matched the O.D. of the round 302-stainless bar? What will happen as that assembly is heated to brazing temperature? The chart in Fig. 1 shows you that because the stainless expands faster than the tungsten carbide, the clearance (braze-gap) between the two parts will shrink (get tighter) as the temperature increases. In fact, the joint may become so tight that no BFM would be able to penetrate through it.

Similarly, if the stainless were the outer cylinder and the carbide was a smoothly fitting insert into the stainless, can you see that the gap clearance between the two surfaces would get wider with increasing temperature? The effect of thermal expansion must be taken into account ahead of time — before the parts are made, so that the metal parts may be appropriately machined before they are assembled for brazing in order to achieve optimal clearance at brazing temperature to let capillary energy do its work.

Problem for Readers to Solve

What room temperature radial clearance should be used for the carbide-stainless combination shown in Fig.2 if it were going to be brazed at 1950F (1065C) in a vacuum furnace, and a 0.0015″ (0.040 mm) radial clearance was desired at brazing temperature?

Next Month: In my next article, I’ll examine the thermal expansion curve for 1018 carbon steel to see why there are strange “reversals” in the thermal expansion curve for that alloy (and for similar metals).

Problem solve and improve with our technical experts.