When metals are heated, they expand (grow). Obviously, then, when they are cooled, they will shrink (contract). Each metal is different due to its chemistry and will thus expand and contract at different rates than any other. Because brazing involves heating metals to high temperatures, there will be significant expansion and contraction of the metals during any brazing cycle. This has led to problems when brazing different metals together, since their expansion rates can be very, very different!
This is illustrated in Figure 1, where a chart shows the different expansion characteristics of a number of different metals from room temperature up to more than 2000°F (1100°C).
If you are brazing different metals together at a temperature of about 1800°F (1000°C), it is apparent from Fig. 1 that significant expansion differences will occur if you are brazing 302 stainless steel to tungsten carbide. Once the assembly is brazed and it is cooled back down to room temperature, the stainless will want to shrink a lot more than the tungsten-carbide piece to which it is brazed. This can lead to distortion of the assembly, or a failure of the part in (or next to) the brazed joint.
This expansion/contraction problem is not just limited to different metals. It may also take place when trying to braze together two parts made from the same metal that have greatly differing mass (weight/thickness). The thinner (lighter) component will become hot much more quickly than a more massive piece. This can lead to distortion since the much hotter part will have expanded much more rapidly than the colder part.
To illustrate this problem, Figure 2 shows an actual example of a braze-joint failure in a carbon-steel fuel-rail assembly where two carbon-steel tubes (one tube was thicker and heavier than the other) were copper-brazed in a continuous-belt furnace at about 2050°F (1120°C).
As you can see in Fig. 1, 1018 carbon steel (and almost all other carbon and alloy steels) will actually go through a temperature range where the steel contracts (shrinks) while still being heated. Then, during cooling, it retraces that curve in the opposite direction and can actually expand when it is being cooled. If the assembly moves too quickly through this temperature range – typically from about 1200-1600°F (650-900°C) – then this can cause part failure (Fig. 2) because of thermal-expansion mismatches that are greater than can be handled by the component design. In the fuel rail shown in Fig. 2, the design was such that the projection-welded bracket on the left side of the photo was not able to handle the large difference in thermal expansion between the thinner tube (top) and the heavier tube (on bottom of photo).
Weren’t both tubes made from the same grade of steel? Then how can this happen? The explanation can be best given by another drawing (Fig. 3), where you see a graphical representation of the expansion of each tube.
As you can readily see, each tube heats up and then goes through its “hiccup” in its thermal-expansion curve (see the 1018 steel curve in Fig. 1). Because of their different masses, however, they go through that “hiccup” at different times. Thus, when the smaller tube wants to shrink, the larger tube is still expanding! This did, in fact, cause such a large difference in their relative expansion/contraction that the welded bracket holding them together broke (Fig. 2), and the tubes pulled apart.
Always remember that different metals expand and contract at different rates. Different masses of the same metal will likewise expand/contract either faster or slower than the other mass made from the same metal. This must ALWAYS be taken into account when designing components for brazing and when setting up the heating/cooling cycle to use for your brazing operations. This becomes especially critical when any of the metal in the brazed assembly goes through a phase transformation during the heating/cooling cycle. If it is not carefully taken into consideration, a lot of scrap and rework can result.