Our last blog entry discusses the importance of understanding the thermal expansion behavior of dissimilar materials when brazing.  This post will illustrate a sample calculation used for determining the nominal dimensions when designing an appropriate braze gap.

CTE is used to calculate the dimensions needed at the time of fabrication, in order to achieve a desirable braze gap (.000” to .005”) at braze temperature, between two dissimilar materials.

For example, if you needed to design a SST weld sleeve to be brazed onto a 6” diameter Titanium tube with a .07” wall thickness, you would determine the fabrication dimensions for the SST weld sleeve as follows: First you would determine the ID of the Titanium at the braze temperature (950º C).

The CTE for Titanium at 950º C = 10.21 μin/in

If the ID of the Titanium Tube at room temperature = Ø5.860”

Titanium ID at 950ºC   =   IDØ + (CTE (IDØ (ΔT) + IDØ))

=   5.860 + (0.00001021 (5.860 (925) + 5.860))

Therefore the Titanium ID at braze temperature would be = Ø5.915”

A desirable braze gap would be ~.005”.  Therefore, we would want the OD of the SST to  be Ø5.910” at braze temperature; where by, the inverse functions are applied. The CTE for SST @ 950ºC = 18.59 μin/in

If the OD of the SST sleeve at braze temperature will be = Ø5.910”

SST OD at room temperature  =  5.910 / (CTE x ΔT) + 1

=  5.910 / (.00001859 x 925) + 1

Therefore the SST OD at room temperature should be =  Ø5.810” The aforementioned example clearly illustrates the SST sleeve will fit very loose at room temperature to achieve an ideal braze gap distance at braze temperature.  It should be noted that this calculation is a numerical example only and is intended to demonstrate the significant thermal expansion changes that can occur among dissimilar materials when brazing.  A designer must also evaluate the stresses that will be apparent during cooling, since a solidified alloy will now be present in the braze gap and the parent materials will no longer be able to “return” to their original dimensions or geometry.  This concept was briefly discussed in the previous post and is an essential component of braze design that must be addressed.  Altair is vastly experienced with creating innovative solutions for such scenarios.