[dankay7]

The Famous Joint-Strength vs. Joint-Clearance Chart

One of the most famous charts ever used in brazing is the famous (infamous?) strength vs. clearance chart created from work done in the Handy & Harman laboratories in Fairfield, Connecticut back in the 1930's.

This chart (attached below) shows that as the joint clearance gets tighter and tighter (moving from right to left along the bottom axis), the strength of the joint gets higher and higher (moving upwards along the left-hand axis). There is a lot of experience with this over the years, and general acceptance of this information is widespread. However, something strange happens at the far left of the chart in the area represented by gap-clearances of about 0.0015" (0.04 mm) or less, i.e., there appears to be a drop-off in strength of the brazed joint.

I heard a PhD metallurgist tell an audience during a conference some years ago that people should not allow their brazements to have gaps tighter than 0.0015" (0.04 mm) because joint strengths weaken when they are tighter than this. I was amazed to hear this erroneous information, and quickly realized that probably many people, in the absence of complete information, might assume the same thing.

The original Handy & Harman report is apparently no longer available, but the data for this chart was generated by flame-brazing (torch-brazing) two pieces of 304 stainless sheet stock together in a butt-joint configuration, using silver-based brazing filler metal (BFM) along with a brazing paste-flux (since it was being brazed in air with a torch).

The test pieces being brazed were apparently designed so that the cross-sectional width (and thickness?) of the stainless on each side of the joint was much greater than that in the area of the braze (thus the test specimen was tapering down as it approached the joint area), so that failure would always occur in the joint, and not in the base metal. Thus, the increased values of "strength" shown in the chart represent the "strength-to-failure" (tensile strength) of the brazing filler metal (BFM) itself, and not that of the "overall joint" which would include the stainless, etc.

It is, therefore, interesting to note that the tensile strength of the silver-based BFM itself is raised by the constraints of the proximity of the sides of the joint. Thus, a silver-based BFM which might have a tensile-strength of about 40,000 psi were a rod of that material pulled apart in a tensile-testing machine, when that same BFM is melted into the confines of a brazed joint the tensile strength of that BFM --in the joint-- is modified by the constraints of faying surfaces on each side of the gap. As the gap-clearance gets tighter and tighter, the normal mode of metallic deformation along preferred slip-planes can no longer effectively take place, since the gap is so narrow, and actual molecular-bonding rupure-mechanisms enter into the picture instead, requiring far higher levels of force to break the joint. Thus, the chart shows higher and higher "strength" levels for the BFM, up to more than 3-times the levels of force required to break the BFM in non-constrained rod-form out in open air.

Flux causing drop in the strength-curve below 0.0015".
Remember that these test pieces were brazed in air with flux. All brazements joined in air using flux WILL (not maybe) contain some flux residues. There is no such thing as a flux-free joint when brazing in air with a flux. Thus, when the test pieces were being torch-brazed with gaps at about 0.0015" (0.04 mm) or less, the inevitable flux voids began to become a noticeable part of the brazed joint, and began to negatively affect the joint strength due to their increased presence (percentage wise). Had the joints been able to be "wiped" (surfaces moved back and forth relative to each other while being heated with the flame) to help remove some of those voids it might have helped, but that was apparently not done.

CONCLUSION: The drop-off in strength-values on that widely-used chart thus has nothing to do with so-called "negative effects of close joint clearances", but only to do with the inevitable presence of flux (and thus flux voids) in the joint. Were these parts to be brazed in a protective inert atmosphere such as nitrogen or argon, there would have been no fall-off of strength values, but instead, they may have continued to rise a bit more on the chart.

[heattreater]

Very interesting and informative post Dan. This is new information to me. Thanks for sharing your expertise with us!!!

[admin]

Dan: I concur with your conclusion that to remove flux voids from the braze will in fact allow one to reduce the gap below .0015". However, in the asymptotic limit of a zero gap, the braze strength must in fact drop to zero (for two perfectly mated surfaces), because there is no longer a brazing joint present. So, your commentary only pertains to redefining the gap distance for maximizing the braze-joint strength under ideal brazing conditions.
Posted by A. Bruce DeWald, Jr.

[dankay7]

Bruce--

Thank you for your comments, but I need to ask you to please further clarify what you are saying.

If a joint clearance approaches "zero clearance", are you then saying that the brazing filler metal (BFM) would find no way to possibly get into the joint (since there is then no clearance between the surfaces)? Copper is commonly used to penetrate press-fits in which the gaps are approaching "zero clearance", and in fact, actually have "negative fits", or press-fits. Of course, due to the normal surface roughness of the parts being joined, even under those press-fit conditions, there will still be enough surface anomalies/unevenness/roughness to allow BFM to find its way into and through the joint.
Now, assuming the surfaces were polished down to perfectly smooth flat surfaces, then as the gap approached zero clearance you would indeed have a gap that could shut off BFM capillary flow completely, as you indicate. But that is theoretical, and not the real world of brazing. That is why I tell people all the time NOT to polish the surfaces smooth, since it can then prevent BFM from getting into the joint by capillary action.
People should use the normal as-received/as-machined surface finish (roughness) of the material being brazed, because when two such surfaces contact each other (i.e, so-called zero-clearance), there is actually still enough open space to allow the BFM to flow through.
If this is not what you were referring to in your note, then please clarify further.
Thanks.