One of the most widely used charts in the field of brazing is the strength vs. clearance chart created from work done in the Handy & Harman laboratories in Fairfield, Connecticut back in the 1930’s. This chart is shown below, in Fig. 1:
Notice that as the joint clearance gets tighter and tighter (moving from right to left along the bottom axis), the tensile strength (as shown on the vertical axis on the left-side of the chart) gets higher and higher. Although there is a lot of experience with this over the years, and general acceptance of this information is widespread, it must be pointed out that this chart is very specific only to the actual testing performed in making this particular chart, and may not be identical to tests performed by others using similar materials or conditions. But the general principal of increased joint strength with tighter gaps can be accepted.
What about that strength “drop-off” below 0.0015” (0.04mm) ?
Please notice that something strange happens at the far left of the chart in the area shown by gap-clearances of about 0.0015″ (0.04 mm) or less. There appears to be a drop-off in the tensile strength when the gaps are tighter than 0.0015″ (0.04 mm).
A number of years ago I attended a brazing conference, and a PhD metallurgist was using this chart in his talk that day. He told the audience that, based on that chart, people should not allow their brazements to have gaps tighter than 0.0015″ (0.04 mm) because joint strength obviously falls-off (gets weaker) when gaps are tighter than this. Unfortunately, the speaker was giving incorrect information to his audience. In reality, there is actually nothing wrong in designing joints to have brazed-clearances tighter than 0.0015″ (0.04 mm).
The original Handy & Harman report is, unfortunately, apparently no longer available. However, we do know enough about those original tests from other subsequent reports and articles to understand that the data for the chart shown in Fig. 1 was generated by flame-brazing (torch-brazing) two pieces of 304 stainless together in a butt-joint configuration, using silver-based brazing filler metal (BFM) and a brazing paste-flux (since it was being brazed in air with a torch).
The stainless steel test pieces being brazed were apparently designed so that the cross-sectional area of the stainless on each side of the joint was much greater than that in the brazed-joint (thus the test specimen was tapering down rapidly as it approached the joint area). Such a test-specimen design would insure that failure was designed to always occur within the joint itself, and not in the stainless base-metals involved. Thus, the increased values of “strength” shown in the chart actually represent the tensile strength of the brazing filler metal (BFM) itself, and not that of the “overall joint” (including the stainless, etc.)! This is very significant, and thus, very revealing about what happens to BFM as the braze-joint gaps get tighter!
Note that the tensile strength of the silver-based BFM itself is increased by the constraints of the proximity of the sides of the joint as the joint clearances get tighter. Thus, a silver-based BFM which might have a tensile-strength of up to 40,000 psi if a rod of that material were pulled apart in a tensile-testing machine, will behave very differently 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 the faying surfaces on each side of the gap. As the gap-clearance gets smaller, it reaches a point where the normal mode of metallic deformation along preferred slip-planes in the BFM can no longer effectively take place. Within very tight joints, it appears that instead of slip-planes operating, deformation can only occur by actual rupture of molecular-bonding within the BFM, requiring far higher levels of force to accomplish that. Thus, the chart shows rapidly higher and higher “strength” levels for the BFM, up to more than 3-times the levels of force that would be required to break that same BFM in a tensile machine were the BFM in a non-constrained rod-form out in open air.
Why did that strength “drop-off” occur below 0.0015” (0.04mm) ?
Remember that the chart in Fig. 1 was created based on test results of tensile specimens that were torch-brazed in air with flux in the joint. All brazements joined in air using flux WILL contain some entrapped flux residues, except under extreme laboratory conditions. There is really no such thing as a “flux-free” joint when production brazing in air with a flux. Yes, it is possible to reduce, to some extent, the amount of any entrapped flux-voids in a brazed-joint by a process called “wiping the joint”, but even then you would not get rid of 100% of all flux voids.
Therefore, when the test pieces used in creating the chart in Fig. 1 were being torch-brazed with gaps at about 0.0015″ (0.04 mm) or less, the inevitable flux voids remaining in the joint began to become a noticeable percentage of the total joint-volume remaining inside the extremely thin brazed joint, and began to negatively affect the joint strength due to their presence (percentage wise). Had the joints been able to be “wiped” thoroughly (the joint surfaces moved back and forth relative to each other while still being heated with the flame), it might have helped to remove some of those entrapped flux voids, but it wouldn’t have removed all of them.
Now, let’s look at a similar chart, as shown in Fig. 2 below:
Notice in Fig. 2 that the tensile strength testing done by T. H. Gray shows increasing strength of four (4) different BFM, each brazed in an inert atmosphere furnace using very dry dissociated ammonia (one part nitrogen, three parts hydrogen), and that the strength continues to increase even with gap clearances of only about 0.0005″ (0.01mm). This represents acceptable joint clearances that are only one-third the joint-thickness of the 0.0015″ (0.04mm) “peak-strength” shown in Fig. 1. Thus, because there is no flux in the braze-joint, there is no “fall-off” of joint strength as the gap gets tighter. Thus, there is really no “minimum required gap size” below which strength would fall off.
Robert Leach, VP of Handy & Harman (Bridgeport, CT) back in 1945, and who was the creator of the chart which is shown above in Fig. 1, wrote to T. H. Gray after reading Gray’s article containing the chart shown in Fig. 2, and said:
“Mr. Gray mentions the results of some experimental work which was done under my direction several years ago on some butt joints with stainless steel sheet 0.065″ thick and a silver brazing alloy which flowed freely at 1175F (635C). The results of that work did show a maximum strength at 0.0015 inch, and the graph of that work has been used by my associates and me as an argument for close spacing. The heating was done with a torch, the joints and brazing alloy were protected with flux and the alloy flowed into the joint by capillarity. I believe that the work justified the recommendation of close spacing and careful fitting for good brazing practice with no thought that undue emphasis should be placed upon 0.0015 inch as an exact spacing which should be accepted as the ideal one for all brazing. It is with considerable satisfaction that I find the author has been able to demonstrate the advantage of a clearance of only 0.0005 inch in his particular testing.”
CONCLUSION: The drop-off in strength-values on the widely-used chart shown in Fig. 1 has nothing to do with so-called “negative effects of tight joint clearances”, but instead, has only to do with the inevitable presence of flux (and thus flux voids) in the brazed-joints used in those specific tests at that time. Were these parts to be brazed in a protective inert atmosphere such as nitrogen or argon, or in a vacuum furnace, there would have been no fall-off of strength values, but instead, they will have continued to increase as the joint-gap got tighter.
CALL FOR TESTING: It would be good if someone reading this article could get some testing funded to repeat these tests, and verify today the results indicated in this article.
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