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Tom,
It is difficult to tell what the exact failure mode is by looking at your photographs. To be sure you would have to do a micro-section of the the weld (a small piece of the weld filler metal, the weld effected zone of the tube, and the tube would have to be encapsulated in epoxy then polished and viewed under a 20x microscope).
However, from your photos it really looks like there was very little weld penetration into the tube material (the filler metal did not melt and mix with the tube material as a result of a low temperature weld,or the duration "dwell time" of the weld was to short to get proper penetration).
Usually when metals work harden you see a lot of exposed grain structure that has been smeared but in this case it is not obvious by the photos.
Hope you feel better soon.
Keith
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