Originally Posted by
rydabent
From the pictures it still looks like there was no weld penetration. Even for good welders it is hard to tell.
If you look at the HT end you can see that the tubes broke behind the welds, not at them as would be caused by poor penetration, so I don't think that was the problem here.
It's very hard to get poor penetration on tubing this thin but it is quite easy to tell how you did by looking inside the HT after the weld for just the right amount of bumpiness on the back side.
They look like good welds to me and my theory is still that the tubes are made of a defective alloy.