This is what happens eventually.
I could give a whole metallurgy class, but I'll leave it at this. Metal has a grain structure to it, everytime you heat cycle your headers you're breaking it down more and more. The area next to the weld, referred to as the HAZ (heat affected zone) will break down and fail first because it was subjected to high temps during welding also. It's not a matter of if, but when will any header begin to crack. These cracks are most commonly on the toe of the weld and grow inward through the base material (pipe).
- Clayton. Official C5Fail4 hater.