Rob, typically the shorter term overheating (10% swelling) occurs locally, where the tube bulges. These types of failures can often be attributed to some immediate problem such as an internal blockage. The overall swelling is indicative of long term overheating, and typically doesn't occur...
In addition to the evaluation method discussed by metengr, another point to consider is that prior to failure, Cr-Mo tubes will commonly swell. The amount of swelling is dependant on the failure cause; a tube that fails due to an internal blockage will swell much more (10% or more) than one that...
