I'm not a physicist, but two thoughts: (a) if there is any adhesive on the joints, heat may loosen it or (b ) the expansion of the different parts may not occur at the same rate, and so the very act of disturbing one part may be enough to loosen it.
Yes and potentially yes to (a) (soften) and (b ) (provided the coefficient of thermal expansion for the inner part is not greater). Where the parts are of the same material, consider that there will be a very small gap between them. However small this gap, for it not to increase with heat would imply that the inner part expands more than the outer to maintain the same physical gap, rather than the gap being proportionately greater. If they are the same material, that makes no sense.
I can work an example if that is not yet clear.
Edited by praxim, 04 October 2017 - 09:37.
"...all observation must be for or against some view if it is to be of any service." Charles Darwin