shrink-ring

from The Century Dictionary.

• noun A ring placed when hot around some other piece, so that in the process of cooling and consequent contraction it will exert a powerful compression and add strength to resist internal strains in the piece.
• noun A ring so located in a structure that all tendencies to contract on cooling shall be concentrated at this point, and can there be compensated or allowed for and their effects reduced or eliminated.
• noun Same as a shrink- or shrunk-collar.