Keilim 11:1

Flat metal utensils and their receptacles are susceptible to ritual impurity; if they break, they are rendered clean. If one re-made them into utensils, they revert to their previous unclean status. Rabban Shimon ben Gamliel says that repaired utensils don’t re-acquire every type of impurity, just corpse impurity.

Keilim 11:2

Any type of metal utensil that has its own name is susceptible to ritual impurity except for a door, a bolt, a lock, a socket, a pin, a beam and a tube (all of these are various parts of a door). These are insusceptible because they are made for use when attached to the ground.