Over time, the extreme heat and rapid change in temperature can warp and damage the fireplace damper, thus needing replacement. A damper must be in one piece that is snug to the flue in order for it to work properly. If the damper cracks while being rapidly heating and cooling, it will no longer work properly. 

What is a Damper?
A damper is a plate or mechanism used to close off the venting system when not in use. A fireplace damper is usually located at the throat of the fireplace, the intersection of the firebox assembly and the smoke chamber assembly. There many types of dampers: Throat dampers; Vestal dampers; Allied/Donley dampers; Majestic dampers; Dampers for steel fireplace; top mounted dampers; barometric dampers etc.