Most residential and commercial keys consist of milled brass, often plated with nickel for corrosion resistance. Brass is intentionally soft. Lock manufacturers prioritize malleable keys so that repeated operation wears down the replaceable key rather than the expensive internal mechanism. That metallurgical choice carries an inherent vulnerability. Every rotation against an unlubricated bolt creates rotational shear along the lowest cut of the key blade, which is precisely where the metal is thinnest.
When a snapped brass key fragment stays behind, the internal physics of the lock shift into lockdown mode. Pin-tumbler locks rely on pairs of pins: upper driver pins and lower key pins. When you insert a complete key, the cut depths lift those pins flush to the shear line, allowing the plug to turn.
If the key snaps mid-turn, the plug stops halfway between locked and unlocked positions. The lock cylinder pins drop out of alignment, jamming into the plug walls and locking the core tight. You cannot extract the blade until the plug returns precisely to its neutral, vertical resting position. Attempting to force the fragment out while the deadbolt keyway sits at an angle will break your tools and gouge the brass housing.
