Keys can break in locks due to weak metal or cuts that are too deep. Removing a broken key from a lock requires patience and the right tools. Professional locksmiths have specialised tools to remove broken key pieces, but household tools can also do the job.
A key extraction tool, which is used by professional locksmiths, is a long, thin piece of strong metal with small teeth or barbs on the end. When the tool is inserted into the lock containing the broken key piece and then pulled back out, the teeth grab the piece and pull it free.
Depending on how far in the lock the break happened, you may be able to pull the broken piece out using a pair of needle nose pliers.
The blade of a coping saw, scroll saw or jigsaw can work to remove a broken key, as the saw blade teeth act much like the teeth from a professional extraction tool. The blade must be thin enough to fit easily into the lock and "bite" into the key piece using its teeth.