What causes work hardening is the heat generated by the cutting tool transferring to the work piece material causing plastic deformation. In fact, it is appears to act like a heat treatment to the work piece but on a lower scale.
To recognize this condition the part being machined will have a very shiny glazed surface and appear slippery. The hardness in the machined part can even realize the same hardness of the cutting tool.
Steps to take to overcome this condition:
When experiencing tap breakage, the cause may not be the tap, but a work hardened hole. Materials likely to work harden are stainless steels and high temperature alloys. Once again, proper tool maintenance will help to reduce work hardening problems.