Work Hardening

Work hardening of materials is a condition to be avoided.

Work hardening is caused by 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.

This condition is often the result of a cutter rubbing against the workpiece instead of actually cutting the workpiece material. This can be caused by dull cutters, too slow a feedrate, or other conditions.

The following steps should be taken to overcome this condition:

Make sure the cutting tools are always sharp!
Run at the recommended feeds and speeds for the material being machined. If incorrect, rubbing vs. cutting will increase heat.
Use coolant-feeding tools where possible. Water based coolant should be used at about 8% to 10% mix.
Do not dwell tool in one position.
When drilling, run with constant feed if possible.
If peck drilling, reduce the number of pecks and withdraw from each one tool diameter.
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.