Material Work Hardening in Machining

Work hardening of materials is a condition to be avoided. 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:

1. Make sure the cutting tools are always sharp!

2. Run at the recommended feeds and speeds for the material being machined. If incorrect, rubbing vs. cutting will increase heat.

3. Use coolant-feeding tools. Water based coolant should be used at about 8% to 10% mix.

4. Do not dwell tool in one position.

5. When drilling, run with constant feed if possible.

6. If peck drilling, reduce number of pecks and withdraw 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.