Plastic deformation: what it looks like, why it happens, how to fix it
The edge bulges or slumps; a bright, rounded land forms that resembles flank wear, the clearance collapses and the edge then fails quickly. Heat plus pressure exceed the hot hardness of the carbide substrate. At roughly 600 to 800 degrees C at the tool-chip interface the cutting wedge creeps and the edge sinks (plastic lowering). This is not wear: no material is removed at first.
| What you see | Mechanism | First fix | Most affected |
|---|---|---|---|
| The edge bulges or slumps; a bright, rounded land forms that resembles flank wear, the clearance collapses and the edge then fails quickly. | Heat plus pressure exceed the hot hardness of the carbide substrate. | Reduce Vc and feed, and use a substrate with higher hot hardness; improve cooling at the edge. | ISO K (cast iron), ISO H (hardened steel), ISO M (stainless steel) |
Why it happens
- Edge temperature too high for the substrate, typically from cutting speed and heavy chip load together.
- Low thermal conductivity work materials (titanium, heat-resistant alloys) keep the heat in the wedge; serrated chips add cyclic load.
- Higher cobalt content in the substrate increases the deformation.
How to measure and judge it
Look for a rounded, shiny edge and a bulge below the cut rather than a flat, matte land. If the shop can measure both radial wear and the flank land, a ratio that drifts over tool life indicates creep; with pure abrasion that ratio stays constant.
Fixes, in the order to try them
- Reduce Vc first, then feed.
- Choose a grade with higher hot hardness (less cobalt, a lower number in the ISO group).
- Keep the clearance angle as small as the setup allows: extra relief removes material that supports the edge and makes it worse.
- Use a chipbreaker rake or an edge chamfer that shortens chip contact and supports the edge.
Do not confuse it with
It looks like flank wear, but the fix is opposite: a "harder, more wear-resistant" grade does not help if the wedge is creeping. See flank wear and plastic lowering in detail.
Other wear modes
Flank wear · Crater wear · Notch wear · Built-up edge (BUE) · Thermal cracking · Edge chipping and fracture · All eight failure modes in one table · Mechanisms in depth
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Frequently asked questions
What is plastic deformation on an insert?
It is the cutting edge sinking or bulging under high temperature and pressure because the carbide substrate creeps. It is not abrasive wear, and it usually ends in a fast collapse of the edge.
How do you stop plastic deformation of the cutting edge?
Lower the cutting speed and feed, use a substrate with higher hot hardness, keep the clearance angle small and use a geometry or edge preparation that supports the edge.