Flank wear is the “normally expected” failure mode for tools to fail when machining steels.
The volume fraction of Manganese Sulfides is a determinant of the tool’s wear rate. “The wear rate of high speed steel tools decreases rapidly up to about one percent volume fraction of MnS and then levels off to a constant wear rate as the volume fraction is increased.“-Roger Joseph and V.A.Tipnis, The Influence of Non-Metallic  Inclusions on the Machinability of Free- Machining Steels.

Manganese and Sulfur have a powerful effect in reducing flank wear on HSS tools
Manganese and Sulfur have a powerful effect in reducing flank wear on HSS tools

As sulfur rises beyond 1% volume fraction, surface finish improves, chips formed are smaller with less radius of curvature, and the friction force between cutting tool and chip decreases due to lower contact area.
Manganese sulfides are a separate internal phase.
Manganese sulfides are a separate internal phase.

How does Manganese Sulfide improve the machinability?

  • The MnS inclusions act as “stress raisers” in the shear zone to initiate microcracks that subsequently lead to fracture of the chip;
  • MnS inclusions  also deposit on the  wear surfaces of the cutting tool as “Built Up Edge (BUE).”
  • BUE reduces friction between the tool and the material being machined. This contributes to lower cutting temperatures.
  • BUE mechanically separates or insulates the tool edge from contact with work material and resulting heat transfer.

This is why resulfurized steels in the 11XX and 12XX series can be cut at much higher surface footage than steels with lower Manganese and Sulfur contents.
More info about Manganese in steel HERE

Cutting fluids contribute 3 ways to our machining processes.

Critical to our shops for three reasons.

1) They provide lubrication. Lubrication reduces friction. Reduced friction means less heat. Less heat means better dimensional capability and faster cycle times are possible.
2) They help remove heat. Even though they provide lubrication, some frictional heat is produced. So the ability to capture, and remove heat is an important property of metalworking fluids.
3) They prevent the welding of workpiece material onto the tool.Pressure of the workpiece against the tool and the heat and temperatures involved contribute to the buildup of workpiece material on the tool. Cutting fluids provide an antiweld film to discourage this.
For a thorough tutorial on the subject of cutting fluids, check out Barbara Donohue’s article  on this subject in Today’s Machining World click this link.
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