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.
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.
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