March 12, 2015
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
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.
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
May 17, 2012
“Slivers are elongated pieces of metal attached to the base metal at one end only. They normally have been hot worked into the surface and are common to low strength grades which are easily torn, especially grades with high sulfur, lead and copper.”- AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
Slivers are loose or torn segments of steel that have been rolled into the surface of the bar.
Slivers may be caused by bar shearing against a guide or collar, incorrect entry into a closed pass, or a tear due to other mechanical causes. Slivers may also be the result of a billet defect that carries through the hot rolling process.
This is my lab notebook sketch for slivers ‘back in the day…’
Slivers often originate from short rolled out point defects or defects which were not removed by conditioning.
Billet conditioning that results in fins or deep ridges have also been found to cause slivers and should be avoided. Feathering of of deep conditioning edges can help to alleviate their occurrence.
Slivers often appeared on mills operating at higher rolling speeds.
When the frequency and severity of sliver occurrence varies between heats, grades, or orders, that is a clue that the slivers probably did not originate in the mill.
This is how Slivers present under the microscope. Note decarburization (white appearance.)
Slivers are often mistaken for shearing, scabs, and laps. We will post about these other defects in the future.