How Chemistry Determines Machinability- Manganese Sulfides

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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

The Point of the Tool Seldom Cuts

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blurry tool copy — My apologies for the scanning lines folks. Click the link below for the video.

In most of this footage you will see that there is a small mass of workpiece material (Built Up Edge or BUE) that is doing the “penetration.”

Video

The fracturing ahead of the tool, and the occassional jamming of material under the tool, and the waviness of the generated surface are concepts to keep in mind when you try to understand why you are getting the finish on the surface that you get.

Also, a good way to visualize how the material is being workhardened by “rubbing” where the material is not separating easily ahead of the tool.

What you are seeing.

Built Up Edge-What It Is And What To Do

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bue mit — Some built up edge (BUE) is normally encountered in machining

Built Up Edge (BUE) is the accumulation of workpiece material onto the rake face of the tool. This material welds under pressure, and is separate from the chip.
In school we were taught that this is because the first material to contact the tool workhardens, and we did hardness tests to confirm this.
Because BUE changes the effective geometry of the tool, it can have either positive or negative effects.
Positive effects

Less tool wear
Lower power requirements
Less contact of the workpiece with the tool (It contacts the BUE instead)
Better surface finish and improved process capability

These effects are only beneficial if the BUE is thin and stable. Machining additives such as sulfur combine with manganese to form manganese sulfides. Manganese sulfide helps to control BUE because of its anti weld properties. On resulfurized steels, BUE is usually stable and not a problem.
Negative effects

Poor tool life
Poor and variable surface finish ( As the BUE sloughs off the tool, it can weld to the workpiece)
Loss of statistical capability on dimensional control
Loss of uptime trying to troubleshoot the process

I have found that BUE is more likely on alloys that work harden.
In order to get BUE under control, the steps that you take depend on the tool material.
For Carbide

Decrease the feed. (Pressure welding usually is the culprit)
Increase the speed
Increase the rake angle or “hook”
Get a better metalworking fluid (including get the fines out of your existing MWF!)
Get a different coating

For High Speed Steel (HSS)

Reduce speed

If the tool is High Speed Steel (HSS) you may think you are in oppositeland when you discover that slowing down the speed reduces the build up. I have found on HSS that as speed (heat increases) so does the tendency to form BUE.
Graphic.

The Chatter About Chatter

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Chatter costs money when you reduce the productivity of the machine by slowing it down to make the vibration go away.

Conventional wisdom states that there are two kinds of chatter- Forced and Self Induced. Some shop guys like to think its caused by the material.
Forced chatter is a result of alternating cutting forces that result from
1) Interrupted cuts (milling);
2) Machine vibrations such as out of balance motors, spindles, gear or shaft irregularities, bad couplings or bearings, (Loose motor mounts and weakened or stretched) couplings;
3) Load on tool / workpiece changing as a result of acceleration or decceleration;
4) Vibrations being transmitted through the machine and foundation from other equipment.
If forced vibration is what you have, confirming the integrity of the machine tool and its power train is a critical first step. Reducing the feed per revolution is one way to determine if it is the variation in the cutting process that is forcing the vibration. Changing the SFM or RPM’s by at least 25% is also something to try (increase or decrease!)
Self excited chatter is induced by a change in the cutting forces themselves, and is where I place the chatter that may be caused by the material. Self excited vibrations can be distinguished from forced vibrations in the machining system because self excited vibrations stop when the cutting does. Forced vibrations are not dependent on the cutting process, and so continue even when the tool is not in the cut. Self excited chatter can be caused by:
1) Change in forces needed to cut caused by differences in the material- Material characteristics (such as workhardening or microstructural differences) that result in variation in chip thickness.
2) Unstable built up edge (BUE) forming then breaking off causing variation in the cutting forces
3) insufficient Stiffness of the workpiece, spindle, tool and tool holding (think deflection and too much length).
To eliminate self excited chatter decrease the length of the tool in the cut, shorten the tool holder, or substitute more rigid tooling and support materials (a carbide boring bar deflects less and can make three or more times heavier a cut than one made of steel for example) .
Still think it’s the material? Here’s my Metallurgist’s tip: Look and see if you have changing build up edge conditions on the tool that exhibits chatter. If the self excited chatter is due to material such as an unstable built up edge (BUE) forming, try increasing the RPMs / SFM. Spindle speeds that are too slow allow workpiece material to weld to the tool edge (pressure weld) and build up.This creates higher forces until it sloughs off. Then forces go back to normal, and build up again until…
Higher RPMs help to keep BUE stable and under control. And they allow you to run faster cycle times, contributing to profits.
Bottom line: Chatter doesn’t always mean you need to slow down.
Photo credit.