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

Why Lead Is NOT An Alloying Element In Steel

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Lead is added to steels to improve their machinability. But Lead is not considered an alloying element.

An Alloying element is “An element which is added to a metal (and which remains within the metal) to effect changes in properties,” according to my copy of the Metals Handbook Desk Edition.

Arrows point to lead.

While lead is an element that is added to a metal:

It does not remain in the metal, it remains separate from and mechanically dispersed in the steel as ‘inclusions’ when it solidifies. It is the dark material on the ends of the manganese sulfides in the photo above.

It does not change mechanical properties of the steel.

“Lead can be added to both carbon and alloy steels to improve machinability…The lead is present as small inclusions that are usually associated with the manganese sulfide inclusions…Lead has no apparent effect on the yield strength, tensile strength, reduction of area, elongation, impact strength, or fatigue strength of steel. “- Cold Finished Steel Bar Handbook
For this reason, the addition to lead to steel is not considered an alloying addition. The addition of lead is a great way to improve the economics of machining and improving the surface finish of complex parts from steel.
Photo from L.E. Samuels Optical Microscopy of Carbon Steels

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.

3 Actions To Minimize Cracks In Our Shops

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Paying attention to draft, chemistry, and steel melt source processes can help you minimize the potential for cracks at your customer after cold work operations.
After a crimping, staking or swaging operation, cracks can develop. This is because the cold work needed to swage, stake, crimp, etc. was greater than the material’s available elasticity. This is the case in the part photographed here.

Cracks can develop after cold work is performed on machined parts.

In order to minimize cracking during or after crimping, or thread rolling, or other substantial cold work, take the following steps:

Specify non-renitrogenized material;
Inform your supplier of your cold work application. They can consider reducing cold draft, or changing suppliers of the hot roll to get basic oxygen process, low residual, low nitrogen steel;
Ask the customer to consider changing the grade. Resulfurized steels are capable of being somewhat cold worked, but their high volume fraction and weight percent of nonmetallic inclusions (What makes them cut so well!) is also what works against successful cold work.

To minimize the occurrence of cracks that are not a result of cold work, try this:

Assure that adequate stock removal is taken in machining;
Buying from reputable sources whose quality systems employ rototesting and eddy current testing;

When cracks are discovered in your shop, what actions do you take?