As Director of Technology and Industry Research for PMPA, Miles brings 38 years of hands-on experience in areas of manufacturing, quality and steelmaking. He helps answer "HOW?","WITH WHAT?" and "REALLY?"
The 0.15- 0.35 weight percent of lead contained in these bars helps them machine 25 percent faster with less power required.
Leaded steel bars historically have been a mainstay raw material in the screw machining industry. As more applications and newer technology move toward non-leaded steel applications, I thought that a brief refresher about lead and its role in shops might be timely.
Leaded steel bars are standard steels and widely available. In the U.S. 12L14 is the predominant grade. 11SMnPb30, 11SMnPb28, 9SMnpb28, and 9SMnPb36 are German designations nominally equivalent to 12L14. The Chinese version of 12L14 is Y15Pb; Japanese nominal equivalents include SUM22L, SUM23L and SUM24L.
Leaded steels are selected for use for the savings achieved in producing parts by machining.
Leaded steels are not appropriate for all parts, and parts with low amounts of stock removal may not create any noticeable savings.
Today’s leaded steels are more consistent, more uniform, than they were when produced by the ingot process.
The decision to use leaded steels for a specific part must be based on the economics for that part—volume, stock removal, part complexity, tolerances required, surface finish needed are all factors that contribute to that economic calculation.
There is no sacrifice in mechanical properties when adding lead to steel. Neither longitudinal nor transvers mechanical properties are affected by the addition of lead to steel.
Leaded steels are currently permitted under European Union regulations covering End of Life Vehicles, RoHS.
The reduction in energy required and time needed (about 25 percent!) to machine a part make leaded steels environmentally friendly by reducing carbon dioxide emissions to create parts compared with using unleaded materials.
In order to be dangerous to humans, lead must be in a soluble form. The lead in steel bars is a separate solid phase. IARC lists lead under its Group 2B category, “possibly carcinogenic to humans.”
Lead, as well as chromium, copper, manganese, nickel, and phosphorous is required to be reported under Sara 313 (40 CFR 372.65) when they are above established thresholds.
Manganese and sulfur have a powerful effect in reducing flank wear on HSS tools.
Manganese sulfides are a separate internal phase.
Flank wear is the “normal” failure mode for tools 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 1 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 because of 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.
My Swedish friend posted this on Facebook, and I did my best to figure out the meaning.
One of my international contacts through the Syndicat International du Decolletage posted an intriguing photo on Facebook. I did my best to try to figure out what “gnosjöandan” means, and eventually, I came up with “entrepreurship.” (The Facebook post was in Swedish.)
So I responded to my friend with “Entrepreneurship?” and waited to see how well I had done. You be the judge, and by the way, don’t be too surprised to find out that you, too, have the “gnosjöandan spirit.”
We are motivated by others’ successes.
We encourage cooperation.
We are ambitious and dare to be great.
We are street smart.
We are doers.
We don’t overcomplicate things.
We are generous and helpful.
We are thrifty, but never stingy.
We dig where we stand.
We are proud of the “gnosjö spirit.”
This reminded me a lot of the entrepreneurial spirit that we find in our shops. What do you think? Do you, too, have the gnosjöandan spirit?
Tool life can vary when machining carbon and alloy steels despite the use of our best technology and our efforts to control our processes. This post discusses factors in the steel that can lead to tool failure. Here are six factors that can affect tool life in your shop:
Variations between suppliers. Suppliers’ melt processes, scrap practices, melt recipes and reduction in cold drawing and straightening practice can significantly affect the way the chip breaks, the resulting built up edge on the tool and the resulting surface finish, even though the grade is the “same.”
Variations in chemistry. A potential subset of variations between suppliers, the fact is that a plain carbon grade with 0.005 weight percent sulfur will not machine at all like the material with 0.025 weight percent sulfur.
Variations in grain size. While this factor is typically more relevant in stainless steels, when machining forgings, blocky structures resulting from excessive forging temperatures can result in inconsistent machining performance.
Variations in microstructure. In this case, it is not so much about the grain size, as it is about the structure present. This is particularly problematic in the ~0.40 weight percent carbon alloy grades such as 4037.
Decarburization or scaling on the work surface. Decarb can result is a carbon poor gummy surface, only to then transition into a fully carbon-containing microstructure. Scale on the work surface can result in excessive tool wear, because of the very high hardnesses of the various iron oxides that may be present (Hematite, the red oxide of iron, Fe2O3, has a microhardness of approximately 1030 DPH.
Deoxidation /high inclusion count. Free machining grades such as 12XX and 11XX steels are typically sold to a “coarse grain practice” with no deliberate additions of grain refiners or deoxidizers. Sometimes, you may find deliberate additions of Silicon to 1144 in order to improve the internal soundness of the steel. The resulting silicates can abrade the edge of the tool when running at the surface feeds expected for a resulfurized steel. The addition of aluminum as a grain refiner can cause rapid edge wear as well. Rarely, very rarely, one might encounter exogenous inclusions entrapped in the steel from melt and casting. These can be real showstoppers.
The takeaway: Purchasing the same item from different suppliers hurls the range of global variation at your machining operations. Standardizing on a single supplier for an item will allow you to get to a steady state in your process.
Our cheat sheet for moving from leaded steel to unleaded steel provides a roadmap for adjusting to unleaded brass.
Unleaded brasses are not necessarily harder to run than leaded brass. They are just different. By recognizing and accommodating for their lack of lead, and the different thermal conductivity, differences in chip forming, and the need to up-tool for heavier feeds rather than higher speeds, your shop can also be successful at making parts from these newer, more challenging grades.
It is widely acknowledged that lead promotes machinability. To get the maximum production from automatic machines, additions of lead have been commonly used in metals, particularly steels and brasses. In brass, dispersed in the grain boundaries, lead acts as an internal lubricant: It reduces friction, and thus heat. By reducing the heat, lead allows the metals to which it has been added to be machined at much higher speeds than the comparable non-leaded grades. These higher speeds (rpm or surface feet per minute) result in shorter cycle times to produce each part. Short cycle times mean less expensive parts.
Leaded brass offered these historical advantages:
Excellent surface finish
Forgiving of machine mis-adjustments
No thermal issues
Fast cycle times
No chip control issues
When machining non-leaded materials, we have to somehow maintain surface finish, get to commercially feasible cycle times, and deal with less than ideal chip characteristics.
What are some strategies for machining the new unleaded brasses?
Increase the feed. Since we lost the lead and the ability to run at higher speeds, increasing the feed can help us get to equivalent cubic inches of removal rates.
Improve the machine rigidity. Heavier feeds mean that your machine needs to be adjusted and solid. It also means more horsepower required— again mandating a rock-solid setup.
Improve the tool. 4 percent lead is very forgiving of tool quality; the new nonleaded grades are the opposite—they present a number of challenges to your tools. Improved materials, geometry and coatings are key to machining unleaded brasses with minimum issues. Also, they will require fewer replacements, helping to get more net production at the end of the shift.
Improve the chip management. Some unleaded grades replace the lead with zinc, resulting in a grade with a type III chip—stringy and bird’s-nest prone. With these grades, pay special attention to drills selected, and try inserts with chip control features to help you manage that chip.
Deal with the increased heat. The lead helped to reduce friction and heat in the leaded grades. With the lead removed, you will have increased heat generated. Carbide is more forgiving of heat, as are tool coatings. Talk to your supplier of metalworking fluids. Chances are, they will have a fluid that will help manage those extra BTUs and maintain your tools’ edges.
Change your ideas about machining brass. Unleaded brass machines more like steel than brass as long as you think of it like leaded brass, you will fight it. Instead, think of it as just a yellow version of 1215 steel or stainless and your expectations will be much closer to reality.
Unleaded brasses are not necessarily harder to run than leaded brass. They are just different. By recognizing and accommodating for their lack of lead and the resultant different thermal conductivity, differences in chip forming, and the need to up the tool for heavier feeds rather than higher speeds, your shop can also be successful at making parts from these newer, more challenging grades.
The market for our precision machined parts continues to be evolve. Evolve your thinking and processing to adjust to the realities of unleaded materials to remain a viable and preferred supplier.