“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 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.
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.
Slivers are often mistaken for shearing, scabs, and laps. We will post about these other defects in the future.
The machinability of steel bars is determined by three primary factors. Those factors are 1) Cold Work; 2) Thermal Treatment; 3) Chemical Composition.
Cold Work improves the machinability of low carbon steels by reducing the high ductility of the hot rolled product. Cold working the steel by die drawing or cold rolling results in chips that are harder, more brittle, and curled, prodcuing less built up edge on the tools cutting edge.. The improved Yield to Tensile Strength ratio means that your tools and machines have less work to do to get the chip to separate. Steels between 0.15- 0.30 wt% carbon are best machining; above 0.30 wt% the machinability decreases as carbon content (and hardness) increase.
Thermal Treatment improves the machinability of steel by reducing stresses, controlling microstructure, and lowering hardness and strength. While this is usually employed in higher carbon steels, sometimes a Spheroidize Anneal is employed in very low carbon steels to improve their formability. Stress Relief Anneal, Lamellar Pearlitic Anneal, and Spheroidize Anneals are the treatments applied to improve machinability in bar steels for machining.
Chemical composition is a major factor that contributes to the steel’s machinability or lack thereof. There are a number of chemical factors that promote machinability including
Carbon- low carbon steels are too ductile, resulting in gummy chips and the build up of workpiece material on the tool edge (BUE). Between 0.15 and 0.30 wt% carbon machinability is at its best; machinability decreases as carbon content increases beyond 0.30.
Additives that promote machining include
Sulfur combines with Manganese to form Manganese Sulfides which help the chip to break and improve surface finish.
Lead is added to steel to reduce friction during cutting by providing an internal lubricant. Lead does not alter the mechanical properties of the steel.
Phosphorus increases the strength of the softer ferrite phase in the steel, resulting in a harder and stronger chip (less ductile) promoting breakage and improved finishes.
Nitrogen can promote a brittle chip as well, making it especially beneificial to internal machining operations like drilling and tapping which constrain the chip’s movement.
(Nitrogen also can make the steel unsuitable for subnsequent cold working operations like thread rolling, crimping, swaging or staking.)
Additives that can have a detrimental effect on machining include deoxidizers and grain refiners.
Deoxidizing and grain refining elements include
These elements reduce machinability by promoting a finer grain structure and increasing the edge breakdown on the tool by abrasion.
Alloying elements can be said to inhibit machinability by their contribution to microstructure and properties, but this is of small impact compared to the factors listed above.
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.
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
Lead is NOT banned by the European Union’s End of Life Vehicles Regulations for machining purposes in steel, aluminum and brass. PDF HERE
Lead is NOT banned by the European Unions Restriction of Hazardous Substances (RoHS) Directive.
The exemption for “Lead as an alloying element in Steel containing up to 0.35% lead by weight, aluminum containing up to 0.4% lead by weight, and as a copper alloy containing up to 4% by weight.” This exemption is located in article 4.2 and Annex, line 6. UK link to RoHS exemptions If even the European Union recognizes that additions of Lead in materials for machining is worthy of exemption, Lead must provide some significant benefits…
“Boosts machinability 25% at lower cost”- Pat Wannell, La Salle Steel April 1994, quoted in Modern Metals Magazine
“Cutting Speeds can normally be increased from 15-25% above those employed for the standard grade”- Monarch Turning Manual
“Lead, found mainly enveloping manganese sulfide inclusions, promotes machinability in two ways, possibly three. By forming a layer of liquid lubricant at the tool chip interface, it reduces the stress required to overcome friction. By acting as an initiator of microcracks and, possibly, by causing some liquid metal embrittlement, it reduces the deformation stress.” American Machinist Special Report 790.
In our experience we have found leaded steels to lower cutting temperatures and reduce wear rates on tools, resulting in greater up time. Surface finish on leaded materials is superior to those on non leaded equivalents.
Increasing speeds and production, reducing power needed (and thus greenhouse gas emissions), and improving surface finish are some powerful advantages that are provided by the addition of lead to materials for precision machining. What’s the down side?
1) Lead is not soluble in iron. It is therefore a separate phase in the steel, usually visible enveloping the manganese sulfides as tails, though sometimes appearing as small particles. 2) Lead has a greater density than iron. This means that it will tend to segregate given enough time while the metal is liquid. 3) Lead has a relatively low melting point (liquidus) compared to steel. This can mean that at processing temperatures for heat treatment, leaded steel parts can ‘exude’ lead
These three factors mean that if you ABSOLUTELY MUST HAVE parts that are free from possible segregation, parts that will not have potential hollows or porosity after being exposed to high temperatures, and absolutely no visible indications of a separate phase in the steel (ie. what the shop guys call “lead stringers,” you probably ought to forego the leaded grade. And forego the 25-30 % savings that it gives you on the piece part machining cost…