July 24, 2012
Shearing occurs when a longitudinal strip of base metal is torn off a bar during rolling. This strip often reattaches as rolling continues, not necessarily to the same bar. Shearing can refer either to the the discontinuity resulting from the detachment or to the subsequent reattachment. There are usually several occurrences of shearing with a single orientation along the bar.–AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
Mill Shearing is commonly mistaken for Slivers, Scabs, Laps, and Seams.
Mill shearing is usually detected visually and appears longer than scabs. also, the surface below the defect is smoother and more uniform than found below scabs.
Excessive rubbing of the steel as it rolls through the mill causes overheating, shearing material off the bar, which is later picked up from mill components on the same or another billet.
Improved guiding, pass design, and better section control can reduce incidents of mill shearing.
- Rolled in material may have come from some source other than the base material.
- If the material which is removed by shearing is not reattached, the remaining gouge in the surface may form other defects upon further rolling.
- Intergranular precipitates or segregation can contribute to mill shearing.
- Adjust mill to reduce sources of friction(al) heating
The rolled in material may have come from some source other than the base metal.
In my experience, mill shearing presents as and is easily confused with laps and slivers. Confirming that it is a piece of foreign material that has been rolled into the product is easily confirmed with a pair of pliers to remove it. Mill shearing almost always is removable by such means, and will show as two completely separate pieces of material in a micro.
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.
May 15, 2012
“Laps are longitudinal crevices at least 30 degrees off radial, created by folding over, but not welding material during hot working (rolling). A longitudinal discontinuity in the bar may exist prior to folding over but the defect generally is developed at the mill.”- AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
Here is my lab notebook entry for a lap back in 1985…
In plain language, a lap is a ‘rolled over condition in a bar where a sharp over fill or fin has been formed and subsequently rolled back into the bar’s surface.’
Photo of a lap from AISI Surface Defects Manual.
An etch of the full section shows what is going on in the mill. Laps were often related to poor section quality on incoming billets, although overfill scratches, conditioning gouges from “chipping” have also been shown to cause laps.
Cross section of steel bar exhibiting laps (white angular linear indications). When two laps are present 180 degrees apart, the depth to which they are folded over can indicate where in the rolling the initial over fill ocurred. White indicates decarburization, which confirms my interpretation that this lapping occurred early in the rolling.
Laps are often confused with slivers, and mill shearing which we shall describe and post soon.
The term ‘lap seam’ is sometimes used, but it is careless usage; it implies the lap is caused by a seam – it is not; a seam is a longitudinally oriented imperfection, and so is used in this mongrel term as a shorthand way of saying ‘longitudinal.’
Modern speakers sometimes try to use the word ‘lamination’ to describe laps but as we will see, not all lamination type imperfections are laps…