“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.
“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
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.’
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.
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…