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“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…

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