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…
July 29, 2009
Paying attention to draft, chemistry, and steel melt source processes can help you minimize the potential for cracks at your customer after cold work operations.
After a crimping, staking or swaging operation, cracks can develop. This is because the cold work needed to swage, stake, crimp, etc. was greater than the material’s available elasticity. This is the case in the part photographed here.
Cracks can develop after cold work is performed on machined parts.
In order to minimize cracking during or after crimping, or thread rolling, or other substantial cold work, take the following steps:
- Specify non-renitrogenized material;
- Inform your supplier of your cold work application. They can consider reducing cold draft, or changing suppliers of the hot roll to get basic oxygen process, low residual, low nitrogen steel;
- Ask the customer to consider changing the grade. Resulfurized steels are capable of being somewhat cold worked, but their high volume fraction and weight percent of nonmetallic inclusions (What makes them cut so well!) is also what works against successful cold work.
To minimize the occurrence of cracks that are not a result of cold work, try this:
- Assure that adequate stock removal is taken in machining;
- Buying from reputable sources whose quality systems employ rototesting and eddy current testing;
When cracks are discovered in your shop, what actions do you take?