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 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
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.
“Seams are longitudinal crevices that are tight or even closed at the surface, but are not welded shut. They are close to radial in orientation and can originate in steelmaking, primary rolling, or on the bar or rod mill.”– AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects
Seams may be present in the billet due to non-metallic inclusions, cracking, tears, subsurface cracking or porosity. During continuous casting loss of mold level control can promote a host of out of control conditions which can reseal while in the mold but leave a weakened surface. Seam frequency is higher in resulfurized steels compared to non-resulfurized grades. Seams are generally less frequent in fully deoxidized steels.
Seams can be detected visually by eye, and magnaglo methods; electronic means involving eddy current (mag testing or rotobar) can find seams both visible and not visible to the naked eye. Magnaflux methods are generally reserved for billet and bloom inspection.
Seams are straight and can vary in length- often the length of several bars- due to elongation of the product (and the initiating imperfection!) during rolling. Bending a bar can reveal the presence of surface defects like seams.
An upset test (compressing a short piece of the steel to expand its diameter) will split longitudinally where a seam is present.
Seams are most frequently confused with scratches which we will describe in a future post.
“These long, straight, tight, linear defects are the result of gasses or bubbles formed when the steel solidified. Rolling causes these to lengthen as the steel is lengthened. Seams are dark, closed, but not welded”- my 1986 Junior Metallurgist definition taken from my lab notebook. We’ve a bit more sophisticated view of the causes now.
The frequency of seams appearing can help to define the cause. Randomly within a rolling, seams are likely due to incoming billets. A definite pattern to the seams indicates that the seams were likely mill induced- as a result of wrinkling associated with the section geometry. However a pattern related to repetitious conditioning could also testify to billet and conditioning causation- failure to remove the original defect, or associated with a repetitive grinding injury or artifact during conditioning.
My rule of thumb was that if it was straight, longitudinal, and when filed showed up dark against the brighter base metal it was a seam.
Rejection criteria are subject to negotiation with your supplier, as are detection limits for various inspection methods, but remember that since seams can occur anywhere on a rolled product, stock removal allowance is applied on a per side basis.
If you absolutely must be seam free, you should order turned and polished or cold drawn, turned and polished material. The stock removal assures that the seamy outer material has been removed.
Metallurgical note: seams can be a result of propogation of cracks formed when the metal soidifies, changes phase or is hot worked. Billet caused seams generally exhibit more pronounced decarburization.