Tag: tears

Scabs On Rolled Steel Products

Posted on by

Scabs are irregularly shaped, flattened protrusions caused by splash, boiling or other problems from teeming, casting, or conditioning.-AISI Technical Committee on Rod and Bar Mills, Detection, Classification, and Elimination of Rod and Bar Surface Defects

Scabs are always present prior to rolling.

(Teeming refers to the process of filling an ingot mold with molten steel from the ladle. We’ll point out some continuous casting analogs  later in this post.)

Scabs have scale and irregular surfaces beneath them; they tend to be round or oval shaped and concentrated to only certain blooms or  billets. Scabs are always the same chemistry as the steel bloom or billet.

(If the  gross irregular surface protrusion characteristic is appearing on all product, it is not likely to be a scab. If the protrusion is a different analysis, it is likely to be mill shearing.)

To differentiate between scabs and rolled in scale,  scabs are ductile when bent while scale is brittle and crumbles.

If the protrusion is brittle, it may be rolled in scale.

Scabs are primarily an ingot process issue related to teeming, but we have seen them on continuous cast  products as a result of mold and tundish anomalies.

Scabs present with scale beneath; Cracks may (but are not always)  be present associated with the scab due to stress concentration causing the material underneath to crack. (Not the crack causing the scab…)

Ingots or blooms showing scabs should be conditioned to remove the scabs. Thermal conditioning of billets (hot scarfing or torch conditioning) can sometimes leave artifacts which present as scabs upon rolling.

While scabs can be confused with slivers, shearing, rolled in scale, or tearing, their ductility eliminates them as rolled in scale. Scabs are distinct from shearing as scabs are isolated by occurrence and have an irregular surface beneath them, while shearing usually presents as multiple instances in a single orientation along the bar. Tearing is characterized by chevron shaped breaks rather than oval shaped protrusions.

Seams On Rolled Steel Products

Posted on by

“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 are longitudinal voids opening radially from the bar section in a very straight line without the presence of deformed material adjacent.

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 are the most common bar defects encountered. Using a file until the seam indication disappears and measuring with a micrometer is how to determine the seam depth.(Sketch from my 1986 lab notebook)

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.