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
(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.