Defects in Rolling

The defects in rolling can be classified as (a) surface defects, and (b) structural defects. The surface defects include rusting and scaling, scratches and cracks on the surface, pits left on the surface due to subsequent detachment or removal of scales. The structural defects are more important rolling defects some of which are difficult to remove. Some common structural defects in rolling are as follows.

Wavy edges and zipper cracks

These defects are caused due to bending of rolls under the rolling pressure [Figure 3.3.1].
Edge cracks and centre split
These defects are caused due to non-homogeneous plastic deformation of metal across the width [Figure 3.3.1].

Alligatoring

Due to friction present between the roll surface and the upper or lower workpiece surface, the elongation on the top and bottom surfaces is less than the deforming material at the centre of thickness. If conditions become severe, it may lead to opening up at the free end of the rolled sheet (like the mouth of an alligator) that is referred to alligatoring [Figure 3.3.3].

Folds

This defect is encountered when the reduction per pass is very low.

Laminations

Laminations mean layers. If the ingot is not sound and has a piping or blow holes and during rolling they do not get completely welded it will cause a defect called laminations