Question

Rolling defect ``alligatoring'' causes

• A. splitting along centerline
• B. surface cracks
• C. edge waves
• D. thickness variation

Hint:

To easily remember rolling defects:
- Alligatoring \(\rightarrow\) Split horizontally along centerline.
- Edge cracking \(\rightarrow\) High lateral spread restriction or low ductility.
- Zipper cracks \(\rightarrow\) High tensile stresses at the center of the sheet width.
- Wavy edges \(\rightarrow\) Thinner edges than center due to roll deflection.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the physical description of the structural failure that occurs when a rolled metal product exhibits the "alligatoring" defect.
Alligatoring is a severe defect in the rolling of plates and slabs.

Step 2: Key Formula or Approach:
This defect is caused by non-uniform deformation throughout the thickness of the material during rolling.
If the surface layers are deformed more than the core (or vice-versa), residual stress states are set up, leading to fracture along the plane of weakest structural strength (the horizontal centerline).

Step 3: Detailed Explanation:

• Stress Distribution during Rolling:
- In slab rolling with high roll-to-workpiece friction and low draft, the deformation is localized heavily near the top and bottom surfaces.
- The surface layers tend to elongate more, putting the central core region of the slab under high longitudinal tensile stress.

• Centerline Separation:
- If there are pre-existing internal defects (such as porosity, casting shrinkage voids, or a weak centerline segregation plane), these tensile stresses will cause a crack to initiate at the slab's mid-thickness plane.
- As the slab exits the roll gap, the top and bottom halves split and curve away from each other due to residual bending moments.
- This split-sheet appearance resembles the open jaws of an alligator, hence the term "alligatoring."

• Other Defects for Comparison:
- Surface cracks are caused by low ductility at rolling temperatures.
- Edge waves are caused by roll bending (camber issues), where the edges are elongated more than the center.


Step 4: Final Answer:
Alligatoring specifically refers to the horizontal splitting of the workpiece along its centerline.
Thus, the correct option is (A).