Mumbai University > Mechanical Engineering > Sem 3 > Production Process 1

Marks: 5M

Year: May 2015

Defects in rolling can be primarily categorized as

a. General

b. Operational

General

The defects may arise due to

• Surface irregularities: The ingot or the raw material may be having irregularities due to scaling which will get trapped in the metal and remain inside the metal surface as laps. This needs to be removed by grinding and there will be metal loss. If the defect is deep and severe the product may get rejected.
• Non-metallic inclusions: The inclusions may results from oxides or nitrides or silicates etc, especially in steels. These are present in the molten metal during the preparation. If less in volume may cause small cracks in the metal and if more in volume will result in severe cracks called crocodile cracks separating the product into two halves.
• Internal Pores: There may be pores in the product due to the presence of gases like hydrogen,oxygen,nitrogen etc., If too much gases are present leads to elongation of the pores and the product may become weaker. Sometimes separation may take place resulting in cracks.

Operational

• Waviness-Varying thickness

  

  Variation in the work across the width in sheet rolling occurs because the roll gap is not perfectly parallel (a). Since width and volume are constant and thickness is varying, the edges elongate more than the center (b). But the sheet is a continuous body; the strains readjust to maintain continuity. Thus the center portion is in tension and the edges are in compression (c). The result is a wavy edge (d)

• Edge Cracking

  

  The length of the center portion increases but the edges are prevented due to frictional force.As a result the material gets rounded off (a). The edges are strained in tension leading to edge cracking along the width of the slab (b). When the difference in the strains become excess i.e. under severe condition, split at the center of the slab occurs (c).