Scab is a defect developed during __________ of metals.

ANS:B - sand casting

Scab defects are indeed more commonly associated with sand casting rather than forging. Sand casting is a popular method for the production of metal components, particularly for complex shapes and large parts. It involves creating a mold made of sand into which molten metal is poured to produce the desired shape. While sand casting offers versatility and cost-effectiveness, it can also be susceptible to certain defects, including scab defects. Here's how scab defects can occur in sand casting:

1. Mold Formation: In sand casting, a pattern of the desired part is used to create a mold cavity in sand. The pattern is typically made of wood, plastic, or metal and is removed once the mold cavity is formed.
2. Molten Metal Pouring: After the mold cavity is prepared, molten metal is poured into the cavity. As the metal flows into the mold, it can come into contact with the sand surface.
3. Interaction with Sand: When the molten metal contacts the sand surface, particularly if the pouring is done at high velocity or if the metal temperature is extremely high, it can cause the sand grains to erode or detach. These sand particles can then become trapped in the metal as it solidifies, forming scab defects on the casting surface.
4. Surface Imperfections: Scab defects appear as rough, irregular patches on the surface of the casting. They can vary in size and severity depending on factors such as the casting process parameters, the condition of the mold, and the characteristics of the molten metal.
5. Prevention and Control: To minimize the occurrence of scab defects in sand casting, several measures can be taken:
   • Proper gating and pouring systems to control the flow of molten metal and minimize turbulence.
   • Proper ventilation and permeability of the mold to allow gases to escape without entrapping sand particles.
   • Careful control of pouring temperature and velocity to reduce erosion of the mold surface.
   • Regular inspection and maintenance of molds to ensure they are in good condition and free from defects that could lead to scab formation.