Question

Which of the following pair is a wrong pair with respect to metal casting process

• A. Hot tears - poor mould collapsibility
• B. sand inclusions - hard ramming of sand
• C. Porosity - gas entrapment
• D. shrinkage cavity - inadequate risering

Hint:

Understanding casting defects is crucial. Remember that hard ramming improves mold strength and reduces sand inclusions due to erosion, but it can lead to other issues like reduced permeability. Sand inclusions are usually caused by mold erosion or soft ramming.

Answer 1

The Correct Option is B

Step 1: Analyze each pair to understand common casting defects and their causes.
Pair 1: Hot tears – poor mould collapsibility
Hot tears (or hot cracks): These are cracks that form in a casting while it is still hot (just after solidification but before it has cooled significantly). They occur because the metal is still weak and ductile at high temperatures, and internal stresses develop due to restricted contraction.
Poor mould collapsibility: If the mold does not collapse sufficiently when the casting shrinks, it restrains the contraction of the casting, leading to tensile stresses that can cause hot tears. This pair is correct. Pair 2: Sand inclusions – hard ramming of sand
Sand inclusions: These are defects where sand particles from the mold or core become embedded in the surface of the casting. They occur when the mold surface erodes or washes away due to the flow of molten metal.
Hard ramming of sand: Hard ramming increases the mold strength and surface stability, making it less susceptible to erosion or sand wash. Poor or insufficient ramming (too soft a mold) can lead to erosion and thus sand inclusions. If the sand is rammed too hard, it can reduce the mold's permeability (leading to gas defects) and collapsibility (leading to hot tears), but it typically prevents sand inclusions from erosion. Therefore, associating hard ramming with sand inclusions (as a cause) is a wrong pair. Sand inclusions are more likely caused by soft ramming, high pouring velocity, or inadequate binding of sand. Pair 3: Porosity – gas entrapment
Porosity: This refers to small holes or voids within the casting.
Gas entrapment: When molten metal is poured, gases (like air from the mold, or gases dissolved in the metal that come out of solution upon cooling) can get trapped in the solidifying metal, forming pores. This pair is correct. Pair 4: Shrinkage cavity – inadequate risering
Shrinkage cavity (or shrink hole, pipe): This is a void or depression in the casting formed during solidification due to the volumetric contraction of the metal. As metal solidifies, it shrinks, and if there's no additional molten metal to feed the shrinking regions, a void forms.
Inadequate risering: Risers are reservoirs of molten metal designed to feed the casting as it solidifies and shrinks. If the risers are too small, improperly placed, or solidify too quickly, they cannot adequately supply molten metal to the main casting, leading to shrinkage cavities. This pair is correct.
Step 4: Identify the wrong pair.
Based on the analysis, the pair "sand inclusions – hard ramming of sand" is the wrong one because hard ramming generally prevents sand inclusions caused by erosion. Sand inclusions are more commonly associated with soft ramming or high molten metal velocity. The final answer is \( \boxed{\text{2}} \).