This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Rate of Solidification”.
1. Which of the following is mostly analyzes during the casting and solidification of casting material?
a) Mould coating
b) Mould material
c) Mould aesthetics
d) Heat transfer
Explanation: The quality of final casting mainly dependent on the rate of solidification as rapid solidification makes fine grains structures with better mechanical properties. The analysis of heat transfer during the casting and solidification of casting is mainly carried out to have better properties of casting material.
2. The solidification time can be increased by increasing the thickness of the mould.
Explanation: The temperature variation with time of the casting is generally recorded by which cooling curves are obtained for the determination of solidification time of the cast. This shows that the moulding thickness increases the solidification time of the casting material during solidification.
3. A large shape of a riser is mainly used for the casting of grey cast iron.
Explanation: Grey cast iron is an interesting exception where solidification occurs in two stages. The shrinkage associated with the first stage can be compensated by the expansion of the casting due to the presence of graphite flakes in the material. Due to this, there is no use of riser in the casting system.
4. Feeding of an alloy in the mould is considered to be difficult if centre-line feeding resistance (CFR) is_____
a) greater than 70%
b) less than 70%
c) less than 20%
Explanation: The difficulty of feeding an alloy in a mould is expressed by a quantity which is commonly known as centre-line feeding resistance (CFR). It is basically defined as the ratio of a time interval between start and end of freezing at centre line to the total solidification time of casting. Normally, feeding is considered to be difficult if CFR is greater than 70%.
5. Which of the following is independent of the solidification time of the casting?
a) Mould material
c) Heat transfer coefficient
d) Mould wall temperature
Explanation: The solidification time of the casting is generally dependent on the various parameters like mould wall temperature, heat transfer coefficient at the metal and mould interface, mould wall thickness and material of the mould. Chaplets are another part used for supporting the core in the mould.
6. Casting which is cooled quickly generally have coarse grain structure.
Explanation: The mould is a cavity where liquid metal is poured to solidify and form the desired shape of the casting. The rate at which castings solidify affects its microstructure, quality, and mechanical properties. Basically, the casting which cools quickly will have a fine grain structure and which cools very slowly will have a coarse grain structure.
7. Refractory coating to the metallic mould helps in reducing the chilling effect on the casting.
Explanation: Metallic mould is made of metal capable of withstand high temperature and coated with refractory wash to reduce its chilling effect on the cast and also facilitates the removal of cast product from the mould cavity. It also helps in better surface finishing of the casting material.
8. The scale of segregation cannot be reduced or avoided by implementing any kind of method.
Explanation: There is very little segregation ahead of dendrite tip because the main partitioning is of solute trapped between the dendrite side arms. The scale of this segregation can be decreased by increasing the solidification rate because the dendrite arm spacing then decreases. Segregation can also be decreased by inducing a greater number density of nuclei.
9. The transfer of heat across a gap (an interface between mould and casting) is given by_____
a) q = h+∆T
b) q = h/∆T
c) q = h*∆T
d) q = h-∆T
Explanation: The transfer of heat across a gap (an interface between mould and casting) is given by q = h*∆T, where q is the heat flux, h is the heat transfer coefficient of the interface and ∆T is the temperature difference between the source and the sink. Values of the heat transfer coefficient may vary from 10 to 102 for large air gap; 103 to 104 for normal castings.
10. What should be the value of biot number (Bi) for chill mould castings?
a) Less than 1
b) Greater than 1
c) Greater than 5
d) Greater than 11
Explanation: Chill mould castings use a massive metal mould with a large thermal capacity or water cooled metal mould, so that biot number (Bi) is always less than 1. When the melt is poured in at a temperature above its melting temperature (Tm), it first cools and then begins solidification when T = Tm.
11. The deduction of cooling rate from a simple heat balance can only be possible when the process is isothermal and the specific heats of the liquid and the solid are same.
Explanation: In the solidification process, extraction of heat can be achieved by applying a suitable cooling to the molten metal in order to create a heat flux. The cooling rate can be deduced from a simple heat balance only if the process is isothermal (low cooling rate) and the specific heats of the liquid and the solid are the same.
12. Increase in mould thickness always decreases the rate of solidification of casting material.
Explanation: The increase in mould thickness can increase the rate of solidification of casting material. This is because the increase in thickness increases the amount of heat absorbed by mould or due to the chilling effect of the mould which is also dependent on the heat content of the molten metal.
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