This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Solidification with Constant Surface Temperature”.
1. A thin water cooled mould always tends to make a casting of soft and ductile nature.
a) True
b) False
View Answer
Explanation: Thin wall of the mould and the water cooling makes the solidification rate fast, which produces fine grain structure of the casting. The fine grain structure of casting is always brittle and hard in nature because the process is somewhat similar to quenching which also produces a casting of brittle nature.
2. Rapid cooling of strand surfaces of casting is mainly done to increase casting speed and productivity.
a) True
b) False
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Explanation: For the casting line productivity and to increase casting speed, rapid cooling of strand surfaces must be employed. It results in the development of thermal stresses and formation of surface cracks if the casting speed and cooling conditions or the arc of casting system are not appropriate for the casting process.
3. In the continuous casting line, the solidification or cooling rate is normally very high.
a) True
b) False
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Explanation: Solidification of metal in the continuous casting line proceeds under non-equilibrium conditions and is specified by the large cooling rate. These can lead to the occurrence of defects at cast strands such as surface or internal cracks, which might also affect the composition of the casting material.
4. The analysis of heat flow is much easy in chilled casting process compared to the other type of castings.
a) True
b) False
View Answer
Explanation: Chilled castings present heat flow analysis much more complex than the normal type of castings. The first step in analyzing is to isolate the metal from the mould, and to consider heat flow in the molten metal separately, using the simplest possible boundary condition such as constant casting surface temperature.
5. The surface temperature of chilled casting is normally lower than the melting point of the metal.
a) True
b) False
View Answer
Explanation: It is an essential feature of chilled castings that the surface temperature is appreciably lower than the melting point of the molten metal, which totally differs from the sand castings and it moderates to steep the temperature gradient exists in the solid metal or casting produced.
6. In any condition, the effects of superheat cannot be considered in the process of chilled castings.
a) True
b) False
View Answer
Explanation: In chilled castings, initially a mass of liquid metal at the melting temperature suddenly cools down to the surface temperature at zero time. The effect of superheat is not considered in chilled casting, but it would be quite possible to consider superheat in the one-dimensional case, but the results obtained may be unrealistic.
7. During solidification of casting material, the successive droplets of the material take a longer time to solidify than the first droplet.
a) True
b) False
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Explanation: During solidification of casting, the successive droplet’s interface has lower temperature gradient when compared to the temperature gradient experienced by the first droplet of the material. Thus, heat transfer occurs at low rates and successive droplets take longer time to solidify that the first droplet of the casting material.
8. Which of the following temperature ranges (in Kelvin) can start the re-melting of droplets of casting material?
a) 400 – 450
b) 450 – 500
c) 500 – 600
d) 600 – 700
View Answer
Explanation: As the first droplet solidified when the successive droplets make an impact to it and the temperature of the pre-solidified droplet is very close to its melting temperature. The droplet temperature is approximately greater or equals to 623 K, at which the pre-solidified droplet re-melts and shows re-melting behaviour at different time instants.
9. Thermal contact resistance can be affected by impact or thermal parameters during the casting process.
a) True
b) False
View Answer
Explanation: The effect of thermal contact resistance on the phase change process is very important to consider during solidification. Thermal contact resistance is varied by changing the heat transfer coefficient at the interface. Thermal contact resistance is always constant throughout the process and cannot be affected by impact or thermal parameters.
10. During solidification, the interface temperature is independent to the magnitude of contact resistance.
a) True
b) False
View Answer
Explanation: During solidification of casting material, the interface temperature which generally prevails in the absence of contact resistance has physical meaning when the contact resistance is in effect. The interface temperature is independent to the magnitude of contact resistance and is dependent only on the thermal properties.
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