This set of Casting, Forming and Welding Multiple Choice Questions & Answers (MCQs) focuses on “Foundry Fluxes and Refractories”.
1. The mixing of flux in the casting material produces endothermic reaction.
a) True
b) False
View Answer
Explanation: The flux is mixed with casting while solidification that basically reacts with a network of oxides and converts them into a powder that floats on the casting so it becomes easier to separate out that part. The whole reaction takes place is exothermic. The flux is normally used after the cleaning process which prevents progressive oxidation.
2. Which of the following materials are commonly used as flux in the casting process to prevent oxidation?
a) Sodium silicate
b) Lead
c) Sodium chloride
d) Sodium carbonate
View Answer
Explanation: Sodium chloride is commonly used as flux in the casting process to prevent oxidation. Sometimes potassium fluoride is also preferred as a flux due to its excellent volatility. It not only prevents the oxidation, the other inclusions are also reduced by using flux. This process avoids metal wastage and avoids several defects formation.
3. Degassing process during the formation of castings can prevent inclusion of hydrogen absolutely.
a) True
b) False
View Answer
Explanation: After degassing process, the hydrogen does not stop coming in the casting material. This can only be controlled by using fluxes in the casting material but it is always recommended to determine the gas quantity before casting process, a device called reduced pressure gas determination equipment is then used for determining the gas content.
4. What should be the melting point (in ⁰C) of flux used for mixing with aluminium casting?
a) 440⁰C
b) 555⁰C
c) 606⁰C
d) 660⁰C
View Answer
Explanation: The melting point for flux to be used for aluminium casting should be approximately 660⁰C. However, the oxidation in aluminum is inevitable but flux is essential to avoid contact of aluminium melt to atmosphere and to get dry dross after skimming process which generally maintains the quality of casting material.
5. The fluxes used in the casting process must have low vapour pressure.
a) True
b) False
View Answer
Explanation: If the flux has high vapour pressure then it will not be easy to control it as vapourization will takes place which will have no any affect on the casting material. So it is always suggested to have low vapour pressure of fluxes that will easily mix with the material and will give excellent results in order to prevent oxidation.
6. Which of the following devices is mainly used for thermal analysis of flux?
a) Thermocouple
b) Thermometer
c) Thermo resister
d) Spiral optical tube
View Answer
Explanation: The flux is first melted inside a crucible and then a thermocouple is inserted into the molten flux. After the cooling process of molten flux, the temperature variations are recorded with respect to time. The melting nature, the variation in viscosity and the melting range of the molten flux with temperature is recorded.
7. Refractory materials are basically used for strengthening the casting materials used in the casting process.
a) True
b) False
View Answer
Explanation: Refractory materials are materials that have high melting points and they basically act as a heat resisting barrier between high and low temperature zones. Refractory materials are useful in constructing structures like boilers, furnaces etc. as they minimize heat losses through the structure during working conditions.
8. Refractories are generally inorganic non-metallic materials which can resist very high temperature.
a) True
b) False
View Answer
Explanation: Refractories are inorganic nonmetallic materials which can resist very high temperature without any physical or chemical variations. It is essential to generate a certain range of refractory materials with different features to be resistant to thermal shock, be chemically inert and to possess good thermal conductivity.
9. Silicon carbide is widely used as refractory material in the mould making for casting processes.
a) True
b) False
View Answer
Explanation: Silicon carbide is a good refractory material, but cannot be used in mould making process because it is highly reactive with oxygen and gets burns itself during oxidation. To avoid such oxidation problems, zirconia is preferred which can also bear very high temperatures and perfectly acts as a refractory material.
10. Refractoriness of a material should always be greater than its application temperatures.
a) True
b) False
View Answer
Explanation: Refractoriness is a property at which a refractory material deforms under its own load. The refractoriness is generally indicated by Pyrometric cone equivalent (PCE). It should always be greater than the application temperatures. Refractoriness is decreased when the refractory material is continuously under loading conditions.
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