1.What are refractories? How are they classified?
Ans: Refractories are materials which can withstand high temperature, without softening, melting or deformation. They are used for lining the furnaces, converters, ladles etc,
2. How refractories are classified ?
Ans. Refractories are classified into three types.
- Acidic refractory – Fire clay, Silica.
- Basic refractory – Dolomite, Magnesite.
- Neutral refractory – Carbon, Chromite.
3. Mention the characteristics of a good refractory.
(i) It should be able to withstand sudden changes of temperature.
(ii) Its dimentional stability should be excellent.
(iii) It should be strong.
(iv) It should be chemically inactive at elevated temperature.
4. What is meant by refractoriness of a refractory?
Ans: It is the ability of the a material to withstand its own weight without deformation under working conditions. It is expressed in terms of pyrometric cone equivalent.
5. What is meant by dimensional stability? Mention their types.
Ans: It is the resistance of a material to any change in volume due to its prolonged exposure to high temperature.
6. What is RUL? How RUL Test is carried out?
Ans: The temperature at which the refractory deforms by 10% is called refractoriness under load (RUL).
The RUL test is conducted by applying a constant load of 3.5 or 1.75 kg/cm2 to the test refractory specimen of size base 5 cm2, and height 75 cm and heating in a furnace at a standard rate of 10°C per minute.
A good refractory should have high RUL value.
7. What is meant by pyrometric cone equivalent (PCE) of a refractory?
Ans: Refractoriness of a refractory is expressed in terms of pyrometric cone equivalent. It is merely a number, which represent the softening temperature of a refractory specimen. It is determined by PCE test.
8. Define porosity of a refractory.
Ans: It is defined as the ratio of the volume of pores of a refractory material to its bulk volume. Materials with high porosity has less strength, poor resistance to abrasion.
9. Mentions any two advantages and disadvantages of high porosity refractories?
Ans: Advantages of high porosity refractory.
- Highly Porous refractory possess lower thermal conductivity due to the presence of more air voids, which act as insultators and hence it can be used for lining in furnaces, ovens etc.
- Highly Porous refractory reduces thermal spalling.
Disadvantages of high porosity refractory.
- It reduces the strength.
- It reduces the resistance to corrosion and to abrasion.
10. What is thermal spalling? How it is minimised?
Ans: Spalling means breaking or fracturing a refractory material at high temperatures. A good refractory material must show a good resistance to spalling. This is due to rapid change in temperature and slag penetration. It can be minimised by avoiding sudden temperature change and by avoiding slag penetrations.
11. What are important properties of high alumina bricks?
- They possess very low coefficient of expansion, high porosity and high temperature load – bearing capacity.
- They are inert to the action of gases like CO2 and natural gas.
- They are very stable to both oxidising and reducing conditions.
12. Explain why silica bricks expand on heating?
Ans: Silica bricks expand on heating because of phase transformation or phase changes from one crystalline form into other having different density.
\underset{(\text { crystalline })}{\text { Quartzite }} \stackrel{870^{\circ} \mathrm{C}}{\rightleftharpoons} \underset{(\alpha \text {-form })}{\text { Tridymite }} \stackrel{1470^{\circ} \mathrm{C}}{\rightleftharpoons} \underset{(\beta \text {-form })}{\text { Cristobalite }}
13. What are cermets?
Ans: They are combination of both refractories and metals. They have high refractoriness and high thermal conductivity. They are used in rockets and jet engine parts.
| Read More Topics |
| Energy Sources – Questions and answers |
| Protective Coatings – Questions and answers |
| Corrosion – Questions and answers |
