Define refractoriness , and type of refractories

 A.  Refractoriness :

1. It is defined as a quality of materials to withstand at high temperatures without decomposing or melting . 
2. The materials which have this property are called refractory materials 
3. Bricks are the most common refractory material .
4. Refractory ceramics are the mostly used ceramics in the industry .
5. Refractory materials have important properties which are :

• 1. To withstand at high temperatures without melting .
• 2. To remain unreactive and inert when exposed to harsh environment .
• 3. To provide thermal insulation . 

B. Classification of Refractories : 

1. On the basis of their compositions refractories can be classified as :

a.   Fire Clay Refractories :

• Fire clay refractories are the mixture of alumina and silica having 25 to 45 % alumina and 70 to 50 % silica and remaining part consists of other oxides of Fe , Ca and Ti .
•  It can withstand at high temperature upto 1860 K. As alumina content increases , it will increase the maximum working temperature with formation of small amount of liquid .

 Application : Fire clay refractories are used to cover the high temperature region as it is used for making the wall of furnaces .

b . Silica Refractories : 

• It is also known as acid refractories . These materials show very good load - bearing capacity at high temperatures . 
• It consists of very small amount of alumina about 0.2 % .
• Addition of small amount of alumina results into decrease in its melting point.
• It can withstand at high temperature upto 1650 ° C . Silica refractories show good resistant towards slag formation . 
• These refractories are used to make arched roofs of steel and glass - making furnace . 

2. On the basis of their chemical behaviour the refractories may be classified as : 

a. acidic refractories : They are reactive towards bases and having silica as main constituent in it

Example : silica, quartz and sand etc. 

Explain Normalization or Hardening

 A.  Normalization 

1. This process consists of heating steel to a point 40 to 50 ° C above its upper critical temperature , holding at that temperature for a short duration and subsequently cooling in still air at room temperature . This is also known as air quenching .
2. This process is suggested for manufacturing operations like hot rolling and forging which are carried out on steels in the austenite range .
3. It is also useful for eliminating coarse - grained structure in castings , removing internal stresses that may have been caused by hot or cold working and improving the mechanical properties of the steel by eliminating the carbide network at the grain boundaries of the steels .
4. Normalizing produces microstructures consisting of ferrite and pearlite for hypoeutectoid steels and pearlite and cementite for hypereutectoid steels .