characteristics of good refractory

A good refractory should have high refractoriness.
1. Refractoriness under load ( RUL) (or) strength
The temperature at which the refractory deforms by 10% is called refractoriness under load (RUL).
Refractories used in industries and in metallurgical operations, should bear varying loads. Hence refractories should have high mechanical strength under operating temperatures. Generally softening temperature decreases with increase of load. The load bearing capacity of a refractory can be measured by RUL test.

RUL Test
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.

2. Porosity
It is defined as the ratio of its pore volume to the bulk volume. Porosity is an important property of refractory bricks, because it affect many other characteristics like chemical stability, strength, abrasion- resistance and thermal conductivity.

Disadvantages of high porosity refractory

(i) It reduces the strength.
(ii) It reduces resistance to abrasion.
(iii) It reduces the resistance to corrosion.

Advantages of high porosity refractory

(i) Highly porous refractory possess lower thermal conductivity. This is due to presence of more air voids, which act as insulators and hence it be used for lining in furnaces, ovens, etc.
(ii) Highly porous refractory reduces thermal spalling.
A good refractory should have low porosity.

4. Thermal spalling
Thermal spelling is the property of breaking, cracking or peeling off a refractory material under high temperature.
Thermal spalling is mainly due to

(i) Rapid change in temperature
This causes uneven expansion and contraction within the mass of a refractory, and leads to development of uneven stresses and strains.

(ii) Slag penetration
This causes variation in the co-effcient of expansion and leads to spalling.
Thermal spalling can be decrased by

(i) Using high porosity , low co-effcient of expansion and good thermal conductivity refractory.
(ii) A voiding sudden temperature changes.
(iii) By modifying the furnace design.
A good refractory must show a very good resistance to thermal splling

5. Dimensional stability
It is the resistance of a refractory to any volume changes, when exposed to high temperature over a prolonged time.

These dimensional changes are of two types
(i) Reversible or (ii) Irreversible.

(i) Reversible dimensional changes
This may result due to the uniform expansion and contraction of a refractory material. So the dimensional changes of a good refractory must be reversible.

(ii) Irreversible dimensional changes
This may result either in the contraction or expansion of a refractory.

Example-1
Magnesite brick shrink in service
Magnesite is an amorphous material (specific gravity is 3.05). On heating it is gradually converted into more dense crystalline form of periclase (Sp. Gravity = 3.54)

Magnesite Periclase (Amorphous) (Crytalline)
Sp.gr = 3.05 sp.gr = 3.54

Example-2
Silica bricks expand in service.
Silica bricks expand on heating due to the transformation of one from to anther forms. This is accompanied by a considerable increase in volume. A good refractory should have high dimentional stability.