Properties Of Refractory Materials

Properties Of Refractory Materials
Quality and suitability for application of refractory material depend upon their physical, thermal and chemical properties. Most common properties of the refractory material are listed below.

Physical properties

The refractory materials commonly characterized by their physical properties.
1. Bulk density
It is a general indication of product quality. The ratio of weight to the volume; measured in g/cm3.
2. Porosity
It is defined as the ratio of its pores volume to its total volume. Porosity is an important property of refractories because it affects many other features such as resistance to abrasion, thermal conductivity. In general, a good refractory must have low porosity. Here are some advantages and disadvantages of high porosity.

Advantages of high porosity
(i)    Highly porous refractory have lower thermal conductivity. This is due to the presence of more air voids, which act as insulation and therefore, be used for lining furnaces.
(ii)    High resistance to the thermal shock and heat spalling.
(iii)    Pores act as a crack inhibitor.

Disadvantages of high porosity
(i)    Reduces the strength.
(ii)    Reduces the resistance to abrasion and corrosion.
(iii)    Lower load bearing capacity.

3. Cold compressive strength
This indicates how refractory material can handle the load of installation or ability to withstand rigours transport, whereas the hot compressive strength shows how they will perform at high temperature. It is measured by a compression test. It is used to ensure that there is no crushing of refractory brick where it is subjected to high pressures. 
4. Flexural strength
Flexural strength;credit:wikimedia

Also known as the “bend strength.” The test consists of subjecting a concentrated central load on a specimen resting on two edges. The specimen is bent until it rapture and highest stress represent flexural strength. 
5. Wear resistance
The refractory walls of furnace and kilns attacked by the shock effect of gases and high-velocity abrasive raw solids flow over them. Therefore, the refractory must have good wear resistance.

Thermal properties

1. Melting point
It should have very high melting point.
2. Thermal expansion
It is the measurement of the linear stability of material when exposed to high temperature. All bodies undergo a reversible change in dimension under the influence of temperature. If refractory material undergoes higher expansion; it will suffer flaking of the surface.
3. Dimensional stability
It is the resistance of a refractory to changes its volume when exposed to high temperatures for a long time.
4. Thermal Shock
Fluctuations in temperature can significantly reduce the strength of refractory surface, causing the breakdown or peeling of layers if it is exposed to no. of thermal cycles. It is important because most process experiences alternative heating and cooling. A good refractory should show a very good resistance to thermal shock.
5. Thermal conductivity
Ability to conduct heat from the hot face to cold face when it is exposed to high temperature. This measurement Important for thermal insulation.
6. Pyrometric cone equivalent (PCE)
It is the measurement of heatwork (it is a combined effect of temperature and time).

Chemical properties

1. Corrosion resistance
The refractory material should have high corrosion resistance since these directly contact with hot gases and slag.

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