Refractory properties and testing methods

 

Refractories are materials with high melting points and qualities that allow them to function as heat-resistant barriers between high and low temperature zones. The physical, thermal, and chemical qualities of refractory materials determine their quality and acceptability for use. 

Here we are going to see the physical properties of refractory and their testing methods. The most typical refractory material qualities and testing methods are mentioned below.

The physical properties of refractory includes: Density, porosity, strength and abrasion.

Disadvantage of open pores: If more interconnected open pores, corrosion will be high.

Advantage of closed pores: More closed pores having better insulation.

A) Density

Density = Mass/Volume

Types of volume

Total volume = Volume of solid+ Volume of open pore+ Volume of closed pore

Apparent volume = Volume of solid+ Volume of open pore

True volume = Volume of solid (actual)

1.     True density

Instrument: Pyconometer

• Refractory powder (300µm) is dried at 110^oC

• Approximately 30 g powder is taken

• The powder was selected by coning and quartering

• This random method of selecting refractory was done upto 8 t0 12g

P – Weight of Pyconometer

P₁ – Weight of Pyconometer + Sample

P₂ –Weight of Pyconometer + Water

P₃ –Weight of Pyconometer+ Sample+ Water

Estimation of bulk density is calculated below:

2.     Bulk density

a)     Boiling method

• One half or one fourth of the refractory brick is taken
• It is dried at 110^oC (w₁ = Dry weight) and boiled in water for three hours
• Then it was cooled to room temperature (w₂ = Soaked weight)
• It was suspended in water using thin wire (w₃ = Suspended weight)

For boiling method,

w₁ = Dry weight

w₂ = Soaked weight

w₃ = Suspended weight

b)     Vaccum method

• One half or one fourth of the refractory brick is taken

• It is dried at 110^oC (w₁) and then placed in vacuum vessel

• Then evacuation is carried out for 15 minutes at 2Kpa vacuum

• Then the medium (water or white kerosene) is filled in vessel to immerse the sample (w₂)

• The sample is evacuated in vacuum in order to bring the vessel to the atmospheric sphere

• The soaking is done for 30 minutes (w₃)

For vacuum method,

s  = Specific gravity

w₁ = Dry weight

w₂ = Soaked weight

w₃ = Suspended weight

Note: Boiling method is suitable for unreactive material and vacuum method for reactive material.

B) Porosity

 

Testing procedures

Permeability

Permeability is the rate of flow of fluid through a porous material through unit area per unit pressure difference per minute.

 

                                  

V – Volume of flowing material in a time t

L – Length of the sample

A – Cross section area of the sample

h – head (constant)

The main factor of permeability is the interconnected open pores. Permeability is high in interconnecting among the pores.

Testing procedures

• The sample dimension is 50 * 50 mm cylinder (interior sample have to take)

• Four mm surface is removed from refractory sample

• The sample is then kept in a gas tight sample holder

• The difference in pressure is measured and noted using manometer                                                    
• If the pressure drop is high, then the sample is more permeable and pressure drop is low, then the sample is less permeable.

 

V – volume of gas flowing in a time t

ᶯ - dynamic viscosity of gas at operating temperature

⸹ - thickness of the sample

A – cross section area of the sample

Ꝭ₁ - gas pressure on entering the sample

Ꝭ₂ – gas pressure on leaving the sample

Ꝭ - absolute gas pressure

Pore size distribution

                       

Instrument: Mercury porosimeter to measure pore size distribution

• The refractory sample is taken in a gas tight container

• The air is evacuated and filled with mercury

• Apply the pressure

• At a particular pressure P1, measure the volume of mercury penetrated

• Then increase the pressure P2 and measure the volume of mercury penetrated

• Repeat the same procedures for different pressures

• Higher the pressure, smaller is the pore size

Assumption: Assumed shape is spherical

Disadvantage of this method

• Pore size obtained is differ from the actual pore size

• Very fine pores and closed pores cannot be measured

Pores are 10-20% is available in the refractory material. Have you ever thought what size of pore will be the advantage? Let’s see the advantage of pore size

Small size pore is having advantages in the refractory field. The advantages are listed here

• Larger pressure for penetration

• Solidification of absorbed material faster

• Minimize the thermal conduction (convection mode of heat transfer takes place in pores)

• Minimize the crack penetration due to thermal shock

                                

Small pores are also called as “CRACK STOPPERS”

C) Strength

Strength is defined as the ability to withstand load without fracture or failure. Here, the load could be compressive, tensile, and shear (rare).

Cold compressive strength (CCS)

• CCS is done at room temperature

• Indication of the extent of sintering

• Indication of abrasion resistance of refractory (Higher the CCS, higher the abrasion resistance of refractory)

Testing procedures

• The sample dimension of 60mm cube is taken with original refractory surface

• Refractory material is placed on a flat surface mainly to bring uniformity to the surface. So, the asbestos board is provided

• Then it is followed by applying load to the material to find out the CCS of refractory material.

Tensile strength

Tensile strength is the resistance of the material to breaking under tension. 

S.no	Brittle	Ductile
1	Sudden failure without neck formation	Failure with neck formation
2	Elastic deformation (ceramics)	Elastic and plastic deformation (metals)
3

Testing procedures

• Take a refractory material and mold it into a dumbbell shape

• The dumbbell-shaped material is placed in between the tensile grips

• Attach the extensiometer to the sample

• Elongation of tensile grips takes place

• Finally, the sample is tested for tensile strength

                                    

For tensile strength, the rod should be perfectly straight, sample preparation is difficult and extensiometer accuracy.

The disadvantage of this test is the dimensional problem and deformation in extensiometer. Hence, tensile strength is not applicable for ceramics.

Modulus of rupture

Modulus of rupture is also called as “BENDING STRENGTH and FLEXURAL STRENGTH”

Testing procedure

Bending tests are accomplished by extending a length of material across a span and pushing down along the span to stretch the materials to failure.

For three-point bending: Placing the material across a span supported on both ends and bringing a point source to the middle of the span and bending the material until failure.

For four-point bending: This test is identical to identical to three-point bending tests, except that instead of one point source being brought down to the centre of the material span, two points slightly spaced from the centre are brought down in contact with the material.

• The sample dimension is 25*25*225mm
• Load is 1.5 Kg/Cm² or 0.5 Kg/Cm²

L – Span

F – Load applied

b – breadth of the sample

d – depth of the sample

ϭ - stress

Length to diameter ratio is > 16 or 32 or 64

There is a concept called “Probability of finding flaw”. If the defect is present in a material, it will fail rapidly. If the defect is not detected, the material will fail gradually. But in the case of 4 point bending, a flaw is present definitely at one particular point. Hence, 3 point bending is higher when compared with 4 point bending according to Probability of finding flaw”.

Another concept called “Load distribution”. In 3 point bending, low load and value is higher. In case of 4 point bending, maximum load with value is lower. Hence, 4 point bending is higher when compared with 3 point bending according to “Load distribution” 

Compression strength > bending strength > tensile strength

Bending strength is two times the tensile strength 

For concrete= 10:2:1

For graphite = 4:2:1

Elastic modulus

Measures substance resistance to elastic deformation, when the force is applied. Apart from elastic modulus, there are shear modulus and bulk modulus are available.

                             

The essential properties of elastic modulus are strength, hardness, abrasion resistance, thermal shock resistance and thermal expansion.

For resonance frequency,

The resonance frequency is also called natural frequency, defined as the frequency at which material prefers to vibrate at a larger amplitude. In this method, applied frequency is equal to natural frequency (natural frequency depends upon the maximum amplitude of the sample)

                   

m – mass

f_f – resonance frequency

w – width

L – length

T – Correction coefficient

t- time

For ultrasonic pulse,

Ultrasonic waves are passed on the refractory material (receiver), which is being tested. Depends upon velocity, propagation will be change. If the higher velocity is obtained, tested material is good in terms of density, uniformity etc., This method is applicable for higher temperatures. At a higher temperature, elastic modulus will change. When temperature increases, elastic modulus decreases (higher the temperature, stiffness of material decreases). Whatever parameter reduces the stiffness, elastic modulus decreases.

                      

V – velocity of ultrasonic waves

Ꝭ - density

D) Abrasion resistance

• Abrasion resistance is resistance to abrasion or friction.

                       

Ϭ_fr - Stress at fracture

ε_fr - Strain at fracture

V = ½ to ¼ E

• Resistance of a material to a abrading force

Factors affecting abrasion reistance

• Sample features: Porosity, density and strength
• Abrasion resistance depends upon grain features. Grain features may be size and hardness
• Angle of impact

Testing procedures

 a) Falling grit test

a.  

                                                                                                           

• Material is placed on the tester

• The rubber wheel is rotating in a clockwise manner

• Then the load is applied to a material

• While applying load, the material with rotating rubber the wheel creates friction between them and falling of small particles takes place

• Finally calculated using the difference (initial weight-final weight of the sample)

• If wear is high, abrasion resistance will be lower.

• Higher the difference, lesser the abrasion resistance

b) Sand blasting test

b.    Instrument: Morgan-Marshall apparatus

• Silicon Carbide grains are taken

• The test is done in a defined pressure for a defined time

• Then measure the change in weight

• Compare this change in weight with the weight loss in standard graphite block.

                        

Calculation of abrasion resistance

                        

During firing, dust particles released from the a furnace which will cause abrasion in a different way are discussed below

i)     Surface/interior: Abrasion resistance is higher at the surface of the material because surfaces have high density than the interior. Hence, resistance of abrasion in higher at the surface.

ii)      Fired refractory/Monolithic refractory:  Abrasion resistance is higher in fired refractory than monolithic refractory

iii)    High temperature/Low temperature: Abrasion resistance is higher at higher temperatures and low abradability index. Abrasion resistance is lower at lower temperatures and at this temperature, abradability index is higher.

iv)   Hardness/toughness: Hardness is resistance to scratching and toughness is resistance to crack propagation. Higher the hardness, higher the abrasion resistance of material than toughness.

 

In this article, we had seen the physical properties of refractory such as density, porosity, strength, and abrasion resistance with their testing methods in an elaborate manner. I hope you gain some knowledge in the physical properties and testing of refractory material.