Types of Testing of Refractories | Refractory Industry Guru

The durability of refractories is a matter which vitally concerns all manufacturers and users since it is intimately connected with economics and efficiency of the process in they are employed. Refractories are characteristically anisotropic in nature which makes it all the more difficult to judge exactly the durability while it is in use. The furnace designer has to choose the refractories according to the data from different tests available to him. That means in order to fulfill the users or various application requirements and to achieve long service lives, refractory products must be assessed by testing.

The main functions of refractories testing may be classified into three types: 

1.Evaluation of new materials before use.

2. Quality control by the manufacturer or user.

3. Post-mortem examination of refractory bricks that behaved differently than the normal.

The testing of refractories can be categorized as:

1. Non-destructive type.
2. Destructive type.

A list including both these types of testing is given below:

• Bulk density
• Apparent porosity 
• Apparent specific gravity
• True specific gravity and true density
• Particle size analysis
• Permeability
• Poresize distribution
• Refractoriness or Pyrometric Cone Equivalent (P.C.E)
• Refractoriness Under Load (R.U.L)
• Cold Crushing Stress (C.C.S)
• Modulus of Rupture (M.O.R)
• Modulus of Elasticity (M.O.E)
• Permanent Linear Change (P.L.C)
• Reversible Thermal Expansion (R.T.E)
• Thermal Conductivity
• Differential Thermal Analysis (DTA)
• Thermo-gravimetric Analysis (TGA)
• Spalling Resistance (water quenching and air spalling)
• Hydration Resistance Test
• Creep in Compression Test
• Abrasion Resistance Test
• Resistance to Carbon Monoxide
• Slag Corrosion Test
• Microstructural analysis under Optical Microscope
• Mineral Phase identification by X-ray Diffractometer (XRD pattern)
• Chemical Analysis

There are several Standard Methods to carry out the above testing which have been accepted and performed globally. Here one must remember that the sampling of the refractories specimen for testing become very important because of several reasons. Methods of carrying out all the tests given in the above list will be discussed individually in separate posts at this site.

Standard Methods for Testing of Refractories (Coming up)

Sampling of the Refractory Specimen for Inspection (Coming up)

Type of Refractory Castables by CaO Content

There are various types of refractory castables:

• Conventional castable 
• Low cement refractory castable
• Ultra low cement refractory castable
• No cement castable

According to ASTM C401-89, based on its CaO content Refractory castables can be classified into the following categories:

Type	CaO Content
CC = Conventional or Regular Castables (Konventionelle Feuerbetone)	CaO = More than  2.5%
LCC = Low Cement Castables (Zementarme Feuerbetone)	CaO = 1 - 2.5%
ULCC = Ultra Low Cement Castables (Ultrazementarme Feuerbetone)	CaO = 0.2 - 1%
NCC = No Cement Castables (Zementfreie Feuerbetone)	CaO = Less than 0.2%

Manipulating the Test Results of Apparent Porosity (AP) During Testing of Refractory Bricks

Apparent porosity (AP) is the percentage ratio of the void space in the refractory specimen to the total bulk volume of the same. AP is one of the most important physical properties for any type of refractory brick that will be certainly mentioned in its specifications. Hence determination of Apparent Porosity is almost a compulsory part of Inspection / Testing of a Refractory Lot. This property becomes more stringent when the refractory brick is of any complicated shape like, checker bricks or nozzle bricks having tongue - groove etc. In such cases often, it remains a cause of worries for the laboratory person conducting inspection.

The results of AP can be manipulated to show less AP% than what the brick has actually and thus, an inspector may be deceived by a laboratory person. But such actions or manipulation of results are undeniably wrong and liable to be penalized or even the whole Refractory Lot may get rejected, if caught red-handed.

Nevertheless, this article is not to discuss the merits-demerits or right-wrong of the action. That is for you to decide. Here we assume that you have made a conscious decision to learn the trick as how to manipulate the results of Apparent Porosity in order to show a better result of a porous brick. Having so decided, below is a guide (trick) on how you can do it:

=>> Apparent Porosity (%) = {(Soaked Wt - Dry Wt) ÷ (Soaked Wt - Suspended Wt)} x 100

=>> To show less AP% we need to increase Dry wt as much as possible.

=>> Make 30% conc. salt solution (i.e. 30gm salt in 100gm water).

=>> Example: To bring down 75 mm std. Brick having actually AP-25% to AP-18%, soak the brick in approximately 30ml quantity of above salt solution so that its Dry Wt. is increased by approx 9 gm. After applying solution from different sides and its complete soaking put the brick in the drier & properly clean itssurfaces after drying.

Better try to know the actual AP before applying/manipulating.

Pore Size Distribution in Refractories

It is highly essential to measure pore size and its distribution to get the desired optimum properties in any refractory brick. Actually ‘pore size’ and ‘pore size distribution’ are two different things. First, let us discuss the difference between these two terms. The ‘pore size’ is a measure of the diameter of the largest pore whereas; the ‘pore size distribution’ is a measure of the range of pore sizes. The range of pore sizes can be normally distributed, and the spread can be quite narrow (e.g. the ratio of largest to smallest may be less than 2). On the other hand, pore size distribution can be very heterogeneous. In the case of large spreads and heterogeneity, which is not desired, the pore size will be far less predictive.

It is measured by analytical instrument known as Porosimeter based on mercury intrusion and capillary flow methods. The volume of mercury penetrating the pores measured directly as function of applied pressure. Pore Size information is best described graphically, rather than with a single number. An alternative method of describing the distribution curve is to use a collection of values that better describe the distribution curve in more detail, i.e., upper limit, lower limit, mean, standard distribution, etc. This P-V information serves as unique characterization of pore structure. This data together with Microstructural analysis and few other test reports of the refractory sample can be of great help in optimizing the properties as well as for any failure analysis.

Suggested Articles: 

• Permeability of Refractory Bricks and Monolithics
• Effects of Compacting Pressure on Sintering and other Properties of Refractory Bricks
• Bulk Density of Refractory Samples

Effects of Compacting Pressure on Sintering and other Properties of Refractory Bricks

11-July-2020

We assume that the reader is already aware with the concept of ‘Sintering’, types of sintering and also the effects of sintering on refractories. In this article we will discuss on the effects of compacting pressure also called forming pressure, on sintering and various other properties of refractory bricks.

It has been established much before by Budnikov and Blyumen that sintering processes and reactions in the solid-state are interrelated and proceed with on the phase boundaries, as in a heterogeneous system. The basis of sintering, according to their broad definition, is the capacity of the solid phase to recrystallize, which, in turn, is related to the physiochemical nature of the crystal. Pressure is said to be an important factor in accelerating reactions in solid state and in facilitating sintering at relatively low temperatures in a refractory brick.

Precautions must be taken to eliminate any pressure variation during compaction of the refractory shape. The main deleterious effect of variation in compacting pressure is the corresponding differences in greenbulk density resulting into non-uniform shrinkage after firing and some sort of distortion of warping is inevitable. The frictional force between the die wall and the powder is directly proportional to the radial stress at the wall. During a uniaxial pressing, the applied stress is in the axial direction and is parallel to the die (mould) wall. For a given axial stress the resultant radial stress depends on the fluidity of the powder under compaction. For example both the radial and axial stresses are equal when a liquid is compacted. However, when a non-elastic and incompressible solid is under axial compaction, there should not be any radial stress. Thus, it is desirable to decrease the powder fluidity in order to minimize the radial and frictional stresses or the density and stress gradients in the refractory brick.

There is no doubt that the forming pressure affects the firing behavior of the refractory materials. Such effects may be due to:

>> Decrease in pore size and better particle contact,

>> Strain energy added due to plastic flow,

>> Strain energy added due to particle interlocking, or

>> Fracture of particles at contact points.

Suggested Article: QualityAssurance during Green Production of Refractory Bricks

In general increasing pressure enhances the Green Density, decreases Shrinkage, and often increases the Fired Density of refractory bricks. Higher compacting pressure (compaction) may cause plastic flow, increased strain energy, or particle fracture, which causes further increase in bulk density in refractory bricks. The effect of these variations on firing properties of a refractory brick depend on the firing time and temperature, and the nature of the refractory aggregates or refractory raw materials used, but in general decreased pore size due to compaction or particle fracture leads to increased density at lower firing temperature in a refractory brick.        

Quality Assurance during Green Production of Refractory Bricks

Quality is defined as what the customer wants in a product (here ‘Refractory’), not just meeting specifications but also ensures customer delight. Consistently achieving the specifications without failure is one of the most important requisites for Quality. Then, a question automatically arises that how can one be effectively consistent in any regular operation. The answer is: one has to plan and follow some Standard Operating Procedures or SOPs, a term more popular among Quality Circles. Standards promote good work habits among all levels of the organization. To standardize is to choose the best method and promote consistency which is essential for quality.

Fig.- Operator Running a Friction Screw Press (FSP)

There has to be SOPs for each and every operation starting from raw material testing, acceptance to brick dispatch in a refractory plant. But merely having SOPs will not help in any respect except for showing them to any customer, if such a situation arises! However, following these SOPs religiously will not only benefit in getting the quality in various refractory products but also, help in increasing the productivity and reducing the rejection percentage at every level thus, ultimately bringing down the cost of production.

Fig.- An Array of Friction Screw Presses in a Refractory Plant

Nevertheless this article is not to discuss the advantages of SOPs or Standardization which will be discussed in a separate post. Here our point of discussion is how to ensure quality in a refractory brick during its green production. For this each and every brick must pass through a process of checking as a part of standard procedure in the process of production. Below is a list of such things (parameters) that have to be checked during green production of refractory bricks as a criterion for their acceptance or before sending them to drier.

GREEN BRICKS CHECKING LIST

1. Size and critical Dimensions as per actual & shrinkage given in the Production Programme (Refer Drawing if required).

2. Quality / Plate mark / Special instructions.

3. Right angle, Centre slope.

4. Warpage, Bulging.

5. Die Plate jam, cleaning.

6. Spongy, Texture.

7. Rags, Corner loose.

8. Lamination, Crack (Hammering).

9. De-airing & Pressing stroke (Set-up approval).

10. Green B.D, Refractory Composition.

11. Sieve analysis report of Powder (Mixture).

12. Free iron in the Powder/Mixrure.