Showing posts with label Inspection (Testing). Show all posts
Showing posts with label Inspection (Testing). Show all posts

November 3, 2009

Cold Crushing Strength (CCS) of Refractory Bricks


Cold crushing strength (CCS) of a refractory brick represents its strength. That is it tells us how much load that refractory can bear in cold conditions. The concept of testing CCS of a refractory material has perhaps, come from metallurgy. This is because for any refractory brick it is rather; rare that it would fail simply due to load on it in cold condition and therefore, the determination of cold crushing strength does not appear to be important from that point of view.
But still testing of this property i.e. knowing the strength of the refractory brick is done to check some other properties which are direct result of strength such as ‘abrasion’. The stronger a material is the greater is the resistance to abrasion. Also stronger refractories are expected to have higher resistance to slag attack. The determination of cold crushing strength (CCS), however, is highly important in case of refractory insulating bricks where bricks have to be porous as well as strong.    
It can be determined by following the steps given in any of the Standard Methods for Refractory Testing like - ASTM, Indian Standards (IS), Ghost, DIN etc. CCS of refractories is determined by placing a suitable Refractory Specimen on a flat surface followed by application of a uniform load to it through a bearing block in a standard mechanical or hydraulic compression testing machine. The load at which cracks appear in the refractory specimen represents the CCS value of the specimen. Load is to be applied uniformly and slowly, depending on the standard testing method followed, with a rate of load varying from 35 – 100 kg/cm2/min. Refractories being anisotropic in nature, the direction of load applied may be stated while reporting the results. The adjacent figure shows an assembly used for conducting the test.

(Fig. Courtsey: Indian Standards Institution, Ref. IS: 1528 of 1974, Part IV)

CCS = Total Load ÷ Total Area
Precaution must be taken that the refractory specimens must have the maximum possible original surfaces, have absolutely parallel and flat faces for applying load, and are free from cracks, voids etc. The size of the refractory test specimen (sample taken for testing) is usually equivalent to a 230 mm standard refractory brick except in case of smaller and other special refractory shapes where the test specimens are of smaller sizes or representative samples of 75 mm cube shape. The value of CCS can be expressed in either lbs. per square inch or kg/cm2.    

May 7, 2009

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 I 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.


Types of Testing of Refractories [Read]




May 6, 2009

Apparent Porosity and True Porosity of Refractory Samples

Porosity is the percentage relationship between the volume of the pore space and the total volume of the refractory sample. Apparent Porosity does not include the volume of the sealed pores. The True Porosity includes the volume of the sealed pores also. The usual difference between the apparent porosity and the true porosity is of the order of 1 to 2 percent unless the proportion of the sealed pores is high. The true porosity figure is the higher than the apparent porosity figure as the true porosity includes the volume of the sealed or closed pores also. The difference between the two values represents the percent volume of closed pores. Porosity can be controlled by the following:

=> By controlling the texture of the brick i.e. by controlling the size distribution of the particles.

=> By the methods of green manufacturing and composition.

=> By controlling the firing temperature, soaking time etc.

=> Quality of raw materials i.e. the inherent grain porosity of the raw materials used.

Higher the porosity, lower will be the strength of the brick. Bricks with lower porosity will have greater resistance to slag attack and more sensitiveness to fluctuations in temperature. Their thermal conductivity will be more.

Hence, apparent porosity is the percentage ratio of the void space in the refractory specimen to the total bulk volume of the same. There are two methods used for the determination of apparent porosity of refractory materials which are:

=> Boiling point method, and

=> Evacuation method.

Samples (Refractory Specimen) measuring 6.5 cm x 6.5 cm x 4 cm is cut from burnt refractory bricks by a cut off wheel from within its core and cleaned any dust or loose particles adhering to its surface and are dried in an oven at 110OC to a constant weight. For graded materials take 3 to 5 mm size grains and dry at 110OC. It can be determined by following the steps given in any of the Standard Methods for Refractory Testing like - ASTM, Indian Standards (IS), Ghost, DIN etc.

True Porosity (%) = {1 - (Apparent Sp. Gr. ÷ True Sp. Gr.)} x 100

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

>> Types of Testing of Refractories [Read]

>> Manipulating the Test Results of Apparent Porosity (AP) During Testing of Refractory Bricks [Read]

April 27, 2009

Bulk Density of Refractory Samples

Bulk Density

This property is important for both insulating and dense refractories (bricks and castables). Bulk density (B.D.) is the ratio of the mass of the refractory specimen to the bulk volume of the same or in other words it is the weight per unit volume the refractory (including the volume of the pore space present in that refractory sample).

There are two methods for determination of Bulk Density in case of refractories:


1. Direct measurement method

B.D.= (Weight of the specimen in gm.) ÷ (Volume of the specimen in cc)


2. Direct volume determination method

B.D (gm/cc) = (Dry weight) ÷ (Soaked weight - Suspended weight)


Out of these two methods the ‘direct volume determination method’, which is generally used for irregular refractories, gives more accurate results. The nearer the Bulk density approaches the Specific Gravity the lower is the Porosity. Lower B.D indicates higher porosity, lower strength of the refractory material. The BD will also affect other properties of the refractory such as the load bearing capacity and thermal conductivity etc. Unless there is any specific reason, the aim of a refractory manufacturer is to produce products of higher Bulk Density. The ultimate Bulk Density of the product will depend on a number of factors like - type of raw materials used and their processing, process control at every step during green manufacturing and firing etc.


Standard Methods for Testing of Refractories [Read]

Types of Testing of Refractories [Read]