CAPITULO IV.- Resultados Específicos por Componentes
2.2.6 INDICADORES PARA LA AUDITORÍA DE GESTIÓN
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smoking. Heating and soaking (holding at temperature for some time) should be applied to allow complete heat diffusion and phase transformations. Once heating is discontinued, the bricks should be allowed to cool with the furnace/kiln to avoid rapid cooling rate (Mark, 2007).
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A wide range of test methods is available to measure all aspects of refractory behaviour. The majority of tests are characterized as national or international standards..
These tests are grouped under
- Characterization/data sheet properties test - Service related test
- Design test.
In characterization test, the first test is the chemical analysis which gives the elemental composition in the refractory. Generally, the quickest and most commonly used method is by x-ray fluorescence. The technique is based on the phenomenon that each element fluoresces characteristically when exposed to x-ray.
The result for each element present is usually reported as oxides. However, the technique is not viable for elements low in the periodic table (from fluorine downward).
The mineralogical analysis is done by the help of x-ray diffraction (XRD) analysis.
This provides a means by which different crystalline phases can be characterized and identified. X-ray diffraction works by diffracting incident x-rays along crystallographic planes. The angle by which x-rays are diffracted is in accordance with Bragg’s law in which
𝑛𝜆 = 2𝑑𝑠𝑖𝑛𝜃
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Where 𝑛 is an integer value
𝜆 is the wavelength of the diffracting x-ray.
𝑑 is the spacing between successive diffracting planes in the crystal.
𝜃 is the angle between the crystallographic plane and the diffracted beam.
The microscopic examination of refractories can range from simple hand lens examination to the use of sophisticated scanning or transmission electron microscopes with magnification in the order of many thousands.
Examination may be conducted on the surface of the refractory, internally to evaluate the normal structure, or across interfaces between materials or material and service environments to evaluate changes that occur. Procedures involved include polishing, which provides greatly improved spatial resolution. This is followed by etching to enhance features like grain boundaries and phase identification.
Evaluating the microstructure of a refractory material is necessary to examine grain structure, grain distribution, bond mechanisms and degree and distribution of porosity which all control the in-service behavior.
Other properties test including design and service properties test are as stated in the literature. (Gilchrist, 1997).
According to the scientist, empirical tests have been developed. The following tests are commonly carried out. British Standard Specifications are available for some
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of them and details can be found in B.SS (1952). Similar series of tests are specified by the American Society of Testing Materials ( ASTM )
2.16.1Test for expansion (or Contraction)
This is determined by heating a suitable sample of bricks for a prolonged time at the proposed working temperature. The cut sample is measured before and after treatment to determine the permanent change in dimensions which should of course be small.
If the expansion (usually a contraction) is too great, more thorough firing is required or another kind of brick. High values lead to severe cracking of furnace walls during use, or to distortion of the structure. A small expansion is preferred to give tight joints and for some special purposes like ladle linings , a fairly high value is deliberately arranged.
2.16.2 Test for refractoriness
The test for refractoriness is to compare the sagging of a “cone” of the material (either cut from the solid or made up from powder and a bond of dextrin) with that of standard Seger cones, when they are heated together, at a standard rate in an oxidizing atmosphere, until the test cone bends over. The number of the best matching seger cone is quoted as the refractoriness of the brick; or its nominal melting point (which can be checked by pyrometer) is referred to as the pyrometric cone equivalent (PCE) temperature.
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2.16.3 Test for hot strength
This is very important in high quality bricks but is not measured directly. Instead it is a temperature that is determined – that at which deformation under standard load is rapid. The test is called the refractoriness under load test (R.U.L.). A constant load of 100 or 200Lb (or 25 or 50Lb/in2) is applied to a prism of brick 2 inches square by 3½inches. The specimen is heated in a carbon granule furnace at a standard rate (100C/min) and a record of its height made, preferably by plotting, until the test piece collapses or sinks to 90 percent of its original length. The refractoriness under load may be quoted as the range of three temperatures:
1. Initial softening 2. Rapid collapse 3. Total collapse.
2.16.4 Test for thermal shock resistance
This is measured by a spalling test. There is no fully standardized test but materials can be compared using constant test conditions and these should be chosen to suit the brick under test and the working conditions under which it is to be used. For example, silica bricks have excellent thermal shock resisting properties above 3000C if properly made, but shatter if cooled quickly below that hence any spalling test on silica must avoid cooling under 3000C.
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Various tests used involve heating either a brick or a prism cut from bricks, slowly up to a working temperature and then cooling it rapidly for 10 minutes in a cold air jet, or on a steel plate, then reheating for 10 minutes in the furnace held at the working temperature. This is repeated until a piece of the brick becomes detached.
The number of cycles to failure is noted. A brick surviving 30 cycles is claimed to be very good and quoted as having a spalling resistance of +30 cycles. A more adequate method of heating involves heating and cooling applied to the face or end only and not along the sides as well.
The result of a spalling test must depend on at least four properties of the brick.
- Its thermal conductivity which determines the temperature gradients set up under the test conditions;
- The coefficient of thermal expansion which determines the strain induced by these thermal gradients.
- The modulus of elasticity.
- The shear strength of the brick substance, which together determine the stresses set up and whether or not they will be relieved by crack formation or propagation. The formation of crack during early cycles would probably modify the temperature distribution during the later cycles in favour of even more severe stresses so that cracks once formed are likely to get worse.
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2.16.5 Test for slag resistance
There are many ways to test this property but the most common one involves drilling holes in the brick and packing these with samples of typical slag likely to be encountered. The assembly is then heated to a working temperature for about an hour, cooled and sectioned. The extent of the penetration of slag into the brick is noted and compared with others. This method takes some account of the effect of brick texture and slag surface tension as they affect reaction rates.
2.16.6 Test for apparent porosity and bulk density
These are determined in one and the same test in accordance with ASTM C 20-80a, standard test methods for apparent porosity, water absorption, apparent specific gravity, and bulk density of refractory bricks and shapes of boiling water (Mark, 2007).
Alternative method is by evacuation in a vacuum dessicator and flooding with paraffin (Gilchrist, 1977). After determining dry weight of brick (Wd), weight when soaked in water (Ws), and weight suspended in air of the previous soaked specimen (Wa), the apparent porosity (Pa) and the bulk density (Db) are calculated from the relationships.
𝑃𝑎, % = {𝑊𝑎−𝑊𝑑
𝑊𝑎−𝑊𝑠} x 100 (2.2) 𝐷𝑏, 𝑔/𝑐𝑚2 = { 𝑊𝑑
𝑊𝑎−𝑊𝑠} (2.3)
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2.16.7 Test for cold crushing strength
The ASTM C-93-67 (reapproved 1977); standard test methods for cold crushing strength and modulus of rupture of insulating firebrick specify this. The testing machine is a mechanical or hydraulic compression-testing machine with a sensitivity of at least 89N in the range 0 to 67kN. The hydraulic type is preferable, though a universal strength-testing machine is the best.
A dial-type micrometer is generally used for obtaining the necessary measurement for calculating the percentage of deformation during the compression. The cold crushing strength, S, is calculated from;
𝑆 = 𝑊
𝐴 (2.4) Where W = total maximum load at 3% deformation or at visible failure, whichever
is small, N;
A = average of the gross areas of two faces of the specimen compressed, mm2. 2.16.8 Test for moisture content of moulding mass
The ASTM C 72-76, standard test methods for sieve analysis and water content of refractory materials, specifies this test. About 50g of material is required. The test sample should be obtained quickly to avoid loss of water. It is weighed to the nearest 0.1g both before and after drying for 3 hours at 105 to 1100C. The moisture content is calculated as a percentage to the nearest 0.1% on the as received basis, thus:
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Moisture content, % = {𝑊𝑒𝑡 𝑀𝑎𝑠𝑠−𝐷𝑟𝑦 𝑚𝑎𝑠𝑠
𝑊𝑒𝑡 𝑚𝑎𝑠𝑠 } x 100 (2.5)
2.16.9 Test for linear shrinkage
This is usually determined by measuring the dimensions of green bricks, and comparing with the dimensions of the fired product. It is expressed in percentage and is calculated using:
Linear shrinkage, = % {𝐿𝑔−𝐿𝑓
𝐿𝑓 } x 100 = ∆𝐿
𝐿𝑓 x 100 (2.6) Where Lg and Lf are the green length and fired length of brick respectively.
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