THE INFLUENCE OF THERMAL SHOCKS ON THE MICROSTRUCTURE AND ELASTIC PROPERTIES OF REFRACTORIES.

Transcription

1 TH INFLUNC OF THRMAL SHOCKS ON TH MICROSTRUCTUR AND LASTIC PROPRTIS OF RFRACTORIS. J. Podwórny, J. Wojsa, T. Wala Institute of lass, Ceramics, Refractory and Construction Materials, Refractory Materials Branch, ul. Toszecka 99, - liwice ABSTRACT Investigations into the microstructure, Young s modulus, shear modulus, internal friction as well as Poisson s ratio changes of selected types of refractories subjected to thermal shocks have been carried out. Mullite and silica fired bricks, high alumina concrete and three types of basic fired materials, i.e. magnesia, magnesia-spinel and magnesiachrome have been tested. The values of Young s and shear moduli, internal friction as well as Poisson ratio were determined by means of RFDA (resonance frequency and damping analyser), using flexural and torsion vibrations. The value of Young s modulus and shear modulus decreased after each thermal shock and the corresponding internal friction increased; in each case the relative drop of Young s modulus was higher than that of shear modulus. This resulted in the decrease of Poisson s ratio of the examined materials. It was found that materials subjected to investigations were characterized by low values of Poisson s ratio, which after thermal shocks in most cases unexpectedly decreased to negative values. In order to explain the observed phenomenon an attempt to correlate the obtained results with the microstructure of materials before and after a series of thermal shocks was undertaken. It was observed that the microstructure of the tested materials after thermal shocks was loosened compared to its initial state. On the basis of the experiments results and literature data, the phenomenon of the existence of porous ceramic materials with negative Poisson ratio was explained and a hypothetical model of such a material was proposed. INTRODUCTION Thermal shock resistance is an important property of refractory materials, which frequently determines the possibility of using a given material in a particular application in a thermal unit. The commonly used method of examining thermal shock resistance, which involves cyclical heating of a material to the set temperature, cooling it with water or compressed air and counting the number of cycles after which the material was not damaged, does not allow prediction the material s behaviour in the conditions of a real thermal shock in a particular thermal unit. In real conditions a different, frequently higher temperature gradient occurs in the material, which results from one-sided heating of the material and higher working temperature. Stresses in the material are also affected by additional factors, such as: mechanical properties resulting from the phase composition and the type of its microstructure, i.e. spatial arrangement of phases, the shape and arrangement of pores, corrosive factors crystallization of corrosion products in the working material s cooler zones as well as thermal shock intensity differing from that observed in thermal shock resistant tests, which is reflected in a different rate and degree of overcooling. The complexity of this problem imposes a necessity of adopting a different approach to investigating the refractories shock resistance and developing a new method of testing this material property, which is so important in practical applications. Despite considerable technological development this problem still remains unsolved, and searches for an appropriate method or group of methods continue. Works aiming to solve this problem are focused on improving the methods of determining or calculating the values connected with thermal shock resistance of a material, such as: the criterion characterising microcracks propagation (RST), work of fracture ( WOF) [], time of thermal stresses relaxation ( after thermal shock [], internal friction (Q-), Young s modulus () and shear modulus () as well as Poisson s ratio ( and then correlating them with the material s thermal shock resistance []. The results presented in this work are a part of comprehensive investigations aiming to solve the signalled problem and include the determination of, modulus, Poisson s ratio as well as internal friction Q- before and after thermal shocks. The aim of this work was to examine the changes caused by thermal shocks to elasticity properties, i.e. and moduli, Poisson s ratio ( ) and internal friction Q- of three types of fired basic refractories characterised by different thermal shock resistance: magnesia, magnesiaspinel, magnesia-chromite as well as mullite, silica material and low-cement chamotte castable fired at three different temperatures. MATRIALS AND XPRIMNTAL PROCDUR Basic properties of the examined materials have been given in table. In the investigations were used samples of industrially produced, formed and fired basic materials, mullite and silica as well as samples of refractory chamotte castable after drying, from which x6x mm plates were cut out. Before starting the experiments only the samples of lowcement chamotte castable were subjected to firing at C, C and C. Thermal shocks were conducted by heating the samples to the temperature of C, at the rate of 8 C/min, which were then annealed at this temperature for hours and next cooled for minutes with compressed air having the pressure of.6 MPa. The experiment was repeated three, four and five times, depending on the sample. The value of and moduli, Poisson s ratio and internal friction Q- before and after each thermal shock was determined by a resonance frequency and damping analyser - RFDA (IMC-Belgia), using the flexural and torsion vibrations. Determinations were carried out according to

2 ,,, - -,,,,, Fig.. Changes of the values of, moduli and the corresponding values of internal friction Q - (damping) of magnesia-spinel material after thermal shocks.,,,,6 Q-(),,, -,,7,,,,,, Q-() - [Pa],, Fig. Changes of the values of, moduli and the corresponding values of internal friction Q - (damping) of magnesia-chromite material after thermal shocks.,, Q-(),,,,, - -, Q () 6, 6,, Q-(), Fig.. Changes of the values of, moduli and the corresponding values of internal friction Q - (damping) of silica material after thermal shocks. 7,, 6,, 6,,,,,, Q-() 7,, Q-(),,, - RSULTS AND DISCUSSION The changes of the values of, moduli as well as the corresponding values of internal friction Q- (damping) for flexural and torsion vibrations after subsequent thermal shocks, measured after hours following the end of shock number i have been shown in figures to 6. The time of h was chosen due to the stable course of dependence (t), (t) and Q-(t) in this area for all the examined samples []. The lines connecting the points in figures to 6 merely illustrate the tendencies of changes of the measured values and do not prove the function continuity. In each case the first thermal shock turned out to be most destructive for the examined materials (figures to 6). This effect was reflected in a drop of the values of and moduli and Poisson s ratio. It is worth emphasising that the drops of modulus values compared to modulus were in each case lower, which may indicate that the tensile and compressive stresses connected with modulus in thermal shock conditions have a more destructive effect in comparison with shear stresses related to modulus. This effect was particularly visible in magnesia material after the second and third thermal shock, where reached 7,6 % and, % respectively, while was, % and, %. [Pa], (9 C) 8, Q-(),,, Fig.. Changes of the values of, moduli and the corresponding values of internal friction Q - (damping) of mullite material after thermal shocks. ( C) ( C) Q-( C),, ( C) Q-( C) Q-( C),, 6,,,,,,,,, ( C) ( C) Q-( C) ( C) Q-( C) Q-( C),6,, ,,8, , 6, , 8, 6.8.,, 7,..,6 -,,.8.9 Q-() Q (), , Q ().6. 6,,,, Thermal shock cycle -.8.9,, 8,,, Q (),, 7,, Q-(),, -, Q () [Pa], Fig.. Changes of the values of, moduli as well as the corresponding values of internal friction Q - (damping) of magnesia material after thermal shocks. [Pa] Open porosity [%] Crushing strength [MPa],9, 7,,6, 6,,8 Q-() 7, [Pa] [Pa] Thermal shock resistance [number of cycles 9 ºC/ water] RST [K m/] Work of rupture WOF [J/m] Thermal conductivity [W/mK] 8 C C Linear thermal expansion [%] C C 9, Q () Table. Basic properties of the examined materials. (Magmagnesia, M-MA-magnesia-spinel, M-MC-magnesia-chrome, Mulmullite, Sil-silica, LC-CC-low cement chamotte castable fired at C) Property Mag MMMul Sil LCMA MC CC Young s modulus , [Pa] ASTM 9 9 standard. The examined plates were placed on two supports, having the shape of -mm diameter cylinders, located at the intersection of the flexural and torsion vibrations nodes. Such location ensures free vibrations of the plate and enables simultaneous determination of and moduli, Poisson ratio and internal friction. The points of inducing the vibrations and registering the frequency were selected so that the amplitudes of both types of induced vibrations were similar. The microstructure of samples in their initial condition as well as after a series of thermal shocks was subjected to observation by examining resin-embedded microsections using the reflected light optical microscopy. An optical microscope MeF with a Leica image analyzer was used in the investigations.,6, 8,, Fig. 6. Changes of the values of, moduli and the corresponding values of internal friction Q - (damping) of

3 low-cement chamotte castable fired at C, C and C after thermal shocks. Different values of and drops are reflected in the change of coefficient, which results from the dependence correlating these three values: ν =. This provides a Poisson ratio basis for concluding that in the conditions of thermal shocks the greater the effect of tensile and compressive stresses compared to shear stresses, the greater is the drop of Poisson s ratio values. Changes of Poisson s ratio values under the influence of thermal shocks for all the examined materials have been given in figure 7.,,,,,, -, -, -, -, -, -, -, Silica Mullite LC chamote concrete ( C) LC chamote concrete ( C) Magnesia-chrome Magnesia-spinel LC chamote concrete ( C) Magnesia Fig. 7. Changes of Poisson s ratio values after thermal shocks. The magnesia-spinel material, having the highest thermal shock resistance among the examined basic materials, was characterised by the greatest changes of and after the first shock. Following the subsequent thermal shocks the changes of and were the smallest. On the other hand, each thermal shock in the magnesia material having the lowest resistance among the examined basic materials, caused similar changes of, whereas the changes of values were the greatest after the first shock. The magnesia-chromite material had intermediate properties. The value of internal friction Q - in this group was observed to increase after each thermal shock. Moreover, it was noticed that the initially low values of positive for the magnesia and magnesia-chromite product and negative for the magnesia-spinel one, dropped after the first thermal shock and decreased further with each subsequent shock. The mullite and silica materials were characterised by the expected values of Poisson s ratio before a thermal shock, which reached,68 for the mullite product and, for the silica one. These values decreased after the first thermal shock, maintaining however a relatively high positive value. After a series of subsequent thermal shocks the mullite material practically did not change its,, Q - and properties (figures and 7), whereas the examined silica material was destroyed after the first thermal shock. The obtained results for the mullite material, characterized by high thermal resistance, and for the silica material, which in fact was not resistant to thermal shocks due to phase transformations, are not surprising. The low-cement chamotte castable was characterised by the most complex course of, and Q- dependencies after thermal shocks among the examined materials. This was chiefly reflected in the drops of and values - lower than expected after the third shock (figure 6), which was accompanied by a very slight increase or even a drop of internal friction value (castable fired at ºC figure 6). The observed phenomenon may be related to the specific microstructure of the material subjected to thermal shocks as well as the processes of material recovery, in which microcracks heal in the conditions of multiple heating of the material. A similar course of internal friction Q - dependence on the number of thermal shocks in corundum low-cement castable was also observed by other authors [], who concluded that this was connected with the formation and propagation of cracks in the materials. The reasons for the falls in Poisson s ratio values after a series of thermal shocks should be sought in the microstructure and phase composition of low-cement chamotte castables. The size of changes to Poisson s ratio values was dependent on the castable firing temperature (figure 7). It was found that this might be related to the growing amount of cristobalite, accompanied by a simultaneous drop in the amount of amorphous phase in the samples of castable after subsequent thermal shocks as well as the amount of residual quartz (figure 8). The smallest increase of cristobalite share was observed in the sample of castable fired at ºC, whereas the highest growth in the sample fired at ºC. Cristobalite at the temperature of ºC undergo an irreversible phase transformation, similarly to quartz at 8ºC. These transformations were accompanied by a considerable change in the volume, which led to a greater number of cracks and loosening of the material s microstructure. As a result the material behaves similarly to silica materials, which are characterized by lack of thermal shock resistance. Fig. 8. Phase composition of low-cement chamotte castable fired at C, C and C after a series of thermal shocks, the main components of which are mullite (M), corundum (Cor), cristobalite (C) and quartz (Q), (Ano anorthite). In figures 9 and magnesia and magnesia-chromite materials have been shown as examples of changes in refractory materials microstructure after thermal shocks. Figure 9 shows the microstructure of the magnesia product before and after thermal shocks. A visible difference could be observed in the channel like pores generated during thermal shocks around magnesia clinker grains, which led to the loosening of the product s microstructure. Similar differences in the general picture of microstructure before and after thermal shocks were observed in the magnesia-chromite material. An increased amount of channel like pores was observed around the chromite grains (figure ).

4 Similarly to the magnesia product, the microstructure of magnesia-spinel material loosened due to the formation of channel like pores around the spinel and magnesia clinker grains. A bigger number of transcrystalline and intercrystalline cracks which joined the pores and intensified the effect of microstructure loosening, the rattle effect was also observed. Similar changes of microstructure loosening were observed in all the examined materials. The resonance method of measuring the values of, moduli, internal friction Q-, and particularly Poisson s ratio, due to its non-destructive character and correlation with the degree of defects, might be more commonly applied in nondestructive investigations into refractory materials. Correlating the changes of damping with the phase composition and microstructure may be used in the designing of new varieties of ceramic materials. CONCLUSIONS In the group of basic materials the differences in the changes of and moduli after a series of thermal shocks correlate with their thermal shock resistance and the criterion characterising microcracks propagation (RST) and work of A B Fig. 9. Fine-crystalline microstructure of the magnesia product (A) before and (B) after thermal shocks with visible microstructure discontinuities in a form of channel like pores on the border of magnesia clinker grains. A B Fig.. Thick-crystalline microstructure of the magnesiachromite product (A) before and (B) after thermal shocks with loosened microstructure. The reasons for low or negative values of Poisson s ratio should be sought in the specific microstructure of refractories, which is characterized by grains of a particular shape, a system of pores, a net of cracks and the presence or lack of direct bonds. The literature quotes examples of materials having a negative Poisson s ratio [, ], which may provide inspiration for explaining this phenomenon in refractories. Most frequently these are materials having a foam structure (high amount of pores) or hierarchical composites. In the above mentioned literature one may read that such behaviour of a material having a negative value may be caused by the amount of voids (discontinuities) and defects, enabling a turn or slight dislocation of the material s grains due to external tensile and compressive stresses. Such an idealised situation which might take place in the defected and porous ceramic material has been shown in figure. Fig.. Hypothetical ceramic material with a negative value of Poisson s ratio, built of identical size grains having the shape of rectangular prisms. Drops in Poisson s ratio to negative values might then be related to voids (pores and cracks) in the material. The obtained results justify such an assumption, as the material s damage, reflected in the drops of and values and the growth of Q- value figures to 6 increases with the higher number of thermal shocks. fracture ( WOF) (table ). As a result of thermal shocks modulus decreases much faster than modulus, which leads to a drop in Poisson s ratio. This proves the stronger effect of tensile and compressive stresses compared to shear stresses on the behaviour of materials in the conditions of thermal shock. Negative values of Poisson s ratio are most probably related to the specific loosened and defected microstructure of a porous material, which may cause a non-homogenous distribution of stresses. Owing to that in the conditions of the applied measuring method that introduces variable tensile, compressive and shear stresses, a turn or dislocation of grains in some limited way in the material is hardly possible. The microscopic observations of refractory samples before and after thermal shocks indicate that thermal shockrelated processes generate the microstructure discontinuity, chiefly in a form of channel like pores around the grains (the so-called the rattle effect ) and cracks related to brittle cracking. This explains the drops in the values of Young s modulus and shear modulus, the increased internal friction and the lower values of Poisson s ratio. Poisson s ratio seems to be a good criterion of evaluating the material s cohesion or the number of direct bonds. The use of Poisson s ratio determinations in nondestructive investigations into refractory materials seems an interesting and prospective direction of research. The study does not explain why a significant increase of internal friction Q- was not observed after the first thermal shock, which resulted in the biggest drops of and moduli s values. This theme as well as the correlation of damping changes with the material s microstructure should become the subject of further investigations. RFRNCS [] J. Wojsa, A. Wrona, K. Czechowska, Zastosowanie kryteriów mechaniki kruchego pękania do przewidywania OWT wyrobów ogniotrwałych. Sprawozdanie IMO 6//DN/BL/BT/ [] J. Podwórny, J. Wojsa, Stress relaxation after thermal shock of basic refractories, Stahl und isen. Special issue November 6, pp.: -8. [] T. Tonnesen, R. Telle, valuation of Thermal Shock Damage in Castables by a Resonant Frequency and Damping Method, Stahl und isen. Special issue November 6, pp.: -6. [] R.S. Lakes, Deformation mechanism in negative Poisson s ratio materials: structural aspects, J. Mat. Sc. 6 (99), 87-9.

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