T he heat-up of refractory castables has been the subject of extensive research

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1 Steel Fibers and Mechanical Behavior of Refractory Castables on Drying The mechanical and drying behavior of refractory castables that contained short and long steel fibers was compared with castables that contained polyaramid fibers. C.M. Peret and V.C. Pandolfelli Materials Microstructural Design Group, Dept. of Materials Engineering, Federal University of São Carlos, São Carlos, S.P., Brazil T he heat-up of refractory castables has been the subject of extensive research because of the importance manufacturers and users attribute to this stage of their processing and its impact on the productivity and quality of end products. The development of compositions capable of withstanding severe heating schedules minimizes the risk of spalling, which decreases the downtime of metallurgical equipment for maintenance and repair. Polymeric fibers are an efficient way to enhance castable resistance to spalling upon drying, because they allow more severe heating schedules under a controlled risk. Two mechanisms are responsible for the benefits conferred by these fibers: an increase in permeability caused by fiber melting, thermal degradation or shrinkage, 1 as in the case of polypropylene (PP) fibers; and mechanical reinforcement, which results especially from the increased energy dissipated during crack propagation, 2 as when polyaramid (PAr) fibers are used. Whenever fibers are added as a reinforcing particle, they should be selected in such a way that their elastic properties are kept on a suitable level within the temperature range where the risk of damage is high. Previous studies 1,3 have shown that, for lowcement high-alumina castables, this range is located between 150 and 200 C, taking as reference the surface temperature of the castable. Therefore, PAr fiber, a fiber that presents thermal degradation only above 300 C, has shown positive results in preventing explosive spalling on drying. 2 In addition to polymers, many other materials have been successfully used as mechanical reinforcements to produce tough cementitious composites for applications such as tunnels, pavements, monolithic and preshaped refractories, and advanced ceramics. In the cases of the construction and refractories industries, carbon, natural and stainless-steel fibers in varying sizes and shapes usually have been applied for reinforcement at room and high temperatures. 4 Steel fibers benefit mechanical reinforcement in many applications, and this mechanism has been identified as the determining factor of spalling resistance on drying. Therefore, the purpose of this work is to determine the efficacy of long (25 mm) and short (2 6 mm) stainless-steel fibers as a drying reinforcement. The American Ceramic Society American Ceramic Society Bulletin January

2 Castable Preparation Table 1. Castable Fiber Length, Diameter, Number A high-alumina ultra-low-cement refractory castable composition was designed for this study, following Andreasen s particle distribution model (q = 0.24). The particle size ranged from 0.1 to 4750 µm, and 2 wt% of calcium aluminate (CA) cement was added to the composition (Fig. 1).To disperse the particles in water (4.2 wt%), 0.05 wt% of citric acid was used. Fiber Polyaramid Gervois Stax 130 Stax 225 Concentration (vol% [wt%]) 0.36 [0.12] 0.36 [0.66] 0.36 [0.66] 0.36 [0.66] Diameter (mm) Length (mm) N/VC (cm 3 ) Daoli 0.36 [0.66] Four types of metal fibers with varying lengths were DuPont AFS (USA). Gervois S.A. (France). Stax AG (Germany). Daoli Ltda (Brazil China). selected for this study. The fibers were made of AISI 434 stainless steel (Gervois, Stax 130 and Stax 225) or 304 stainless steel (Daoli). However, the fibers were produced by different Table 2. Castable Mixing Procedure processes: the Gervois, Stax 130 and Stax 225 fibers were made of Mixing time Event chopped drawn wire, whereas the Daoli fiber was obtained by melt 0 s Start extraction (Table 1 and Fig. 2). 60 s First addition of water For comparison, a 6 mm long PAr fiber with a 20 µm diameter also was used. These fibers have a high thermal stability, because their elastic modulus is not decreased by degradation in the critical drying temperature range ( C), as shown in a previous report. 2 (3.6 wt% at constant rate) s Addition of remaining water (total of 4.5 wt%) 60 s after finishing End of mixing water addition (total of s) All the fibers were added to the castable at a volumetric ratio of 0.36%, before the addition of water, to inhibit fiber agglomeration. The mixing and water addition steps took place in a rheometer for castables developed by the authors research group (GEMM). 5 It consisted of the following procedure (Table 2): after 60 s of dry-mixing time, 3.6 wt% (12.5 vol%) of water was added at a constant rate; the remaining water, which amounted to a total of 4.5 wt% (15 vol%), was added only after a constant mixing torque (~160 s of mixing time) was attained. Specimens were cast by vibration in the following geometries: 40 mm high 40 mm diameter cylinders for drying and splitting tensile strength; mm prismatic specimens with an embedded notch 1 mm deep 1 mm wide for work-of-fracture measurements; and 75 mm diameter 22 mm high cylinders for permeability measurements. The specimens were cured at 8 C for 7 d in a moisturized environment. However, the specimens for permeability testing were cured for the same time and at the same humidity, but at 50 C. The temperature of 8 C was chosen because hydration of the cement phases leads to the formation of CAH 10 (CaO Al 2 O 3 10H 2 O), which has a higher molar volume. As a consequence, the castable permeability is decreased and a critical condition for drying is attained. The purpose of the permeability measurements in this work was only to verify the effect of the addition of various fibers and the relative difference between the compositions before and after a heat treatment. Therefore, curing of the specimens for permeability tests was conducted at 50 C for 7 d. Figure 2 Length statistical distribution for the steel fibers. A mechanical evaluation was made based on splitting tensile strength (ASTM C406-96) and work-of-fracture (γ wof ) measurements, both at room The American Ceramic Society American Ceramic Society Bulletin January

3 temperature. These tests were conducted immediately after curing, without previous drying, to better simulate the true conditions of the material when saturated with water. The splitting tensile strength (in MPa) is given by the following equation: σ f = (2/π)(P/dh) (1) where P is the ultimate load (in newtons), and d and h the specimen diameter and height (both in millimeters), respectively. The work-of-fracture was calculated by integrating the curve of load, P, as a function of the actuator displacement, δ, in a three-point bending test, under a constant displacement rate of 20 µm/min: γ wof = (1/2A) P dδ (2) where A is the projected fracture surface area. The non-darcyan permeability constant, k 2,was calculated by measuring the flow rate of compressed air through a cylindrical specimen of the material, as a function of the inlet air pressure (which varied from 0 to 5 bar (0 to 500 kpa)). The data were treated according to Forchheimer s equation: 6 [(P 2 i P 2 o )/2P o L] = (µ/k 1 )V S + (ρ/k 2 )V 2 S (3) where P i and P o are the inlet and outlet (atmosphere) air pressures, respectively; L the height of the specimen; µ and ρ the air viscosity and density, respectively; and V S the speed of the air through the material, calculated by the measured air flow rate and the cross-sectional area of the specimen. 6,7 Unlike the mechanical measurements, the permeability was measured in specimens previously dried at 50 C, and then repeated with the same specimens after they were heat-treated at 800 C for 12 h. The mass loss on drying was evaluated using a thermogravimetric apparatus developed by the authors research group. 8 This device consisted of an electrical furnace in which a specimen holder was connected to a balance. The furnace heating elements were shielded against spattering fragments by enclosing the specimen holder in a stainless-steel cage. The specimen was heated at 20 C/min, from room temperature to 600 C. The test can be interpreted using the W(t) evaluation, which is a measure of the cumulative fraction of the water that already has been released from the specimen at time t, in comparison with the total water initially present: W(t) = 100[(m 0 m(t))/(m 0 m f )] (4) where m(t), m 0 and m f are the specimen mass at time t, the initial mass and the dry-specimen mass, respectively. For the analysis, the instant drying rate was calculated by deriving W(t) as a function of time and plotted against the furnace temperature. Figure 3 Torque vs mixing time for the fiber-containing refractory castables (Ref. means reference, i.e., without fibers). Rheological Behavior The torque profile with the mixing time (Fig. 3) presents some differences when comparing compositions containing steel or PAr fibers against compositions without fibers. The mixing profile changes slightly with the addition of steel fibers. The Stax 130 and Stax 225 The American Ceramic Society American Ceramic Society Bulletin January

4 steel fibers present a decrease of ~13% in the time it took to reach the maximum torque, and an increase of ~10% in the final torque was attained during mixing. These changes are minimal in comparison with those observed from the addition of PAr fibers. 9 For these polymeric fibers, the torque is ~33% higher than that of the reference material. The average mixing effort can be calculated by integrating the torque profile curves as a function of time (Fig. 4). A higher mixing effort value was verified when processing the PAr-fiber-containing castable. These polymeric fibers were 6 mm long which is in the same order of some steel fibers studied but presented a diameter of ~20 µm. The average number of fibers per unit volume of castable, N/V c,can be calculated approximately by dividing the volumetric ratio of fibers in the castable, p,by the volume of a single fiber, estimated by considering it a perfect cylinder with diameter d and length L: N/V C = p/v single fiber = 4p/πd 2 L (5) Figure 4 Average mixing effort for the fiber-containing castables. According to their geometrical features, the average number of fibers per unit volume of castable, for a constant volumetric ratio of (0.36%), may be 468, 68, 24 and 0.9 fibers/cm 3 for the Gervois, Stax 130, Stax 225 and Daoli fibers, respectively. On the other hand, with the PAr fiber, this value increases to ~1909 fibers/cm 3.Thus, the distance between the fibers is shorter for the polymeric fibers, and the probability of collisions and interference during mixing and molding is increased. The number of fibers, therefore, is an important factor determining the rheological behavior of the castable during the mixing step. This statement can be extrapolated to material behavior during other steps of rheological processing, such as pumping and shotcreting. Effect of Fibers on Permeability No significant differences in the values of the permeability constants were found among the castables (Fig. 5) when evaluated in the green stage. After a heat treatment at 800 C for 12 h, the permeability increased in the following order: Gervois < Stax 130 < Stax 225 < Daoli. This result possibly was caused by the mismatch of the thermal expansion coefficient and the difficulties to accommodate these stresses, depending on the fiber geometry. Figure 5 Non-Darcyan permeability constant (k 2 ) of the green and fired fiber-containing refractory castables. In the PAr-fiber-containing castable, the increase in permeability after thermal treatment was 10-fold greater than that of the steel-fiber-containing castables, because the thermal degradation generated many permeable channels through the microstructure. 1 The American Ceramic Society American Ceramic Society Bulletin January

5 Effect of Fibers on Mechanical Properties The splitting tensile strength of the materials varied from 0.8 to 1.4 MPa (Fig. 6). The Daoli fiber, which is the longest of the steel fibers evaluated, conferred the greatest improvement in strength on the material. However, the addition of PAr fiber had little effect on this property. These results are in agreement with the general literature, which shows that the volume of short fibers that can be added to ductile- and brittle-matrix composites is insufficient to produce a significant improvement in their elastic modulus and strength. 4,10,11 On the other hand, the mechanical behavior of the castables after the beginning of crack propagation is greatly influenced by the fibers, as indicated by the γ wof results (Fig. 7). Among the steel-fiber refractories, the γ wof scaled up with the average length of the fibers, although the value for the castable containing Gervois fibers was not altered in comparison with the reference material. The other steel fibers Stax 130, Stax 225 and Daoli produced composites apparently tougher than the one with PAr fibers. The relative increase of this property, obtained with the addition of Daoli fibers, was ~900%, whereas that of PAr fibers was only 250%. Figure 6 Splitting tensile strength for the refractory castables. The increase in energy consumption after the beginning of propagation in the fibercontaining composites is driven by the bridging phenomenon.this effect is caused mainly by a crack-wake compressive force generated by fiber anchoring and pullout. 12 The low γ wof attained with Gervois fiber indicates that there may be a minimum Figure 7 Work-of-fracture of the refractory castables containing steel or length for fibers to provide enough anchoring in the castable matrix, as well as suffi- PAr fibers. cient interfacial friction during pullout to contribute to the dissipation of the stored elastic energy. Drying Behavior Explosive spalling occurred in all the steel-fiber-containing castables tested at 20 C/min (Fig. 8). On the other hand, the material containing PAr fibers survived heating at that rate, attesting to the greater efficiency of these fibers in enhancing refractory resistance to drying and explosion. The thermogravimetric analysis revealed no significant differences among the mass loss rate profiles of the materials. Indeed, the structure of the permeable channels through which the water vapor flows was not greatly modified by the presence of the fibers studied. Neither melting of PAr and steel fibers nor degradation of PAr fiber occurred below the critical temperature range involved in the drying process. The greater resistance to damage on drying displayed by the PAr-fiber-containing material indicates that a more severe heating rate can be applied to the material, thereby lessening the overall time needed for the completion of heat-up. The American Ceramic Society American Ceramic Society Bulletin January

6 Effect of Distance between Fibers The mechanical effect of fibers upon the drying damage resistance is related to the increase in energy dissipation with crack propagation through a bridging and pullout mechanism. The extent of such nonlinear fracture behavior can be inferred by the castables γ wof. 13 Nevertheless, the mechanical reinforcement potential can be activated only if many fibers cross the crack propagation front. Therefore, the distance between fibers should be designed to be compatible with the scale of the cracks formed during the first stages of damage. According to Betterman et al. 14 closer fibers (microfibers) are capable of lessening the propagation of microcracks that precede catastrophic failure, because the mean distance that a crack propagates before being crossed by a fiber is, at most, half of that between the fibers. The distance between fibers is determined mainly by their geometry and volumetric ratio in the castable. Considering a homogeneous, cubic distribution of fibers in the microstructure, the shortest distance, S,between them can be estimated by S = (V C /N) 1/3 = (πd 2 L/4p) 1/3 (6) Equation (6) originates from a model that represents the fibers by their middle points and takes into account neither the natural variation of diameter and length nor the typical entanglement of more flexible, higher-aspect-ratio fibers, such as PAr fiber. According to Eq. (6), the distance between the fibers in the present work follows the order PAr < Stax 130 < Stax 225 < Daoli (Table 3). This trend was confirmed through visual evaluations of the fracture surfaces of the materials (Fig. 9). According to Griffith s theory, unstable crack propagation is followed by an increase of kinetic energy.to prevent catastrophic failure, the propagating front must be crossed by the fibers while the strain energy release rate is low.therefore, to enhance the drying performance of refractory castables, in which only minor microcracking is allowed before the material explodes, the fibers must be distributed as close as possible to each other to provide a useful reinforcement. Table 3. Distance between Fibers Fiber Distance, S (mm) Polyaramid 10.3 Gervois 3.5 Stax Stax Daoli 0.8 Containing 0.36 vol% of steel or polyaramid fibers. That is why PAr fibers were the only ones able to inhibit explosive spalling in the present study. Our results show that 25 mm long steel fibers, which are efficient to enhance refractory thermal shock and fatigue resistance, are not suitable for protecting castables from vapor pressurization at low volume fractions. 15 If there are points in the castable microstructure that are unprotected by the mechanical action of fibers, they are vulnerable to explosion. Fiber Selection Factors Although steel fibers caused significant modifications in castable work-of-fracture and splitting tensile strength, especially when compared with the polyaramid-containing material, this benefit could not ensure their survival under severe drying heating schedules. Before explosive spalling occurs, microcracks grow and merge in the castable microstructure. Thus, to prevent such critical propagation, the fibers should be distributed as close as possible to each other, thus increasing the drying resistance of the material. This is not achieved using longer or thicker fibers, such as the steel fibers studied here. However, better results may be obtained by increasing their volumetric content. Our results indicate that values of mechanical strength and work-of-fracture are not the only properties that should be considered when evaluating castables for their resistance to spalling upon drying. The American Ceramic Society American Ceramic Society Bulletin January

7 Fiber reinforcement can offer advantages provided selection takes into account the following factors: The fibers must have a minimum length to provide anchoring in the castable matrix. The unit volume and volumetric ratio of the fibers should be designed so that the distance between them is compatible with the spatial scale of the mechanical loads to which the material may be subjected. The mechanical strength and elasticity of the reinforcing particles (i.e., fibers) should be kept within a suitable range at the temperature at which the stress is at its greatest (in the case of castable drying, in the range C, considering the surface temperature). Acknowledgments The authors are grateful to FAPESP, Magnesita S.A. and ALCOA-Brazil for their financial support of this work, and to Daoli Ltd. (Brazil China), Stax AG (Germany), Gervois S.A. (France) and DuPont AFS (USA) for providing the steel and polymeric fibers used. References 1 M.D.M. Innocentini, C. Ribeiro, R. Salomão, V.C. Pandolfelli and L.R.M. Bittencourt, Assessment of Mass Loss and Permeability Changes during the Dewatering Process of Refractory Castables Containing Polypropylene Fibers, J. Am. Ceram. Soc., 85 [8] (2002). 2 C.M. Peret, R. Salomão and V.C. Pandolfelli, Polymeric Fibers as Additives for the Drying of Refractory Castables, J. Tech. Assoc. Refract., Jpn. (Taikabutsu Overseas), 24 [2] (2004). 3 M.D.M. Innocentini, M.F.S. Miranda, F.A. Cardoso and V.C. Pandolfelli, Vaporization Processes and Pressure Buildup during Dewatering of Dense Refractory Castables, J. Am. Ceram. Soc., 86 [9] (2003). 4 H.G. Jiang, J.A. Valdez, Y.T. Zhu, I.J. Beyerlein and T.C. Lowe, The Strength and Toughness of Cement Reinforced with Bone-Shaped Steel Wires, Compos. Sci. Technol., 60 [9] (2000). 5 R.G. Pileggi, V.C. Pandolfelli, A.E.M. Paiva and J. Gallo, Novel Rheometer for Refractory Castables, Am. Ceram. Soc. Bull., 79 [1] (2000). 6 M.D.M. Innocentini, A.R.F. Pardo, V.R. Salvini and V.C. Pandolfelli, How Accurate is Darcy s Law for Refractories? Am. Ceram. Soc. Bull., 78 [11] (1999). 7 M.D.M. Innocentini, A.R.F. Pardo and V.C. Pandolfelli, Influence of Air Compressibility on the Permeability Evaluation of Refractory Castables, J. Am. Ceram. Soc., 83 [6] (2000). 8 M.D.M. Innocentini, C. Ribeiro, J. Yamamoto, A.E.M. Paiva, V.C. Pandolfelli, L.R.M. Bittencourt and R.P. Rettore, Drying Behavior of Refractory Castables, Am. Ceram. Soc. Bull., 80 [11] (2001). 9 R. Salomão, V. Domiciano, C. Isaac, R.G. Pillegi and V.C. Pandolfelli, Mixing Step and Permeability of Polymeric-Fiber-Containing Refractory Castables, Am. Ceram. Soc. Bull., 83 [1] (2004). 10 E. Absi (org.), Béton de Fibres: Synthèse dês Etudes et Recherches Réalisées au CEBTPæ; pp , Annalles de l Institut Technique du Batîment et des Travaux Publics, No. 250, Jan A.M. Pallière (org.), Le Béton de Fibres Métalliques: Etat Actuel des Connaissances ; pp , Annales de l Institut Technique du Bâtiment et des Travaux Publics No. 515, July Aug R.W. Steinbrecht, Toughening Mechanisms for Ceramic Materials, J. Eur. Ceram. Soc., 10 [3] (1992). 13 S.M. Barinov and M.M. Sakai, The Work-of-Fracture of Brittle Materials: Principles, Determination and Applications, J. Mater. Res., 9 [6] (1994). 14 L.R. Betterman, C. Ouyang and S.P. Shah, Fiber Matrix Interaction in Microfiber-Reinforced Mortar, Adv. Cem. Based Mater., 2 [2] (1995). 15 P.C. Tatnall, Shotcrete in Fires: Effects of Fibers on Explosive Spalling, Shotcrete,[Fall] (2002). The American Ceramic Society American Ceramic Society Bulletin January

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