Thermal Shock Resistance Of Refractories Research Articles

Preparation and thermal shock behavior of MgAl2O4–C refractories by designing buffering gap around aggregates

a new refractory microstructure with thermal stress buffering gap around aggregates was designed. PVA (Polyvinyl alcohol)-coated MgAl2O4 (MA) aggregates were prepared by spray coating technology. The obtained PVA coated MA aggregates were adopted in MgAl2O4–C (MA-C) refractories to research its effect on properties of refractories. The results showed that a uniform PVA layer could be coated on the surface of MA aggregates by spraying coating technique. In the heat treatment process, the PVA around MA aggregates was decomposed and leaves gap between aggregates and matrix, which could offset the aggregates expansion. Due to the reducing thermal expansion coefficient, the thermal shock resistance of refractories was significantly improved. The COMR after thermal shock and the residual strength retention rate of the sample with stress buffering gap were 6.02 MPa and 77.87 %, while that of the sample without stress buffering gap were 5.26 MPa and of 60.32 %.

Enhanced thermal shock resistance of low-carbon Al2O3-C refractories by aggregate/matrix interface design with MgAl2O4/CNTs layer

Aggregate/matrix interface microstructure design to guide more cracks propagating within it favors improved thermal shock resistance of refractories. A MgAl2O4/CNTs interfacial layer was formed between the aggregate and matrix in low-carbon Al2O3-C refractories fabricated by core-shell corundum aggregates. The microstructural evolution of aggregate/matrix interface with increased contents of nickel nitrate hexahydrate catalyst in the interfacial layer and its effects on the thermal shock resistance of refractories were investigated. The results show that the highest amount of CNTs within the interfacial layer was obtained with the catalyst concentration of 0.5 wt%. The catalyst also promoted the growth of AlN whiskers that bridging the aggregate and matrix. Cracks impinging the interface are prone to be deflected by the interfacial layer and propagate within it. The deflection of cracks by the MgAl2O4 layer, the bridging and pulling out mechanism of the CNTS and AlN whiskers all contribute to the enhanced thermal shock resistance.

Enhanced thermal shock resistance of low‐carbon Al2O3‐C refractories via CNTs/MgAl2O4 whiskers composite reinforcement

AbstractThe CNTs/MgAl2O4 whiskers composite reinforcement was prepared by catalytic carbon‐bed sintering and added to low‐carbon Al2O3‐C refractories to enhance the thermal shock resistance of refractories. The effects of Ni(NO3)2·6H2O content on the phase and microstructure of CNTs/MgAl2O4 were investigated. The nano‐indentation technology was used to quantitatively evaluate the effect of CNTs/MgAl2O4 on the thermal shock resistance of low‐carbon Al2O3‐C refractories, and the toughening mechanism of CNTs/MgAl2O4 on refractories was further investigated. The results revealed that the inner and outer diameters of the CNTs were approximately 5.69 and 16.26 nm, respectively. The CNTs winded around interlocking structural MgAl2O4 whiskers with a high aspect ratio (>100), which was beneficial to the dispersion of CNTs in the refractory matrix. The thermal shock resistance of low‐carbon Al2O3‐C refractories was significantly improved by adding 3.0 wt.% of CNTs/MgAl2O4 (named C3). The specific fracture energy of refractory matrix and residual strength ratio of C3 reached 141 N·m−1 and 39.2%, which were 29.4% and 97.0% higher than those of the blank sample (109 N·m−1 and 19.9%), respectively. This is because the refractory matrix was significantly reinforced by CNTs and MgAl2O4 whiskers, promoting the occurrence of “bridging” and “crack deflection” and the generation of microcracks in the refractory matrix.

Brittleness reduction of low-carbon Al2O3-C refractories with in-situ formation MgAl2O4/CNTs layer between aggregate and matrix

Crack propagation at the aggregate/matrix interfaces of refractories favors their brittleness reduction and better thermal shock resistance. Design of suitable interfaces is highly desirable in refractories. With coating of magnesium citrate and nickel nitrate hexahydrate catalyst on corundum aggregates, a thin MgAl2O4/CNTs interfacial layer of porous structure was formed between aggregate and matrix in low-carbon Al2O3-C refractories. The formation mechanisms of this interfacial layer are pyrolysis of magnesium citrate, reaction with corundum into MgAl2O4 and growth of CNTs. Comparison study of mechanical results shows that this compound interfacial layer contributes to an increase in cold crushing strength, and brittleness reduction assessed with wedge splitting tests. The 3-times water quenching tests testified that brittleness reduction brings about a mild loss of cold modulus of rupture. The results support the conclusion that aggregate/matrix interface design is an effective method to reducing the brittleness and eventually improving the thermal shock resistance of refractories.

Effect of C/MgO nanocomposite powders on the properties of low-carbon MgO–C refractories

To produce high performance low carbon MgO–C refractories, the combustion synthesized MgO nanofibers containing C/MgO composite powders (SCM) were introduced. The effect of SCM on the mechanical properties, thermal shock resistance, oxidation resistance and slag resistance of MgO–C refractories has been studied, along with exploring the toughening mechanism on thermal shock resistance. The results showed that the refractories containing 6 wt% SCM (M6) exhibited the most favorable comprehensive properties. Compared to reference sample M0, M6 showed significant enhancements in thermal shock resistance, oxidation resistance, and slag resistance by 38.6%, 29.8%, and 23.9%, respectively. Notably, the unique microstructure of MgO nanofibers and carbon encapsulated MgO played a crucial role in facilitating crack deflection and branching, which promote the generation of microcracks and the consumption of strain energy. These mechanisms contributed to the improvement in thermal shock resistance of refractories.

Nanoscale toughening of low carbon Al2O3‐C refractories by means of SiCnw/Al2O3 composite reinforcement

AbstractTo improve the thermal shock resistance of low carbon Al2O3‐C refractories, SiC nanowires (SiCnw) containing SiCnw/Al2O3 composite reinforcement were introduced. The specific fracture energy of the Al2O3‐C refractory matrix was obtained by statistical grid nano‐indentation. The reinforcement mechanism of SiCnw/Al2O3 on thermal shock resistance of refractories was investigated. The results revealed that the matrix‐specific fracture energy of A6 (6 wt% SiCnw/Al2O3 added) was 217 N/m, which was 58.4% higher than reference sample A0 (137 N/m) and 18.6% higher than MA6 (183 N/m, 6 wt% SiC/Al2O3 added). A6 showed the highest residual strength ratio of 49.8%, which was 114.7 % higher than A0 (23.2%) and 82.4 % higher than MA6 (27.3%). The components with different morphology in SiCnw/Al2O3 cluster, especially SiC nanowires, promote the generation of microcracks, crack multi‐deflection, and branching, which toughen the matrix and improve the thermal shock resistance of refractories. In comparison to the literature, A6 showed a higher rising in residual strength ratio than those with higher graphite content (4 wt% and 20 wt%), which will greatly reduce the consumption of carbon‐containing refractories and contribute to the reduction of CO2 emission.

A new approach for the determination of thermal shock resistance of refractories

The life cycle of refractory materials is affected by a variety of factors. Depending on the type of industrial application, they must be resistant to extreme temperatures, mechanical loads, chemical corrosion and large temperature changes when in contact with molten metal and slag. Correct estimation of durability is important in the choice of material and for determining the service life of individual parts of the structure. Together with the expected maintenance cycles, this influences the choice of material for optimum structural design. In practice, refractories are very often subject to large temperature variations and thermal cycling. Thermal shock is thus a predominant factor that shortens the service life of monolithic refractory products. Therefore, the main objective of refractory manufacturers is to research and develop refractories with higher resistance to thermal shock and to analyse, compare and evaluate them effectively. This paper deals with the issue of testing the resistance of refractory materials, respectively analysis and comparison of standard and new test methods for accelerated thermal shock resistance testing.

Effect of particle grading on fracture behavior and thermal shock resistance of MgO-C refractories

Two types of MgO-C refractories with tight particle grading and non-tight particle grading were prepared according to Andreasen's continuous packing theory. Fracture behaviors were investigated using wedge splitting tests combined with digital image correlation method and acoustic emission techniques. The results indicated that MgO-C refractory with non-tight particle grading treated at 1400 ℃ had more in situ phases (e.g., AlN and MgAl2O4) and exhibited less brittleness than specimens with tight particle grading even though they had similar nominal tensile strengths. In contrast, specimens with non-tight particle grading had greater horizontal strain under various loading stages, reflecting their better ability to resist rupture deformation. In addition, more microcracks were initiated earlier in the pre-peak region, and more energy was consumed. The decrease in coarse particles and corresponding increase in fine powder content increased the interface between particles, benefiting for reducing the local stress concentration and improving the thermal shock resistance of refractories.

The Effect of Carbon Sources on the Thermal Shock Properties of MgO-C Refractories

Thermal shock resistance of MgO-C refractories, which were used in the iron and steel industry, by incorporation of pyrolytic carbon black obtained by waste tire pyrolysis was investigated. The effect of porosity on the thermal shock resistance of those refractories as a function of carbon source (graphite or pyrolytic carbon black) was also examined in the current study. The microstructure and fracture surfaces were characterized using the Scanning Electron Microscopy (SEM). Experimental studies showed that refractories produced by the use of pyrolytic carbon black had higher amount of porosity and lower thermal shock resistance than refractories containing flake graphite.

Combination of Brazilian test and digital image correlation for mechanical characterization of refractory materials

Flexibility of refractories is an interesting property to improve thermal shock resistance of refractories. It can often be obtained by promoting a nonlinear mechanical behaviour which reduces brittleness. Among industrial refractory materials developed in this aim, magnesia hercynite is of a particular interest for cement industry. As linings for rotary kilns, magnesia hercynite can be submitted to tensile and compressive stresses. Since tensile strength is usually much lower than compressive one for brittle materials, the mechanical characterization is, in such case, more significant in tension than in compression. To overcome difficulties involved by direct tensile test, an indirect tensile test (Brazilian test) has been applied here and combined to digital image correlation in order to measure kinematic fields on the surface of the sample during loading. This combination has allowed to accurately measure initial elastic properties (Young’s modulus and Poisson ratio), to detect crack initiation and to analyze fracture process.

Development of Multi-Walled Carbon Nanotube and Processed Graphite Incorporated Hydraulically Bonded Refractory Castable

The incorporation of carbon materials is known to significantly enhance both corrosion and thermal shock resistance of refractories. For mixing and placement of refractory castables, water is the most suitable liquid media due to its availability, cost, non-hazardous characteristics and ease of handling. The main factors limiting the development and use of carbon-containing castables are the inferior wettability of carbon by water. Dispersion of carbon-containing materials in water with and without addition of surfactant was tested. By varying the carbon source and keeping the quality and quantity of all other raw materials constant, zero cement castable (ZCC) compositions were designed. The samples were coked and physical properties, oxidation resistance, and pore sizes were measured and FESEM studies carried out. The properties of castables containing processed graphite (PG) and multi-walled carbon nanotubes (MWCNT) were found to be most favourable for the iron and steel industry.

Theoretical Analysis on Thermal Shock Resistance of Refractories

So far, thermal shock theories have not been well verified in refractories. This paper compared hypothesis and failure mechanism of two typical thermal shock theories, analyzed the applications of two typical theories in refractories from working environment, failure process and microstructure differences. And thinks that the critical stress fracture theory is suitable for nearly ideal brittle refractory with ceramic bond and used in thermal shock extremely harsh environment; thermal shock damage theory suites to evaluate multiple loops thermal shock. Analyzed the factors influencing the thermal shock parameters, and discussed Ways to improve the thermal shock resistance of refractories from destructive force and resistive force of refractories. Conclusions are that choosing small swelling expansion rate and high thermal conductivity material, reducing the micro stress concentration and adding flexible phase, these three methods can be used to make refractories with good resistance to thermal shock.

Tensile behaviour of magnesia-spinel refractories: Comparison of tensile and wedge splitting tests

In some industrial applications, the need to improve the thermal shock resistance of refractories by optimisation of their microstructural design is of major importance. Refractories with enhanced thermal shock resistance usually present a rather low resistance to crack initiation but high resistance to crack propagation (rising R-curves), as well as a mechanical behaviour deviating from pure linear elastic fracture mechanics (LEFM), often qualified as nonlinear. The present work aimed at studying the influence of thermal micro-damage within the microstructure released during the cooling process on the nonlinearity of the mechanical behaviour in tension. The two-phase composites considered were magnesia-spinel refractories with different spinel inclusions content allowing to modulate the micro-damage level. Two different destructive mechanical tests, namely tensile and wedge splitting tests, were performed and their results were compared. The influence of thermal damage on different relevant mechanical parameters was investigated, and a quantitative correlation analysis between these parameters was proposed.

Microstructural effects associated to CTE mismatch for enhancing the thermal shock resistance of refractories

This work is devoted to the study of thermomechanical properties of several industrial and model refractory materials in relation with the evolution of their microstructure during thermal treatments. The aim is, in particular, to highlight the role of thermal expansion mismatches existing between phases which can induce damage at local scale. The resulting network of microcracks is well known to improve thermal shock resistance of materials, since it usually involves a significant decrease in elastic properties. Moreover, this network of microcracks can strongly affect the thermal expansion at low temperature and the stress-strain behaviour in tension. Even if these two last aspects are not so much documented in the literature, they certainly also constitute key points for the improvement of the thermal shock resistance of refractory materials. Evolution of damage during thermal cycling has been monitored by a specific ultrasonic device at high temperature. Beyond its influence on Young's modulus, this damage also allows to decrease the thermal expansion and to improve the non-linear character of the stress-strain curves determined in tension. The large increase in strain to rupture, which results from this non-linearity, is of great interest for thermal shock application.

The deformability characteristics as a factor in the calculation of the criteria of the thermal-shock resistance of refractories

The characteristics of the behavior of the material after the load application must be taken into account in a nondimensional assessment of the thermal-shock resistance. For materials that are deformed nonlinearly the calculation of the thermal-shock resistance criteria which define the ability of the material to resist cracking must be based on the measured deformation, and that of the criteria which define the ability of the material to resist destruction on the effective elastic energy stored in the material up to the onset of destruction.

Studies on Thermal-Shock Resistance of Alumino-silicate Refractories in Relation to Strength, Thermal Expansion and Modulus of Elasticity

The effects of mechanical strength, linear thermal expansion and modulus of elasticity on thermal-shock resistance of alumino-silicate refractories have been studied and the results have been statistically analysed. An equation correlating thermal-shock resistance of these refractories to their strength, thermal expansion and modulus of elasticity has been suggested on the basis of the observations made.

Thermal shock resistance of refractories

Thermal shock resistance of refractories