by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : September 2006
76 R2=R1λ Considering a constant heat transfer on the surface - R2 is the maximum allowable heat flux through body under conditions of steady heat flow (W/m) R3=R1a R3 is the maximum allowable rate of surface heating. σ = tensile strengths μ = Poisson ratio (transversal contraction index) α = linear thermal expansion E = Young's modulus (static: determined as V-modulus) λ = thermal conductivity a = λ / (cp ρ) = thermal diffusivity Based on the stress parameter, an ideal thermal shock resistant material should possess the following thermo- mechanical properties: ■ low thermal expansion ■ high thermal conductivity ■ high strengths under consideration of the stress/strain relationship. (A high σ/ E ratio leads to good thermal shock properties) Furthermore the tile design is important: ■ the wall thickness should be uniform and reasonably thin ■ the overall dimensions of the tile or pieces of the tile should be small ■ the geometry of the tile should avoid sharp angular shapes MATERIAL CHOICE FOR BURNER TILE APPLICATION Castable materials have been traditionally used for burner tiles because of the advantage in minimizing costs when manufacturing complex shapes of limited quantities. Placement of castables, when compared to pressed- or rammed products, is easier and the bodies show a more isotropic structure which is advantageous in relation to thermal shock properties. One difference between fired products and castable refractories is the higher shrinkage of castables at elevated temperatures which affects the geometry of the body at service temperature. Thus the interaction between permanent linear change and thermal expansion needs to be considered as an important factor for the design of burner tiles. Cement bonded materials In the past 30 years cement bonded materials have evolved from high cement conventional formulations to low and ultra low cement materials with an improvement in placement properties and high temperature stability. This evolution has been assisted by the development of additives and fillers in order to achieve water reduction to decrease porosity and increase mechanical strengths (4) Binder characteristic and thermomechanical properties of cement bonded castables The binder characteristic of calcium aluminate bonded castable materials has been clearly explained by numerous authors.(5,6,7,8) A simplification of the complex setting procedure can be explained as follows: High alumina cement CA, CA2, C12A7 etc.) + additives + water ----> calcium aluminate hydrates (CAH10, C3AH6, C2AH8, etc.) + additives + water + heat + residual water. (9) Due to the high quantity of water and the low permeability of cement bonded materials, a rapid temperature increase may result in explosive spalling or steam explosion. Factors that can contribute to explosive spalling include curing conditions, heating and dry-out method, and physical properties of the material (10). Due to these factors pre-cast burner tiles require accurate dry out or pre-firing before the installation. Usually thermo-mechanical data of cement bonded materials includes refractoriness under load or hot modulus of rupture data. However, compressive stress/strain data of castable materials which are necessary to predict strain controlled loads are not often published. C.A. Schacht reported that the chemistry of the refractory material has a significant influence on the stiffness of the castable. The lower the cement content and the higher the alumina content the higher the stiffness.(11) Due to the high strength of the low-cement castables the maximum strain is similar to conventional castables Phosphate bonded materials Experience has shown that phosphate bonded burner tiles of complicated shapes can hold up, in some critical process conditions, much better than conventional burner tiles made of cement bonded materials. Since not all phosphate bonded castable materials possess comparable properties it is important to refer to the distinguishing characteristics. Binder Characteristic and thermo-mechanical properties Phosphate bonded refractory castable products are available with various binder systems which can be differentiated as follows: ■ heat setting ■ cold setting + setting additive ■ one component dry material with water addition ■ two component mix with phosphoric acid solution or monoaluminium phosphate solution + dry component The chemical compositions of cold setting phosphate bonded castables are based on the system Al2O3 - SiO2 - P2O5 - MxO (MxO is alkali- or alkali earth oxide). Heat setting phosphate bonded castables are based on the system Al2O3 - SiO2 - P2O5. The P2O5 content ranges from approximately 3% -- 6% and the MxO content from approximately 0.5% - 6%. A great number of works have been reported about the reaction between alumina and phosphoric acid (12,13,14) www.metals.rala.com.au