Compositions of batches for obtaining porous cermets on the basis of alloyed steel scale with addition of ferrosilicon have been developed. Roentgenograms of the obtained alloys were shown, mechanical durability and porosity of the obtained materials were investigated. The model of formation of porous cermets was offere

Lebedeva, O. A.

Tubalov, N. P.

Electronic archive of Tomsk Polytechnic University

Introduction Porous cermets (PC) form the basis of many devicesand aggregates of modern engineering. Variety of constructive solutions and PC operating conditions [1] supposes the presence of different types of materials of thisclass directed to their optimal use for concrete aimstechnically and economically.The existing methods of obtaining PC are far frombeing perfect. They are characterized by considerableenergy and material expenditures as well as low capacity. The method of selfpropagating hightemperaturesynthesis (SHS) developed in our country has great abilities at PC development [2].SHS as a method of obtaining porous materials [3]includes low energy capacity, wastelessness and possibility of dynamic variation of structural and other properties of products. These prerequisites allow using economy and simple technology of obtaining porous items.Selection of batch original components has the great value for obtaining product of high quality. In thiswork the basis of the batch for obtaining porous materials is industrial wastes of engineering enterprises beingmetal oxides (alloyed steel scale) and metal powders.Development of SHSmaterials on the basis of metal oxides (alloyed steel scale) has ecologyeconomicsignificance besides scientific novelty and urgency. Therefore, the aim of this work consists in determining lawsof formation of structures and composition of productson the basis of binary and multicomponent compounds;and development on their basis of new methods andtechnologies of obtaining porous cermets for their further use as fine filters of gaseous and liquid media in engineering and transport.It is known that in powder media with porosity morethan 30 % (that is typical for the majority of original SHSsystems) mass transfer is implemented through pores.The existing techniques of experimental diagnosticsallow studying to a large extent the SHS method butrequire perfecting, development of both research devices and upgrading methodological basis.Structuralmethodological diagram of the work isgiven in Fig. 1.Industrial wastes of engineering enterprises beingmetal oxides and metal powders form the basis of thebatch for obtaining porous permeable materials.Fig. 1. Structuralmethodological diagram of researchesWe used materials on the basis of scale of alloyed steel hot forging with addition of chromium, chrome oxide and ferrosilicon.So, for example, in reacting system containing to65 wt. % of steel scale, to18 wt. % of chrome oxide (CrO2)and 17 wt. % of Al the SHsynthesis is possible with obtaining porous permeable material the structure of whichis formed spontaneously at synthesis in general owing tomelt redistribution in reaction zone and gas desorptionfrom the surface of powder reacting materials.Studying the structures of reaction products in thesystem of chrome oxide (IV) (CrO2) – Al which is themost exothermal of reactions occurring in the compound system the following things were ascertained: intensive reacting of components with product melting; simultaneous phase separation («rise» of oxide product);further crystallization in the form of ingot covered withloose layer of oxide slag. Metal chromium, intermetalliccompound Cr5Al8 as well as aluminum oxide of composition Al2O3 α and χmodification (crystal and amorphous structure) form the basis of the product.The structure of the product is given in Fig. 2. Lightinclusions in product structure are identified by the analysis as metal chrome: microhardness of this phase is4,4...8,6 GPa, (black spots are pores, grey fields are oxide impurity of unreacting particles).The pattern of distribution of regions of reconstructed chrome allows supposing that metal constituent iswell seen in the form of nuclei of average size1,2...2 mkm where further growth of metal fragmentsoccurs as at substrates. Simultaneously at growth of reconstructed chrome regions the phase separation occurs– chromium motion on discontinuity flaw of porousExamination of properties of original powder materialsSimulation of structures of porous materials  Development of batch compositions and selection of original powder materialsInvestigation of dependence of properties of porous materials on properties of original powder ones Pilot tests and introduction of the results of investigationAnalysis of the obtained results and determination of laws and criteria of synthesis and formation of properties of porous materials of powder componentsMathematics and mechanics. Physics131UDC 66.796.2STRUCTURE AND OPERATIONAL PROPERTIES OF POROUS CERMETS ON THE BASIS OF ALLOYED STEEL SCALEN.P. Tubalov, O.A. LebedevaI.I. Polzunov’s Altai state technical university, BarnaulEmail: drulya@inbox.ruCompositions of batches for obtaining porous cermets on the basis of alloyed steel scale with addition of ferrosilicon have been developed. Roentgenograms of the obtained alloys were shown, mechanical durability and porosity of the obtained materials were investigated. The model of formation of porous cermets was offered.aluminum oxide (Fig. 3). Movement pattern of the formed chromium is shown.Fig. 2. Microstructure of reaction product in the system chromeoxide–aluminumIntermetallic compound Cr5Al8 is formed in the regions of reconstructed metal chromium where concentration of dissolved aluminum corresponds to stoichiometry of the given compound that is determined by theparameter of crystal lattice of solid solution aluminumchromium in the neighborhood of intermetallide extractions.Oxide Al2O3 with parameters of lattice close to equilibrium and microhardness 18,2...20 GPa is the mainphase of production as well.Fig. 3. Path of phase separation in structure of reaction productof the system chrome oxide–aluminumStructure of reaction products in the system of steel scale – aluminum Interaction of aluminum with steel scale occurs atlow efficiency and temperature than alumothermic reduction CrO2 (IV). It is conditioned by scale phasecomposition including the lowest and nonstoichiometric iron oxides characterized by lower exothermicity ofreduction reaction. Significant content of iron (ferrite)forming 40...48 wt. % affects the thermokinetic parameters of reduction reaction as well.In the structure of end product ferrite attends in theform of fields with the size from 10 to 300 mkm the structure of which is similar to the structure of chromium reduction regions in products of reduction reaction ofchrome oxide (IV). Mechanism of formation of ferrite fields includes stages of melting scale original ferrite, formation of primary nuclei as a result of reduction reaction,their growth and coalescence into extended regions atphase separation, as it may be concluded from the investigation of the structure of the product. Ferrite fields arecharacterized by finegrained equilibrium structure ofcrystal grains, field microhardness changes weakly bymicrosection area amounting to 1,8...2,1 GPa (Fig. 4).Fig. 4. Microstructure of reaction product of alloyed steel scale– aluminumFig. 5. Particles of aluminum and iron oxide in reaction productstructure in the system of alloyed steel scale – aluminumOxide matrix the basis of which is aluminum oxidewith inclusion of residual iron oxides Fe2O3 and whichhas accurate perfect faceting is also presented in productstructure (Fig. 5). There, where aluminum oxide composition differs from stoichiometric and there are ironimpurities and its compounds in oxide the type of oxidematerial differs considerably (Fig. 6).Matrix on the basis of ferric oxides the compositionof which changes from Fe2O3 to FeO is typical constituent of reaction product in complex system. Oxides areformed, obviously, at metalreduction reaction in sequence Fe2O3, Fe3O4 – FeO. In this case plates of FeOgrow owing to reactionary reduction of disperse particles of magnetite Fe3O4 being a constituent phase of theoriginal scale.Parallel reactions between aluminum and CrO2 (IV)as well as between aluminum and scale results in formation of compound oxides of composition FeO(Al)2O3 inproduct structure; they are presented in the form of par5·10-6 ?3·10-6 ?3·10-6?1·10-5 ?Bulletin of the Томsк Pоlytеchnic University. 2007. V. 311. № 2132ticles of complex faceting and occupy to 80 % of a sample (Fig. 7). Space change of spinel particles faceting indicating the variable composition of compound oxidesis typical. Spinels are the hardest component of material – particles, microhardness amounts to 7,5...8,4 GPaand depends on their composition.Fig. 6. Particles of nonstoichiometric aluminum oxideFig. 7. Microstructure of reaction product of the system scale –chrome oxide – aluminumPhase inhomogeneity of the obtained product aswell as very nonuniform distribution of constituentphases by volume of material samples caused by phaseseparation at structureformation are the factors capable of negatively affecting the material service propertiesfirst of all mechanical strength (owing to inhomogeneity of destruction type) and corrosion resistance (by thereason of possibility of formation of microgalvanic pairsin the frame). In order to improve material propertiesthe experiments on formation of more homogeneousstructure of material were carried out.One of procedures was introduction of powderychromium into original reaction mixture. It is capable ofhomogenizing solidliquid reactionary medium due tospreading at combustion temperature as well as screening local areas of intensive heatsegregation owing tosignificant heat capacity. Melting temperature of chromium higher than that of iron allows narrowing the interval of existence of solidliquid medium raising its solidus and increasing melt viscosity. Thus, there is kineticresistance to medium redistribution at phase separation.As the experiments showed chromium introductioninto the mixture in the amount to14 wt. % does not causeprinciple change of product phase composition. However, expansion of regions of solid solution on the basis ofironchromium accompanying by increase of microhardness of fields of solid solution to 9,7...11,1 GPa was observed. It was ascertained that chromium introductioninto the mixture results in more uniform distribution ofmetal component in frame structure, alignment of microhardness and crystal lattice parameter of solid solutionby microsection area and therefore, solid solution homogenization. Decrease of sizes of oxide matrix fieldswas noted. In the whole structure of the material containing chromium is characterized by higher uniformitythan structure of basic samples (Fig. 8 and 9) wherestructures of sample frames of basic system and systemexpanded by chromium introduction are compared.Fig. 8. Microstructure of reaction product in threecomponentsystem of steel scale – chrome oxide (IV) – aluminumThus, structure and phase composition of SHSsynthesis reaction products in the compound system atmetalreduction reactions were studied.Fig. 9. Microstructure of reaction product in threecomponentsystem of steel scale – chrome oxide – aluminum expanded by chromium introductionIntroduction of powdery ferrosilicon FS70SS14157 in the amount of 1...5 wt. % containing70...75 wt. % of silicon into the batch increases combustion temperature of mixture and implies increase of liquid phase volume and as a result significant growth ofmechanical compressive and bending strength. Ferrosilicon deoxidizes the obtained metalceramic basis ofmaterial frame due to binding silicon with oxygen i. e.results in considerable increase of mechanical strengthin 8...10 times. In this case, silica covers pore walls decreasing material conductance to zero.1·10-5 ?1·10-5 ?3·10-6 ?3·10-6 ?Mathematics and mechanics. Physics133Increase of mechanical strength expands sphere ofapplication of items of porous permeable material andincreases their resistance to dynamic and static loads.Properties of PC with different admixtures changingoperating characteristics are given in Fig. 10.Conclusion 1. Phenomenological model of mechanism of formation of porous cermets on the basis of oxides of ferricand aluminum – aluminum and alloy additionsconsisting in the following was developed:• at the primary stage of SHsynthesis fragile ceramic frame of Al2O3 is formed;• reduction of iron in reaction Fe2O3 + Al2O3 + Aland coalescence of liquid iron melt round solidframe of Al2O3 forming crystallizable drops (globules) which change their form, increase size ofpores between them under the influence of expanding gas pressure at intensive heat extraction.2. It was ascertained that introduction of ferrosiliconinto the original batch results at synthesis in increasing liquid phase volume, its spreading and furtheroxidation of iron free surface, increasing wettabilityat interfaces of globule adjacent particles.3. It was proved that introduction of 8...10 % of aluminum into the system ferric oxide (scale) – aluminum oxide supports occurrence of selfpropagatinghightemperature synthesis due to iron reduction ofoxides. The selected concentration range of reactionsystem components determined steadystate combustion of mixture obtaining porous material. Porous items with specified properties were obtained:average size of pores is 100...150 mkm, porosity45...60 %, fraction of oneside open and close porosity at its value less than 0,18 amounts to 2...5 % oftotal porosity of material, mechanical compressivestress varies from 15,4 to 1,7 MPa and bending stressvaries from 4,2 to 1,0 MPa.Bulletin of the Томsк Pоlytеchnic University. 2007. V. 311. № 2134Fig. 10. Properties of cermets with different admixturesScale Fe (40…45 %) + Al2O3 (40…45 %) + Al (13…15 %) Cr (5…10) % Ni (5…20) % SiO2 (5…40) % Increase of compressivestrength(11…15) % CorrosionresistanceThermal stability (more than 900 °?)Decrease of pore diameter and their distribution(10…12) % CorrosionresistanceIncrease of compressivestrength (15…18) % Decrease of pore effectivediameter(10…12) %Pore size uniformityFilling material andeconomyAl2O3Increase of compressivestrength (5…10) % Decrease of pore effectivediameter (16…18) % FeSi (1…5) % Increase of dynamic load resistance(to 30) % Increase of compressivestrength (70…75) %Pore size uniform distributionIncrease of pore size by4 % REFERENCES1. Volkov V.I. Filtering turbulence mode // Increase of environmentalsafety of automotive engineering: Article collection. – Barnaul,1999. – P. 72–77.2. Evstigneev V.V., Volpe B.M., Gorkol D.A. Development of principles of generalized SHStechnologies of dispersed composite material // Altai STU works. – 1993. – Issue 1. – P. 3–18.3 Tubalov N.P., Lebedeva O.A., Vereschagin V.I. Porous composite ceramic materials obtained by selfpropagating hightemperaturesynthesis in the system F2O3Аl2O3Al // Novye Ogneupory. – 2003.– № 9. – P. 40.Received on 14.11.2006Mathematics and mechanics. Physics135On the basis of selfpropagating hightemperaturesynthesis (SHS) the technology of obtaining porouspermeable materials which may be used for fine purification of liquids and gases was proposed [1–3]. Scale ofalloyed steel, metal powders and ferrosilicon were usedas original components of batch [4].The principle stages of the process of obtaining porous materials is the diagram, Fig. 1, including grindingoriginal materials, powder screening, batch preparing,component matching and gauging , synthesizing andproduct improvement.Fig. 1. Diagram of technology of obtaining porous permeableproduct by SHS methodTechnological experiments showed that the highestefficiency of grinding is achieved at its carrying out intwo stages; at the first stage the foregrinding occurs, atthe second stage fine grinding with obtaining rather narrow required fraction (through the sieve) occurs.Investigation of scale powders obtained by processingoriginal material at the grinder КID100, showed that theprimary type of material decomposition is intercrystallinefracture; particles of the obtained powder of various fractions have similar morphology repeating morphology ofcrystal grains of ferrite of original material.On the basis of the extracted fractions of steel scalepowder the reaction mixtures of constant concentrationcomposition were prepared; samples of porous materialwere later prepared from them.Filters for purification of gas mediaThe determining properties of material for gas purification are average pore size (Fig. 2) and mechanicalstrength at compression and bending (Fig. 3). This implies that at increase of steel scale concentration from45 to 60 % by mass the mechanical strength decreases.Filtration of exhaust gases in porous materials maybe divided into several stages. Firstly, solid particleswhich should be separated from the flow of exhaust gases are supplied to the surface of filter, then they abut tofilter surface and at the next stage penetrate filter surface and are trapped in pores. The larger particle size thehigh the probability of its trapping in porous wall. Theabsorption factor of solid particles by porous SHSfilterowing to engagement may be presented as:(1)223 ,2 3ñò ò÷ï ñòñò ñëñërδδ τ υ δ δδ⎛ ⎞= ⋅ +⎜ ⎟+⋅ ⎝ ⎠Original powders of metals and nonmetalsBatching and mixingFilling the formCompressionSynthesisExtraction from the formFlattering and partingoffControl of items: – visual examination– density (porosity) control– pore controlPreparation of formPowders: Aluminum АSD1Chromium PH1Nickel PNК1Chrome oxideOxide of aluminum МRTU609Oxide of silicon SS 907782Grinding ScreeningScale of steel18x2H4MAUDC 66.796.2PURIFICATION OF GAS AND LIQUID MEDIA BY METALCERAMIC SHSFILTERSA.A. Geyneman, V.D. Goncharov, A.L. Novoselov, N.Yu. ShchetinkinaI.I. Polzunov’s Altai state technical university, BarnaulEmail: drulya@inbox.ruIndustrial samples of fine filters of gas and liquid media from mechanical microparticles have been developed. Porous permeable cermets obtained by selfpropagating hightemperature synthesis are the basis of filters.Drying 

Structure and operational properties of porous cermets on the basis of alloyed steel scale

Abstract

Similar works

Full text

Available Versions

Electronic archive of Tomsk Polytechnic University