Grigorovich K.V. Baikov Institute of Metallurgy and Materials Science, RAS, Leninskii pr., 49, Moscow Russia, ,

Transcription

1 DEVELOPMENT OF NON ISOTHERMAL HOT EXTRACTION METHODS FOR DETERMIHATION OF FORMS OF THE EXISTENSE OF OXYGEN AND NITROGEN IN METALS AND NANO-SIZED POWDERS ABSTRACT Grigorovich K.V. Baikov Institute of Metallurgy and Materials Science, RAS, Leninskii pr., 49, Moscow Russia, , The fractional gas analysis method and procedure were developed. The OxSeP original software allowing us to process FGA data has been realized for the modern TC-600 LECO gas analyzer. A considerable discrepancy between the equilibrium reduction temperatures of oxides calculated by thermodynamics and real values is observed. The identification OxID software, which includes a thermodynamic model of carbon reduction of oxide inclusions and dissociation of nitrides during the analysis, was developed. It was demonstrated that numerous FGA results of alloys and different steels and nano sized powders were in a good agreement with experimental. The analysis of metals, steels and powder materials has shown that the FGA method and software developed can be successfully used in different fields of metallurgy and materials science. INTRODUCTION The useful properties of metals and alloys, including steels, are determined by the combination of their physical (in particular, mechanical) characteristics. These characteristics, in turn, depend on the chemical composition, homogeneity of the distribution of alloying elements and impurities, and the amount and character of the distribution of nonmetallic inclusions. Light elements are characterized by high solubility in liquid metals, which are several orders of magnitude greater than their solubility in same solid. These elements can also form various oxides, carbides, nitrides, which are nonuniformly distributed over the volume and significantly modify the strength and plasticity of the material. Principal features of nano-sized powders consist in the high both specific surface area ( S SP from 2 to 100 m 2 /g) and chemical reactivity. The oxygen content in metallic powders is higher than those of all other impurities by an order of magnitude and determines the total purity of a material. The oxygen content in ultrafine powders (S SP 2-3 m 2 /g) is no less than wt %. In nano-sized powders with S SP m 2 /g, it can be reach 2-10 wt %. That is why, the gas-forming impurities (C, O, N), which are present in the powders in various forms, are subject to required quality control. The control of light elements speciation allows to predict the metal properties however modern methods of analysis are highly labor and time-consuming. The amount, size, and form of nonmetallic inclusions are now determined using automated methods of quantitative metallographic analysis. The morphology and composition of inclusions are usually studied using local X-ray microanalysis. Alternatively, the inclusions can be isolated by electrochemical methods with subsequent chemical analysis. All these methods are 259

2 rather laborious and time-consuming. Examination of a representative area on a polished sample section using scanning electron microscope (SEM) with image analysis takes several hours. The results of such investigations are usually weakly correlated with data on the oxygen and nitrogen content obtained by means of gas analysis. Carrier gas hot extraction analysis (CGHE) in inert gas is one of the thermal evolution methods, routinely used to determine the oxygen and nitrogen content of metal products. This technique involves sample fusion in a graphite crucible under flowing helium, oxygen extraction from a melt as CO, catalytic oxidation of CO to CO 2 over heated rare earth copper oxide and detection of CO 2 by the infrared (IR) absorption and nitrogen by thermo conductivity cell (TCC). Total oxygen and nitrogen content is determined in the isothermal mode at temperatures above 2500 K. The complete simultaneous extraction of all oxygen and nitrogen species from a sample usually proceeds in seconds. The procedure is developed in details and a large number of reference materials are available. Fractional Gas Analysis method (FGA) is a modified oxygen and nitrogen determination method realized under non-isothermal conditions. It is based on the difference in the thermodynamic stability of oxides and nitrides. It provides a possibility to separate and identify the oxygen and nitrogen chemical forms in metals and powders. The sequence of reduction of different oxides and nitrides in a carbon saturated melt was predetermined by the standard Gibbs energy of their formation. The progress of earlier works (1-2) was not, however, succeeded. This fact was mainly attributed to two problems. The first one was the absence of numerical algorithm and software for processing of non isothermal kinetic data. The second problem was the problem of oxides and nitrides identification during the analysis procedure. The above problems have been worked on in recent years. First, to process the results of temperature ramped analysis an OxSeP original software has been developed and implemented on the modern TC-600 LECO gas analyzer (3-4). The numerical procedure involved consecutive separation and subtraction of individual peaks from the total evolution curve followed by minimization of the sum of squared residuals. The temperature-dependent background evolutions as well as mixing effects in a gas system of analyzer were also treated by this model. Second, a thermodynamic model of carbon reduction of oxides in a molten sample, saturated with graphite during FGA was developed (5-7). The present study was aimed to development of the fractional gas analysis method (FGA) and to demonstrate possibilities of this method for analysis of inclusions in steels and metals and nano sized powders. EXPERIMENTAL Figure 1 shows results of typical FGA data processing for a carbon steel sample. The curve of CO evolution from the specimen is a sum of peaks. Each peak results from the reduction of a particular kind of oxide inclusions. The oxide reduction processes in carbon saturated iron- and nickel-based melts were investigated. It was established that the process of carbon reduction of oxides during the heating of sample in a carbon crucible of analyzer could be divided into two stages, which differ in the reaction conditions. With increasing temperature, the oxide particles R x O y present in the melt are reduced with carbon with formation of CO bubbles nucleated on the particle surfaces. The reduction of the oxide in the carbon-saturated melt with the 260

3 oxide-forming component passing into the solution develops according to the following reaction: R x O y (s) + yc(gr) = xr + yco(g), K p a X R Y C a p a Y CO ; where R is the deoxidizing element; x, y are the stoichiometric coefficients, K is the reaction equilibrium constant characterizing the stability of p oxides; a C, a RxOy, a R are carbon, oxide and deoxidizer activities, respectively and p CO is the partial pressure of carbon monoxide. Based on the equilibrium constant for considered reaction it is feasible to evaluate actual temperatures of oxide reduction beginning in the melt. The equality of the chemical potentials of graphite of crucible and of carbon dissolved in the liquid corresponds to the attainment of equilibrium of the reaction between the graphite crucible and the molten specimen. According to the Gibbs phase rule, the system consists of four phases ( ), i.e., oxide, graphite, melt, and gas, and five components (C), namely, oxide - R x O y, M, graphite - C, oxide-forming component - R, and CO, between which one independent reaction (r) is possible. Thus, the number of degrees of freedom of the system is equal to two, i.e., = 2 + (C r) = 2. Thus, at a constant pressure, the temperature of oxide reduction is unambiguously predetermined by the concentration of the element R in the analytical melt. RxOy Fig. 1 Fractional gas analysis result for a carbon steel sample. The arrows indicate OxID calculated parameters: Ts is the start temperature of the oxide reduction and Тmax is the temperature of peak maximum. The oxide reduction in the presence of a strong carbide forming elements can be accompanied by the formation of corresponding carbide phases. There is a point in the phase equilibrium diagram where in this case, in a presence of the five phases (oxide, graphite, melt, and gas R x O y, M, graphite C, component - R, CO and carbide R i C k ) coexists with six components (R x O y, M, C, R, CO and carbide R i C k ) and one independent reaction are possible. In this case at a given pressure the variance of the system is equal to zero. Then for alloys with R concentration more than R kx, oxide is reduced at the temperature independent on the R concentration. 261

4 The next fundamental principles of oxide identification in the FGA method can be specified as follows: - for a given melt composition, each oxide has own temperature field of the carbon reduction. The lower temperature of this field (T s ) can be thermodynamically calculated as a temperature of the start of reduction. Based on the Gibbs energy equation, we can, in the absence of oxide-metal mutual solubility, estimate directly the temperature, at which the carbon monoxide vapor pressure reaches a desired value: G y( G 0 CO + RTlnp CO ) + RTxln(X R R ) - G RxOy =0, (2) where X R, R are the mole fraction and activity coefficient of deoxidizer in the melt; T (K) is a temperature; G 0 CO is the change in the standard free energy of the CO formation (J/mole); G RxOy is the standard free energy of the oxide formation from pure liquid metal and gaseous oxygen (J/mole). The thermodynamic estimation of the T s for the FGA carbon reduction of silica, alumna and titanium oxides in iron and nickel- based alloys depending on their concentrations was compared with results of experiments (6). The oxides in samples were determined using wet chemical analysis and SEM equipped with X-ray microprobe analyzer. The calculated Ts increases with increasing Si, Al and Ti concentrations in analytical melt. Comparison of experimental (points) and calculated results shows their good agreement. The shape of the FGA peaks was especially investigated using different iron based samples. In the experiments, the samples were heated in the carbon crucible of furnace of analyzer with a heating rate of 1,7-1,9 K/s. (Fig. 2) presents the analytical curves of carbon reduction of alumna(1) and silica(2) inclusions in iron base samples; and (3) alumna in stainless steel samples v(t) v'(t) t - t Ì, sec. Figure 2. FGA curves of alumna (1) and silica (2) inclusions in iron-based samples; (3) alumna in stainless steel samples plotted on nondimensional coordinates. v(t) is the relative intensity; t, t M time coordinats, s; (4)- v (t) derivativ of (1). 262

5 All of the curves are plotted on nondimensional coordinates: v(t) is the conversion rate equal to conversion rate divided on the oxygen quantity; t, t M are time coordinates,( s); and (4) is v (t) derivative. It was established that the similarity of the different peaks and conversion rate v(t) is independent on the oxide composition, aluminum and silica concentrations in the melt and oxygen content (total surface of peaks). The analytical curve can be approximated by a sum of individual peaks. It was established that the shape of each individual peak was modeled in terms of the following formulae: T Tm T k E RT I ( T ) / ( ) IМ exp E e d T T m Tm - where, T m is the temperature of peak maximum and I M -is the peak height. The k, E are the model parameters, such as the reaction rate constant and activation energy, were calculated for the real curve I(T) on the dedicated section near the maximum by the equation. The analytical curve I s (t) was approximated by the sum of the selected peaks: s p I ( t) I ( t, t, k, E, j 1 м, j j м, j j j ) - where p is the peaks number in the analytical curve. The Iм, j optimum values, were calculated by minimization of the functional: 2 n p м i (I ) I s I м, j φ j ( ti ) i 1 j 1 It was found that the reliability of the OxSeP and OXID algorithms provide the good repeatability of the experimental results. Using the FGA data, it can rapidly determine the volume fraction of oxide inclusions in the steels. This parameter characterizing the metallurgical purity of steel, is estimated by quantitative metallography, and is controlled by a number of documents such as ASTM-E45, method D. Since FGA can quantitatively determine the oxygen content in each type of inclusions, it can easily show that the volume fractions of oxide inclusions can be calculated to a higher accuracy as compared to that providing by metallographic methods using formula: n steel OОX МОX Voxides 100 i 1 ОX ymo, where steel and OX are the density of steel and oxides of a given composition, respectively; M O is the atomic mass of oxygen; Mox is the oxide molecular mass; y is the stoichiometric coefficient outside oxygen atom in the oxide formula; and Oox is the FGA determined content of oxygen (wt %) fixed in the oxide of this type of inclusions. The volume fractions of inclusions determined by FGA usually more precisely than those obtained by quantitative metallography. This is related to the fact that the metallographic sensitivity is restricted by the resolution of an optical microscope and rate of deformation of inclusions in steel. Based on the equilibrium constant for considered reaction, it is feasible to evaluate the actual temperatures of oxide reduction starting in the melt. The OxID software developed calculates identification parameters - the temperature of the oxide reduction start -Ts, temperature of peak maximum Tmax - to define certain kind of supposed oxide inclusions in the steel melt. As a final result of the real experimental data treatment by "OxSeP", one can obtain the total oxygen and surface oxygen contents, oxygen content in oxides as a sum of oxygen corresponding to peaks and a set of oxygen content for each peak having its own model temperatures. 263

6 RESULTS AND DISCUSSION Steel samples To improve the quality of steel, it is of importance to develop methods allowing the control of quantitative and qualitative compositions of nonmetallic inclusions. It is known that than more quantity of harmful inclusion in a metal, than higher the probability of fatigue defects formation near the rolling surface of a rail, wheel and bearing. Thus, the amount of oxygen in an FGA curve for a certain oxide peak characterizes the amount of inclusions of this type and, hence, their possible affect on the fatigue properties of the steels. The rail-wheel, railway, bearing and tire cord steels sampled in the course of ladle treatment processes were investigated by the FGA method. The content of non-deformable inclusions in these steel samples has been determined using the TC-600 LECO analyzer and the original OxSeP software. The FGA results were compared with data on inclusions control obtained by Image analysis on IA-32 LECO Analyzer and X-ray microprobe analysis. The volume fraction of inclusions clearly illustrates the contamination of a metal. Figure 3 presents the results of the FGA control of different tire cord rod samples, namely, mean values and standard deviation. The tire cord rod samples produced by the Mechel Steel Work (Russia), Oskolskii Steel Work (OESW -Russia), Saar Stahl Plant (Germany), Nippon Steel Co. (NS- Japan), Byelorussian metallurgical plant BMZ, and Moldavian metallurgical plant (MMZ) were controlled. Figure 3 FGA results for cord steel samples (mean values and standard deviation). The FGA peaks were divided into the three groups according to the chemical composition of oxides and bulk analysis results for the metal and original OXID software calculations. The first group includes peaks with Tmax < K being attributed to silica and manganese silicates. The second group peaks are attributed to alumina - hard deformable and very harmful inclusions for this steel grade. The third group of peaks is attributed to complex (Ca, Al and Mg-rich) complex spinels. 264

7 It was found that oxide peaks spectra were similar for all tested samples. The oxide spectra in rod samples of the Saar Stahl Plant (Germany), Nippon steel Co. (NS Japan), Byelorussian metallurgical plant BMZ are generally highly similar. This means that the amount of alumina inclusions is very low and most of inclusions are deformable silicates. Figure 4 presents the FGA results the mean values and Standard deviation for steel probes that were sampled from the ladle furnace (LF) and the ladle vacuum degasser (LD) during the ladle and vacuum treatment of wheel steels. All the peaks were divided into three groups according to the chemical composition of oxides identified using by the original OXID software. The first group of peaks with Tmax < K was attributed according thermodynamics calculations to silica and manganese silicates. The second group of peaks was attributed to aluminates more harmful hard deformable inclusions. The third group of peaks was attributed to complex (Mg,, Ca, -reach ) - spinel. Figure 4 allow us to estimate the influence of ferroalloys and deoxidizers additions during ladle treatment on the total oxygen and nitrogen content as well as the amount and content of different oxide inclusions in steel melt. Fig. 4 Quantitative changes of total oxygen and nitrogen content and oxygen content in different oxide groups according FGA results during the ladle (LF) and vacuum treatment (VD) of wheel steels mean values and Standard deviation. The oxygen content in the form of calcium aluminates and magnesium spinels increases (Probe 5) from 13.8 to 42.2 ppm just after the SiCa wire infeeding in the steel melt. The total oxygen content in the form of aluminates and Ca, Mg rich oxides decreases simultaneously from 42.8 to 8.2 ppm as a result of inclusion modification and assimilation with the slag. The oxygen content in the form of aluminates abruptly decreased from 21.3 to 7.8 ppm (Probes 4 and 7) as a consequence of vacuum degasing and inclusion modification with SiCa wire. As seen in Figure 4, that after the vacuum treatment in the process of casting of metal on CCM, the secondary oxidation occurs that result high-alumina inclusions can formed with the aluminium content in 265

8 the melt of 0.003% (by weight). The main indicator of the processes of secondary oxidation may be an increase of nitrogen content in the metal during the casting, which characterized the difference in nitrogen content between samples picked after vacuum treatment (VD) (7), at the casting (8) or from ready wheels (9). Thus, the results of analyses by FGA of non metallic inclusions in samples allows us to formulate some of the corrective actions in ladle treatment technology that necessary to improve the cleanness of steel. Nano sized powders Carbon, nitrogen and oxygen can present in nano-sized powders in the form of weakly fixed, absorbed «surface», dissolved in solid solution («lattice»), and in the form of compounds. Changes in the quantity and distribution of these forms in different powder materials produced by plasma and chemical methods affect substantially the quality of powders. The hardness and quality of tungsten-carbide process tools and multiphase powder materials with the high specific surface depend on the quantity and forms of light elements, such as free and fixed carbon, nitrogen, and oxygen that has to be controlled. For example, the oxygen content in WC-powders can reach up to 5%. High thermal conductivity is one of attractive features of AlN and Si 3 N 4 ceramics produced by sintering of micron- and nano-sized powders. A quantitative correlation between the concentration of oxygen dissolved in the lattice of AlN and Si 3 N 4 crystals (that is associated with vacancies) and thermal conductivity has been shown in several studies (8-9). Thus, it is of great interest whether oxygen dissolves in the crystal lattice and, if so, how much. The hot currier gas extraction method was applied for oxygen analysis of AlN and Si 3 N 4 ceramics because it as shown to be capable to distinguish surface, grain-boundary, and lattice oxygen (10-11). The carbon, nitrogen and oxygen contents bound in compounds can be calculated directly from the quantitative X-ray analysis results. The accuracy of this method, however, is rather limited if several compounds are present or their concentrations are low. It is impossible completely if the particles are nano-sized and phases are amorphous in term of X-ray diffraction. High errors can result from, consequently, if the dissolved element content is calculated from different total gasanalysis results and X-ray bound one derived from such data. Using FGA method, the contents of different oxygen forms in nano-sized metal powders such as W, Mo and Ni; refractory compounds such as carbides WC, NbC, SiC and nitrides AlN, Si 3 N 4, were analyzed. The nano-sized W-C powders produced in IMET RAS by two-stage technology (carbon reduction of tungsten oxides in plasma followed by low temperature synthesis) and commercially available powders WC (manufacturer: H.C. Stark, Gosar, Germany) were used for the investigations. The powders were preliminary studied using BET method, chemical analysis, X-ray scanning diffractometry and scanning electron microscopy equipped with an energy dispersive X-ray detector (SEM/EDX). The typical oxide species were analyzed using Leo 430 microscope with an Oxford ISIS EDX analysis system. Measurements were made simultaneously on TC-600 and RC-412 (LECO, USA) commercial analyzers. The specific surface of multiphase W-C powders (W 2 C, WC 1-x, W, WC, WO 3 ) varied from 20 to 24 m 2 /g was measured by BET method and the oxygen content of % was analyzed by FGA after the first - plasma reduction stage and following low temperature synthesis in a hydrogen atmosphere. 266

9 The technique analysis on the RC-412 analyzer involves a sample heating with a giving rate in a silica crucible under flowing oxygen, carbon and water extraction from samples as H 2 O, CO, CO 2. Then, the catalytic oxidation of CO to CO 2 over heated rare-earth copper oxide and detection of H 2 O and CO 2 by the infrared (IR) absorption cells were performed. The technique analysis in the TC-600 analyzer involves sample heating with a giving rate in a carbon crucible under helium flow, oxygen extraction from samples as CO and CO 2 and detection of CO and CO 2 by the infrared (IR) absorption cells. Double crucible technique was applied. The outer crucible played a role of a heater providing a uniform temperature field. The inner crucible served as a reactor. The multiphase W-C powders (W 2 C, WC 1-x, W, WC, WO 3 ) with the specific surface varied from 20 to 24 m 2 /g and the oxygen content of % were investigated after the first - plasma reduction stage. The maximum oxygen content varied from 0.1 to 0.4 % corresponds to oxygen present in the form of WO 3 particles. The Figure 5 presents the comparison of FGA analysis results for three WC nanosized powders fresh (after evacuation from the plasma reactor and followed passivation in argon) and after 3 and 7 days storing in wet air, respectively. It can be seen that the only single peak was obtained in the FGA evolution curve of the fresh WC powder. That peak can be attributed to WO 3 virgin raw oxide particles. It was confirmed by X-ray analysis of these powders. There were two extracted peaks found on the FGA curves of the WC powders after 3 and 7 days oxidation. All of the additional oxygen in these WC powders was associated with the first, lowtemperature peaks ( К maximum temperature) in the FGA curves. Besides, the intensity of the high temperature WO 3 oxide reduction peak remaines the same without any changes. It can be proposed that the first peak with the maximum temperature 150K lower than following one can be attributed to the oxide layers arising on the surface of nano sized particles. 267

10 Figure 5. Comparison of FGA analysis results of the WC nano-sized powders: fresh (just after the production in a plasma reactor) and after 3 and 7 days oxidation in wet air. Using our experimental and literature data, a dependence of the oxygen content on the increasing specific surface of powder was found (12). The oxide layer was recalculated from the FGA results for the first peaks on the evolution curve. We calculated that, for powders differing in the particle size, the thickness of the surface oxygen layer varies from 1.5 to 2.5 monatomic layers (a monatomic layer is at. O/cm 2 ) or from 0.04 to 0.07 µg/cm 2, respectively. Results of the study performed show that the oxygen content (ignoring absorbed water) in the WC nano-sized powders exposed to air can be considered standard value if it is equal to µg/cm 2 per unit surface. This value corresponds to the covering of powder particles with 2 ± 0.5 monatomic layers of oxygen. For nanosized powders with a particle size of nm, the oxygen content can be limited by 0.5 wt %.It was shown that the content of oxygen adsorbed on the surface of nanosized powders (in the form of H 2 O) can reach up to 50% of the total oxygen content. The correctness of identification of oxygen forms in the materials was confirmed by thermodynamic calculations and by X-ray diffraction analysis data. Experimental data on the oxidation of powders during storage are given and discussed. The original technique developed for the determination of weight percentage of oxides in nano-sized tungsten and tungsten monocarbide powders was certificated in the Test Analytical and Certification Center. The dependence of the oxygen content on the specific surface of the nickel powders produced by plasma method was investigated by FGA method. It was established that the oxygen content in powders increases with decreasing the particles size. At the same time, the most part of oxygen corresponds to the covering of powder particles with oxide films. The analysis of metals, steels and powder materials has shown that the FGA method and software developed can be successfully used in different fields of metallurgy and materials science. REFERENCES 1. Prumbaum R., Orths K.: Gießerei Forschung, 31(1979), No. 2/3, Sommer D., Ohls K.: Fresenius Z. Anal Chem, 313 (1982), Grigorovitch K.V., Katsnelson A.M., Krylov A.S., Vvedenskii AV.: Proceedings Analytical Chemistry in the Steel and Metal Industries, 4th Intern. Conf. Proc., Luxembourg, 1995, p Grigorovitch K.V., Krasovskii P.V., Isakov S.A., Gorokhov A.A., Krylov A.S.: Industrial Lab., 68 (2002), 9, Krasovskii P.V., Grigorovich K.V.: Metally, 2002 (2002), 2, Krasovskii P.V., Grigorovich K.V.: Metally, 2001 (2001), 4, Krasovskii P.V., Grigorovich K.V.: Industrial Lab., 68 (2002), 10, Slack G.A., Tanzilly R.A., Pochl R.O. and Vandersande R.O.: J. Phys. Chem. Solids, 48 (1987), Buchr H., Muller G., Wiggers H., Aldinger F., Foley P., Roosen A.: J. Am. Ceram. Soc., 74, (1991), Thomas A., G. Muller: J. Eur. Ceram. Soc., 8, (1991),

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