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Steel refining by chemically active plasma
Journal of Materials Processing Technology 78 (1998) 36 – 42
Steel refining by chemically active plasma V. Dembovsky´ * Department of Materials Engineering, VS& B-Technical Uni6ersity, 17. listopadu 15, Ostra6a, Czech Republic
Abstract Heating in a 100 kW plasma furnace equipped with an arc plasma torch and an anode embedded cool crucible made it possible for pronounced decarburisation, deoxidation, and reducing the sulphur content in a low-carbon steel, C& SN 41 2013. A prerequisite for successful refining by argon–oxygen or argon–hydrogen plasma is a clean, slag-free surface in the bath. In the case of critical carbon content, an argon–oxygen plasma can contribute to the substantial acceleration of the deoxidation reaction. The values of sulphur content reduction attained are interesting. This above is also applicable to the refining of alloyed and stainless chrome steels. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Particles recombination; Activating energy; Kinetics; Critical concentration; Steel refining; Ladle
1. Introduction The basis of what we call plasma metallurgy rests in the surface reactions of reducing and oxidising gases in excited, dissociated-atomic, and ionised states. The high values of the specific enthalpy of the plasma enables us to keep the process in thermal balance. The current designs of arc plasma torches (PT), reaching an overall output up to several MW [1], enable us to generate temperatures up to 20000 K. Such temperatures ensure the almost complete ionisation of the gases entering the extraction and refining processes. The heightened potential energies of the particles constituting the thermal plasma demonstrate, in relation to the gases formed by the molecules in their basic energy state, different physical and chemical properties [2]. Fig. 1 demonstrates the generally valid shift of activating energies of the chemical, and hence also the metallurgical, surface reaction with particles present. As a consequence of the low levels of activating energies with the participation of gases in the plasma state, a shift in the chemical reaction in favour of the product and the elimination of the dependency of the reaction velocity on temperature occurs. * Tel.: + 420 69 6994407; fax: + 420 69 6994401; e-mail: [email protected] 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(97)00460-3
Fig. 1. Maxwell-Boltzmann distribution of particles translation energies, E, inside the thermal plasma, and, and the different level of activating energies, Eact, of heterogeneous surface reactions which are valid for the presence of particles with lower and higher potential energy: n, the total the particles taking part in the surface reaction; m, the weight of the particles; 6, the particles translation velocity. Reaction in the presence of dissociated atomic particles demonstrate very low or zero activation energies [3]. All particles demonstrating the value Eact =0 lead after the collision with the surface atoms of the element reacting to the formation of a new compound.
V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
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Fig. 2. Decrease of plasma temperature, Tp, within the thermally unstable system with the marked region of particle recombination at the surface of the refined metal with temperature Ts. The case is valid for the electrically insulated surface with the flow potential conducting away.
Inside the plasma layer adjacent to the relatively cool surface of the solid or liquid phase, a thermal gradient is formed with the consequent recombination and partial relaxation of the original particles constituting the plasma. The decrease in the plasma temperature at the marked region of particle recombination in the thermal boundary layer, dt, at the surface of the refined metal is shown in Fig. 2. The low activating energies of the oxidising and reducing gases in the plasma state ensure a substantially higher efficiency and an increase of an order in the surface reaction velocity of the decarburisation, deoxidation, desulphurisation, and degassing of the bath. A prerequisite for the successful refining of liquid metals by a chemically active plasma is a clean drossfree surface on the bath. The current design of plasma re-melting equipment
within which arc PTs with tungsten cathodes have been installed do not enable, due to their high oxidation rate and the thermal erosion of the electrodes, the direct generation of a plasma of metallurgically active gases, e.g. oxygen or hydrogen. One possibility for metal refining in a plasma remelting ladle fitted with tungsten cathodes PTs is to feed oxidising and reducing gases into the operative area of the discharge chamber of the PT [4]. Neither tungsten nor PT hollow copper electrodes cause—in contrast to graphite electrodes used in common arc heating—an increase in the steel carbon content. The functional ability of the plasma heating in practically any controlled atmosphere offers the possibility of refining steel by employing the effect of a chemically inert, oxidising or reducing plasma. A high temperature at the place of PT arc action creates conditions for an intensive course of decarburisation.
V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
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2. Experiment arrangement In order to verify the possibilities of low carbon steel refining by argon, argon – oxygen, and argon – hydrogen plasma, experiments have been carried out in a laboratory plasma furnace under conditions which have excluded the impact of contact between the steel and the crucible oxidising material [5]. An experiment with heat carried out in a water cooled copper crucible excluded the possibility of establishing the volume temperature of the refined metal. We can only estimate the temperature gradient between the metal surface touching the plasma, and the surface temperature of metal which rests against the wall of the cool crucible. The heat conditions were experimentally chosen so as to achieve full melting of the metal in the region of metal–cool crucible contact. The surface of the anode embedded in the metal is exposed to the influence of the arc plasma column with radial distributed temperatures [6]. The temperature gradient from the centre to the area covered by the plasma column is also constituted at the surface of the anode embedded in the liquid metal. On the basis of surface temperature measurements in the region adjacent to the plasma layer with the consolidated anode drop, it was established, using an electronic pyrometer, that in the cases of Ni, Co, Fe, Ti, Mo, and W and with the heat and intensity of the current fed to a PT with a nozzle diameter of 16 mm, the surface temperature did not exceed the metal melting point by more than 300°C. The values established are in accordance with temperature measurements carried out by different methods [7]. It follows from the above that in the case of plasma re-melting and in refining technologies using arc PTs, spatially directed thermal gradients arise both in the anode region and the liquid phase volume. The experimental melts, whose objective was to shadow the refining effects of chemically inert argon and chemTable 1 Initial composition of steel, C& SN 41 2013,used to investigation refining by argon and chemically active plasma Element
Weight (%)
C Ni Cr Mn Cu Si S P Al [O]
0.07 0.03 0.08 0.28 0.08 0.4 0.019 0.01 0.096 0.007
The weight of button samples, 30 mm in diameter, was 40 grams in all cases investigated; the diameter of PT nozzle: 16 mm; argon flow rate: 20 Nl min−1.
ically active Ar–O and Ar–H plasma, were carried out on steel C& SN 41 2013, with the initial composition given in Table 1. For the melts themselves, an arc PT with a tungsten electrode enabling operation with continuous control of current feed up to 1000 A was been employed. The arc discharge was stabilised by argon at 99.95% purity. Refining gases were fed outside the PT inside the vacuum tight operating area of the plasma furnace.
3. Equilibrium condition and kinetics of steel decarburisation The process of carbon oxidation is usually viewed as an aggregate of subsequent mass transfer stages including adsorption and desorption of the reacting substances and products, as well as related chemical transformations. Usually no allowance is made for, in parallel running, the oxidising processes of slag forming admixtures in the melted metal (Si, Mn, etc.) or for the oxidation of the iron itself. The important property of the decarburisation process, which makes it different from other processes of oxidation refining, is the origin of new gaseous phase. Carbon monoxide can arise along the following lines: (1)
(2) Both reactions are of a heterogeneous character, as the CO molecule existing in the liquid iron is rather improbable. The reaction progresses at the surface between the liquid metal and the oxidising, chemically neutral, or reducing gas. The reactions involve the state of desorption, CO(ads) CO(g), and are conditioned by the simultaneous presence of carbon and oxygen at the phase interfaces. The difference between the reactions in Eq. (1) and Eq. (2) rests in the fact that in the first instance, the oxygen is being adsorbed at the surface of the liquid phase from the interacting plasma, and that, in the second case, the oxygen diffuses to the surface along with the carbon from the volume of the liquid metal:
(3)
(4)
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39
The analysis of the decarburisation process [8] demonstrates that the rate coefficient of the surface chemical reaction exceeds for more than one order the rate coefficient of mass transfer of the reacting elements. In the diffuse regime, the resulting reaction rate is controlled by the element whose diffuse flow is the lowest. In the region of greater carbon concentration, the oxygen diffuse flow is lowest. This is valid for the situation in which the carbon mass transfer into the reacting surface layer exceeds that of the oxygen in the same area of the reaction [9], and therefore we can maintain: [C] \ [C]kr;
dn dt
B
O
dn dt
(5)
C
The decarburisation reaction by the critical carbon content in the reaction in Eq. (5) is controlled by the outer diffusion regime of oxygen transfer with a renewable source of oxygen particles. The carbon concentration that is called critical responds to the equivalent mass transfer of the carbon and oxygen in the reaction in Eq. (3) or Eq. (4). The carbon concentration, [C], which is lower than the critical carbon concentration, [C]kr, controls the diffuse transfer of carbon in the liquid metal (9): [C] B[C]kr;
dn dt
B
O
dn dt
(6)
C
The reaction in Eq. (6) is controlled by the volume diffusion of carbon into the reaction zone without renewing the source of carbon. The theory of critical concentrations enables us to establish the laws of carbon, and other admixtures in the liquid metal phase, oxidation and to utilise individual factors for the regulation of the oxidation rate [10].
4. Experimental verification of the refining effect of argon and chemically active plasma The results of the experimental verification of the steel decarburisation by the oxygen dissolved in the metal, [O], as in the reaction in Eq. (2), and outlined in the reaction in Eq. (4), are represented in Fig. 3. The content of oxygen, which decreases follows the reaction of decarburisation, attains equilibrium at a value which, in keeping with the physical analogy of the experiment, relates to the partial pressure of oxygen in the argon atmosphere of the furnace. The kinetics of decarburisation including the transport of oxygen from the Ar – O plasma to the surface of the liquid metal bath with different concentrations of oxygen diffusing into the arc plasma column from the operative area of melting plasma furnace is illustrated in Fig. 4.
Fig. 3. Time behaviour of decarburisation and oxygen content decreased by low-carbon steel, C& SN 41 2013. Furnace atmosphere: argon of 3n5 purity; furnace pressure p = 105 Pa. This illustrates the kinetics of steel decarberisation by argon plasma from the original 0.075 weight, [C], and establishment of equilibrium at 0.025% weight, [C], after about 100 s.
The course of change in the oxygen content in the refined steel, which relates to the decarburisation kinetics in Fig. 4, is illustrated in Fig. 5. The kinetic independence of the decarburisation from the oxygen transfer to the surface of the decarburised steel further corroborates (including Fig. 3) the reaction process in the subcritical region of carbon, [C]B[C]kr. The character of the course of the oxygen content temporal changes in Fig. 5 defines, in relation to the decarburisation curves belonging to the chosen oxygen content in the atmosphere of the furnace, four zones: I. Adsorption of oxygen, O2(g) O2(ads) 2O(ads); O(ads) [O] at a supersaturated solution of Fe–O; Development of decarburisation,
the
heterogeneous
reaction
of
CO(g) J [C]+ [O](ads) CO(ads) at the arising nuclei of FeO; II. Concentration increase of FeO nuclei, their coagulation and emergence on the liquid steel surface, continuation of decarburisation reaction;
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V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
Fig. 6. Temporal course of sulphur content decrease for steel C& Sn 41 2013, registered during the refining argon plasma and by the content of 10 and 20% vol. H2 in the operative area of the plasma furnace. Other conditions as in Fig. 3. Fig. 4. Time behaviour of decarburisation in a laboratory plasma furnace with different oxygen content in the argon atmosphere of the furnace, steel 41 2013.
III. Oxygen content, [O], increases in the bath after decelerating the decarburisation reaction; IV. Release of oxygen dissolved in phase I to III in consequence of the desorption dominance over adsorp-
tion; at the moment in which, under the surface of the oxidation slag formed, the direct absorption of oxygen from the plasma is excluded. From Table 1 and Fig. 6, it is evident that by utilising the argon plasma, a decrease in the sulphur content, [S], in the steel occurs which is most probably caused by the reaction of the initial oxygen and carbon content in the steel: (7) In comparison with the decarburisation and deoxidation reaction illustrated in Fig. 3, it is evident that the desulphurisation was terminated after exhaustion of the oxygen by the carbon reaction. Tab. 1 and Fig. 6 further illustrate the decrease in using an argon–hydrogen furnace atmosphere. The primary reaction in which H2S is created is [3,11]: (8)
Fig. 5. Temporal course of oxygen content changes in steel C& Sn 41 2013, during refining in the laboratory plasma furnace. Furnace atmosphere: Ar + 1% vol. O2; Ar +2% vol. O2; Ar + 3% vol. O2. Other conditions as for Figs. 3 and 4.
In spite of the dispersion variance in the values of [S] it is possible to observe the ‘insensitivity’ of the desulphurisation reaction kinetics to the transfer of hydrogen in the reaction zone which indicates that a heterogeneous reaction with subcritical values, [S]B [S]kr, is controlled by sulphur diffusion to the reacting surface. The sulphur content equilibrium states established depend on the hydrogen content in the atmosphere of the plasma furnace.
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Fig. 7. Temporal courses of [C], [S] and [O] content decrease for steel C& Sn 41 2013, noted in the course of argon plasma refining. The content of hydrogen in the operative area of the plasma furnace was kept at 20% vol. Other conditions as in Fig. 3.
The curve courses in Fig. 7 enable us to make a mental picture of the time dependent courses of decarburisation, deoxidation, and desulphurisation by the argon plasma within the furnace atmosphere constituted by the mixture of Ar +20% H2, and, at the same time, it illustrates the time shift or superimposition and termination of particular processes in steel refining. The independence of steel decarburisation kinetics to hydrogen content in the furnace atmosphere indicates a reaction controlled by the diffusion of carbon to the reacting surface. From the point of view of the thermodynamics, it is the formation of acetylene which takes the greatest part in the decarburisation reaction: (9)
Experimental melts of 13% Cr steel employing an Ar – C –H plasma— outside the framework of the results established—have corroborated the possibility of decreasing the phosphorus content by 50%. Further increases in steel refining efficiency by a chemically active plasma can be expected from processes employing an anode PT which enables cathode incorporation in the liquid steel bath, and utilisation of
the refining gases’ positive ions in the electric field of the cathode potential drop.
5. Practical application of the data obtained A steel maker will hardly be likely to be happy with the idea of slag-free refining in which hydrogen is absorbed by the liquid steel. Some sorts of steel exist which enable the hydrogen absorbed to be reduced by ladle vacuum degassing to acceptable values. Nonetheless, for electrotechnical steel and steel for enamelling, customers demand that the upper limit of carbon content be 0.005% or even 0.006%. For stainless steel the carbon content limit is usually determined by demanding CB 0.02%. Apart from that for stainless steel, high micro-purity—at which the limit of particular inclusions is followed—is required. Taking into account the high affinity of chromium for oxygen, blasts of oxygen or oxygen rich argon lead to the formation of chromium oxides, Cr2O3, or FeO.Cr2O3 which constitute an undesirable formation of inclusions. Based on the four decarburisation stages illustrated in Fig. 5, the employment of an Ar-plasma, with an admixture of oxygen excluding supersaturation of the
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V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
At subcritical carbon concentrations, a pre-requisite for deep decarburisation is to keep the oxygen concentration in the bath under its solubility limit. A further increase in the decarburisation effect of a chemically active plasma can be expected by the use of anode plasma torches that enable the generation of plasma by the direct supply of active gases into the region of the arc discharge, and use of the cathode voltage drop for ion transport to the bath surface. Experimental melts accompanied by evaluating the refining effect of active oxygen and hydrogen provide data for the design of ladle equipment with plasma torches.
References
Fig. 8. Layout of the proposed equipment for the plasma extra heating and the ladle steel refining in the regime [C] B [C]kr.
liquid phase over the limit of oxygen solubility, seems to be more advantageous. Using the oxygen jet process of steel refining, metallurgical deficiencies can be removed by use of the principles of argon plasma refining and feeding an Ar – O2 mixture into the operating area of the refining equipment. The diagram of the equipment proposed is illustrated in Fig. 8.
6. Conclusions At supercritical carbon concentrations, it is possible to shorten the decarburisation time by increasing the oxygen transport intensity into the region of argon plasma action.
.
[1] R. Knight, R.W. Smith, D. Apelian, Int. Mater. Rev. 36 (6) (1991) 221. [2] I. Boulos, P. Fauchais, E. Pfender, Thermal Plasmas —Fundamentals and Applications, Plenum Press, New York, 1994. [3] K. McTaggart, Plasma Chemistry in Electrical Discharges, Elsevier Amsterdam, 1967. [4] V. Dembovsky´, Rea´lne´ moznosti plazmove´ metalurgie (Real Possibilities of Plasma Metallurgy), Proc. Metal 96, Tanger Ostrava, 1996. [5] V. Dembovsky´, Pec pro tavenı´ kovu˚ a pr' ´ıpravu materia´lu˚ s vysoky´m bodem ta´nı´, C& eskoslovensky´ Patent No. 177255, (1971), (A Furnace for Metal Heats and Preparation of Materials with High Melting Point, Czechoslovak Patent No. 177255). [6] Plasma Technology in Metallurgical Processing, The Iron and Steel Society, USA, 1987. [7] V. Donskoj, V.S. Klubnikin, Electro-plasma Processes and Regulations in Mechanical Engineering, Leningrad, 1979 (in Russian). [8] S. Filipov, The Theory of Metallurgical Processes, Mir, Moscow, 1975. [9] D.I. Rysjenko et al., Theory of Metallurgical Processes, Moskow, 1989 (in Russian). [10] A. Grigoryan, L.N. Belyannschikov, A.J. Stomachin, Theoretical Foundations of Electro-stelmaking, Metallurgizdat, Moskow, 1987 (in Russian). [11] M. Smirnov, Plasma Chemistry, vol. 3, Atomizdat, Moskow 1974 (in Russian).
Steel refining by chemically active plasma V. Dembovsky´ * Department of Materials Engineering, VS& B-Technical Uni6ersity, 17. listopadu 15, Ostra6a, Czech Republic
Abstract Heating in a 100 kW plasma furnace equipped with an arc plasma torch and an anode embedded cool crucible made it possible for pronounced decarburisation, deoxidation, and reducing the sulphur content in a low-carbon steel, C& SN 41 2013. A prerequisite for successful refining by argon–oxygen or argon–hydrogen plasma is a clean, slag-free surface in the bath. In the case of critical carbon content, an argon–oxygen plasma can contribute to the substantial acceleration of the deoxidation reaction. The values of sulphur content reduction attained are interesting. This above is also applicable to the refining of alloyed and stainless chrome steels. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Particles recombination; Activating energy; Kinetics; Critical concentration; Steel refining; Ladle
1. Introduction The basis of what we call plasma metallurgy rests in the surface reactions of reducing and oxidising gases in excited, dissociated-atomic, and ionised states. The high values of the specific enthalpy of the plasma enables us to keep the process in thermal balance. The current designs of arc plasma torches (PT), reaching an overall output up to several MW [1], enable us to generate temperatures up to 20000 K. Such temperatures ensure the almost complete ionisation of the gases entering the extraction and refining processes. The heightened potential energies of the particles constituting the thermal plasma demonstrate, in relation to the gases formed by the molecules in their basic energy state, different physical and chemical properties [2]. Fig. 1 demonstrates the generally valid shift of activating energies of the chemical, and hence also the metallurgical, surface reaction with particles present. As a consequence of the low levels of activating energies with the participation of gases in the plasma state, a shift in the chemical reaction in favour of the product and the elimination of the dependency of the reaction velocity on temperature occurs. * Tel.: + 420 69 6994407; fax: + 420 69 6994401; e-mail: [email protected] 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(97)00460-3
Fig. 1. Maxwell-Boltzmann distribution of particles translation energies, E, inside the thermal plasma, and, and the different level of activating energies, Eact, of heterogeneous surface reactions which are valid for the presence of particles with lower and higher potential energy: n, the total the particles taking part in the surface reaction; m, the weight of the particles; 6, the particles translation velocity. Reaction in the presence of dissociated atomic particles demonstrate very low or zero activation energies [3]. All particles demonstrating the value Eact =0 lead after the collision with the surface atoms of the element reacting to the formation of a new compound.
V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
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Fig. 2. Decrease of plasma temperature, Tp, within the thermally unstable system with the marked region of particle recombination at the surface of the refined metal with temperature Ts. The case is valid for the electrically insulated surface with the flow potential conducting away.
Inside the plasma layer adjacent to the relatively cool surface of the solid or liquid phase, a thermal gradient is formed with the consequent recombination and partial relaxation of the original particles constituting the plasma. The decrease in the plasma temperature at the marked region of particle recombination in the thermal boundary layer, dt, at the surface of the refined metal is shown in Fig. 2. The low activating energies of the oxidising and reducing gases in the plasma state ensure a substantially higher efficiency and an increase of an order in the surface reaction velocity of the decarburisation, deoxidation, desulphurisation, and degassing of the bath. A prerequisite for the successful refining of liquid metals by a chemically active plasma is a clean drossfree surface on the bath. The current design of plasma re-melting equipment
within which arc PTs with tungsten cathodes have been installed do not enable, due to their high oxidation rate and the thermal erosion of the electrodes, the direct generation of a plasma of metallurgically active gases, e.g. oxygen or hydrogen. One possibility for metal refining in a plasma remelting ladle fitted with tungsten cathodes PTs is to feed oxidising and reducing gases into the operative area of the discharge chamber of the PT [4]. Neither tungsten nor PT hollow copper electrodes cause—in contrast to graphite electrodes used in common arc heating—an increase in the steel carbon content. The functional ability of the plasma heating in practically any controlled atmosphere offers the possibility of refining steel by employing the effect of a chemically inert, oxidising or reducing plasma. A high temperature at the place of PT arc action creates conditions for an intensive course of decarburisation.
V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
38
2. Experiment arrangement In order to verify the possibilities of low carbon steel refining by argon, argon – oxygen, and argon – hydrogen plasma, experiments have been carried out in a laboratory plasma furnace under conditions which have excluded the impact of contact between the steel and the crucible oxidising material [5]. An experiment with heat carried out in a water cooled copper crucible excluded the possibility of establishing the volume temperature of the refined metal. We can only estimate the temperature gradient between the metal surface touching the plasma, and the surface temperature of metal which rests against the wall of the cool crucible. The heat conditions were experimentally chosen so as to achieve full melting of the metal in the region of metal–cool crucible contact. The surface of the anode embedded in the metal is exposed to the influence of the arc plasma column with radial distributed temperatures [6]. The temperature gradient from the centre to the area covered by the plasma column is also constituted at the surface of the anode embedded in the liquid metal. On the basis of surface temperature measurements in the region adjacent to the plasma layer with the consolidated anode drop, it was established, using an electronic pyrometer, that in the cases of Ni, Co, Fe, Ti, Mo, and W and with the heat and intensity of the current fed to a PT with a nozzle diameter of 16 mm, the surface temperature did not exceed the metal melting point by more than 300°C. The values established are in accordance with temperature measurements carried out by different methods [7]. It follows from the above that in the case of plasma re-melting and in refining technologies using arc PTs, spatially directed thermal gradients arise both in the anode region and the liquid phase volume. The experimental melts, whose objective was to shadow the refining effects of chemically inert argon and chemTable 1 Initial composition of steel, C& SN 41 2013,used to investigation refining by argon and chemically active plasma Element
Weight (%)
C Ni Cr Mn Cu Si S P Al [O]
0.07 0.03 0.08 0.28 0.08 0.4 0.019 0.01 0.096 0.007
The weight of button samples, 30 mm in diameter, was 40 grams in all cases investigated; the diameter of PT nozzle: 16 mm; argon flow rate: 20 Nl min−1.
ically active Ar–O and Ar–H plasma, were carried out on steel C& SN 41 2013, with the initial composition given in Table 1. For the melts themselves, an arc PT with a tungsten electrode enabling operation with continuous control of current feed up to 1000 A was been employed. The arc discharge was stabilised by argon at 99.95% purity. Refining gases were fed outside the PT inside the vacuum tight operating area of the plasma furnace.
3. Equilibrium condition and kinetics of steel decarburisation The process of carbon oxidation is usually viewed as an aggregate of subsequent mass transfer stages including adsorption and desorption of the reacting substances and products, as well as related chemical transformations. Usually no allowance is made for, in parallel running, the oxidising processes of slag forming admixtures in the melted metal (Si, Mn, etc.) or for the oxidation of the iron itself. The important property of the decarburisation process, which makes it different from other processes of oxidation refining, is the origin of new gaseous phase. Carbon monoxide can arise along the following lines: (1)
(2) Both reactions are of a heterogeneous character, as the CO molecule existing in the liquid iron is rather improbable. The reaction progresses at the surface between the liquid metal and the oxidising, chemically neutral, or reducing gas. The reactions involve the state of desorption, CO(ads) CO(g), and are conditioned by the simultaneous presence of carbon and oxygen at the phase interfaces. The difference between the reactions in Eq. (1) and Eq. (2) rests in the fact that in the first instance, the oxygen is being adsorbed at the surface of the liquid phase from the interacting plasma, and that, in the second case, the oxygen diffuses to the surface along with the carbon from the volume of the liquid metal:
(3)
(4)
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The analysis of the decarburisation process [8] demonstrates that the rate coefficient of the surface chemical reaction exceeds for more than one order the rate coefficient of mass transfer of the reacting elements. In the diffuse regime, the resulting reaction rate is controlled by the element whose diffuse flow is the lowest. In the region of greater carbon concentration, the oxygen diffuse flow is lowest. This is valid for the situation in which the carbon mass transfer into the reacting surface layer exceeds that of the oxygen in the same area of the reaction [9], and therefore we can maintain: [C] \ [C]kr;
dn dt
B
O
dn dt
(5)
C
The decarburisation reaction by the critical carbon content in the reaction in Eq. (5) is controlled by the outer diffusion regime of oxygen transfer with a renewable source of oxygen particles. The carbon concentration that is called critical responds to the equivalent mass transfer of the carbon and oxygen in the reaction in Eq. (3) or Eq. (4). The carbon concentration, [C], which is lower than the critical carbon concentration, [C]kr, controls the diffuse transfer of carbon in the liquid metal (9): [C] B[C]kr;
dn dt
B
O
dn dt
(6)
C
The reaction in Eq. (6) is controlled by the volume diffusion of carbon into the reaction zone without renewing the source of carbon. The theory of critical concentrations enables us to establish the laws of carbon, and other admixtures in the liquid metal phase, oxidation and to utilise individual factors for the regulation of the oxidation rate [10].
4. Experimental verification of the refining effect of argon and chemically active plasma The results of the experimental verification of the steel decarburisation by the oxygen dissolved in the metal, [O], as in the reaction in Eq. (2), and outlined in the reaction in Eq. (4), are represented in Fig. 3. The content of oxygen, which decreases follows the reaction of decarburisation, attains equilibrium at a value which, in keeping with the physical analogy of the experiment, relates to the partial pressure of oxygen in the argon atmosphere of the furnace. The kinetics of decarburisation including the transport of oxygen from the Ar – O plasma to the surface of the liquid metal bath with different concentrations of oxygen diffusing into the arc plasma column from the operative area of melting plasma furnace is illustrated in Fig. 4.
Fig. 3. Time behaviour of decarburisation and oxygen content decreased by low-carbon steel, C& SN 41 2013. Furnace atmosphere: argon of 3n5 purity; furnace pressure p = 105 Pa. This illustrates the kinetics of steel decarberisation by argon plasma from the original 0.075 weight, [C], and establishment of equilibrium at 0.025% weight, [C], after about 100 s.
The course of change in the oxygen content in the refined steel, which relates to the decarburisation kinetics in Fig. 4, is illustrated in Fig. 5. The kinetic independence of the decarburisation from the oxygen transfer to the surface of the decarburised steel further corroborates (including Fig. 3) the reaction process in the subcritical region of carbon, [C]B[C]kr. The character of the course of the oxygen content temporal changes in Fig. 5 defines, in relation to the decarburisation curves belonging to the chosen oxygen content in the atmosphere of the furnace, four zones: I. Adsorption of oxygen, O2(g) O2(ads) 2O(ads); O(ads) [O] at a supersaturated solution of Fe–O; Development of decarburisation,
the
heterogeneous
reaction
of
CO(g) J [C]+ [O](ads) CO(ads) at the arising nuclei of FeO; II. Concentration increase of FeO nuclei, their coagulation and emergence on the liquid steel surface, continuation of decarburisation reaction;
40
V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
Fig. 6. Temporal course of sulphur content decrease for steel C& Sn 41 2013, registered during the refining argon plasma and by the content of 10 and 20% vol. H2 in the operative area of the plasma furnace. Other conditions as in Fig. 3. Fig. 4. Time behaviour of decarburisation in a laboratory plasma furnace with different oxygen content in the argon atmosphere of the furnace, steel 41 2013.
III. Oxygen content, [O], increases in the bath after decelerating the decarburisation reaction; IV. Release of oxygen dissolved in phase I to III in consequence of the desorption dominance over adsorp-
tion; at the moment in which, under the surface of the oxidation slag formed, the direct absorption of oxygen from the plasma is excluded. From Table 1 and Fig. 6, it is evident that by utilising the argon plasma, a decrease in the sulphur content, [S], in the steel occurs which is most probably caused by the reaction of the initial oxygen and carbon content in the steel: (7) In comparison with the decarburisation and deoxidation reaction illustrated in Fig. 3, it is evident that the desulphurisation was terminated after exhaustion of the oxygen by the carbon reaction. Tab. 1 and Fig. 6 further illustrate the decrease in using an argon–hydrogen furnace atmosphere. The primary reaction in which H2S is created is [3,11]: (8)
Fig. 5. Temporal course of oxygen content changes in steel C& Sn 41 2013, during refining in the laboratory plasma furnace. Furnace atmosphere: Ar + 1% vol. O2; Ar +2% vol. O2; Ar + 3% vol. O2. Other conditions as for Figs. 3 and 4.
In spite of the dispersion variance in the values of [S] it is possible to observe the ‘insensitivity’ of the desulphurisation reaction kinetics to the transfer of hydrogen in the reaction zone which indicates that a heterogeneous reaction with subcritical values, [S]B [S]kr, is controlled by sulphur diffusion to the reacting surface. The sulphur content equilibrium states established depend on the hydrogen content in the atmosphere of the plasma furnace.
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41
Fig. 7. Temporal courses of [C], [S] and [O] content decrease for steel C& Sn 41 2013, noted in the course of argon plasma refining. The content of hydrogen in the operative area of the plasma furnace was kept at 20% vol. Other conditions as in Fig. 3.
The curve courses in Fig. 7 enable us to make a mental picture of the time dependent courses of decarburisation, deoxidation, and desulphurisation by the argon plasma within the furnace atmosphere constituted by the mixture of Ar +20% H2, and, at the same time, it illustrates the time shift or superimposition and termination of particular processes in steel refining. The independence of steel decarburisation kinetics to hydrogen content in the furnace atmosphere indicates a reaction controlled by the diffusion of carbon to the reacting surface. From the point of view of the thermodynamics, it is the formation of acetylene which takes the greatest part in the decarburisation reaction: (9)
Experimental melts of 13% Cr steel employing an Ar – C –H plasma— outside the framework of the results established—have corroborated the possibility of decreasing the phosphorus content by 50%. Further increases in steel refining efficiency by a chemically active plasma can be expected from processes employing an anode PT which enables cathode incorporation in the liquid steel bath, and utilisation of
the refining gases’ positive ions in the electric field of the cathode potential drop.
5. Practical application of the data obtained A steel maker will hardly be likely to be happy with the idea of slag-free refining in which hydrogen is absorbed by the liquid steel. Some sorts of steel exist which enable the hydrogen absorbed to be reduced by ladle vacuum degassing to acceptable values. Nonetheless, for electrotechnical steel and steel for enamelling, customers demand that the upper limit of carbon content be 0.005% or even 0.006%. For stainless steel the carbon content limit is usually determined by demanding CB 0.02%. Apart from that for stainless steel, high micro-purity—at which the limit of particular inclusions is followed—is required. Taking into account the high affinity of chromium for oxygen, blasts of oxygen or oxygen rich argon lead to the formation of chromium oxides, Cr2O3, or FeO.Cr2O3 which constitute an undesirable formation of inclusions. Based on the four decarburisation stages illustrated in Fig. 5, the employment of an Ar-plasma, with an admixture of oxygen excluding supersaturation of the
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V. Dembo6sky´ / Journal of Materials Processing Technology 78 (1998) 36–42
At subcritical carbon concentrations, a pre-requisite for deep decarburisation is to keep the oxygen concentration in the bath under its solubility limit. A further increase in the decarburisation effect of a chemically active plasma can be expected by the use of anode plasma torches that enable the generation of plasma by the direct supply of active gases into the region of the arc discharge, and use of the cathode voltage drop for ion transport to the bath surface. Experimental melts accompanied by evaluating the refining effect of active oxygen and hydrogen provide data for the design of ladle equipment with plasma torches.
References
Fig. 8. Layout of the proposed equipment for the plasma extra heating and the ladle steel refining in the regime [C] B [C]kr.
liquid phase over the limit of oxygen solubility, seems to be more advantageous. Using the oxygen jet process of steel refining, metallurgical deficiencies can be removed by use of the principles of argon plasma refining and feeding an Ar – O2 mixture into the operating area of the refining equipment. The diagram of the equipment proposed is illustrated in Fig. 8.
6. Conclusions At supercritical carbon concentrations, it is possible to shorten the decarburisation time by increasing the oxygen transport intensity into the region of argon plasma action.
.
[1] R. Knight, R.W. Smith, D. Apelian, Int. Mater. Rev. 36 (6) (1991) 221. [2] I. Boulos, P. Fauchais, E. Pfender, Thermal Plasmas —Fundamentals and Applications, Plenum Press, New York, 1994. [3] K. McTaggart, Plasma Chemistry in Electrical Discharges, Elsevier Amsterdam, 1967. [4] V. Dembovsky´, Rea´lne´ moznosti plazmove´ metalurgie (Real Possibilities of Plasma Metallurgy), Proc. Metal 96, Tanger Ostrava, 1996. [5] V. Dembovsky´, Pec pro tavenı´ kovu˚ a pr' ´ıpravu materia´lu˚ s vysoky´m bodem ta´nı´, C& eskoslovensky´ Patent No. 177255, (1971), (A Furnace for Metal Heats and Preparation of Materials with High Melting Point, Czechoslovak Patent No. 177255). [6] Plasma Technology in Metallurgical Processing, The Iron and Steel Society, USA, 1987. [7] V. Donskoj, V.S. Klubnikin, Electro-plasma Processes and Regulations in Mechanical Engineering, Leningrad, 1979 (in Russian). [8] S. Filipov, The Theory of Metallurgical Processes, Mir, Moscow, 1975. [9] D.I. Rysjenko et al., Theory of Metallurgical Processes, Moskow, 1989 (in Russian). [10] A. Grigoryan, L.N. Belyannschikov, A.J. Stomachin, Theoretical Foundations of Electro-stelmaking, Metallurgizdat, Moskow, 1987 (in Russian). [11] M. Smirnov, Plasma Chemistry, vol. 3, Atomizdat, Moskow 1974 (in Russian).