Predicting of Reactions During Carburization and Decarburization of Steels in Controlled Atmospheres Napovedovanje reakcij, ki potekajo med naogljičenjem in razogljičenjem jekla v kontroliranih atmosferah B. Koroušič1, IMT Ljubljana, Slovenija M. Stupnišek, Faculty of Mechanical Engineering and Naval Architecture, Zagreb Uni-versity, Croatia Prejem rokopisa - received: 1996-10-01; sprejem za objavo - accepted for publication: 1996-11-04 The knovvledge of the thermodynamics of complexe systems consisting of gases and metal should be valuable for the control of industrlal processes. The Gibbs energy minimization model has been implemented in the softvvare program GPftO® and associated with a powerfull and reliable database. The computer package can perform computation of the equilibrium composition in very complex chemical and metallurgical systems. Some examples in this paper illustrate the simplicity of the computation and the use of the program m the field of some typical metallurgical applications. Key words: equilibrium reactions, NOx modelling, combustion of fossil fuels, active gas-atmospheres, decarburizing of non-oriented electrical steels, carburizing of alloyed steels with in situ produced atmospheres Poznavanje termodinamičnih odnosov v kompleksnih sistemih plin - kovina ima lahko izreden oomen za kontrolo industrijskih procesov. Gibbsov model o minimizaciji energije je implementiran v programsko opremo GPRCP, ki mu služi kot osnova močna baza verificiranih termodinamičnih podatkov. Programska oprema omogoča izračunavanja ravnotežnih sestav v zelo kompleksnih kemijskih in metalurških sistemih. Navedeni primeri v tem članku ilustrirajo enostavnost izračunavanj in način uporabe programa na področju metalurških reakcij, ki jih večinoma izvajajo strokovnjaki na tem področju. Ključne besede: ravnotežne reakcije, tvorba NOx, zgorevanje fosilnih goriv, aktivne plinske atmosfere, razogljičenje neorientirane elektropločevine, naogljičenje legiranih jekel 1 Introduction The application of thermodynamics to a system gas/solid enables to calculate the composition at equilib-rium, direction and extent of change vvhich can take plače under specified conditions. Rapid developments have taken plače in recent years in efficiency of thermodynamics in the engineering as thermodynamic can be defined as being the meeting point between physical - chemical principles and practi-cal applications1. In this paper an attempt has been made to demonstrate use of a personal computer softvvare program as an ellegant and sensitive method for numerous metallurgical applications especially for the analysis of gaseous systems. It is hoped, that users of this method will be in a good position to go more deeply into learn-ing thermodynamic laws. 2 Principles of the Gibbs method In the fields of heat treatment of metals like anneall-ing, carburizing, decarburizing, nitrocarburizing and many other operations, the metallurgist is concerned not with the pure gases but with the mixture of various spe-cies (gaseous and solids) vvhich form the atmosphere in the furnace. Prof. Dr. Blaženko KOROUŠIČ Inštitut za kovinske materiale in tehnologije 1000 Ljubljana. Lepi pot 11. Slovenija The thermodynamics of sueh complex systems can be treated by two methods: - The classical method of numerical solution of an equilibrium problem when the equilibrium constant (Kr) or free energy change AG° of the involved reactions are known. - The general Gibbs method for the numerical solution of an equilibrium. The problem is to determine the values of the species vvhich minimize the state of total free energy at the given temperature and pressure. Both treatments are thermodynamically equivalent, hovvever, it seems that the later method has significant advantages for calculating the equilibrium conditions in complex systems, in mixtures containing both gaseous and condensed species. During the last 20 years, SOLGASMIX computer program, as the method of attacking chemical and metallurgical problems, has influenced our approach to the study of a braneh of scientific knovvledge in physical chemistry. There can be no doubt that to attack sueh a complex application of thermodynamics is only possible vvith the use of computer technology. 3 Description of the method used for the calculation of complex equilibrium conditions Several excellent softvvare programs for calculating equilibria reactions at high temperatures, have been de-veloped in the last two decades (SOLGASMIX, THER-MOCALC, FACT, CHEMSAGE...)23. However, most of them are designed and vvritten in a complex form using very strong computer units, while few are intented as a simply a tool to be applied for the purposes of solving practical problems. Therefore, it seemed worthwile to develop a program which would combine these two computer program designs. The new software program, called GPRO is based on the method of free energy mini-mization and extended to systems containing numerous gaseous and condensed phases in accordance with SOL-GASMIX-principles. GPRO-program is dimensioned for 16 elements and 100 species. If necessary, this figure can be increased or new included datasets, vvhich if neces-sary are vvritten by the user (private databases are open and can be easily included also). 3.1 Thermodynamical approaches to the Gibbs-method The povver of Gibbs method energy minimization lies in its simplicity for the description of chemical reactions in complex systems, and its ability to facilitate the determination of the effect, on equilibrium state, of changes in the external influences vvhich can be brought to bear on the system. In our softvvare program, the user needs only to specify the type, the species present and the conditions (for example: temperature of the system) for the calculation. The program vvill automatically perform equilibrium thermodynamic computions typically associ-ated vvith complex chemical equilibria from a defined database. With the aid of the GPRO-program, a user is able to perform most of the follovving operations: 1. The energy for pre-heating the initial mixture from the initial temperature T0 to the reaction temperature T, 2. The reaction heat, 3. The computation of the complex chemical equili-bria in gaseous mixtures and activity of solid compounds, 4. Displaying and printing data for compounds and solutions at selected temperature and composition. An additional scientific and engineering benefit of this program is the softvvare able to develop a more basic understanding of chemical equilibria at high temperatures and its applications. Although the povver softvvare program vvill automatically perform the thermodynami-cal computation (no danger of pluging vvrong numbers in vvrong equations), hovvever, the user must have some knovvledge of the chemical nature of the considered sys-tem. In this paper the attempt is made to demonstrate the breadth and diversity of the modern softvvare program in simple way so that a user may be able to understand the thermodynamical method and apply it to metallurgical problems. Most of the examples are chosen vvith the aim to shovv superiority of the computer program, over tradi-tionally manual methods, vvhich are particularly stressed for the engineers and students. 3.2 Databases associated for the equilibrium thermody- namic comptutations From many excellent standard treatises on thermody-namics it is knovvn, that vvithout reliable thermodynamic data most equations are ineffective and the numerical an-svvers vvill be therefore vvrong. GPRO softvvare program is based on the use of both the expressions for calcula-tions of the standard Gibbs energies of the formation of a selected phase: in the form: AG°t = Y + B + CT + Dj2 + Ej3 + + FTlnT or using thermodynamical data on enthalpy AH°t, en-tropy AS°t and heat capacity CP(T): rT rTC (T) AG°t = AH°298 + J Cp(T)dt -TAS°298 -Tj -^dT t t 'o o Both methods used from the database involve the search for a minimum value of free energy AG of a sys-tem and give an equivalent result. Hovvever, the last method using enthalpy AH°t, entropy AS°t and heat ca-pacity CP(T) has more advantages because it combines heat and equilibrium calculations. A typical example is the determination of the adiabatic flame temperature, vvhere enthalpy of reaction serves as the criterion of the heat balance. 4 Exploiting the GPRO-program for complex equili-bria calculations Modelling Mechanism of Formation Nitrogenous Oxides by the Combustion of Fossil Fuels Modern combustion processes of fossil fuels meet the relevant requirements for cost-effective operation and avoidance of enviromental pollution. In article some results of the basic study of the formation and reduetion NOx in high temperature combustion processes are pre-sented. The obtained results demonstrate the use of the sophisticated methods of thermodynamics as one of the most important tools by the study of the combustion processes for a better understanding of the mechanism of formation of nitrogen oxides, one of the most important pollutants in combustion of fossil fuels8"16. Example 1: In this example is a demonstration of the use of the GPRO-softvvare program as method for predietion of complex combustion reactions and equilibrium gas composition including NO-oxydes formation. The high temperature furnace is fired vvith natural gas and air (no air preheating). The question vvas: calculate Table 1: Results of GPRO-analysis of the natural gas combustion by different air - index and without air preheating Equilibrium data for methane combustion (A,=0,74...2,2)' CH4 + 2^.0 2 + 7,52XN 2 Air index 0.74 0.84 0,94 1,00 1,10 1,30 1,60 1.80 2,00 2.20 CO (%) 6,90 4,44 1.95 0,95 0,30 0,04 0.002 0,0 0,0 0,0 CO2 (%) 4.77 6,29 7.69 8,49 8,40 7,44 6,16 5,51 4,99 4,71 NO (%) 0,0021 0,017 0,112 0,23 0,345 0,334 0,200 0,130 0,087 0,066 H20 (%) 18,09 18,99 19,01 18,52 17,28 14,94 12,32 11,03 9,98 9,42 O (ppm) 1,04 14,20 133 258 282 111 15 5 1 0 02 m 0,00 0,00 0,127 0,53 1,75 4,32 7,28 8,75 9,93 10,56 N2 (%) 64,97 67,79 70,02 70,89 71,78 72,90 74,03 74,58 75,02 75,25 H, (%) 5.27 2,47 0.81 0.37 0,11 0,02 0,00 0,00 0,00 0,00 SfinpuD (mole) 8.04 9,00 9,25 10,52 11,47 13,37 16,23 18,14 20,04 21,24 If«.»ii (mole) 8,56 9,32 10,10 10,59 11,49 13,37 16,23 18,14 20,04 21,24 TadbiK)_2023 2143 2233 2231 2151 1955 1712 1584 1482 1421 1 air index the equilibrium gas composition and the adiabatic flame temperature for the air-index in range 0,74 < X < 2,2 and compare the obtained results of the flame temperature vvith similar reference data knovvn in the literature (nor-mally presented in graphically form). In table 1 and figure 1 the computed values for the gas equilibrium are given. The adiabatic flame temperature calculation shovv values slightly above the compared data. Thermodynamic evaluation of carburizing atmospheres The accuracy of the gaseous atmosphere control in the steel carburizing furnaces has been remarkably im-proved ovving to the application of the computer control system and the development of nevv measuring tech- niques, for example: oxygen and/or carbon sensors. The atmosphere in carburizing furnaces are consists of: air + methane or other hydrocarbons and involves the gases CO, CO2, H2, H2O, N2. The four first gases are interde-pendent in a reversible reaction, commonly called the vvater-gas reaction: CO + H20 = C02 + H2 The ratio: K,„ — Pco; • Ph, Pco ' Ph,o (D is a constant, the value of vvhich depend on the temperature. The carburizing of steel, i.e. carbon content increas-ing on the steel surface, occurs through the reaction: CH4+2n02+7.52nN2 TTiermal NOx . dNO/di (t-o) (»l %/•) air inde* ■ . ............. i 105 CH4+2n02*7.62nN2 Th*rm«l HCM IdNO/dtKt-O) IvdVil air hxtax ■ 105, .............i. / ........ ...........f........ T .... i ....... Q»yo«nWi + (volH) CH4*2n02«7.52nN2 Thflrm«! NOx CH4*2n02*7.62nN2 Thtrmal NO* IdNO/dlKl-O) (votV«) Figure 1: The formation of the nitro-genous oxide NO (model simulation) Slika 1: Tvorba dušičnega oksida NO (modelne simulacije) .dNO/dt (t-O).(vcl v«) no aoo Alf tompTahira C air index ■ 2CO = C + CO, (2) For any given temperature, the coresponding equilib-rium constant of Boudouard's reaction will determine the carburizing potential of the atmosphere: K: Pco ■ (Pcc/Pc (3) The carbon potential of on atmosphere is simple to determine if the partial pressure of CO and ratio (pco/pco:) is known. From EMF measurement (electro-motive force) with the oxygen probe, considering the furnace temperature (pco/pco2) or (ph2/Ph2o) and measuring the CO - content in the atmosphere is possible to en-sure the control of the carburizing process. Example 2: The carbon activity in a steel depends on the content of alloying elements, thus every steel composition will have determined carbon potential which corresponding to the atmosphere composition. In next example three type steels were treated vvith air + methane atmosphere vvith the aim to obtain a constant carbon content near the surface of about 1 wt.% C). Data in table 2 shovv, that small deviations in the gas atmospheres (or the change of air + methane ratio) have a remarkable effect on the carbon activity. This model simulation is in good agreement vvith practical data. Table 2: Influence of the steel chemistry on the process parameters (Simulation made by GPRO programme by T = 1223K) Chemistry (%) Fe+l%C+ l%Si Fe+ 1%C Fe+l%C+ l%Cr CO 19.27 19.25 19.24 co2 0.0629 0.0717 0.078 h2o 0.199 0.227 0.248 ch4 5.17 5.15 5.13 n2 36.83 36.88 36.92 h2 38.46 38.42 38.38 02 (bar) 9.4 10"21 1.16 lO'20 1.46 lO"2« EMF(mV) 1173 1167 1162 Tdp*'(°C) -13 -11 -10 ac 0.818 0.715 0.654 %c 1.03 1.06 1.08 Qgas(m3/h) 1.1 1.1 1.1 Oair(m3/h) 2.094 2.100 2.1045 > Tdp = Dew point temperature Example 3: The carburizing of steel is a continuously process vvithin vvhich - due to the kinetics of various reactions -damming up effects may occur leading to non equilib-rium CH4-contents in the furnace atmosphere. In this čase the carburizing reactions under non-equi-librium conditions are modelled. A mixture of natural gas and air at 1 bar total pressure is introduced into the carburizing furnace heated to 1223 K5. The quantity of natural gas and air are 3,1.10"4 m3/s and 4,2.10"4 m3/s. Calculate the gaseous equilib-rium composition in the furnace atmosphere and the carbon activity assuming graphite as standard state. If the air flovv suddenly increased from 4,2.10~4 m3/s to 5,8.10"4 m3/s by constant natural gas flovv 3,1.10"4 m3/s in the in-let mixture, determine the new gas equilibrium composition and carbon activity! Table 3 shovvs the computed results for gas non-equi-librium composition and obtained energy changes, the preheating energy H°t - H°289, the heat of reaction H°r and H°totai the total heat of the system. Table 3: Example of input and output of a non-equilibrium composition by the production of endothermic gas from methane and air by t = 1223 K _1,1CH4 + 2A.02 + 7,52XN2_ X = 0,157 X = 0,221 Air/ 4,2.10(-4)/3,1.10(-4' 5,8.10('4V3,1.10("4) methane (m3/s) ot1 'melhane = 0,575 aVthane = 0,795 Xinn (mole) Xoul voI.(%) Xinp (mole) X0Ui vol.(%) CO 0,0000 17,58 0,0000 19,25 C02 0.0000 4,28.10"2 0,0000 7,19.10"2 CH4 1,1000 13,40 1,1000 5,80 h2o 0,0000 0,135 0,0000 0,227 HCN 0,0000 3,76. lO"3 0,0000 2,65.10'3 h2 0,0000 35,29 0,0000 38,41 n2 1,1890 33,33 1,6590 36,22 o2 0.3150 (5,45.10"21)2' 0,4410 (1.28.10"20)2' a.-3' 1,0000 0,714 X(mole) 2,604 3,5397 3,200 4,4962 (H2/C)4) 2.00 2,00 2,00 2,00 H°t-H°298 (kJ/mol) 92,95 116,77 H°r (kJ/mol) - 14,79 - 22,24 H°tolyl (kJ/mol) 78,15 94,53 CH4(c) " ^methane = CH4(c) = fully cracked CH4 and CH4(t) CH4(t) = total CH4 2) p02 in bar, 3) ac = carbon activity referred to graphite as standard state 4. H2 H, + H,O + 2CH4 ' — =-, A. = air index c co + co2 + ch4 Calculation of the decarbirization process of silicon al-loyed steels The use of gaseous atmospheres vvith a vvell-defined oxygen potential for the decarburization of low carbon iron-silicon steels in continuous furnaces can be simu-lated using a thermodynamical model. Equilibrium cal-culations and practical measurements shovv that the solu-bility and carbon activity in Fe-C-Si steels depend on the gaseous atmosphere, temperature and steel composition. Silicon-iron alloys containing 1 - 3% Si and 0,3 - 1% Al are typical steels for non-oriented sheets and a strict Fe + C + 3 % Si Dynamo steel 0.03%C, 2.7%Si, T=840 C, 6t/h TdpCC) EMF(mV) (H20/H2)_eq n95tor09 Figure 2: Plot of thermodynamical data for Fe2SiO-i as a function temperature calculated with GPRO-program Slika 2: Diagram termodinamihnih podatkov Fe2SiC>4 kot funkcija temperature izrahunano s GPRO-programom control of the decarburization and surface reactions is re-quired. The optimum properties for an electrical steel normally include high permeability with low core loss and minimal aging effects. An important factor in a process control is the formation of a high quality glassy film which is developed through a complex series of processing steps. Example 4: In order to clarify the relation between the decarburization atmosphere for the carbon removal during the an-nealing, the thermodynamical reactions and formation of different oxide phases in the scale have been studied. The first task was the determination of the carbon activ-ity in the decarburization gas atmosphere containing at the start H2 + N2 + H2O in temperature range 600 -1000°C. The mathematical model GPRO allows an easy use of thermodynamical data to predict the equilibrium carbon content in electrical steels. It is convenient to use the carbon activity in the gas atmosphere by different partial pressures of CO to present the conditions for the formation of FeO and Fe2SiC>4 by the different temperature. Figure 2 shows the results obtained. Having these curves available, it is possible to determine the dew point temperature as the function of the partial pressure ratio H2O/H2. Figure 3: Equilibrium oxide-formation by the decarburization of non-oriented electrical sheets in gaseous atmosphere H2 + H2O + N2 (Tdp - dew point temperature, EMF (mV) = electromotive force, (H20/H2)-eq = equilibrium pressure ratio) Slika 3: Ravnotežni pogoji tvorbe oksidov med razogljičenjem neorintitane pločevine v plinski atmosferi H2 + H2O + N2 (Tdp - točka rosišča, EMF (mV) = elektromotorna napetost, (H20/H2)-eq = ravnotežno razmerje plinov) As shown on Figure 3, the ratio of H2O/H2 at which the formation of fayalite actually disappeared is near H2O/H2 = 0,24 at 840°C. It is obvious that the pressure ratio H2O/H2 and CO2/CO is interchangeable with the partial pressure of oxygen - po3 and finally also by means of the relation: log po, = log A Pco ac v / 11854 - 9,090 (4) which allows the application of the oxygen (carbon) sensor signal (EMF). 5 Conclusions The use of thermodynamic predictive model offers many advantages over conventional gas atmosphere cal-culations because of the simplicity for description of chemical reactions in complex systems, the automatic performance of equilibrium computations, of the avoid-ance plugging wrong numbers in wrong equations and so on. The rational and theoretical basis for the Gibbs en-ergy model used was presented elsewhere5'17-19. To sum-marise, the key features of model calculations for the ni-trocarburizing atmospheres are as follows: - Modern combustion processes of fossil fuels meet strict requirements for cost-effective operation and avoidance of enviromental pollution. This article presents the first results of study into formation and reduction NOx in high temperature combustion processes. - The obtained results demonstrate the use of the so-phisticated methods of thermodynamics as one of the most important tools for the study of combustion processes to understanding better the mechanism of formation of nitrogen oxides, as one of the most important pollutants in fossil fuels combustion. - Little is given in disponible references on use of thermodynamical models in the field of active at-mospheres. Such mixtures containing both gaseous and condensed components for example: Fe + C + O + H + N are extremly complicated for the numerical calculations. Detailed experimental studies are diffi-cult and also thermodynamical results are mostly presented in the graphical form, which are very use-ful in research work but of little effectivness in searching solutions for a current practical operation. - To obtain equilibrium compositions in the real gaseous mixtures by high temperatures, taking into ac-count both energy and material balances, the development of new approaches are strongly required. 6 References 'D. R. Gaskel: Introduction to Metallurgical Thermodynamic McGravv-Hill Book Company, (Washington, D.C.), 1973 2 W. B. Christopher, G. Erriksson: Metallurgical Thermochemical Data-bases, Canadian Metallurg. Quat., 29, 1990, 2, 105-132 3 G. Eriksson, K. Hack: ChemSage - A Computer Program for the Cal-culation of Coinplex Chemical Equilibria, Metallurgical Transactions B, 21B, 1990, 1013-1023 4 F. D. Richardson, J. H. E. Jeffes: J. Iron Steel Inst., 160, 1948. 261-270 5B. Koroušic: Contributions to the Computer Predictions of the Homogene and Heterogene Equilibrium Compositions for Gaseous Atmos-pheres, Veitsch-Radex-Rundschau, 1994, 1-2, 465-542 6R. Hoffmann: Aspekte des Kurzzeitnitrierens, HTM, 31, 1976, 152-157 7H. Henrich, W. Kozlowski, W. Liere: Energieeinsparung und Schad-stoffreduzierung an Beheizungseinrichtungen in der Industrie, Gaswarme International, 41, 1992, 2/3, 89-100 8 F. A. \Villiams: Combustion Theory, Benjamin Cummings, Menlo Park, 1985 'K. Hein: Fossil Fuel Utilisation, Combust. Sci. and Tech., 93, 1993, 61, 27-39 10 R. Haupt, R. Oppenber: Feuerungen in Dampf - und Heisswasserer-zeugern, Gaswarme International, 41, 1992, 10, 445-456 "B. Bonn. H. Baumann: Kenntnisstand der N2O Bildung in ver-schiedenen Feuerungsanlagen, VDI Berichte, 922, 1991, 17, 625-633 12 H. Schuster: Minderung der NO* - Emissionen aus Kraftvverksfeuerungen, VDI-Kolloquium Emissionsminderung bei Feuerungsanlagen SO2 - NOx - Staub Essen, 10-11 November 1983, VDI-Bericht, Nr. 495 13 Y. H. Song, J. M. Beer, A.F. Sarofim: Reduction of Nitric Oxide by Coal Char at Temperatures of 1250 - 1750 K, Comb. Sci. Techn., 25, 1981,237 14 H. Schulz, H. Kremer: Bildung von Stickstoffoxiden bei der Kohlen-staubverbrennung, Brennstoff Warme Kraft BWK, 37, 1985, 1/2, 29-35 15 J. Zel'dovich: The oxidation of nitrogen in combustion and explo-sions, Acta Physicochimica URSS, 21, 1946, 4, 577-628 16 Y. B. Zefdovidh, P. Y. Sadovnik, D. A. Frank-Kamentku: Oxidation of Nitrogen in Combustion, (translated by Shelef), Academy of Sciences ofUSSR, 1947 17 B. Koroušic: Fundamental thermodynamic aspects of the Ca0-Ah03-S1O2 system, Steel res., 62, 1991, 7, 285-289 18 B. Koroušic: Study of equilibrium reactions in gaseous nixtures (Part 1. Protective atmospheres), Rudarsko-Metalurški zbornik, 1993, 1-2, 5-17 " B Koroušic, M. Stupnišek: A thermodynamic evaluation of nitrocar-burizing atmospheres, Steel res., 66, 1995, 8, 349-352