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Теоритичен доклад относно усвояване на азот от сплави на желязма основа изнесен на международна конференция, публикуван в сборник.
ON SOME THEORETICAL PROBLEMS RELATED TO NITROGEN ABSORPTION BY FERRO-CARBON BASED ALLOYS UNDER PRESSURE
Dr.-Eng. Bogomil Velikov Kolev
Institute of Metal Science, Bulgarian Academy of Sciences 67 Shipchensky prohod St., 1574 Sofia, Bulgaria
Summary The paper presents an attempt to provide a physical explanation of the observed deviations from Sievert "s law. The analyses are built on the basis of own experimental observations as well as on other Bulgarian researchers studies. The approach is completely different from that of the latter and is based mainly on some general characteristics of the liquid-solid physical state. 1. Introduction Nitrogen (N) solubility subjects to Sievert"s law in compliance with the opinions existing until recently. According to this law the increase of two-atoms gas content in the Fe base melt S2CN2) is directly proportional to squire root of its partial pressure over it: S(N) = kJPа (i) Most of the researchers think that Sievert"s law is valid in all cases [1-7]. The analysis of the deviations observed show that they do not differ from the error values or are connected with method errors. In contrast to [5-9] where the deviations are looked for in the exponent Pa (a=0,5 - 1,02), it is noted in [10-11] that the deviations should not be reflected in the exponent but in the activity coefficient. More characteristic although not confirmed differences are observed in chromium alloyed and other alloyed alloys [10-13]. In the analysis by different scientists of the data of nitrogen activity in [8] it is noted that in studying N solubility in Fe, Co and Ni deviations from Sievert"s law are not found (for moderate pressures) while at N dissolution in Cr, Mn, V and some other elements deviations from this law are observed still at comparatively low pressures of the gas phase (N). That according to [8] is a result of the contradictory N solubility in alloys. There are no confirmations for the deviations noted in [10-11]. There are no satisfactory explanations for them, too. 2. Results, analysis and discussion Taking into account the available publications, own experiments and observations related to noted deviations from Sievert"s law, Fig. 1 [14-15], as well as the results of Prof. D.Sc. T. V. Rashev concerning some alloys of the Fe-Mn-N, Fe-Cr-Mn-N, Fe-V-N systems (stainless and other steels, Fig. 2, 3 [10-11] and the contradictions between individual researchers, the necessity occurred to attempt a new theoretical explanation of the facts on the basis of some physical parameters, laws and scientific achievements. It is seen in Fig. 1-3 that with the increase of the absolute values of pressure, the relative efficiency of its effect in N saturation decreases. In other words, the law for increasing the N content with P of the gas phase does not obey quantitatively to Sievert"s law. According to Prof. D.Sc. T. Rashev and his collaborators it may reach up to 110%. In [10] it is reported that as N2 is a two-atoms gas, from thermodynamic point of view, the stoichiometry of the reactions with the alloying elements in which it takes part should be changed. And as the stoichiometry is determined not by P but by the concentration of the alloying elements (in this case by the N content) in the opinion of Prof. D.Sc T. Rashev and his collaborators it follows that the deviations from Sievert"s law should not be reflected in the exponent but in the activity coefficient, fjvj, in the parameters of interaction, respectively [10]. Observing the generally accepted form to take into account the effect of the individual components contents on the nitrogen activity, ffsf, the deviation from Sieverts"s law could be expressed by introducing the parameter of interaction "nitrogen-nitrogen" (eN ) which they introduce [10-11] by equation (2):
lg[%N] + eNN [%N] = lgN0 + 1/2 lg Pn 2 - Ig f J (2) Through regression analysis in case of Cr-Mn alloys, equation (3): Ig[%N]+O,13[%N]=l,38 - 0,51gPN 2 + 0,046[%Cr] + 0,02[%Mn] (3) N Cr In compliance with [8] the parameters eN and eN are related to the function and argument respectively, and therefore can not be simultaneously defined as regression coefficients. A conclusion is reached that this is the probable reason for the contradictory data in the publications for the possibility such a parameter to be used at all. The analysis of the equation concerning the dependence of the interaction parameter on the pressure P and N content shows that the parameter calculated in this way should depend on the alloy composition as the N content depends on the alloying N element. It is reported in [8] that interaction parameter Ј v valid for any system on Fe base can not be introduced. That means that in he alloys of the Fe-Cr-Mn system the parameter eN will depend on the Mn/Cr relation which has been proved by R. Ivanov. The major approach of the researchers in determining N solubility is based on the characteristics of only the liquid physical state, leaning on the generally accepted norms. Quite sophisticated equations are given. The deviations from Sievert"s law observed in own investigations [14, 15] and from investigations in [10, 11] could obtain a new theoretical physical explanation relying on the specific structure of the liquid and solid physical states, on some similarities in them expressed by common characteristics, on the processes related to crystallization, on the polymorphous and other transformations (assumed in the melt, too), on the qualitative and quantitative effects, respectively, of the alloying elements on the structure and characteristics of the two physical states, and on the effects of P and T°C on them, taking into account also possible evaporation, on the utilization of various N sources and various ways of their introduction, on various methods, inherited qualities of the materials studied, purity and degree of gas phase saturation, etc. [15-19]. This approach could be applied for physical explanation without rejecting the approach of Prof. D.Sc. T Rashev and his collaborators. Deviations from the law are alsc observed in case of additional introduction of N-containins substances (ferrous alloys nitrided) [14, 15], Fig. 1. Therefore, i could be assumed in the most general case that the stoichiometric content of the N source probably determines the exponent (Pa) ir Sievert"s law where a = 1/2 to 1. It is indicated in [8] that in alloy: of the Fe-Cr system the exponent "a" at 50% Cr in the alloy i: 1,02 and in alloys of the Fe-Ni system a = 0,35. Sievert"s law i: deduced and proved for saturation of the melt by molecula nitrogen (N2). The same refers also for the drops method in [10 11]. If we consider the kinetic problems [20] it is found that N i: absorbed by the melt (particularly non-alloyed) throug! T"p dissociation , N2 2N. This means that a high dissociatioi energy is required which depends on the alloying. Botl adsorption and absorption are impeded. In other words, with sucl a source (two atoms molecule) the exponent a = 1/2 or it can bi assumed to be close to 1/2. When using N bearers which ii contact with the melt very easily disintegrate in atomic N or ii case of blowing N2 over the melt in glowing discharge, electrii
arc furnaces [15], N is absorbed easier although in the second case the dissociation is low and partial. In this case at complete dissociation (N containing substances) the exponent changes a = 1. That is the case of introducing: nitrided ferrous alloys, various metal and non-metal N containing substances, NH3, etc. [14-20]. Naturally, in introducing compound N bearers the other components shall also have effect, particularly if they contain other gases: O2, H2, etc. Theoretically it can be assumed that with increasing P and the alloying the N dissociation energy decreases, i. e. the processes are facilitated (activated) depending on the alloying. But that is related to kinetics [15, 20]. Proceeding from the above said we should have in view that the N solubility in the melt is assessed by what is registered in the crystallized (solid) mass. The N found in the solid state is assumed also for the liquid state which has some similar but also some different characteristics in the non-identical temperature range of the conditional state diagram comprising also the other physical states, in this case up to the vapor. The capabilities of the ferrite, ferrite-austenite and austenite non-alloyed or alloyed phases are different. Alloy of Fe-Mn-N system with 2% of Mn is ferrite and with Mn above 8 - 12% - austenite. Alloy with 0-9% of Cr in the Fe-Cr system is ferro-magnetic. At Cr above 12%, high P and absorbed N it may become austenite. N absorption passes also through interval of crystallization that creates a number of peculiarities in gases (N) alloying unlike the other alloying elements [14-20]. The formation of carbide, nitride or carbide-nitride phase independently of the rapid quenching by fixing the high temperature condition of the drop also affects the absorption. The possibilities of N absorption (implementation) in the main matrix of solid solution and in the carbide phase are different [15-20]. Carbide and nitride phases (reducing the over-all mass) are observed in quenching a drop in copper mold by the method of [10-11] on the basis of own studies. All that probably affects the absorbed N content, Fig. 4. In other words, with increasing P, the introduced N content respectively, the vacancies in the face-centered cubic lattice/body-centered cubic lattice of the solid state, which are quantitatively different, decrease. This refers also to the vacancies in the melt clusters, that at temperatures close to the liquid retain the characteristic features of the solid phase. Both in liquid and solid these vacancies decrease with the increase of P and of the N absorbed under its effect [15-20]. P and N affect also the density (grows), viscosity, diffusion capabilities, etc. [15-20], The resistance i.e. stresses, to the N introduction increase with the increase of P. Moreover, with Fe-C based alloys the C and N atoms interfere during their introduction [15-20]. All that undoubtedly affects the solubility, i. e. Sievert"s law. If we proceed from the dependencies for: the number of vacancies in the liquid physical state (by Frenkel [15-20]) which grow with the increase of T°C/n =me"E/RT | where m - number of atoms, E -activation energy, R - general gas constant, density pjvf = m/n (diffusion coefficient D = Doe-E/RT) and viscosity (v), v=v oe~ E/RT and assume the values of activation energy at elevated pressure E = PV0, then the solubility according to Sievert"s law can be presented in several forms: N = Nopae-PV0/aRT; if a = 1 lhen N = N0Pe-PVQ/RT (4) P"
N = N0Pan/m = A^ ------ , expression with n, m(pm) (5) P» N = N0PaD/D0; expression with the coefficient "D" (6) N = NoPaVo/v"; expression with the viscosity "v" (7) where Do, No, v0 - coefficients, Vo - free volume, a =1/2-1 (if in (5) we express the density, pm, by the mass m and volume V then N = N0Pafn/v In compliance with the equations (4) - (7) on the one hand with the increase of P the N solubility increases. With the increase of
T°C the N solubility should grow to a certain critical value of T°C in dependence of the alloy composition, after which it should start to decrease as the inter-atomic interactions decrease (reduce) [17-20]. In spite that P acts in contradiction, increasing the solubility, the number of vacancies "n" at certain T°C starts to grow again and the density pm to decrease. On the other hand, with the increase of pressure, P, the number of vacancies, n, and the diffusion coefficient, D, decrease while the viscosity, v, and the density, pm = m/v = m/n, increase. The influence of T°C is opposite to that of P [19 - 20]. As a result of P increase (T°C = const.) the absorption starts to be impeded, i. e. to deviate from Sievert"s law (that is probably deduced at P = Patm)- Therefore, it is seen that with P increase the N solubility is a sum result of the effects of: P, T°C, n, m, pm, D, v. The values of n, m, pm, v, D and v change under the influence of P and T°C as the equilibrium curves in the phase diagram also change [20], The qualitative and quantitative relationship of the additives (alloying elements) affect the N solubility because it affects the values of: n, m, pnl, v, D, N, a, in a different way at that. Therefore, the type of the elements and their quantity is of importance, too. For example, Mn slightly decreases pm while W increases it [21]. The kinematics viscosity, v, decreases at dissolving 0-14% Mn, Q-22% Cr, up to 5% mass Si or up to 2% mass Ni as a result of interaction between the Fe atoms and the atoms of dissolving element(s) and loosen the bonds between the atoms of Fe(Me). In case of Si above 5% the kinematics coefficient increases as a result of the formation of Fe-Si groups [21]. The same is the effect on the diffusion coefficient, D [21]. For example, at 1600°C Dpe = l,3-6.10-9m2/s, DMn = 0,3.10-9m2/s, Dc - 1,59-3,46.10-9m2/s, D>j = 5,56.10-9m2/s. DP = 5,37.10-8m/2s, D0= 1,33.10-8m2/s, DH = 2,712.10-7m2/s, DS=1.10-8-l,9.10-9m2/s. Proceeding from the known dependence for D and v at elevated P [16 - 20], D = RT/6v, v = voePVo/RT (D = RT/8voePVo/RT then for the N solubility we obtain: N = No Pa/pm; "f Pm=m/V (mass/volume) N=N0 Pa. .e-pv0/RT (8)
N = NopaRT/Do5voePVo/RT (9) where 8 - length of atoms free run, m - number of atoms or mass, V - volume. In equation (9) the value of RT could be probably more precise by new values under the effect of P, in accordance with [23], who have carried out not only thermodynamic calculation of the phase equilibrium in the Al-Si system but they have proved that by experimental results. Because in doing this the equation (9) gets a very sophisticated form we shall not present here the analysis connected with additional characterization of RT by characteristics such as: energy of blending, entropy, phase transition temperature, change of volume during phase transition, concentration of components, pressure. general gas constant. Moreover, the calculations in [23] refer to binary Al-Si systems. Undoubtedly, the pressure, P. affects a number of thermal-physical parameters [15-20]. It is seen in formula (9) that the sum effect of the pressure, P, and temperature, T°C, affects both positively and negatively the N solubility in the corresponding thermal-dynamic conditions. In other words, equations (8) and (9) take into account the melt structure which undoubtedly changes under the action of P and T°C. Unfortunately, the liquid "phase diagram" is not sufficiently well known and yet less the effects of P and T°C on it are known. Still, at this development stage of the knowledge about the melt, from the obtained formulae (8) and (9) and the somewhat simplified ones (4) to (7) it can be seen the effects of the diffusion coefficient, number of atoms (mass), volume, viscosity, density, vacancies, that change under the action of P and T°C. For technological purposes, when the above said formulae are used, for simplification a=l. In the most general case, in compliance with Sievert"s law, with the increase of T°C the N solubility (two atoms gases) increases. It is above mentioned that on reaching certain critical T°C for any composition and P in spite of the
tendency for compacting and filling in with atoms (including N) the vacancies in the melt clusters transform in free gas space (bubble) [17-20]. A homogeneous origination of gas phases in the melt starts. The solubility of two atoms gases (N) begins to decrease. Example: Alloy containing 15%Mn, 1,84%C, 20%Cr, at P=1.10"5Pa and Ptotal=30.10-5Pa. Above 1940°C the N solubility in this alloy decreases: 1940°C - 0,429%N, 1960°C -0,235%N, 2000°C - 196%N. The data refer to own investigations for determination of N solubility by drops method used by Prof. D.Sc. T. Rashev and collaborators. It should not be forgotten that Mn is a volatile element. Problems of vapor pressure interfere. By the above said equations and most of all by equation (5) certain physical explanations in relation to the deviations from Sievert"s law can be given at this stage. Undoubtedly, with the progress of the knowledge for the liquid phase as physical state more precise equations and explanations in connection with the gas solubility will be achieved. It is supposed that these equations could serve also as a base for development of methods to determine the solubility of the two atoms (and other) gases. The above said could serve as a base for development of old and working out new theories and methods without negation and rejection of the available ones. Conclusion An attempt is made for a new theoretical grounding of the nitrogen solubility (absorption) in Fe based melt at variation of the partial gas (N) pressure in the gas phase and variation of the T°C as well as of the observed deviations from Sievert"s law through personal studies and studies of other Bulgarian scientists. For that purpose, in the classical equation of the Sievert"s law new characteristics are introduced which connect the liquid and solid physical state: number of vacancies, viscosity, density, diffusion coefficient, surface tension and others that strongly change under the action of pressure, P, and temperature, T°C, the increasing nitrogen content and the qualitative and quantitative presence of the other additions and alloying elements. The alloying elements affect the above said parameters in different ways at the respective P and N and this affects the N solubility. The latter is a result of the sum effect of the above said factors. References l.Essin O.A.., P.V.Geld. Fizitcheskaia chimia pirometallurgit-ches-kih processov. Metallurgizdat, Sverdlovsk, 1954, str. 606. 2.Levy L.I. Azot v tchugune dlia otlivok, Mashinostroenie. M. 1964. 3.Pridantchev M.A. i dr., Vaissokoprotchnaie austenitnaie stalli. Metallurgia. M.,1969. 4,Averin V.V.i dr., Azot v metallah, Metallurgia , M., 1973. 5.Pehlke R.D., Y.F. Elliot. Trans. AIME, 1960, v.218, p. 1076, 1088-1108. 6.Ficher W.O., O. Hofman. Archiv Eisen nhuttenwessen, 1960, 31, N4, 215-219, 1962, 33, N9, 583-598, 29, 1959,30. 7.Kasamatsu I., S.Matoba. Tetsu to Hagane.Journal Iron and steel inst.Japan , 1959, N2, 100, 1957, 43, 925-926. 8.Erinin Chr.D. Metallurgia na stomanata, C, Tchnika, 1990. 9.1vanov E., i dr., Izv. VUZ ,TcH. M., 1966 , 9, 5-7, 1966, 5, 57-61 10.Rashev Ts.V.Dobivane na legirany stomani. Izd. BAN, Sofia,
1978. 11.Rashev Ts.V.Vaisokoazotistaie stally pod davleniem. Izd. BAN, prof. M.Drinov, Sofia, 1995. 12.Pomarin Iu. M., Absorbtcia azota v vaissokochromistaimi rasplavami (Fe +Cr>30-40%) I jidkih Cr pri povaishennaim davleniem. Metallai, N5, 1990, str .40. 13.Pomarin Iu. M., G. I. Grigorenko. Uravnenie dlia raztcheta raztvorimosti azota v splavah Fe. Metallai, N4, 1998, str. 40. 14.Dimov I.N., Kolev B.V. Po niakoi termodinamichni vaprossi na usvoiavane na N v liati splavi ot systemata Fe-Cr-Mn-C. 1984. Materialoznanie i technologia, 13, 1984, str. 44--S5. 15.Kolev B.V. Vazmojnosty za polutchavane, structuruobrazuvane i svoistva na niakoi leiarski splavi na Fe-C osnova , legirani sas azot. Dissertatcia, IM-BAN, Sofia 1985. 16.Kolev B.V.Production of Fe-based N-alloyed cast aslloys. 3rd Intern. Conf. on HNS 93, Kiev. Ukraine, 1993. 17.Kolev B.V. Osnovni resultati, svarzani s polutchavaneto . izsledvaniata i prilojenieto na novi splavi na Fe osnova legirani s azot. Perspectivi i nasoki na razvitie prez sledvashtia 21 vek.NT Conf. s mejd. utchastie. Metaloznanie, metaloleene i termichno obrabotvane. Dokladi, chast I, str. 42-52, str. 218-224, str.225- 232. 18.Kolev B.V. New Fe-based N and other volatile elem. alloys, General results, Development, Future prospects. Intern, congr. Mech. Engin. Techn.97, Procedings, 2, p. 118-124. Sofia, 1997? 19.Kolev B.V. The fundamental philosoph. resourses of material science and Metallurgy in explanation of some processes and phenomena in supreme modes of the matter.s motion. Development of Metallurgy in the Balkans at the begining of the XXI st Century, Repports .p. 296-313, Varna , 1996. 20.Kolev B.V.,T.Trendafilov .Niakoi osobenosty na legiranetp na sivi doevtectichni tchuguni s azot. Mashinostroene, 3, str. 122- 126. Sofia, 1981. 21.1vanchev I.Osnovi na stomanodobivnite protcessi, Techika. Sofia, 1990. 22.Frenkel Ia.N. Vvedenie v teoriu metallov. Nauka. L., 1972. 23.Shineev A.la., D.B.Tchernov. Primenenie vaissokogo davlenia dlia povaishenia raztvorimosti metallov (Al). Techn. leghkih metallov. 1976, 11, str.3-5. FIGURES:
10 5 ,Pal/2), 0, 6-0, 8%C, 16-18%Mn. 5. 8-
7, 2%Cr, 0,9-l,2%Si; 1. Calculated straight line by Sievert"s law. 2. Experimental results.
Fig. 2. N = (fJPN . MPa) in alloyed alloys a), dependence in
principle 1. Straight line by Sievert"s law 2. Experimental dependence [11]. Investigations under pressure of [11] in the Fe system, b) 1. By Sievert"s law 2. For 41%Cr, 3.-35%Cr: 4.-30%Cr, 5.-21%Cr, 6.-14,5%Cr, 7.-10%Cr, 8.-0%Cr
Fig. 3. N = f( JPNl MPa1/2. Alloys on Fe-N base. 1 .-3%Mn. 2.-
7,65%Mn, 3.-ll,8%Mn, 4.-17,5%Mn, 5.-24%Mn, 6.-32.5%Mn
[11].
Fig. 4. N = f(Cr)
NOTE: The work has been carried out under Contract
TH717/97 of the Ministry of Education and Science.
Annex
Summary The paper presents an attempt to provide a physical explanation of the observed deviations from Sievert "s law. The analyses are built on the basis of own experimental observations as well as on other Bulgarian researchers studies. The approach is completely different from that of the latter and is based mainly on some general characteristics of the liquid-solid physical state. 1. Introduction Nitrogen (N) solubility subjects to Sievert"s law in compliance with the opinions existing until recently. According to this law the increase of two-atoms gas content in the Fe base melt S2CN2) is directly proportional to squire root of its partial pressure over it: S(N) = kJPа (i) Most of the researchers think that Sievert"s law is valid in all cases [1-7]. The analysis of the deviations observed show that they do not differ from the error values or are connected with method errors. In contrast to [5-9] where the deviations are looked for in the exponent Pa (a=0,5 - 1,02), it is noted in [10-11] that the deviations should not be reflected in the exponent but in the activity coefficient. More characteristic although not confirmed differences are observed in chromium alloyed and other alloyed alloys [10-13]. In the analysis by different scientists of the data of nitrogen activity in [8] it is noted that in studying N solubility in Fe, Co and Ni deviations from Sievert"s law are not found (for moderate pressures) while at N dissolution in Cr, Mn, V and some other elements deviations from this law are observed still at comparatively low pressures of the gas phase (N). That according to [8] is a result of the contradictory N solubility in alloys. There are no confirmations for the deviations noted in [10-11]. There are no satisfactory explanations for them, too. 2. Results, analysis and discussion Taking into account the available publications, own experiments and observations related to noted deviations from Sievert"s law, Fig. 1 [14-15], as well as the results of Prof. D.Sc. T. V. Rashev concerning some alloys of the Fe-Mn-N, Fe-Cr-Mn-N, Fe-V-N systems (stainless and other steels, Fig. 2, 3 [10-11] and the contradictions between individual researchers, the necessity occurred to attempt a new theoretical explanation of the facts on the basis of some physical parameters, laws and scientific achievements. It is seen in Fig. 1-3 that with the increase of the absolute values of pressure, the relative efficiency of its effect in N saturation decreases. In other words, the law for increasing the N content with P of the gas phase does not obey quantitatively to Sievert"s law. According to Prof. D.Sc. T. Rashev and his collaborators it may reach up to 110%. In [10] it is reported that as N2 is a two-atoms gas, from thermodynamic point of view, the stoichiometry of the reactions with the alloying elements in which it takes part should be changed. And as the stoichiometry is determined not by P but by the concentration of the alloying elements (in this case by the N content) in the opinion of Prof. D.Sc T. Rashev and his collaborators it follows that the deviations from Sievert"s law should not be reflected in the exponent but in the activity coefficient, fjvj, in the parameters of interaction, respectively [10]. Observing the generally accepted form to take into account the effect of the individual components contents on the nitrogen activity, ffsf, the deviation from Sieverts"s law could be expressed by introducing the parameter of interaction "nitrogen-nitrogen" (eN ) which they introduce [10-11] by equation (2):
lg[%N] + eNN [%N] = lgN0 + 1/2 lg Pn 2 - Ig f J (2) Through regression analysis in case of Cr-Mn alloys, equation (3): Ig[%N]+O,13[%N]=l,38 - 0,51gPN 2 + 0,046[%Cr] + 0,02[%Mn] (3) N Cr In compliance with [8] the parameters eN and eN are related to the function and argument respectively, and therefore can not be simultaneously defined as regression coefficients. A conclusion is reached that this is the probable reason for the contradictory data in the publications for the possibility such a parameter to be used at all. The analysis of the equation concerning the dependence of the interaction parameter on the pressure P and N content shows that the parameter calculated in this way should depend on the alloy composition as the N content depends on the alloying N element. It is reported in [8] that interaction parameter Ј v valid for any system on Fe base can not be introduced. That means that in he alloys of the Fe-Cr-Mn system the parameter eN will depend on the Mn/Cr relation which has been proved by R. Ivanov. The major approach of the researchers in determining N solubility is based on the characteristics of only the liquid physical state, leaning on the generally accepted norms. Quite sophisticated equations are given. The deviations from Sievert"s law observed in own investigations [14, 15] and from investigations in [10, 11] could obtain a new theoretical physical explanation relying on the specific structure of the liquid and solid physical states, on some similarities in them expressed by common characteristics, on the processes related to crystallization, on the polymorphous and other transformations (assumed in the melt, too), on the qualitative and quantitative effects, respectively, of the alloying elements on the structure and characteristics of the two physical states, and on the effects of P and T°C on them, taking into account also possible evaporation, on the utilization of various N sources and various ways of their introduction, on various methods, inherited qualities of the materials studied, purity and degree of gas phase saturation, etc. [15-19]. This approach could be applied for physical explanation without rejecting the approach of Prof. D.Sc. T Rashev and his collaborators. Deviations from the law are alsc observed in case of additional introduction of N-containins substances (ferrous alloys nitrided) [14, 15], Fig. 1. Therefore, i could be assumed in the most general case that the stoichiometric content of the N source probably determines the exponent (Pa) ir Sievert"s law where a = 1/2 to 1. It is indicated in [8] that in alloy: of the Fe-Cr system the exponent "a" at 50% Cr in the alloy i: 1,02 and in alloys of the Fe-Ni system a = 0,35. Sievert"s law i: deduced and proved for saturation of the melt by molecula nitrogen (N2). The same refers also for the drops method in [10 11]. If we consider the kinetic problems [20] it is found that N i: absorbed by the melt (particularly non-alloyed) throug! T"p dissociation , N2 2N. This means that a high dissociatioi energy is required which depends on the alloying. Botl adsorption and absorption are impeded. In other words, with sucl a source (two atoms molecule) the exponent a = 1/2 or it can bi assumed to be close to 1/2. When using N bearers which ii contact with the melt very easily disintegrate in atomic N or ii case of blowing N2 over the melt in glowing discharge, electrii
arc furnaces [15], N is absorbed easier although in the second case the dissociation is low and partial. In this case at complete dissociation (N containing substances) the exponent changes a = 1. That is the case of introducing: nitrided ferrous alloys, various metal and non-metal N containing substances, NH3, etc. [14-20]. Naturally, in introducing compound N bearers the other components shall also have effect, particularly if they contain other gases: O2, H2, etc. Theoretically it can be assumed that with increasing P and the alloying the N dissociation energy decreases, i. e. the processes are facilitated (activated) depending on the alloying. But that is related to kinetics [15, 20]. Proceeding from the above said we should have in view that the N solubility in the melt is assessed by what is registered in the crystallized (solid) mass. The N found in the solid state is assumed also for the liquid state which has some similar but also some different characteristics in the non-identical temperature range of the conditional state diagram comprising also the other physical states, in this case up to the vapor. The capabilities of the ferrite, ferrite-austenite and austenite non-alloyed or alloyed phases are different. Alloy of Fe-Mn-N system with 2% of Mn is ferrite and with Mn above 8 - 12% - austenite. Alloy with 0-9% of Cr in the Fe-Cr system is ferro-magnetic. At Cr above 12%, high P and absorbed N it may become austenite. N absorption passes also through interval of crystallization that creates a number of peculiarities in gases (N) alloying unlike the other alloying elements [14-20]. The formation of carbide, nitride or carbide-nitride phase independently of the rapid quenching by fixing the high temperature condition of the drop also affects the absorption. The possibilities of N absorption (implementation) in the main matrix of solid solution and in the carbide phase are different [15-20]. Carbide and nitride phases (reducing the over-all mass) are observed in quenching a drop in copper mold by the method of [10-11] on the basis of own studies. All that probably affects the absorbed N content, Fig. 4. In other words, with increasing P, the introduced N content respectively, the vacancies in the face-centered cubic lattice/body-centered cubic lattice of the solid state, which are quantitatively different, decrease. This refers also to the vacancies in the melt clusters, that at temperatures close to the liquid retain the characteristic features of the solid phase. Both in liquid and solid these vacancies decrease with the increase of P and of the N absorbed under its effect [15-20]. P and N affect also the density (grows), viscosity, diffusion capabilities, etc. [15-20], The resistance i.e. stresses, to the N introduction increase with the increase of P. Moreover, with Fe-C based alloys the C and N atoms interfere during their introduction [15-20]. All that undoubtedly affects the solubility, i. e. Sievert"s law. If we proceed from the dependencies for: the number of vacancies in the liquid physical state (by Frenkel [15-20]) which grow with the increase of T°C/n =me"E/RT | where m - number of atoms, E -activation energy, R - general gas constant, density pjvf = m/n (diffusion coefficient D = Doe-E/RT) and viscosity (v), v=v oe~ E/RT and assume the values of activation energy at elevated pressure E = PV0, then the solubility according to Sievert"s law can be presented in several forms: N = Nopae-PV0/aRT; if a = 1 lhen N = N0Pe-PVQ/RT (4) P"
N = N0Pan/m = A^ ------ , expression with n, m(pm) (5) P» N = N0PaD/D0; expression with the coefficient "D" (6) N = NoPaVo/v"; expression with the viscosity "v" (7) where Do, No, v0 - coefficients, Vo - free volume, a =1/2-1 (if in (5) we express the density, pm, by the mass m and volume V then N = N0Pafn/v In compliance with the equations (4) - (7) on the one hand with the increase of P the N solubility increases. With the increase of
T°C the N solubility should grow to a certain critical value of T°C in dependence of the alloy composition, after which it should start to decrease as the inter-atomic interactions decrease (reduce) [17-20]. In spite that P acts in contradiction, increasing the solubility, the number of vacancies "n" at certain T°C starts to grow again and the density pm to decrease. On the other hand, with the increase of pressure, P, the number of vacancies, n, and the diffusion coefficient, D, decrease while the viscosity, v, and the density, pm = m/v = m/n, increase. The influence of T°C is opposite to that of P [19 - 20]. As a result of P increase (T°C = const.) the absorption starts to be impeded, i. e. to deviate from Sievert"s law (that is probably deduced at P = Patm)- Therefore, it is seen that with P increase the N solubility is a sum result of the effects of: P, T°C, n, m, pm, D, v. The values of n, m, pm, v, D and v change under the influence of P and T°C as the equilibrium curves in the phase diagram also change [20], The qualitative and quantitative relationship of the additives (alloying elements) affect the N solubility because it affects the values of: n, m, pnl, v, D, N, a, in a different way at that. Therefore, the type of the elements and their quantity is of importance, too. For example, Mn slightly decreases pm while W increases it [21]. The kinematics viscosity, v, decreases at dissolving 0-14% Mn, Q-22% Cr, up to 5% mass Si or up to 2% mass Ni as a result of interaction between the Fe atoms and the atoms of dissolving element(s) and loosen the bonds between the atoms of Fe(Me). In case of Si above 5% the kinematics coefficient increases as a result of the formation of Fe-Si groups [21]. The same is the effect on the diffusion coefficient, D [21]. For example, at 1600°C Dpe = l,3-6.10-9m2/s, DMn = 0,3.10-9m2/s, Dc - 1,59-3,46.10-9m2/s, D>j = 5,56.10-9m2/s. DP = 5,37.10-8m/2s, D0= 1,33.10-8m2/s, DH = 2,712.10-7m2/s, DS=1.10-8-l,9.10-9m2/s. Proceeding from the known dependence for D and v at elevated P [16 - 20], D = RT/6v, v = voePVo/RT (D = RT/8voePVo/RT then for the N solubility we obtain: N = No Pa/pm; "f Pm=m/V (mass/volume) N=N0 Pa. .e-pv0/RT (8)
N = NopaRT/Do5voePVo/RT (9) where 8 - length of atoms free run, m - number of atoms or mass, V - volume. In equation (9) the value of RT could be probably more precise by new values under the effect of P, in accordance with [23], who have carried out not only thermodynamic calculation of the phase equilibrium in the Al-Si system but they have proved that by experimental results. Because in doing this the equation (9) gets a very sophisticated form we shall not present here the analysis connected with additional characterization of RT by characteristics such as: energy of blending, entropy, phase transition temperature, change of volume during phase transition, concentration of components, pressure. general gas constant. Moreover, the calculations in [23] refer to binary Al-Si systems. Undoubtedly, the pressure, P. affects a number of thermal-physical parameters [15-20]. It is seen in formula (9) that the sum effect of the pressure, P, and temperature, T°C, affects both positively and negatively the N solubility in the corresponding thermal-dynamic conditions. In other words, equations (8) and (9) take into account the melt structure which undoubtedly changes under the action of P and T°C. Unfortunately, the liquid "phase diagram" is not sufficiently well known and yet less the effects of P and T°C on it are known. Still, at this development stage of the knowledge about the melt, from the obtained formulae (8) and (9) and the somewhat simplified ones (4) to (7) it can be seen the effects of the diffusion coefficient, number of atoms (mass), volume, viscosity, density, vacancies, that change under the action of P and T°C. For technological purposes, when the above said formulae are used, for simplification a=l. In the most general case, in compliance with Sievert"s law, with the increase of T°C the N solubility (two atoms gases) increases. It is above mentioned that on reaching certain critical T°C for any composition and P in spite of the
tendency for compacting and filling in with atoms (including N) the vacancies in the melt clusters transform in free gas space (bubble) [17-20]. A homogeneous origination of gas phases in the melt starts. The solubility of two atoms gases (N) begins to decrease. Example: Alloy containing 15%Mn, 1,84%C, 20%Cr, at P=1.10"5Pa and Ptotal=30.10-5Pa. Above 1940°C the N solubility in this alloy decreases: 1940°C - 0,429%N, 1960°C -0,235%N, 2000°C - 196%N. The data refer to own investigations for determination of N solubility by drops method used by Prof. D.Sc. T. Rashev and collaborators. It should not be forgotten that Mn is a volatile element. Problems of vapor pressure interfere. By the above said equations and most of all by equation (5) certain physical explanations in relation to the deviations from Sievert"s law can be given at this stage. Undoubtedly, with the progress of the knowledge for the liquid phase as physical state more precise equations and explanations in connection with the gas solubility will be achieved. It is supposed that these equations could serve also as a base for development of methods to determine the solubility of the two atoms (and other) gases. The above said could serve as a base for development of old and working out new theories and methods without negation and rejection of the available ones. Conclusion An attempt is made for a new theoretical grounding of the nitrogen solubility (absorption) in Fe based melt at variation of the partial gas (N) pressure in the gas phase and variation of the T°C as well as of the observed deviations from Sievert"s law through personal studies and studies of other Bulgarian scientists. For that purpose, in the classical equation of the Sievert"s law new characteristics are introduced which connect the liquid and solid physical state: number of vacancies, viscosity, density, diffusion coefficient, surface tension and others that strongly change under the action of pressure, P, and temperature, T°C, the increasing nitrogen content and the qualitative and quantitative presence of the other additions and alloying elements. The alloying elements affect the above said parameters in different ways at the respective P and N and this affects the N solubility. The latter is a result of the sum effect of the above said factors. References l.Essin O.A.., P.V.Geld. Fizitcheskaia chimia pirometallurgit-ches-kih processov. Metallurgizdat, Sverdlovsk, 1954, str. 606. 2.Levy L.I. Azot v tchugune dlia otlivok, Mashinostroenie. M. 1964. 3.Pridantchev M.A. i dr., Vaissokoprotchnaie austenitnaie stalli. Metallurgia. M.,1969. 4,Averin V.V.i dr., Azot v metallah, Metallurgia , M., 1973. 5.Pehlke R.D., Y.F. Elliot. Trans. AIME, 1960, v.218, p. 1076, 1088-1108. 6.Ficher W.O., O. Hofman. Archiv Eisen nhuttenwessen, 1960, 31, N4, 215-219, 1962, 33, N9, 583-598, 29, 1959,30. 7.Kasamatsu I., S.Matoba. Tetsu to Hagane.Journal Iron and steel inst.Japan , 1959, N2, 100, 1957, 43, 925-926. 8.Erinin Chr.D. Metallurgia na stomanata, C, Tchnika, 1990. 9.1vanov E., i dr., Izv. VUZ ,TcH. M., 1966 , 9, 5-7, 1966, 5, 57-61 10.Rashev Ts.V.Dobivane na legirany stomani. Izd. BAN, Sofia,
1978. 11.Rashev Ts.V.Vaisokoazotistaie stally pod davleniem. Izd. BAN, prof. M.Drinov, Sofia, 1995. 12.Pomarin Iu. M., Absorbtcia azota v vaissokochromistaimi rasplavami (Fe +Cr>30-40%) I jidkih Cr pri povaishennaim davleniem. Metallai, N5, 1990, str .40. 13.Pomarin Iu. M., G. I. Grigorenko. Uravnenie dlia raztcheta raztvorimosti azota v splavah Fe. Metallai, N4, 1998, str. 40. 14.Dimov I.N., Kolev B.V. Po niakoi termodinamichni vaprossi na usvoiavane na N v liati splavi ot systemata Fe-Cr-Mn-C. 1984. Materialoznanie i technologia, 13, 1984, str. 44--S5. 15.Kolev B.V. Vazmojnosty za polutchavane, structuruobrazuvane i svoistva na niakoi leiarski splavi na Fe-C osnova , legirani sas azot. Dissertatcia, IM-BAN, Sofia 1985. 16.Kolev B.V.Production of Fe-based N-alloyed cast aslloys. 3rd Intern. Conf. on HNS 93, Kiev. Ukraine, 1993. 17.Kolev B.V. Osnovni resultati, svarzani s polutchavaneto . izsledvaniata i prilojenieto na novi splavi na Fe osnova legirani s azot. Perspectivi i nasoki na razvitie prez sledvashtia 21 vek.NT Conf. s mejd. utchastie. Metaloznanie, metaloleene i termichno obrabotvane. Dokladi, chast I, str. 42-52, str. 218-224, str.225- 232. 18.Kolev B.V. New Fe-based N and other volatile elem. alloys, General results, Development, Future prospects. Intern, congr. Mech. Engin. Techn.97, Procedings, 2, p. 118-124. Sofia, 1997? 19.Kolev B.V. The fundamental philosoph. resourses of material science and Metallurgy in explanation of some processes and phenomena in supreme modes of the matter.s motion. Development of Metallurgy in the Balkans at the begining of the XXI st Century, Repports .p. 296-313, Varna , 1996. 20.Kolev B.V.,T.Trendafilov .Niakoi osobenosty na legiranetp na sivi doevtectichni tchuguni s azot. Mashinostroene, 3, str. 122- 126. Sofia, 1981. 21.1vanchev I.Osnovi na stomanodobivnite protcessi, Techika. Sofia, 1990. 22.Frenkel Ia.N. Vvedenie v teoriu metallov. Nauka. L., 1972. 23.Shineev A.la., D.B.Tchernov. Primenenie vaissokogo davlenia dlia povaishenia raztvorimosti metallov (Al). Techn. leghkih metallov. 1976, 11, str.3-5. FIGURES:
Fig. |
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