EP0024106A1 - Procédé de traitement thermique de métaux ferreux - Google Patents

Procédé de traitement thermique de métaux ferreux Download PDF

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Publication number
EP0024106A1
EP0024106A1 EP80302236A EP80302236A EP0024106A1 EP 0024106 A1 EP0024106 A1 EP 0024106A1 EP 80302236 A EP80302236 A EP 80302236A EP 80302236 A EP80302236 A EP 80302236A EP 0024106 A1 EP0024106 A1 EP 0024106A1
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EP
European Patent Office
Prior art keywords
gas
furnace
air
atmosphere
chamber
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP80302236A
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German (de)
English (en)
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EP0024106B1 (fr
Inventor
Charles Arthur Stickles
Claude Melvin Mack
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Ford Werke GmbH
Ford France SA
Ford Motor Co Ltd
Original Assignee
Ford Werke GmbH
Ford France SA
Ford Motor Co Ltd
Ford Motor Co
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Publication date
Application filed by Ford Werke GmbH, Ford France SA, Ford Motor Co Ltd, Ford Motor Co filed Critical Ford Werke GmbH
Publication of EP0024106A1 publication Critical patent/EP0024106A1/fr
Application granted granted Critical
Publication of EP0024106B1 publication Critical patent/EP0024106B1/fr
Expired legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces

Definitions

  • This invention relates to method of heat treating ferrous workpieces.
  • furnace atmosphere serves to protect the steel parts from carburization or decarburization.
  • carburizing operations propane or other hydrocarbon gas which is the source of the carbon supplied to the steel from the furnace atmosphere.
  • the endothermic gas is produced in a gas generator, separate from the heat treatment furnace itself.
  • the gas is produced at elevated temperatures, cooled to ambient temperatures, then reheated again in the heat treatment furnace. No provision is made for storing the generated gas, thus, if the generator output cannot be fully utilized nt any time, the excess gas is simply flared. This entire mode of operation is inefficient in its use of hydrocarbon gas.
  • Endothermic gas is usually produced at 1900-2000°F., from methane or propane according to the following approximate reaction:
  • the principal constituents of endothermic gas are C0, H 2 and N 2 with minor amounts of CO 2 , H 2 O and CH 4 .
  • the proportions of CO, H 2 and N 2 vary with the C/H ratio of the hydrocarbon used as feed stock.
  • Heat must be supplied to an endothermic gas generator to sustain the reaction of a hydrocarbon with quantities of air substantially less than that needed for complete combustion.
  • a catalyst is therefore used in the generator by the prior art.
  • the composition of endothermic gas is modulated by varying the ratio of air and hydrocarbon fed to the generator. By this means, it is possible to produce gases which are neutral to (that is, will not carburize or decarburize) steel of a certain carbon content at a particular temperature.
  • Air/ Methane ratios of about 2.5 and air/propane ratios of about 7.5 are commonly used when. methane or propane is fed to the gas generator.
  • endothermic gas is enriched with, typically, a 3-12% methane addition at the carburizing furnace (or an equivalent amount of other hydrocarbon gas) so that the overall air/hydrocarbon ratio used to produce carburizing atmospheres may be as low as 1.6 when methane is used, or as low as 6.0 when propane is used.
  • Control of the air/hydrocarbon ratio for either neutral hardening, annealing or carburizing furnace atmospheres is usually based on an analysis of the amount of CO or H 2 0 in the furnace atmosphere. If the constituents of the furnace atmosphere are assumed to be in thermodynamic equilibrium, the carburizing tendency of the furnace atmosphere can be related to its C0 2 or H 2 0 content. Operation of endothermic gas generators and their control is described in detail in the 8th Edition of the Metals Handbook, Volume 2, pp. 67-92 published by the American Society of Metals in 1964.
  • endothermic gas generators are inefficient from the standpoint of energy consumption because after reacting air and hydrocarbon in the generator, the reacted gas is cooled to room temperature, piped to the heat treatment furnace, then reheated again when it enters the furnace.
  • furnace atmospheres for neutral hardening, annealing and carburizing coulu be generated within the heat treatment furnace itself. It has been proposed by the prior art, in certain instances, that the endothermic gas be produced directly in the actual furnace used for treatment of metal parts. However, when the process was conducted, undesirable carbon black formed on the surfaces of the work pieces which rendered the surfaces of the work pieces inactive. To solve this problem one approach suggested in U.S. Patents 3,519,257 and 3,620,518, employed a catalyst on the walls of the furnace in which the gas atmosphere was to be generated in situ. Furnace temperatures of 870°C (for carbonitriding) and 900°C (for carburizing) are mentioned.
  • a method of heat treating ferrous based workpieces in a furnace chamber by heating said workpieces therein to the temperature range of 1500-2000°F (800-1100°C) while in the presence of an endothermic type gas, characterised in that the gas is passed through the said chamber at a low flow rate.
  • ferrous based workpieces are subjected to a heated furnace chamber maintained at heat treating temperature (1500-2000°F) while introducing a supply of air and hydrocarbon gas into the furnace chamber at a predetermined ratio which, when heated by the furnace chamber, chemically reacts to form an endothermic type gas, the endothermic type gas being controlled to flow through the furnace chamber at a low flow rate which preferrably maintains the average residency time of the endothermic type gas in said furnace at least 0.2 hours (12 minutes).
  • the air/hydrocarbon ratio be 1.6-2.4 when methane is selected and 6.0-7.2 when propane is selected. With such air/hydrocarbon ratios, soot-free carburization can be accomplished using the in situ generated atmosphere at lower temperatures without the necessity for special catalysts.
  • the process becomes more sensitive to air contamination by leakage into the furnace chamber or by being carried into the furnace chamber on or in the workpiece.
  • the carburizing or decarburizing potential of the endothermic atmosphere will be detrimentally affected if the air/hydrocarbon gas supply is not variably adjusted. It is preferred therefore to introduce the air component for the air/ hydrocarbon gas mixture at a constant flow rate and to automatically vary the hydrocarbon gas supply to maintain a constant value of CO 2 and/or oxygen potential.
  • the oxygen potential if used as a reference, is preferably measured by a zirconia oxygen sensor device.
  • Figures 1-4 are graphical illustrations of various gas furnace atmosphere characteristics when the furnace temperature is maintained at 927°C. and the gas flow rate therethrough is 15 liters per minute (after allowing for the volume expansion which occurs when air and propane react).
  • Figure 1 depicts average weight gain in the carburized article after 2.5 hours as a function of air-propane ratic
  • Figure 2 depicts the C0 2 gas constituent as a function of air-propane ratio
  • Figure 3 depicts CH 4 content as a function of air-propane ratio
  • Figure 4 depicts the carbon content as a function of distance inwardly from the outer surface of the test samples;
  • the method comprises supplying air and hydrocarbon gas to a furnace chamber at a predetermined ratio where the heat of the furnace chamber (maintained at a heat treating temperature of 1500-2000°F) causes the gases to react and produce in situ an endothermic type gas atmosphere.
  • the endothermic type gas atmosphere is caused to flow through the furnace chamber at a low flow rate and the generation of the atmosphere can preferably be variably controlled to overcome the sensitivity of the method to impurities at such low flow rate.
  • the air and hydrocarbon gas reacts rapidly to produce C0, H 2 , CO 2 , H 2 O, CH 4 and N 2 .
  • the proportions of these molecular constituents arc not the proportions expected at thermodynamic equilibrium.
  • the minor constituents of the initially reacted gas, CO 2 , CO1, H 2 O and CH 4 are invariably present in much greater quantity than is expected at equilibrium. If the reacted gas in allowed to rumain in the furnace, the C0 2 and H 2 O are slowly reduced by the methane by reactions such as
  • Carbcn is transferred from the furnace atmosphere to ferrous workpiece or vice-versa, by reactions such as
  • the first two of the above reactions are known to be much faster than the third reaction.
  • the result of this behaviour is that the carburizing/decarburizing tendency of the furnace atmosphere is strongly affected by the H 2 0 and CO 2 contents of the atmosphere, and only weakly affected by the methane content. If the C0 2 , H 2 0 and CH 4 contents of the atmosphere are all much higher than the equilibrium amounts, the atmosphere will be more decarburizing than it would be if the gaseous constituents were in equilibrium.
  • the carburizing effect of the high methane content does not offs.et the decarburizing effect of the high CO 2 and H 2 0 contents.
  • an endothermic type gas is defined to mean one where the air and hydrocarbon gas are reacted to produce C0, H 2 , C0 2 , H 2 0, CH 4 and N 2 .
  • the proportions of C0, H 2 , CO 2 and H 2 0 are substantially the same at thermodynamic equilibrium as for an independently generated erdothermic gas, but the proportion of methane is typically 2-3 times higher.
  • This invention has provided a way of obtaining soot-free carburizing without the necessity for catalyst or pre-heating of the oxygen supply, and yet save energy up to 75% over comparable energy units used by the state of the art carburizing techniques. This is based on the appreciation that if air/hydrocarbon blends similar to those used in endothermic gas-base atmospheres are permitted a long residence time in the heat treatment furnace at temperature by using very low inlet gas flow rates, a satisfactory carburizing atmosphere can be produced.
  • Low flow rate or slow flow of air/hydrocarbon gas herein shall mean a gas movement which is sufficiently long to permit the immediate reaction products of air and hydrocarbon gas at heat treating temperature to additionally react to lower the C0 2 and H 2 0 content of the gas to substantially thermodynamic equilibrium amounts.
  • Low flow rate can also be defined as that rate of gas movement which allows the mean residency time for all molecules of the gas reaction products to be in the heat treating chamber for at least 0.2 hours (12 minutes).
  • the preselected air/hydrocarbon gas ratio will control the character of the equilibrium atmosphere as to being carburizing, neutral or decarburizing for purposes of hardening, annealing or carburizing.
  • a first series of heat treat experiments were run to determine if carburization by an in situ generated endothermic gas atmosphere at low flow rates can in fact take place, and if so, can be controlled by reflating the proportions of air and hydrocarbon gas entering the furnace.
  • Figures 2 and 3 The significance of Figures 2 and 3 is that while thermodynamic equilibrium is not achieved, it is approached reasonably closely so that the process is controllable using C0 2 analysis if that is desired. At high flow rates with the same gas blends, weight gains would be low, and the CO 2 and CH 4 contents much higher, far from the equilibrium values. Furthermore, at high flow rates carburizing is not uniform. Parts near the gas inlet in the furnace chamber will carburize less than parts located at some distance from the gas inlet.
  • Figure 4 shows the gradient of carbon content measured by electron microprobe analysis for samples from several of these trials. Figure 4 demonstrates that tho inventive process can obtain the same carbon increases as would the prior art at about the same air-propane ratios, except that it is accomplished without prior reaction of the air and propane in a gas generator.
  • the automatic control system is designed so that the total reacted gas flow does not change appreciably as the inlet air/hydrocarbon ratio changes. Ideally, this can be done by regulating the flows of both air and hydrocarbon gas. However, if just the hydrocarbon flow is altered, with.the air flow held constant, the variation in reacted gas flaw (and residence time of the gases within the furnace) is small enough so that it does not appreciably affect process control.. Table 1 shows that the computed flow of reacted gas varies only 20% for air/propane ratios from 3 to 9 and a constant air flow.
  • test samples were run at 927°C and 843°C as in the previous example.
  • Figure 9 shows that the weight gain due to carburization after 2.5 hours at 927°C increases systematically as the set oxygen sensor voltage is increased.
  • the surface carbon content of samples determined by microprobo analysis, also increases systematically as the oxygen sensor voltage increases.
  • the air flow rate employed was chosen to give approximately the same residence time for gases within the furnace as in the previous example, Figures 1 - 4.
  • Figure 10 shows similar results for samples carburized for 6 hours at 84°C. Again, the air flow rate was chosen to give approximately the same residence time for gases within the furnace as in the previous example, Figures 5 - 8.
  • Example I samples were held in the furnace vestibule for several hours while the furnace and vestibule were purged in order to minimize the entry of air into the furnace chamber when the samples were charged into the furnace. A long purging time was necessary because the flow rates employed were low.
  • Example II no special effort was made to avoid entry of air into the furnace chamber. Samples were held in the furnace vestibule for about 15 minutes before charging into the furnace; this holding time in the vestibule is typical of commercial practice with endothermic gas-base atmospheres.
  • All gas molecules entering a furnace chamber do not remain in the chamber for the same length of time. At any fixed inlet gas flow rate there is a distribution of residence time for the molecules .. The mean residence time for all the gas molecules can be readily defined and measured.
  • the mean residence time can always be found by a method of graphical or numerical integration, The calculation of mean residence time will be simpler if a mathematical model for the furnace is used. For example, if the furnace chamber has a Volume V and the flow rate of gas into and out of the furnace occurs at a rate f, then if perfect mixing occurs in the furnace chamber, it can be shown that and the moan residence time is
  • the steep line in each graph at short times represents the influence of the volume of the main furnace chamber, and the shallow line for longer times represents the influence of the volume of the vestibule chamber. It is very difficult to theoretically calculate ahead of time the mean residence time.
  • the volumes of such chambers can be directly measured but the rate of recirculation of gases between the furnace chamber and the vestibule cannot be predicted. Therefore, an experimental measurement of mean residence time is needed to determine appropriate flow rates.
  • appropriate flow rates can be found by progressively lowering the flow rates and simultaneously monitoring furnace gas ccmposition until the furnace gas is close to the equilibrium composition.
  • Figure 15 gives typical means residence times needed to produce satisfactory furnace atmospheres for neutral hardening, annealing or carburizinr by this invention for temperatures from 800 to 1000°C.
  • the amount of air and propane admitted to the furnace is regulated by motorized valves 12 and 13, respectively, which are controlled to operate to maintain a constant air/propane ratio.
  • the ratio is preset in controller 14 and variances in the flow ratio as sensed by flow meter s 15 and 16 causes the individual controllers 17 or 18 to maintain the preset ratio in controller 14.
  • the atmosphere composition may be controlled automatically by monitoring the furnace atmosphere C0 2 content by infrared gas analysis or by measuring the oxygen potential of the atmosphere by means of a zirconia oxygen sensor.
  • the hydrocarbon gas addition is automatically regulated to maintain predetermined levels of C0 2 content or oxygen potential.
  • a suitable system for automatic atmosphere control is shown schematically in Figure 17.
  • the valve controller 20 and 21 adjust the opening of the respective motorized valve 22 and 23 to match the voltage output of the respective flowmeter 24 and 25 to the control voltage.
  • the control voltage is set by adjusting a potentiometer on the valve controller 21.
  • the control voltage is derived from the proportional controller 26. The output of the proportional controller depends on the difference between the signal received from the zirconia oxygen sensor 27 and a reference voltage obtained by setting a potentiometer. The necessary voltage-to-voltage and voltage-to-current converters are not shown.
  • the: necessary flow may be estimated by requiring that the flow ratio (furnace chamber volume in cubic feet divided by the flow rate in cubic feet per hour measured at the furnace temperature) be greater than about 0.2 hours.
  • the allowable flow rates are higher, but must be determined either by trial or by a direct measurement of residence time of the gases.
  • a slow flow rate would be about 400 standard cubic feet/ hour for a flow ratio of 0.25 hours.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
EP19800302236 1979-07-09 1980-07-02 Procédé de traitement thermique de métaux ferreux Expired EP0024106B1 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US5585379A 1979-07-09 1979-07-09
US55853 1979-07-09
US9943979A 1979-12-03 1979-12-03
US99439 1979-12-03

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EP0024106A1 true EP0024106A1 (fr) 1981-02-25
EP0024106B1 EP0024106B1 (fr) 1986-01-02

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DE (1) DE3071318D1 (fr)
ES (1) ES493253A0 (fr)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5194228A (en) * 1990-10-12 1993-03-16 General Signal Corporation Fluidized bed apparatus for chemically treating workpieces
BE1006007A3 (fr) * 1989-12-26 1994-04-19 Tong Yang Nylon Co Ltd Procede pour le reglage de l'atmosphere dans un four pour traitement thermique.
EP0751234A1 (fr) * 1995-06-30 1997-01-02 CARL AUG. PICARD GMBH & CO. KG. Matériaux de base pour la fabrication de lames de scie pour scie circulaire, scies à cadre, meules tronçonneuses et de dispositifs de coupage ou de raclage
US5827375A (en) * 1993-07-23 1998-10-27 Barbour; George E. Process for carburizing ferrous metal parts
WO2001014611A1 (fr) * 1999-08-25 2001-03-01 Messer Griesheim Gmbh Procede pour la nitro-carburation de pieces metalliques
EP2578704A1 (fr) * 2011-10-07 2013-04-10 Linde Aktiengesellschaft Procédé et système de carburation ou carbonitruration d'un composant et composant traité correspondant

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19738653A1 (de) * 1997-09-04 1999-03-11 Messer Griesheim Gmbh Verfahren und Vorrichtung zur Wärmebehandlung von Teilen

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3413161A (en) * 1963-09-21 1968-11-26 Goehring Werner Process for the generation and utilization of furnace atmospheres for the heat treatment of metals, especially of steel
US3519257A (en) * 1967-03-23 1970-07-07 Degussa Process for the treatment of surfaces of workpieces in an annealing furnace
DE1918923B1 (de) * 1969-04-15 1970-11-12 Indugas Ges Fuer Ind Gasverwen Verfahren zur Aufkohlung und Entkohlung von Stahlgegenstaenden
US3620518A (en) * 1967-03-23 1971-11-16 Degussa Process and device for the treatment of surfaces of workpieces in an annealing furnace
US4049473A (en) * 1976-03-11 1977-09-20 Airco, Inc. Methods for carburizing steel parts
US4049472A (en) * 1975-12-22 1977-09-20 Air Products And Chemicals, Inc. Atmosphere compositions and methods of using same for surface treating ferrous metals
GB2016698A (en) * 1978-03-21 1979-09-26 Ipsen Ind Int Gmbh Method and apparatus for measuring carbon level of nonequilibrium gas mixture

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3413161A (en) * 1963-09-21 1968-11-26 Goehring Werner Process for the generation and utilization of furnace atmospheres for the heat treatment of metals, especially of steel
US3519257A (en) * 1967-03-23 1970-07-07 Degussa Process for the treatment of surfaces of workpieces in an annealing furnace
US3620518A (en) * 1967-03-23 1971-11-16 Degussa Process and device for the treatment of surfaces of workpieces in an annealing furnace
DE1918923B1 (de) * 1969-04-15 1970-11-12 Indugas Ges Fuer Ind Gasverwen Verfahren zur Aufkohlung und Entkohlung von Stahlgegenstaenden
US4049472A (en) * 1975-12-22 1977-09-20 Air Products And Chemicals, Inc. Atmosphere compositions and methods of using same for surface treating ferrous metals
US4049473A (en) * 1976-03-11 1977-09-20 Airco, Inc. Methods for carburizing steel parts
GB2016698A (en) * 1978-03-21 1979-09-26 Ipsen Ind Int Gmbh Method and apparatus for measuring carbon level of nonequilibrium gas mixture

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
METAL PROGRESS, Vol. 113, No. 4, April 1978, K.D. GLADDEN et al. "Furnace Atmosphere Control by the Oxigen Potential Method", pages 40 to 44. *

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
BE1006007A3 (fr) * 1989-12-26 1994-04-19 Tong Yang Nylon Co Ltd Procede pour le reglage de l'atmosphere dans un four pour traitement thermique.
US5194228A (en) * 1990-10-12 1993-03-16 General Signal Corporation Fluidized bed apparatus for chemically treating workpieces
US5827375A (en) * 1993-07-23 1998-10-27 Barbour; George E. Process for carburizing ferrous metal parts
EP0751234A1 (fr) * 1995-06-30 1997-01-02 CARL AUG. PICARD GMBH & CO. KG. Matériaux de base pour la fabrication de lames de scie pour scie circulaire, scies à cadre, meules tronçonneuses et de dispositifs de coupage ou de raclage
WO1997002367A1 (fr) * 1995-06-30 1997-01-23 Carl Aug. Picard Gmbh & Co. Kg Materiau de base utilise pour produire des lames d'origine pour scies circulaires, meules tronçonneuses, scies alternatives a cadre, ainsi que pour dispositifs de coupe et de raclage
US6375762B1 (en) 1995-06-30 2002-04-23 Carl Aug. Picard Gmbh & Co. Kg Base material for producing blades for circular saws, cutting-off wheels, mill saws as well as cutting and scraping devices
WO2001014611A1 (fr) * 1999-08-25 2001-03-01 Messer Griesheim Gmbh Procede pour la nitro-carburation de pieces metalliques
EP2578704A1 (fr) * 2011-10-07 2013-04-10 Linde Aktiengesellschaft Procédé et système de carburation ou carbonitruration d'un composant et composant traité correspondant

Also Published As

Publication number Publication date
ES8106559A1 (es) 1981-07-01
EP0024106B1 (fr) 1986-01-02
DE3071318D1 (en) 1986-02-13
ES493253A0 (es) 1981-07-01

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