US3605484A - Method and apparatus to determine carbon potential in the atmosphere of treatment furnaces - Google Patents

Method and apparatus to determine carbon potential in the atmosphere of treatment furnaces Download PDF

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US3605484A
US3605484A US784496*A US3605484DA US3605484A US 3605484 A US3605484 A US 3605484A US 3605484D A US3605484D A US 3605484DA US 3605484 A US3605484 A US 3605484A
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furnace
atmosphere
carbon
gas
temperature
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Joachim Wunning
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/0004—Gaseous mixtures, e.g. polluted air
    • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
    • G01N33/0062—General constructional details of gas analysers, e.g. portable test equipment concerning the measuring method or the display, e.g. intermittent measurement or digital display

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  • the present invention relates to a method, and to apparatus to determine the carbon potential of the atmosphere in furnaces for the treatment of metals, particularly ferrous metals, and more particularly relates to a method and apparatus to determine the concentration of a component of the gas Within the atmosphere, for example carbon dioxide (CO or water vapor or steam.
  • CO carbon dioxide
  • water vapor or steam a component of the gas within the atmosphere
  • German Pat. 1,071,378 (corresponding to U.S. Pat. No. 2,935,866) contains a discussion related to measuring and controlling the addition, or removal of carbon from carbon soluble work pieces, such as iron, or iron alloys, in annealing furnaces. As set forth in this patent, it is necessary when controlling the carbon content within the work piece to provide for a means to measure increase or decrease of carbon content accurately. Carbon is absorbed. or removed from the work piece, until an equilibrium condition exists between the carbon concentration within the work piece and the mixture of the atmosphere surrounding the work piece which includes carbon atoms itself.
  • the carbon content which a sample consisting of pure iron can absorb, at a given temperature, when in equilibrium with an atmosphere within the oven in generally referred to as the carbon potential or the C-potential of the atmosphere within the furnace.
  • Various arrangements and systems to measure this carbon potential are known, in order to adjust the atmosphere within the furnace to a specific carbon potential; reference may be made to HTM, Part A, Volume 16 (1961), Issue 1, page 7. Two methods of measuring, the direct and the indirect methods, have to be distinguished.
  • Direct measuring of the carbon potential is done placing 3,605,484. Patented Sept. 20, 1971 a sample in the furnace, bringing the atmosphere within the furnace into equilibrium therewith, and then determining the carbon content of the sample either, after cooling or quenching by analysis, or by determination of electrical conductivity within the furnace itself (see, for example, the aforementioned German 1,071,378, U .8. Pat. No. 2,935,866).
  • the indirect method the composition of the gaseous atmosphere within the furnace itself is analyzed and the carbon potential is then calculated from known equilibrium conditions of the gaseous components within the furnace atmosphere.
  • the direct method of determining the carbon potential has the advantage of accuracy and simplicity; yet, it has substantial disadvantages which prevent its use in many instances.
  • the method of analysis for example, by analyzing foils, cannot be done continuously and automatically; when introducing a separate test wire within the carbon atmosphere in order to make an electrical conductivity determination, the presence of such test wire may be objectionable and further, a high carbon potential which reaches the region of iron carbides, cannot be accurately measured.
  • Such a high carbon potential, which is desired when forcing increased carbon content has a deleterious effect on the sensing wire and may cause suflicient damage to prevent good measurement.
  • the indirect method of measuring carbon potential is generally used in industry. It is known to utilize the components of the gases within the furnace atmosphere as test samples. As can be shown, it is practically only useful to use the CO CH or the H O components as a representative quantity to determine the carbon potential of the atmosphere within the furnace. The concentration of the particular component is determined. The CH component is, due to the slow change of equilibrium condition, not used much in practice, so that as measuring samples the CO and the H 0 components are usually the only ones which can effectively be utilized. Today, the 00 or H O concentration is almost exclusively determined by means of measuring the dew point, for which a lithium chloride measuring unit is used; alternatively, infrared analysis may be employed.
  • the present invention departs from the prior method and provides for temperature compensation, the temperature being determined from the gases in the oven themselves. For accurate determination, the temperature sensing elements must also be accurate. Their calibration must be accurately known, and they should not be subject to drift; Further, thermal dynamic equilibrium within the furnace must obtain, so that temperature measurements are made under equilibrium conditions and not during deviations from an average value.
  • gas is removed from a given location within the furnace, which given location is also utilized to measure the temperature within the furnace.
  • the gas is analyzed and a value is determined proportional to the concentra tion of the gas component under analysis; this value is made approximately equal to the logarithm of the partial pressure of the gaseous component and this, so transformed measuring value is added to a measuring value derived from the temperature, to add a temperature correction factor thereto; from this combined added value, the carbon potential can be directly indicated.
  • Other correction factors representative for example of composition of the atmosphere within the furnace, the absolute pressure within the furnace, the particular alloy composition of the work piece, etc. can readily be added to the measured value.
  • a correction factor is obtained representative of drift of the temperature measuring element, or of lack of equilibrium of the carbon potential, by making a determination from a test sample, from time to time, and compensating, electrically, for any deviations between indicated, and actually measured values.
  • an electrical signal (current, or voltage) is derived representative of the concentration of the, particular gas, which signal is made proportional to the logarithm of the partial pressure thereof.
  • A' correction signal derived from a thermocouple is added to the signal representative of gas concentration.
  • Correction for instrumentation drift, or lack of equilibrium is obtained by adding another electrical correction signal to the signal representative of gas concentration.
  • This signal can be varied, from time to time, in accordance with analysis results obtained from test samples, and comparing these results with the value indicated by the instrument.
  • the additional signal is then added in (in positive, or negative direction) in order to have the indication on the "instrument agree with actual conditions as determined from analysis of the test sample.
  • the carbon possible to utilize a regulator which adjusts the composition of the atmosphere within the furnace.
  • carbon potential can be determined without the use of complicated diagrams, or computer apparatus replacing. diagrams. It has been found, suprisingly, that the heat of mixing of gamma iron is equal to zero.
  • the method and apparatus of the present invention provides for technical utilization of this discovery.
  • AG AH-TAS wherein AG is the reaction enthalpy
  • AH is the heat of reaction
  • AS is the reaction entropy
  • the water vapor reaction is considered in the following relationship 10g pflzFlog (1:58 o.
  • the partial pressures p or P112 respectively are made proportional to electrical currents or potentials. Correction is then particularly simple because it can be entered by manually settable potentiometers. Any one of the correction factors has a potentiometer assigned thereto, the scale of which can be directly readable in the correction factor, so that the use of additional diagrams is unnecessary. Further, automatic compensation of at least some of the correction factors can be carried out automatically very simply, because within a certain range a shift of the zero or null point is proportional to the specific quantity to be measured, so that the correction factors to be entered can remain constant and need not be changed. I
  • the method according to the present invention can be carried out readily by means of an apparatus which, in accordance with another feature of the present invention, consists of a sampling assembly, introduced into the furnace, which contains a thermo-electric element within a protective cover and, an immediately adjacently arranged suction tube having at least on its interior surface a ceramic coating, from which gas is removed by suction from the furnace. Concentration of the gas so removed is determined in an analysis apparatus, which may be any form of apparatus well known in the art itself. The analysis apparatus is so arranged that it delivers a signal at least substantially proportional to the logarithm of the partial pressure of the gas component under consideration, in the analysis apparatus. Electrical signals so obtained are then added in an adding network to a potential proportional to the output of the thermo-electric elements.
  • FIG. 1 is a partially vertically sectional view of a test assembly in combination with the apparatus, shown partly in schematic form;
  • FIG. 2 is an electrical circuit diagram, in schematic form, indicating measuring of water vapor concentration
  • FIG. 3 is a modified circuit diagram, particularly suited for measuring carbon dioxide concentration within the atmosphere in the furnace, in schematic form.
  • FIG. 1 Referring now to the drawings and particularly to FIG. 1:
  • the furnace not forming part of the invention, is not illustrated; its wall 2 is formed with an opening into which a sampling and test assembly generally shown at 1 can be inserted.
  • the test assembly 1 includes a gas suction tube 3, 4, formed of two telescoping tubes; the outer one, tube 3, may consist of chromium nickel steel, and the inner one, tube 4, may be of ceramic material in order to prevent changes'in the gas being removed which might arise if metallic surfaces, at the temperature of the oven, have catalytic effects thereon.
  • the gas inlet is shown at 5.
  • a double-walled protective tube 7 containing a thermo element Located adjacent the gas inlet 5, and within the common sampling assembly 1 and secure to a common sampling assembly head 11, is a double-walled protective tube 7 containing a thermo element, the active point 6 of which is located adjacent the suction inlet 5 of gas tubes 3, 4. Since both tubes 3, 4 and the tube 7,
  • the sampling assembly head 11 contains a third tube 8, extending within the interior of the furnace, the interior of which communicates with the furnace interior by means of perforations 9 formed in the Wall therein. Tube 8 is adapted to hold work samples, shown at 10, which can be introduced into the furnace to initially adjust, and calibrate the instrument, as will be discussed in detail below.
  • the sampling head 11 is formed with a removable cover 20 and each one of the tubes extending within the furnace itself is covered by a plug 21.
  • the analysis apparatus 14 which will be discussed in detail below, supplies an electrical voltage (or current) to operate indicator 15 (FIG. 1).
  • This indicator can be directly set to indicate percent of carbon potential. It is arranged in a cabinet 23 and contains the circuits further illustrated in FIGS. 2 or 3. A value representative of temperature is applied thereto over conductors 140, connected to the active point 6 of the thermo electric element. Potentiometers 16, 17 and 18, are manually settable in accordance with directly indicating scales 24 likewise arranged on cabinet 23 in order to compensate for parameters or ambient conditions existing in the oven, such as the particular composition of the atmosphere in the oven, pressure within the oven, or composition or particular alloy of the work piece within the furnace.
  • the partial H O pressure is determined by a lithium chloride humidity-measuring element and, in this case, forming the analysis instrument 14.
  • a humidity sensing resistance 14a is included in one branch of a bridge network 25, one diagonal connection of which is fed by a rectifier 26 supplied by potential from one winding of a transformer 50.
  • the structure of the rectifier itself need not further be explained and it may be an ordinary kind of a bridge rectifier; it is indicated schematically only in FIG. 2.
  • the other diagonal portion of the bridge 25 is connected in series to a potential which is determined by the temperature, as obtained from thermo element 6.
  • the conductors from thermo element 6 are connected across a variable resistance 27 which is again fed by potential from a rectifier 28 connected to the transformer 50.
  • the circuit according to FIG. 2 is arranged to permit the addition of fixed values to the output obtained from analysis instrument 14 and the thermo electric element 6.
  • the analysis instrument 14 is so arranged that the output obtained therefrom corresponds to the relationship indicated in Equation 2 above. That is, an output potential proportional to log PHZO.
  • Equation 2 shows that, when the average operating conditions are approximately within a medium range, which is usually the case in actual practice, that the other factors within Equation 2 can be so transformed so that constant correction factors can be added therein.
  • the composition of the work piece has to be compensated for; ordinarily it is customary to define the carbon potential with respect to pure iron.
  • the effects of deviations due to activity of the material are within the limit of tolerance of the equipment.
  • the addition of alloying materials even in only little alloyed steels have an influence which can no longer be ignored.
  • the activity of carbon in the gamma-crystal can be sufiicient so that the carbon content may change up to 20 percent from the desired value, if the alloy content is not considered. Alloying elements which form carbides have an even greater effect.
  • the third tube 8 includes a test. sample 10 consisting of pure, that is, unalloyed iron; further, the test sample consisting of the work piece is inserted therein. Both test samples are exposed to the temperature and atmosphere of the furnace for a sufficient time in order to obtain equilibrium. Thereafter, the test samples are removed from tube 8, quenched and analyzed for carbon content.
  • the coelficient to be determined then is proportional to the quotient of the carbon content of the unalloyed, that is, pure sample (which is the same as that of the carbon potential for pure iron, of course) and of the alloyed work piece or sample.
  • pure sample which is the same as that of the carbon potential for pure iron, of course
  • alloyed work piece or sample the relationship is percent C (pure iron sample) percent 0 (alloy sample) which, of course, corresponds to the definition given above.
  • the scale of the correction potentiometer which is intended to correct for the composition of the alloy, may contain not only the coefficient above-referred to but may further contain indications showing the particular types of steel to be treated. This greatly simplifies setting by even unskilled personnel.
  • FIG. 3 this figure, basically corresponds to the circuit of FIG. 2.
  • a single bridge network that is bridge 29, has been illustrated in order to consider the correction factors previously discussed.
  • various serially connected bridge circuits can be used.
  • a further bridge network 37 is added to the bridge networks 29, in similar manner.
  • a source of electrical potential 137 is connected across one of the diagonal cross con nections of bridge 37, the other diagonal cross connection being placed in series with bridge circuit 29 and the connection to element 32;
  • a variable potentiometer 38 is adjusted in order to introduce a correction potential, the value of which is determined by deviation between indicated value, that is value indicated by measuring instrument 15 and actual value as determined from analysis of a sample of iron strip, or foil introduced into the furnace.
  • the additional compensation compensates for aging and drift of the thermal elements providing for temperature measurement, as well as for errors which arise due to lack of thermo dynamic equilibrium within the furnace.
  • the accuracy of measurement obtained is thus, practically, limited only by the possible errors in analysis of the iron sample strip. It has been found in actual practice that adjustment of potentiometer 38, to compensate for variations in thermo dynamic equilibrium, and in calibration of the temperature measuring elements is only necessary once or twice weekly. Changes in calibration of the measuring instrument, and changes in thermo dynamic equilibrium within the furnace occur only slowly.
  • the anaylsis instrument 14 instead of containing a humidity sensitive resistance, may be made to be sensitive to the partial pressure of the carbon dioxide component.
  • the analysis instrument 14 then contains an infra-red analysis device 240, operating in accordance with infra-red absorption and delivering a potential which is dependent on the partial pressure of the carbon dioxide component introduced therein.
  • the output is made approximately proportional to the logarithm of the partial pressure of the carbon dioxide, that is, mathematically to log p
  • the relationship of the output will then be in accordance with Formula 1 above.
  • the output should relate only to a particular component within the gaseous mixtures introduced thereto, the composition of the gas, in the aggregate, has a substantial influence on the result of the output from analysis apparatus 240.
  • cross-sensitivity exists which in the past 7 (percent 0) I has been compensated by special arrangements within the instrument itself, which introduced complications.
  • the particular gas to be considered can be absorbed specially; or, the undesired gases can be introduced into a special comparison or test vessel for sampling; or additional and known gases can be introduced or used for comparison.
  • such undesired cross-sensitivity can be used directly for correction and calibration of the measuring instrument.
  • the indicator 15 can be formed with maximum and minimum contacts, activated by the indicator pointer. Alternatively, a continously effective comparator can also be utilized.
  • a manually settable potentiometer 52 is adjusted for a certain carbon potential value; it controls a potential which is compared in comparator 51 with the ouput from amplifier 32., also applied to indicator 15.
  • Comparator 51 then controls a valving arrangement, consisting of valves 53, 54 which, respectively, introduce propane or air into the atmosphere of the furnace. For example, if the carbon potential value to too low, propane is added by opening valve 53; if the carbon potential is too high, a small quantity of air is added by opening of valve 54.
  • the determination as to whether to measure the partial pressure of water vapor, or carbon dioxide depends on the general ambient conditions of each application. Measuring the water vapor pressure has the advantage that the measuring instrument 14 is comparatively inexpensive, because an infrared analysis device need not be used. Measurements of water vapor pressure usually are based on measurements of steam pressure, so that the result obtained is usually generally substantially proportional to the Water vapor partial pressure (p Transducers to determine the dew point, or humdity, such as lithium chloride are comparatively simple and cheap, particularly in comparison with analysis apparatus for carbon dioxide.
  • p Transducers to determine the dew point, or humdity, such as lithium chloride are comparatively simple and cheap, particularly in comparison with analysis apparatus for carbon dioxide.
  • One of the disadvantages of measuring the water vapor partial pressure is, that the vapor may condense within the ducting 13 (FIG. 1) when the dew point of the sampling gas exceeds the ambient temperature.
  • the filter 12 in the sampling assembly head 11, in accordance with the present invention and as shown in FIG. 1 avoids the necessity of heating the filter itself and avoids the major diificulties with condensation.
  • the transformation temperature t of lithium chloride is measured directly, which is directly proportional to the water vapor partial pressure p
  • a pair of potentiometers can be adjusted and utilized to introduce correction factors: one for variations of ambient atmospheric pressure, and the other for variations of the product of the volumetric portions of carbon monoxide and hydrogen. Since the atmospheric pressure variations are usually slow, correction twice daily in accordance with barometric pressure is sufficient.
  • the scale on the potentiometer can thus be set to indicate from 700 to 780 torr. Calibration is in accordance with the Equation 2 above.
  • Apparatus for determining the carbon potential of the atmosphere in a furnace comprising:
  • c in atomic fractions
  • p in atmospheres, is equal to partial pressure of the gas com-ponent in the atmosphere
  • T in degrees K. equals absolute temperature in the furnace
  • 7 is a correction coefiicient for a specific alloy composition of a work piece in the furnace; means continuously electrically adding said temperature signal and said gas characteristic signal; means indicating a characteristic of said combined,
  • Apparatus as claimed in claim 1 including signal generating means connected to said adding means to add further electrical signals to said added temperature and gas-characteristics signals, said further electrical signals being representative of additional correction factors obtained during operation of said furnace.
  • Apparatus for determining the carbon potential of the atmosphere in a furnace comprising:
  • c in atomic fractions
  • p in atmospheres, is equal to partial pressure of the gas component in the atmosphere
  • T in degrees K. equals absolute temperature in the furnace; and is a correction coeflicient for a specific alloy composition of a Work piece in the furnace; means continuously electrically adding said temperature signal and said gas characteristic signal; means indicating a characteristic of said combined,
  • Apparatus as claimed in claim 4 including signal generating means connected to said adding means to add further electrical signals to said added temperature and gas-characteristics signals, said further electrical signals being representative of additional correction factors obtained during operation of said furnace.
  • Apparatus to determine the carbon potential in the atmosphere of a furnace comprising means to sense the condition of the atmosphere in the furnace, said means being located within said furnace and including a thermocouple (6, 7) to determine the temperature within a point in the furnace and supplying a temperature signal;
  • a suction tube and means to suck gas from the furnace through the tube said tube having a suction inlet located immediately adjacent the position of the thermocouple within the furnace;
  • a gas analysis apparatus connected to said suction tube to analyze gas sucked therefrom, said apparatus including means to sense the partial pressure of the component of the gas sucked through the tube and providing an electrical gascharacteristic signal;
  • a function generator having said gas characteristic signal and said temperature signal applied thereto and providing a transformed gas-characteristic output signal, for the gas being C wherein c (in atomic fractions) is equal to concentration of carbon in gamma-iron; v p, in atmospheres, is equal to partial pressure of the gas component in the atmosphere; T in K.
  • manually settable electrical potential generating means (29, 30, 31, 37) generating adjustment signals to represent'fixed factors and to represent the error value between the carbon potential of a standard material sample, as derived from an intermittently analyzed sample and the carbon potential as indicated; an adding circuit connected to and electrically adding said transformed gas-characteristic signahsaid' tem- 12 perature signal and at least one of said adjustment signals and providing an indicating signal; and an indicating means (15) connected to said adding circuit and having said indicated signal applied thereto and being directly readable in percent of carbon potential.
  • said sensin means includes a gas filter (12) located immediately adjacent the inlet and of said suction tube.
  • said sensing means includes a sensing assembly introduced into said furnace; and said sensing assembly includes a test sample tube to hold a standard sample, said tube having perforations to establish communication between the interior of the test sample tube and said furnace to expose said test sample to the atmosphere inside the furnace.
  • said manually settable potential generating means includes sources of potential and manually settable Potentiometers (16, 17, 18, 38).
  • said manually settable potential generating means includes means generating a fixed potential (33, 34, 35, 37); seriesconnected bridge circuits (29, 30, 31, 37) having one diagonal connection connected to said sources of fixed potential and the other diagonal connections connected in series; and manually settable potentiometers (16, 17, 18, 38) in a branch of each of said bridge circuits.
  • said means to sense the partial CO pressure includes an infrared spectrum-absorption apparatus.
  • control means connected to said indicating means, valve means regulating the composition of the atmosphere within said furnace; said control means controlling the valve means to keep the carbon potential Within the furnace at a desired point by addition of air or gas capable of changing the carbon content of the atmosphere within the furnace.
  • Apparatus to determine the carbon potential in the atmosphere of a furnace comprising means to sense the condition of the atmosphere in the furnace, said means being located within said furnace and including a thermocouple (6, 7) to determine the temperature within a point in the furnace and supplying a temperature signal;
  • a suction tube and means to suck gas from the furnace through the tube said tube having a suction inlet located immediately adjacent the position of the thermocouple within the furnace;
  • a gas analysis apparatus connected to said suction tube to analyze gas sucked therefrom, said apparatus including means to sense the partial pressure of the component of the gas sucked through the tube and providing an electrical gas-characteristic signal;
  • c in atomic fractions
  • p in atmospheres, is equal to partial pressure of the gas component in the atmosphere
  • T in K. equals absolute temperature in the furnace
  • 'y is a correction coefficient for a specific alloy composition of a work piece in the furnace
  • manually settable electrical potential generating means (29,-30, 31, 37) generating adjustment signals to represent fixed factors and to represent the error value between the carbon potential of a standard material sample, as derived fronr-and intermittent- 13 ly analyzed sample and the carbon potential as indicated
  • an adding circuit connected to and electrically adding said transformed gas-characteristic signal, said temperature signal and at least one of said adjustment 5 signals and providing an indicating signal;
  • an indicating means (15) connected to said adding circuit and having said indicated signal applied thereto and being directly readable in percent of carbon potential.
  • said means to sense the partial H O pressure is a lithium chloride humidity indicator and includes a bridge network (25) having one branch thereof formed by a humidity sensitive resistance (14a); said adding circuit including connection means interconnecting one diagonal connection of said bridge circuit with said thermocouple.

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US784496*A 1966-04-21 1968-10-03 Method and apparatus to determine carbon potential in the atmosphere of treatment furnaces Expired - Lifetime US3605484A (en)

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Application Number Priority Date Filing Date Title
DE1673308A DE1673308C3 (de) 1966-04-21 1966-04-21 Verfahren und Vorrichtung zur Bestimmung des Kohlenstoffpotentials von Ofenatmosphären bei Glühöfen
DE1673328 1967-10-05

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US (1) US3605484A (ru)
AT (1) AT294462B (ru)
CS (1) CS151481B2 (ru)
DE (1) DE1673308C3 (ru)
FR (2) FR1526196A (ru)
GB (2) GB1135306A (ru)
SE (1) SE347817B (ru)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3727450A (en) * 1970-01-28 1973-04-17 Rech Metallurg Centre Nat Gas composition analysis
US3871228A (en) * 1973-05-14 1975-03-18 Us Navy Permeable membrane gas saturometer
US3927555A (en) * 1973-10-15 1975-12-23 Gen Electric Hydrogen detector system
US20120160468A1 (en) * 2009-07-20 2012-06-28 Sinvent As Local Thermal Management

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3727450A (en) * 1970-01-28 1973-04-17 Rech Metallurg Centre Nat Gas composition analysis
US3871228A (en) * 1973-05-14 1975-03-18 Us Navy Permeable membrane gas saturometer
US3927555A (en) * 1973-10-15 1975-12-23 Gen Electric Hydrogen detector system
US20120160468A1 (en) * 2009-07-20 2012-06-28 Sinvent As Local Thermal Management

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DE1673308C3 (de) 1978-10-12
GB1229467A (ru) 1971-04-21
AT294462B (de) 1971-11-25
GB1135306A (en) 1968-12-04
SE347817B (ru) 1972-08-14
FR95622E (fr) 1971-03-26
DE1673308B2 (de) 1975-02-27
DE1673308A1 (de) 1971-06-16
FR1526196A (fr) 1968-05-24
CS151481B2 (ru) 1973-10-19

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