US1988929A - Metallurgical furnace gas and method of controlling composition - Google Patents

Description

Jan. 22, 1935. F. P. WILSON, JR 1,988,929

METALLURGICAL FURNACE GAS AND METHOD OF CONTROLLING COMPOSITION Filed March 6, 1930 Inventor: Frederick PWilSoTmJP,

by MW H i s After-neg.

Patented Jan. 22, 1935 umrao STATES METALLURGICAL FURNACE GAS AND DIETHOD OF CONTROLLING COMPOSI- TION Frederick P. Wilson, Jr., Schenectady, N. Y.,

assignor to General Electric Company, a corporation of New York Application March 6, 1930, Serial No. 433,726

2 Claims.

The present invention relates to metallurgical operations, such, for example, as the annealing and brazing of metals, and in particular to the enveloping of steel and other carbon-containing metals during metallurgical operations with a gaseous atmosphere which is adapted to protect the metals from oxidation and to preserve or modify the carbon content of the metal.

The present invention comprises a method of controlling the carbon-reactive property of a gaseous mixture so as to make the same carbu rizing, neutral or decarburizing at will.

In accordance with my invention a gaseous mixture comprising a decarburizing gas, such as hydrogen, or carbon monoxide or both and a carburizing gas, such as methane, ethane, propane or butane/is subjected prior to its introduction into a metallurgical furnace or the like to a temperature chosen and controlled to cause said gas to have a desired carbon-reactive eifect, that is, to be carburizing, decarburizing to a desired degree or neutral, as may be desired. The carbon content of steel, or other ferrous metal, may be preserved during a brazing operation in an enveloping atmosphere which otherwise would lower the carbon content in accordance with my invention by the presence of a relatively small amount of hydrocarbon gas in the brazing atmosphere, for example, about one per cent by volume of methane. I prefer also to have present some carbon monoxide in the brazing gas as the presence of water vapor has less deleterious effect on it than on hydrogen.

The novel features of my invention will be pointed out with greater particularity in the appended claims.

The appended drawing shows in Fig. 1, in, somewhat diagrammatic form, and partly in section, an apparatus suitable for carrying out my invention: Fig. 2 is a longitudinal section of an adjunct suitable for use together with the apparatus shown in Fig. 1; and Fig. 3 somewhat conventionally illustrates a modification.

Referring to Fig. 1 of the drawing, a carbon-, containing gas, such as a fuel gas of the character commonly employed for city gas supply, is introduced by a supply pipe 1 to a mixing chamber 2. As examples of a hydrocarbon fuel gas, coal gas, natural gas, coke oven gas and producer gas may be mentioned. Such a fuel gas usually comprises hydrogen, carbon mon-.

oxide, methane and higher normally gaseous hydrocarbons, commonly known as illuminan A hydrocarbon, such as butane, also may be used. In carrying out my invention, steam may be admixed with the hydrocarbon gas to combine with dissociated carbon although the presence of steam is not a necessary feature of my invention. The steam is admitted to the mixing device May a supply pipe 3, the steam flowing through a valve 4, which is controlled by a temperature-responsive device 5' whereby the amount of steam admixed with the hydrocarbon gas is controlled. The proportion of steam to be added to the gaseous mixture may be determined by trial and analysis. Ordinarily,

steam should be added in such stoichiometrical proportion that carbon freed in the reaction chamber is reacted upon substantially completely to form hydrogen and carbon monoxide gas.

The device 5 operates the valve 4 to vary the steam supply in response to 'the temperature of the gas and steam mixture in the mixing chamber 2 so as to maintain a constant ratio of combustible gas and steam in the reaction mixture delivered to a reaction chamber 6 by the pipe 7, emerging from the mixing chamber. I may use for elements 4 and 5 a regulator sold by the Taylor Instrument Company of Rochester, N. Y. under the trade designation Tycos Temperature Regulator. This device comprises a steam valve operated by a mechanism which is responsive to the temperature of a medium (in this case the gas-steam mixture in 2) through the intermediary of a gas or other fluid under pressure, which communicates with a bellows device (not shown) whereby motion is transmitted to the steam valve to vary its setting. With a proper setting of the regulating device, the quantitative ratio of 'gas and steam is maintained constant regardless of variations of gas or steam pressure.

steam to be admitted. Conversely a rise insteam ratio will result in an increased liberation of heat in the mixing chamber and cause the device 5 to reduce the flow of steam by reducing the aperture of the steam valve. Other known suitable, regulating devices responsive to temperature of a medium may be employed, as for example, an electrically controlled device having a thermostatic element.

The gas mixture passes from the pipe through a heat interchanger 7a to the reaction chamber 6 where it is subjected to a high temperature varying with the conditions and the results desired but in general being within the-limits of about 700 to 1100 C.

As will'be hereinafter more fully explained the temperature in the reaction chamber is maintained at such value with respect to the temperature and other conditions of the treating zone that the content of hydrocarbon gas in the product will cause it to have a desired chemical effect with respect to the metal charge in the furnace, that is, to be decarburizing, carburiz ing or neutral as may be desired.

The hydrocarbon gas which in the examples given is in excess-of the combining proportion required to eliminate such water vapor as may be present will be decomposed in the reaction zone, liberating carbon and causing an equilibrium to be established between the gas and the carbon at the reaction temperature. If water vapor is present the carbon will combine with the water vapor. If water vapor is absent, the carbon will be deposited as lampblack in the reaction chamber.- In that event the reaction chamber should be periodically cleaned out.

The reaction chamber comprises a metal shell 8 within which is located a fire-brick wall 9, resting on a base 10. Mounted on the wall 9 is a distributing chamber 11 which communicates by a conduit 12 with the heat interchanger 7a. The shell 8 is lined with a heat insulator 13 consisting of finely divided material, such as silocel. An electric resistance heater 14 is supported within the chamber 6 from the wall.

This resistor preferably comprises a ribbon of nickel-chromium alloy which incidentallyhas a catalyzing effect upon the gaseous reaction occurring in the reaction chamber. Its main function is to maintain a desired temperature in the reaction chamber. Suitable external regulating devices (not shown) may be employed to control the temperature of the heater.

The gas mixture entering the chamber passes through the iron tubes 15 to' a region near the bottom of the chamber andpasses upward and emerges from the reaction chamber by the conduit 16 to an electric furnace, or oven 17 in which brazing, heat treating, or other metallurgical or chemical process is being carried out requiring a gas atmosphere. Examples of brazing furnaces are shown in United States Patents 1,536,944, issued May 5, 1925 to Christian Steenstrup, and 1,610,809, issued December 14, 1926 :to D. F. Newman.

I The gaseous product delivered to the furnace 17 is constituted of a preponderant proportion of hydrogen, a lesser proportion of carbon monoxide together with a minor content of inactive gas, such as nitrogen and carbon dioxide. A

small content of methane, say about one per eifectiveness of the hydrogen to prevent oxida-' tion disappears. This is not true of carbon monoxide. If suflicient carbon monoxide is present to be in chemical equilibrium with carbon dioxide, oxidation will not take place as the temperature is lowered.

As the brazing gas produced by the above described process comprises mainly hydrogen and carbon monoxide and a lesser amount of carbon dioxide, oxidation of the charge will not occur during cooling. If reduction of oxide occurs in the brazing or annealing furnace, or in any way oxygen is-introduced both water vapo'r and carbon dioxide firstare formed, but water vapor then reacts with the methane forming hydrogen and carbon monoxide.

At a given temperature prevailing in the reaction chamber, the hydrogen and me methane are in chemical equilibrium. Such mixture, is in chemical equilibrium with respect to the carbon of the steel in a brazing furnace at a temperature not far removed from the reaction temperature depending on the amount of carbon in the steel and other conditions, including the presence of water vapor in the brazing furnace.

If it is desired to carry out a metallurgical operation such as annealing of steel with an atmosphere which is neutral, that is neither carburizing nor decarburizing with respect to the steel, then by trial and physical or chemical determination of the carbon content of the steel before and after treatment, the conditions of the gas converter are so adjusted that a gas is produced which is in chemical equilibrium with the steel or other charge.

By trial the various factors, such as temperature, rate of flow, water vapor content, composition of the hydrocarbon gas fed into the converter may be adjusted to produce a neutral gas.

Ordinarily, it is preferable to carry out metallurg-ical operation involving ferrous metals by the use of a protective atmosphere which is slightly decarburizing. To accomplish this one of two courses may be pursued.

(1) The reaction chamber temperature may be increased to completely decompose or crack the methane, or other hydrocarbon gas.

(2) The gas mixture may be varied to produce a small percentage of carbon dioxide in the gas fed to the brazing or other metallurgical furnace. For example, the steamcontent of the mixture fed into the reaction chamber may be increased.

If it is desired to operate a brazing furnace at say 1100 0., containing a slight amount of of this water vapor by the presence of a some.-

what larger amount of hydrocarbon in the gas. The reaction chamber temperature may be somewhat lowered by trial until the desired content of methane in the gas is obtained. For many brazing operations about one per cent of methane is desirable.

If a carburizing effect is desired, as when the gaseous products from the converter are to be used in a case-hardening furnace, then the conditions are chosen to produce a substantial ex-.- cess of methane in the gaseous product. Ordinarily this will be done by operating the reaction chamber at,a still lower temperature, in some cases as low as 700 0., at "which temperature a substantial proportion, say about 15%,

' portion of nitrogen varying from about 5% to 95%. The presence of the nitrogen renders an accidental explosion in the furnace less violent.

in fact, a mixture containing inert gas, such as nitrogen, in proportion as high as 95%, is noninflammable while still having a reducing effect upon metal if the mixture is dry.

If it is desired to produce a gaseous mixture for metallurgical operations having a substantial content of inactive gas, such as nitrogen, and carbon monoxide some of the fuel gas fed to the converter is first burned by mixing it with such amount of air that incomplete combustion is produced. In that event a modified gas feeding system is employed as is illustrated in Fig. 3.

this figure a combustion chamber 29 is inserted into the gas supply line 1 by closing the valve 21 in the feed pipe 1, and opening the valve 22 in a supply pipe 23 of 'the combustion chamber, which is also supplied with oxygen, air or other combustion supporting gas under pressure through a pipe 24. As shown in Fig. 2 an oxygen flame is produced at the orifice of the air supply pipe 24 which burns in the surrounding atmosphere of fuel gas in the combustion chamber 20 and consumes part of the fuel gas forming carbon dioxida'carbon monoxide, and water vapor. These products, together with the nitrogen introduced as part of the air, are carried through the pipe 25 into the mixing chamber 2, and from thence into the reaction chamber 6. It is desirable to remove the water vapor from the gaseous mixture in pipe 25. This can be done in any convenient way, as for example by passing the gases through a condenser 26 and drawing ofi the condensed water. In some cases the water vapor may be retained in the gaseous mixture and a correspondingly lesser quantity of steam admitted to the mixing chamber, so as not to disturb the gas and steam ratio. The gases entering the reaction chamber from the pipe 25 'will comprise hydrogen, carbon monoxide, hydrocarbon gas, such as methane, nitrogen, and carbon dioxide. This gaseous mixture is further admixed with steam depending on the amount of hydrocarbon present and the reaction then proceeds as above described, the product containing substantial admixtures of nitrogen depending on the amount of air introduced in the combustion chamber.

When the metallurgical furnace is first started up it is desirable to sweep out air from the interior of the furnace by operating the combustion chamber 20 with such proportions of combustible gas and air that the products consist wholly of inert gas, such as nitrogen and carbon dioxide. It is also desirable during the shutting down period after the metallurgical operation in the furnace 1'7 has been completed, or in fact at any time when it is desired to shut down the furnace 17, to purge the interior of'the furnace of' combustible gases so as to avoid an explosion. This may be done at any time by closing the valve 21 supplying fuel gas, opening the valve 22 and supplying suflicient air through the supply pipe 24 to cause complete combustion as above described.

It is possible to cause a combustion chamber 20 to assume also the function of the reaction chamber 6, the operation being carried out in such a way that cracking or decomposition of unburned hydrocarbon gas occurs in the com bustion chamber. The product from the combustion chamber then comprises carbon dioxide, nitrogen, and carbon monoxide, and if desired, a small amount of hydrocarbon, this gaseous mixture then being supplied to the metallur-r gical furnace without further change or reaction. When this result is desired enriched air or pure oxygen is supplied to the combustion chamber by the pipe 24 and the combustion chamber is constructed of refractory heat insulating ma= terial so as to conserve the heat evolved thereinv by combustion. With the combustion chamber operating at a high temperature, decomposition of either a large proportion or all of the hydrocarbon gas occurs. In that case the gases need not be passed through the reaction chamber, which may be omitted. For convenience, if such operation is intermittent, the reaction chamber may be kept in the system but the resistance unit therein is not brought up to operating temperature.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. The method of protecting ferrous metal while being brazed at a temperature of about 1100 C. which consists in enveloping said metal with gaseous products obtained by heating a mixture of a hydrocarbon gas and water vapor to a temperature of about 1050 C. and removing water vapor from the gases produced by such heating before such gases come in contact with said ferrous metal.

2. The step in a metallurgical operation such as carburizing or brazing, involving heating solid ferrous metal to an elevated temperature at which the carbon content of said metal is subject tochange which consists in enveloping said metal during such operation with a gas obtained by heating a mixture of an oxidizing gas and a chemical excess of a hydrocarbon gas to a reaction temperature within the range of about 700 to 1050 C. the reaction temperature being maintained at least 50 C. lower than the temperature at which said metallurgical operation is carried out, thereby maintaining a predetermined quantity of hydrocarbon gas in said enveloping gas.

' FREDERICK P. WILSON, JR.

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