US2085587A - Method of brazing - Google Patents

Description

`lune 29, 1937a` A. G. HoTcHKlss METHOD OF BRAZING Filed May 29, 1935 Irverwtcnr:

Allem G. Hotchkiss. @LJ ff 19M His ttorrweg.

Patented Jlune 29, 1937 to General Electric C New York Application May 29,

3 Claims.

My invention relates to the metallurgical operation of copper brazing metal parts, and particularly to the production of an improved gaseous reducing atmosphere for enveloping such parts during the brazing operation.

Heretofore rcopper brazing operations generally have involved the use of large volumes of expensive gas, such as commercial hydrogen. Use of hydrogen-containing mixtures less dangerous to handle than undiluted hydrogen has required an increase in total gas volume almost inversely proportional to` the hydrogen content, with little eventual saving in cost.

In accordance with my present invention gaseous products of the exothermic combustion of relatively cheap hydrocarbon materials may be used to provide an atmosphere for copper brazing which yields results entirely unpredictable on the basis of prior knowledge. For example, with a given quantity of reduction mixtures resulting from the burning of ordinary city gas, I have been able to accomplish brazing operations normally requiring an equal volume of commercial hydrogen and a much larger volumeof diluted hydrogen. Not only is this much less costly than previous methods, but due to the smallamount of gas needed the brazing furnace is cooled to a correspondingly slighter degree. Consequently, the whole operation can be carried out with an economy of time and power greatlyin excess of that attained by any of the methods of the prior art.

In carrying out my invention, the percentage of air in the combustion mixture is varied until a product of desired composition is obtained. This will occur when at the operating temperature of the furnace a state of chemical equilibrium is approached such that the various constituents are present in reducing proportions. 4In the usual case such an equilibrium obtains when the percentages of hydrogen, Water vapor, carbon monoxide and carbon dioxide in the furnace atmosphere are such that there is no tendencyfor an oxidizingV reaction to take place at brazing temperature. When this condition is reached, effective uxing of the copper and good brazing will result.

ompany, a corporation og 1935, Serial N0. 24,007

in section of certain portions of the apparatus which are to be used for the combustion and cooling of the gaseous mixtures.

Referring to Fig. 1, hydrocarbon materials, such as city or natural gas, or butane, may be admitted through an adjustable inlet valve I to a suitable mixing valve 2. This valve is of conventional design and is provided with an inlet conduit 3 for supplying air in a combustion-supporting ratio. An easily adjustable control 4 is attached by means of which the ratio of the mixture may be varied at will.

A preheater 6 is provided which may be used in some cases for raising the combustion mixture to a combustion-supporting temperature as it ows through conduit 5 to a combustion chamber designated as a whole by the numeral '1. This latter is enclosed in a flanged metal cylinder 8 which is sealed at its ends by metal plates 9 and I U respec tively. As appears more clearly in Fig. 2, the cylinder 8 is provided with a refractory lining II to conserve heat and to protect the metal parts from the destructive eiects of the combustible gases.V The combustion space maybe lled, at least in part, with a quantity of granular material I2, such as alumina or magnesia, which serves as a catalyzing agent to insure complete exothermic combustion of the gaseous mixture.

After combustion takes place the hot gaseous products may be conducted through a purier I3 which eliminates any oxygen or unsaturated hydrocarbons, such as ethylene, which may remain. For this purpose the purier may contain copper or iron turnings.

For controlling the moisture contained in the products of combustion, I have shown a. cooling unit I4 which comprises a Water tank I5 having connections I6 and Il for circulation of tap Water, or other suitable cooling fluids. Enclosed Within the tank I5 and so positioned as to be surrounded by the cooling medium is mounted a condensing coil I8 having at' its lowest point a lead-oir I9 which communicates with a drip valve 20 to eliminate condensed moisture. It is alternatively pos- ,sible to reduce the temperature of the hot gases by passing them through a cooling tower (not shown) in direct contact with a cooling uid. For certain types of work this latter method has the advantage of removing soluble impurities and solid particles carried over from the combustion chamber.

The combustion products, which consist essentially of a mixture of hydrogen, carbon monoxide, carbon dioxide, and Water vapor in reducing proportions With a residuum of nitrogen, are led fil through a conduit 2l to a brazing furnace 22. This latter is of conventional construction and comprises a gas-tight structure lined with a refractory material. It is, of course, provided with heat-generating means operative to raise its internal temperature to about 1100 C., which is the level required for brazing, and embodies a structure suitable for supporting the work pieces to be subjected to the brazing operation. Examples of such a furnace are shown in U. S. Patent No. 1,536,944, issued May 5, 1925, to Christian Steensti-up, and Patent No. 1,610,809, issued December 14, 1926 to D. F. Newman.

The brazing operation consists essentially in joining metal parts by fusion with an alloying metal, copper being the material generally considered most suitable for this use. As is well understood bythose skilled in the art, however,

successful copper brazing requires the use of a reducing atmosphere, for which purpose hydrogen has been most frequently employed. When used in this way, the hydrogen apparently acts as a ilux for cleaning the metal surfaces and causing them to be Wet by the copper, thus resulting in an extremely perfect joint. Silver is an equivalent for copper in brazing for most p'urposes.

In accordance with m-y invention combustion products of hydrocarbon materials and air, produced according to the method outlined above, may be used as a substitute for pure hydrogen as an enveloping gas. To be suitable for this purpose, however, it is essential that the proportions of water vapor, carbon monoxide, hydrogen and carbon dioxide should be such that the resulting mixture is of a reducing nature.

In determining the existence of this condition, the following three reactions are those principally involvedzl It is essential for successful brazing that these reactions, which are reversible, shall produce a net reducing effect as the various reactions approach chemical equilibrium. In practice, using partially combusted hydrocarbon 'gas from which a large part of the Water formed has been removed, the concentration of the gases will be such that reaction (3) proceeds in a direction from left to right at operating temperatures. It is not necessary, however, that reactions.(1) and (-2) both proceed Iin the reducing direction (from right to left) as long as their combined effect is preponderantly reducing.

At a brazing temperature of about 1100 C. chemical equilibrium will be substantially obtained when the partial vapor pressure of the various constituents are in a relation which satisfies the following equation:

(C0) X (H20) (C02) X (H2) For the furnace atmosphere to have an inherently reducing tendency, it is necessary that certain limiting conditions exist as the above dened chemical equilibrium is approached. In general, the required conditions will be fulfilled when, at a brazing temperature of 1100 C., the ratio of the partial pressure of CO2 to that of CO is less than'about .35 and the ratio of the partial pressure H2O to that of H2 is less than about .73.

A typical gas mixture which meets the required conditions and which is suitable for brazing includes about 61% nitrogen; 15% hydrogen; 9% carbon monoxide; 5% carbon dioxide, and 10% Water vapor. The percentages given, however, should not be regarded as limiting and are merely exemplary of a particular combination chosen at random from those which have been found practicable.

I have found that a considerable number of hydrocarbon and air mixtures may be used as combustion substances for the production of gaseous reducing atmospheres which possess the advantages peculiar to my invention. As a practical matter, however, the proportions selected should be those which are able to support combustion.

The amount of air required to maintain a combustible ratio will necessarily vary greatly with the particular source of hydrocarbon used. With a mixture of air with the so-called coke oven gas or ordinary city gas, for example, I have obtained satisfactory results when the ratio of air to gas is as low as 1 to 1.

When natural gas is used, this being quite as suitable as city gas, the percentage of air should be increased until it preponderates in the ratio of about 4 to l. Successful results may also be obtained by the use of pure hydrocarbons such as butane and propane, provided the proportions are changed so that from 12 to 16 parts of air are used to one of the hydrocarbons. To be sure that combustion will prove self-sustaining in each cf the above cases means for preheating the combustion mixture should be employed when the lower air ratios are used. As pointed out in connection with Fig. 1, this may comprise an elementary heater located adjacent the main combustion chamber.

It is not practicable to assign exact upper limits for the amount of air which it is permissible to admix with the combustible hydrocarbon. In fact, however, a restriction is imposed by the requirement that the hydrocarbon should be in excess of a complete combustion ratio with respect'to the admixed air by a value such that substantial amounts of hydrogen and carbon monoxidewill be provided by partial combustion and thermal decomposition. The quantities of these gases should be sufficient to reduce oxidizing impurities occurring on the brazing surfaces and thus eiectively to flux the copper.

The conditions under which this requirement will be fulfilled may be determined readily by empirical'tests and will depend largely on the composition of the hydrocarbon. For city gas most satisfactory results have been obtained with a ratio of air to gas at least as low as 2% to 1. Natural gas, on the other hand, permits a preponderance of air in the neighborhood of 5 to 1, While correspondingly higher ratios will be proper if butane or propane is used.

The amount of water vapor in the furnace atmosphere is not critical as far as the brazing operation itself is concerned. It is desirable, however, to keep its proportion 10W in order to avoid condensation of moisture in the furnace. Furthermore, where a bright surface is required to be maintained on the finished product, the water vapor content should be below that at which oxidation or decarburization can take place during cooling. This factor may be regulated conveniently by partially drying the combusted'gases in the cooling unit I4. The temperature and rate of iiow of the cooling liquid may be adjusted to provide any chosen degree of drying.

With the hydrocarbon materials which I consider most suitable for practice of my invention, in addition to Water vapor, hydrogen, carbon monoxide and carbon dioxide the combustion products Will contain a residuum chiefly composed of nitrogen. Slight percentages of methane may also be present depending on the composition of the hydrocarbon chosen.

In. order to prevent blackening or carbonization of the metal parts desired to be brazed, particularly ii these are of ferrous content, it is advisable to avoid the possibility of any dissociated carbon being carried over into the furnace. This may be accomplished by maintaining temperature of combustion above that at which free carbon can occur. Practical experience shows that thisdependson maintaining a sufficiently rapid ow of combustible gas so that the temperature of the combustion chamber Will be kept above about 980 C.

While decarburization is a relatively unimportant factor in the brazing operations with which my invention is concerned, where it is desirable to do so, its effects may be reduced to a minimum by decreasing the amount of air fed into the mixing chamber to the least value suiicient to support combustion.

Gaseous combustion mixtures prepared in accordance with the method which I have outlined above have been found to be much safer for factory use than undiluted hydrogen due to the fact that they are of a considerably less explosive nature. Furthermore, as has already been pointed out, the cheapness and accessibility of the raw materials required renders their production a matter of very little difficulty.

It will be seen that my invention makes plossible the practice of brazing operations in an atmosphere of reducing gases obtained by the exothermic combustion of inexpensive materials. The fact that mixtures are used which are capable of supporting combustion avoids the necessity for complicated and costly reaction equipment and a separate source of heat supply. The economy of this method is accentuated when it is realized that the volume of the combustion products is at least twice as great as that of the hydrocarbon gas required. This increase in volume is of course explainable by the fact that a second reaction material, air, is added to the hydrocarbon.

From another point of view experience has .shown that it is an extremely important advantage of my invention that the amount of gas required to be -introduced into the brazing furnace is but very slightly in excess of that needed when pure hydrogen is used as the enveloping medium. This is believed to be due to the fact that the heavier gaseous mixtures which I employ have much less tendency to escape from the furnace than does the more volatile hydrogen. Accordingly, both Waste of gas and danger of an explosion are greatly diminished.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not Wish to be limited thereto since many modifications in the structure may be made, and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

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

l. A method comprising the step of brazing metal parts in an enveloping gaseous atmosphere containing Water vapor, hydrogen, carbon dioxide and carbon monoxide so proportioned that said gaseous atmosphere is reducing at brazing temperature.

2. A method comprising the step of brazing metal parts in an enveloping gaseous reducing atmosphere containing water vapor, hydrogen, carbon dioxide and carbon monoxide and wherein, at a brazing temperature of about 1100 C., the ratio of the partial pressure of the Water vapor to that of the hydrogen is less than about 0.73 and the ratio of the partial pressure of the carbon dioxide to that of the carbon monoxide is less than about 0.35.

3. A method comprising the step of copper brazing ferrous metal parts in an enveloping gaseous reducing atmosphere consisting mainly of nitrogen, Water vapor, hydrogen, carbon dioxide and carbon monoxide and wherein, at a brazing temperature of about 1100 C., the ratio of the partial pressure of the water vapor to that of the hydrogen is less than about 0.73 and the ratio of the partial pressure of the carbon dioxide to that of the carbon monoxide is less than about l0.35.

ALLEN G. HOTCHKISS.

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