Method of brazing

Abstract

Claims

1. AN IMPROVED METHOD OF BRAZING STRUCTURAL METAL PARTS, THE BASE METAL OF WHICH IS SELECTED FROM THE GROUP CONSISTING OF IRON AND NICKEL, COMPRISING THE STEPS OF DISPOSING THE SURFACES OF SAID PARTS TO BE BRAZED IN CLOSELY-SPACED RELATIONSHIP, PLACING AT THE RESULTING GAP AREA BETWEEN THE SURFACES A MIXTURE CONSISTING OF DIBASIC AMMONIUM PHOSPHATE, CHROMIUM AND A NICKELIFEROUS MATERIAL SELECTED FROM THE GROUP CONSISTING OF NICKEL AND NICKEL OXIDE IN A PROPORTION YIELDING AN ALLOY COMPRISING APPROXIMATELY 80% NICKEL, 10% PHOSPHORUS AND 10% CHROMIUM, THEN HEATING SAID METAL PARTS TO A TEMPERATURE OF AT LEAST 1800*F. AND MAINTAINING SAID METAL PARTS IN A REDUCING ATMOSPHERE OF DRY HYDROGEN DURING THE BRAZING OPERATION.
- velopment. ,be easily fabricated using welding techniques. 7 complex and delicate components such as fin-to-tube heat- United States A 3,065,537 METHOD OF BRAZING Peter Patriarca, Knoxville, and Gerald M. Slaughter, Oak Ridge, Tenn., assignors to the United States of America, as represented by the United States Atomic Energy Commission N Drawing. Filed Mar. 27, 1958, Ser. No. 724,486 3 Claims. (Cl. 29-494) Our invention relates to an improved brazing composition which is suitable for use in a high temperature and corrosive environment, and to a method for preplacing said brazing composition. Molten metals and molten salts find considerable use in industry as coolants due to their high heat capacity, high heat transfer coefficients, and low vapor pressures at elevated temperatures. Increasing interest in the use of those materials is being shown in nuclear reactor de- For example, sodium and sodium-potassium alloys are being used in reactor heat-exchange circuits. However, these media are corrosive and present difficult problems in the fabrication of reactor components. In the fabrication of components for heat-exchange systems, items such as flanges, piping, and tanks may However, exchangers are not amenable to fabrication by welding. Efforts have been directed, therefore, towards fabrication of the more complex reactor components using brazing methods, and stringent requirements must be met. For example, many commercial brazing compositions and fluxes are eliminated from consideration because of high thermal neutron cross-sections. Others are severely attacked by corrosive media and nuclear radiations. Another serious difficulty is in the preplacement of the brazing alloy on the surfaces to be joined. Electrolytic preplacing of brazing materials is not generally satisfactory because of inability to deposit a uniform plate on a complex surface. One object of our invention, therefore, is to provide an improved method of joining metal surfaces for use in high-temperature, corrosive media. Another object is to provide an improved brazing composition and method of preplacing said brazing composition in the joining of metal surfaces, wherein the resulting joint is resistant to oxidative and corrosive attack. Another object is to provide a method in which said composition may be preplaced by a non-electrolytic method. Another object is to provide a brazing composition that displays adequate mechanical strength and heat transfer characteristics and does not have a high total neutron absorption cross-section. Still another object is to provide a method of performing said brazing in a relatively simple and rapid manner. Other objects and advantages of our invention will become apparent from the following detailed description and the claims appended hereto. In accordance with our present invention, metal surfaces may be joined by preplacing a brazing composition comprised of a major portion of nickel, a minor portion of phosphorous and a minor portion of chromium on said surfaces, and brazing together the resulting surfaces. The resulting alloy displays superior oxidation and corrosion resistance at elevated temperatures under prolonged exposure to molten metals; the metallurgical bond possesses adequate mechanical strength and heat transfer characteristics; and the alloy is satisfactory from the thermal neutron cross-section view-point. The brazing composition may be evenly preplaced on the surfaces to be joined, and during brazing displays excellent flow, wetability, and filleting properties. The composition of the brazing alloy may vary over a considerable range, although the major constituent must be nickel. We generally find that the nickel content of the alloy should be at least approximately 70% by weight, a particularly suitable alloy comprising, for example, approximately 5%15% phosphorous, by weight, and approximately 5%-15%, by weight, of chromium, the remainder being nickel. Our preferred brazing composition comprises approximately: nickel, 10% phosphorous and 10% chromium. This composition has a flow point in the region of l750l800 F. The brazing composition may be preplaced upon the metal surfaces to be joined in a variety of ways, and the exact method employed is not critical. One suitable method involves the chemical deposition of nickel and phosphorous and the subsequent electroplating of chromium on the deposited nickel and phosporous. The preferred method involves the application of a slurry (actually a paste) of the brazing composition upon the surfaces. In the first method, nickel and phosphorous are first deposited from an aqueous bath by reaction between nickel ions and an anionic complex of phosphorous and oxygen, such as phosphite and hypophosphite. In one such reaction, employing nickel chloride and sodium hypophosphite, the hypophosphite is converted to the phosphite with subsequent deposition of nickel, perhaps in accordance with the reaction: Since the deposited nickel is found to contain phosphorous, secondary reactions of unknown mechanisms probably play a significant role in the deposition. The bath should be operated at close to a neutral pH and at an elevated temperature, say about 95 C. The rate of deposition is slow, being only about 0.00035 inch per hour, and to get a plate of the desired thickness, approximately 0.001 inch, may take several hours. Upon completion of the nickel-phosphorous deposition, the resulting plated surfaces may be made the cathode of a conventional electrolytic cell, and a very thin layer of chromium electroplated thereon. For the preferred composition the chromium layer should be approximately one-tenth the thickness of the nickel-phosphorous layer. In the second, non-electrolytic preplacernent method, the brazing compositon may be formed by mixing a comminuted nickelous material selected from the group consisting of metallic nickel and nickel oxide with chromium powder and with a readily reduced phosphorous compound, forming either an aqueous or organic slurry of the resulting mixture, applying the slurry upon the surfaces to be joined and permitting the coating to dry before brazing. Especially suitable phosphorous compounds are alkali metal compounds of the phosphorous complexes previously referred to, the preferred compound being dibasic ammonium phosphate. The nickel and chromium powder size may vary considerably, but approximately 200-500 mesh size is preferred. In addition to water, a number of organic carriers, readily volatilized in the brazing operation, are available for the slurry formation. An acetone-cellulose mixture may be used, for example. Another very suitable binder is a methyl methacrylate composition sold under the trade name of Nicrobraz Cement by the Wall Colmonoy Corporation of Detroit, Michigan. While the ratio of the organic or water carrier to the nickel-phosphorous-chromium composition may satisfactorily vary, approximately 1 part carrier to 2 parts brazing composition, by volume, is quite suitable. The slurry may be easily applied on the surfaces to be joined by a number of different ways, one method being simply by brushing. In the brazing of finto-tube heat exchangers-and other like devices the heat surfaces to be joined, the brazingoperation may be. performed in a variety of conventionalways, and the exloHCl to maintain the pH. About two cc. hypophosphite solutionwas added every thirty minutes. The rateof depositionwas approximately 0.00035 inch p er hour, and j the deposition vvas continued until a thicknessof about hon-oxidizing atmosphere, preferably a hydrogen atmos- I y 0.020 of an inch was obtained.v Ontothis plateapproxia p phcrle, for about 10 minutes at atemperature above the i 'mately 0 023 16 1 12 chr mium. Was platedv byconvene 1- .tional electrodeposition means. Theresulting fins were duces an alloy of the brazing constituents't: Forthe phosphorous, 10. percent chromium the preferred brazing i to form a uniform fillet-along the tulllengthof the ioi and Chromels. I in greater detail. 'prisedot approximately phosphorous, 10% chro- :ratio of binder to brazing,compositionwas' approximate-ly' (one totwop After dry g, a brazingtcomposition layer 445. fins were then assembled with type 316; stainless steel punching 0.190 inch holes in 0.010qinchtype 304'stainless' I I 'steel sheet A lip was left toproyidealifin per inch r spacing. These fins wereplated'with ainick.elgxhosphorousw follows: 27 grams sodium carbonatewere dissolved in. nema exchanger may be assembled pieceaby-piece, and the a slurry applied to each surface during the assembling operation. Since. such an assembly usually has enough wascooled', and then filtered; Distilled water was added to the filtrate to make about liter and 30 grams of temporarily, the entirebrazingoperation, may be per formed upon completion ofthe assembly: v Once the Ni-Pecr mixture has been applied to the, resulting solutio-n-"'as filtered and enough distilled water was added tothe filtrate to'make 1 liter. The bath. was act method employed is not critical to the success of our invention. One satisfactory method comprises placing the nickel-phosphowns-chromium covered surfaces in a dry, melting point of the brazing compositiomand this pr0- I I then fittedionto 7 inch 0D. tubes having a Wall thick? preferred composition of 80 percent :nickc :0 percent phosphorous-chromium mixture meltsjt seeks thenearest 1 A. portion of jthefispecimenwas then evaluated forcorcapillary, and adequatcly fills unplatedmetal alongajoint 'rosior resistancein molten sodium at 1500 Ft The ex- I tentef; the attackl after 1.00flhours exposure. time was negligible; and'thefilletremained relatively soundwith ing a wide variety of metals with amelting point high'er only minor attack atthefillet sodium interface. Another 1 portion of the assembly was exposed to static air at 1500*" 'F. for 500 hours and virtually no oxidation was revealed, The above examples are merely illustrative and should Our brazing method maybe -satisfactorily used in join-3 if than the melting point of the brazing cornposition.fAll nickel and iron-basealloys may be joined using our method. Typical of the commercially available alloys which may be joined by our method areall the stainless not be construed as limiting the scope of our invention. steels and the alloys sold as the Nichrornesfl lncohels 3 It should be apparent that those. skilled in -metallurgy may. a i -make variations in our method without departingflfrom, ' Therefore; our invention. should be limited only: as indicatediby the appended The following examples willillustrate ourdnventionu thespirir of our invention claims.'."' y i 7 Having thusdescribed our invention, we" claim; y 1 Example] -A series of stainless steel radial fins was preparedhy punching 0.190 inoh ho les in 0.010 inch type 304 stainless steel sheet, leaving a lip to, provide-1a 15 fin per. inch: spacing. These, fins were brusheswwith a mixture of; .Nicrobraz Cement, nickel and chromium powder, and 40 dibasic ammonium phosphate. The mixture was I com consistingohiron and nickel, comprisingthe steps of mium and 80% nickel; by Weight and the volumetric 'xammoniumphosphami chrpm'iumzand'anickefifemus approximately 0.0015 inch; thick was measuredigThe tubing having a inchoutsidediameter andOfiZinch wall i thickness, thereby making a small heatg exchang bra mg operatic in a globar furnace having a dry: hydrogen atmospher Afteri removal-from the furnace itiwas noted that. ade ilfyemus" mamrialsivs the farm at a Slurry quate fiow,,wetting; and filleting had occurred. -Agporj l a i 'tion of a sectioned fin-to-tube ioint was then subjected to fan oxidationtest instatic air at 1500 for 100 hours. .55 A photomicrograph of the joint after exposure revealed that the extent of; corrosive attack was negligible. it l m m if composition, using the following procedhre.,The fins 3 1 15 11. 3 1 3,1953 2 were degreased, anodically etched in 37% I-ICl for ,one 651 2 71' 4 76 gb m i u 9, "1 r minute at room temperature using '6. volts, in order to 23515340 Amef 11 "195 clean thesurface. The resulting fins. werejgiven a nickel 1 32 3 2 7 HOrVitZ Fab strike for five minutes at 2 volts,-at a temperature of 2 8 4 r h h t ah 1958. -80.? 'F. and at pH 0.5ina bath containing one gallon 23545 scape-steal J; V -1 195g 7 acid. The fins were rinsed, washed in 37% 'HCl and K June 13 233L817- immediately placed m mekel hypophosphiebath n a e as A ,FOREIGNCPATENTS" distilled water and hc 'solution was boiledwiththeslow (487 263) addition of glycolic acid to .give pH 7.' The solution V nickel'chloride were dissolved therein. 10 cc. hypophosmechanical. strength to retain its poper configuration 5 phite solution, preparedby dissolving 100 grams sodium .hypophosphite ill 100 cc. distilled water, were added, the H ness'ot 0.020 inch; This assembly was brazed at 1800" I for '10 mimitesin. a dry hydrogen atmosphere, ,and temperature is approximately 1800"?) I Asdthe' nickek 20 'exce11entj floW,Wetability and filletin g wereflobserved, l improved method of brazingstruetural metal". parts,- the base metalofwhichis selected from thegroup disposing thesurfaces of said parts to be brazed in close-f. ly-spaeedrelationship, placing at the resulting gap; area ,i 3 between the surfaces at mixture/{consisting of ,dibasici. terial'selectedtrom thegroup. consisting of nickel and a nickel. oxide in a proportion yielding an alloy comprisi ing] approximately nickel, l0%phosphorus and 10% lchromiunnthen heating said metal partsto a tempjra ture of at least 1800 maintaining said metal parts in a" reducin'g'atmosphere of dry hydrogen-during" 'basic"ammoniumphosphate; .ehromium; and thenichel BJThemethod'pfclaim zlwherein' the slurry is f ormed b'y-g mixing an; organic binder in approximate volumetric C roportions: of onefpa'rt binder totwo parts nickebphosa ntrsssess' oasein them areas patent Q v I W 3 311935 ew, 1941: Fen-l5, 19 49;

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