Volume 06 - Welding, Brazing and Soldering Part 15 ppsx

4 HEAT-EXCHANGER ASSEMBLY IN BRAZING FIXTURE AND DETAIL OF JOINTS BRAZED WITH GOLD BRAZING FILLER METAL Silver and copper brazing filler metals could not be used, because of their inco

FURNACE BRAZING IN DRY HYDROGEN

HYDROGEN DEW POINTS, °C (°F) -75 (-100)(B); -60 (-70)(C)

HYDROGEN FLOW RATE, M3/H (FT3/H) 11 (400)

CONVEYOR TRAVEL SPEED, M/H (FT/H) 9 (30)

TIME AT BRAZING TEMPERATURE, MIN(E) 5

(A) ELECTRICITY HEATED (60 KW), CONSTRUCTED WITH HEATING CHAMBER HIGHER THAN ENTRANCE AND DISCHARGE ENDS

(B) INCOMING

(C) EXHAUST

(D) IN FORM OF 1 MM (0.040 IN.) DIAMETER WIRE-RING PREFORMS

FIG 3 RETAINER ASSEMBLY FURNACE BRAZED WITH BAG-13 FILLER METAL

The components were vapor degreased, assembled (keeping their outside diameters concentric), and spot welded (to make them self-jigging) at four locations 90° apart (Fig 3) A ring of brazing filler metal wire in a 1 mm (0.040 in.) diameter was preplaced at the joint, and assemblies were loaded two-across on the mesh belt of a conveyor-type furnace The heating chamber of the furnace was elevated from the entrance and discharge level to conserve the lighter-than-air hydrogen and to prevent oxygen in the atmosphere from mixing with the hydrogen, which could either raise the dew point

or cause an explosion

Quality standards for brazed assemblies, which were checked by 100% visual inspection, required that the joint exhibit full braze penetration (360° fillets on both sides of the joint) and be pressure-tight

Example 4: Use of a Gold Brazing Filler Metal for Brazing in an Aerospace Heat Exchanger

In the fabrication of a high-reliability heat exchanger for manned space flights, 2552 fins of 0.10 mm (0.004 in.) thick type 347 stainless steel were brazed to 0.64 mm (0.025 in.) thick type 347 stainless steel side panels, as shown in Fig 4 The 5104 fin-to-panel joints had to be strong and corrosion resistant

FURNACE BRAZING IN DRY HYDROGEN

HYDROGEN DEW POINT (MAX), °C (°F) -60 (-80)(B)

PROCESSING TIME PER ASSEMBLY

ASSEMBLE COMPONENTS IN FIXTURE, H 4

TIME AT BRAZING TEMPERATURE, MIN 7-10

(A) ELECTRICALLY HEATED, WITH 36 IN DIAM RETORT WITH WATER-COOLED RUBBER SEALS

(B) HYDROGEN WAS PURCHASED AS CYLINDER HYDROGEN, THEN PASSED THROUGH AN ELECTROLYTIC DRIER

(C) NUMBER OF VOLUME CHANGES IN RETORT

(D) 200-MESH POWDER SUSPENDED IN AN ORGANIC BINDER

(E) INCLUDING COOLING TO 150 °C (300 °F) IN RETORT, WHICH WAS PURGED WITH ARGON BEFORE BEING OPENED

FIG 4 HEAT-EXCHANGER ASSEMBLY IN BRAZING FIXTURE AND DETAIL OF JOINTS BRAZED WITH GOLD

BRAZING FILLER METAL

Silver and copper brazing filler metals could not be used, because of their incompatibility with sulfur-bearing rocket fuel The BNi series of nickel brazing filler metals had the necessary compatibility, but made nonductile joints that were unreliable under tension peel stress Therefore, gold brazing filler metals were used The necessary brazing characteristics for the fin-to-panel joints were present in BAu-4 (nominal composition, 82Au-18Ni) The strength and ductility of the resulting brazed joints justified the high cost of this particular brazing filler metal

The fins and side panels were cleaned by vapor degreasing The side panels were then pickled, rinsed in clean water, and dried The brazing filler metal was deposited on the panels in the form of a powder suspended in an organic binder Multiple lap joints were made between the flat-crown-hairpin ends of the fins and the flat side panels The assembly was placed in a fixture (Fig 4), and then the entire assembly and fixture were placed in the retort of a bell-type furnace and sealed The sealed retort was purged with a volume of hydrogen that was equivalent to five times that of the retort The retort was then heated to a brazing temperature of 1020 °C (1860 °F) and held for 7 to 10 min The joint gaps at brazing temperature ranged from 0.000 to 0.254 mm (0.010 in.) After brazing, the retort was purged with argon while being cooled to 150 °C (300 °F) and was not opened until after purging and cooling

The joints brazed by this procedure were the final brazed joints in the assembly In a prior brazing operation, tubes had been joined to the fins (Fig 4) by brazing at 1080 °C (1970 °F) using a higher-melting-point gold brazing filler metal of 70Au-22Ni-8Pd

The completed assemblies were visually inspected and pressure tested at pressures that far exceeded those of the intended service environment: 1.86 MPa (270 psi) on the outside of the tubes and 11.8 MPa (1710 psi) on the inside Acceptance pressure test values were 3.7 MPa (540 psi) on the outside and 15.7 MPa (2275 psi) on the inside of the tubes Selected brazed assemblies were tested to bursting These assemblies were required to withstand at least three times the service pressures before bursting The assemblies that were brazed with gold brazing filler metals passed all tests and had three times the bursting strength of the assemblies brazed with undiffused nickel brazing filler metal

Example 5: Combination Brazing and Solution Heat Treatment of an Assembly of Three Types of Stainless Steel

Three different stainless steels were selected to make the cover for a hermetically sealed switch The switching action had

to be transmitted through the cover without breaking the seal This was accomplished by providing a diaphragm through which a shoulder pin was inserted, as shown in Fig 5 The switch was actuated by depressing the pin, which in turn deflected the diaphragm The pin (type 303), the diaphragm (PH 15-7 Mo), and the cover (type 305) were assembled as shown in Fig 3, and then brazed using a silver-base filler metal in a furnace with dry hydrogen

FURNACE BRAZING IN DRY HYDROGEN

BRAZING TEMPERATURE, °C (°F) 955 ± 8 (1750 - 15)

TIME AT BRAZING TEMPERATURE, MIN 10

TIME IN FIRST COOLING ZONE(D), MIN 5

TIME IN FINAL COOLING ZONE(D), MIN 5

(A) THREE-ZONE FURNACE WITH A HIGH-HEAT ZONE 125 MM (5 IN.) IN DIAMETER BY 460

(E) WATER-COOLED ZONE, IN WHICH ASSEMBLY WAS COOLED TO ROOM TEMPERATURE

TO COMPLETE HEAT TREATMENT OF THE PH 15-7 MO DIAPHRAGM, ASSEMBLY WAS

FIG 5 THREE-STEEL SWITCH-COVER ASSEMBLY THAT UTILIZED BRAZING TEMPERATURE AS PART OF

SOLUTION HEAT TREATMENT

Silver brazing filler metal BAg-19 was chosen because it flowed at a temperature that coincided with the solution treating temperature for the PH 15-7 Mo diaphragm (950 °C, or 1750 °F) A holding fixture was needed to keep the PH 15-7 Mo diaphragm in position during the brazing cycle To avoid carburizing the diaphragm, the material selected for the fixture was stainless steel, rather than graphite The furnace was a batch-type tube furnace with a 120 mm (5 in.) diameter high-heat zone that was 460 mm (18 in.) long The moisture content of the hydrogen atmosphere was carefully controlled, because the lithium-containing filler metal flowed too freely when the atmosphere was too dry, and it did not seal the joints

heat-After being cleaned, the components were assembled with two preform rings of BAg-19 wire Tweezers were used to avoid contaminating the cleaned surfaces Each assembly was held in a stainless steel fixture, which in turn was placed on

a stainless steel furnace sled The sled was pushed into the high-heat zone of the furnace and held at 950 °C (1750 °F) for

10 min, before being pulled into an intermediate cooling zone at 1830 °C (1000 °F) and held for 5 min Finally, it was pulled to the water-cooled zone, where it cooled to room temperature The brazing of the two joints and the solution treating of the PH 15-7 Mo diaphragm were accomplished simultaneously at the brazing temperature of 950 °C (1750 °F)

To complete the heat-treating process, the assembly was cooled to -70 °C (-100 °F), held for 8 h, and then aged at 510 °C (950 °F) for 1 h

A 25 mm (1 in.) square piece of PH 15-7 Mo was processed with each batch of cover assemblies and used as a hardness test specimen to verify that the diaphragms had been correctly heat treated Brazed assemblies were inspected by the brazing operator The joints were required to be fully sealed and to not have any voids The pins were required to be perpendicular within 4° Perpendicularity was measured on a comparator Randomly selected samples were given a push-out test, in which joints had to withstand a push of 60 N (14 lbf) All assemblies were given 100% visual inspection at high magnification

Example 6: Simultaneous Brazing of a Heat-Exchanger Assembly

An air-to-air heat-exchanger assembly, shown in Fig 6, consisted of 185 thin-walled (0.20 mm or 0.008 in.) tubes and two 1.6 mm ( 1

16 in.) thick headers All components were made of type 347 stainless steel The tubes were assembled with the headers by flaring the tube ends to lock them in place and provide metal-to-metal contact for the brazing filler metal All 370 joints were brazed during a single pass through a continuous conveyor-type electric furnace

FURNACE BRAZING IN DRY HYDROGEN

FURNACE TEMPERATURE, °C (°F) 1120 ± 5 (2050 ± 10)

BRAZING TEMPERATURE, °C (°F) 1065 ± 5 (1950 ± 10)

HYDROGEN DEW POINTS, °C (°F) -75 (-100)(C); -60 (-70)(D)

HYDROGEN FLOW RATE, M3/H (FT3/H) 17 (600)

CONVEYOR TRAVEL SPEED, M3/H (FT3/H) 9 (30)

(A) ELECTRICALLY HEATED (60 KW), CONSTRUCTED WITH HEATING CHAMBER HIGHER THAN ENTRANCE AND DISCHARGE ENDS

(B) HOLDING FIXTURE FABRICATED FROM 3.2 MM (1

8 IN.) THICK SHEET

Although there are nickel brazing filler metals that contain silicon in place of boron, they generally require much higher brazing temperatures, which can result in grain coarsening in the base metal Therefore, after numerous tests, BNi-3 filler metal was selected on the basis of its brazing temperature and excellent fluidity The problem of applying the correct amount of filler metal to avoid erosion was solved by preparing a slurry from an accurately controlled mixture of filler-metal powder, acrylic-resin binder, and xylene thinner

Before the filler metal was applied, the heat-exchanger assembly was cleaned ultrasonically in acetone and carefully weighed to determine the proportionate weight of filler metal that would be required Half of the total amount of filler metal was then applied to one end of the assembly by spraying The assembly was reweighed, and the remaining filler metal was applied to the opposite end At all stages of processing, the assembly was handled by operators wearing clean, lint-free cotton gloves

The assembly was placed on a holding fixture made of stainless steel sheet, on which a stop-off compound had been applied to prevent the assembly from brazing to the fixture if the brazing filler metal flowed excessively Assemblies were placed 300 mm (12 in.) apart on the conveyor, as they traveled through the furnace at 9 m/h (30 ft/h) under the protection

of dry hydrogen

After brazing, each side was subjected to 100% visual inspection to detect the presence of fillets, and the assemblies were pressure tested in accordance with customer requirements Because of the thin-walled (0.20 mm, or 0.008 in.) tubing, this assembly was brazed more consistently and at a lower cost than could have been achieved by other joining processes

Example 7: Combined Brazing and Hardening of a Shaft Assembly

The shaft assembly shown in Fig 7 consists of three bars or screw machine products (a shaft, a drive pin, and a guide pin) and two stampings (upper and lower mounting plates), all made from type 410 stainless steel and furnace brazed together using four joints By brazing with copper filler metal at 1120 °C (2050 °F), it was possible to austenitize and harden the assembly to the required minimum hardness value of 40 HRC during the brazing and cooling operations, thereby avoiding separate hardening operations after brazing

FURNACE BRAZING IN DRY HYDROGEN

HYDROGEN FLOW RATE, M3/H (FT3/H) 11 (400)

CONVEYOR TRAVEL SPEED, M/H (FT/H) 6 (20)

TIME AT BRAZING TEMPERATURE (F), MIN 8

(A) ELECTRICALLY HEATED (60 KW), CONSTRUCTED WITH HEATING CHAMBER HIGHER THAN ENTRANCE AND DISCHARGE ENDS

(B) COMPONENTS WERE STAKED, FOR SELF-FIXTURING ASSEMBLIES, SUPPORTED BY

CERAMIC SPACERS TO KEEP SHAFT END UP, WERE BRAZED ON TRAYS

(C) INCOMING

(D) EXHAUST

ONE END OF DRIVE PIN, BOTH ENDS OF GUIDE PIN

(F) ASSEMBLIES WERE IN HIGH HEAT ZONE FOR ABOUT 10 MIN

FIG 7 FOUR-JOINT SHAFT ASSEMBLY THAT WAS SIMULTANEOUSLY FURNACE BRAZED AND HEATED FOR

HARDENING

Because the joints were all relatively short, an interference fit of 0.000 to 0.025 mm (0.001 in.) was satisfactory Typically, a clearance fit between mating parts is required with longer joints in stainless steel The automatic staking of components was used to make the assembly self-fixturing

As shown in Fig 7, a full ring of 0.50 mm (0.020 in.) diameter BCu-1 copper wire was preplaced around the 13 mm (1

2

in.) diameter shaft to braze the shaft to the upper and lower mounting plates A small amount of BCu-2 copper paste was applied at one end of the drive pin to braze it to the two mounting plates Because of the separation between the two plates on the guide-pin side, a small amount of BCu-2 copper paste was manually applied on each end of the guide pin The assemblies were placed in brazing trays, with the shaft in a vertical position, and were supported in this position by ceramic spacers

The brazing trays were then placed on the mesh belt of a continuous-type conveyor furnace containing a dry hydrogen atmosphere They were transported up an incline to the horizontal preheat and high-heat chambers at a speed of 6 m/h (20 ft/h) Because the assemblies were small, they became heated to the brazing temperature in about 2 min After 8 min at the brazing temperature, the assemblies were conveyed into water-jacketed cooling chambers, where they cooled rapidly

in the hydrogen atmosphere to room temperature Brazed assemblies that were bright and oxidation-free emerged from the exit end of the furnace

The brazed assemblies were 100% visually inspected for complete joint coverage Hardness tests on a sampling basis were used to determine whether the assemblies had responded properly to hardening Tempering to the desired final hardness followed the simultaneous brazing and hardening operation

Example 8: Medical Device Brazed, Rather than Welded, in Hydrogen

Because of the need for strong, corrosion-resistant, and leak-proof joints in a stainless steel blood-cell washer (Fig 8), the process that was selected was hydrogen furnace brazing with BNi-7 brazing filler metal The devices are used to expedite and standardize cell-washing procedures in blood banks and hematology laboratories Therefore, neither voids nor cracks could be tolerated, because the possibility of breeding bacteria in the devices had to be avoided

FIG 8 MANIFOLD AND CANNULAS TUBE ASSEMBLY OF A BLOOD-CELL WASHER THAT WAS HYDROGEN

FURNACE BRAZED WITH BNI-7 BRAZING FILLER METAL AT 1040 °C (1900 °F) COURTESY OF WALL

COLMONOY CORPORATION

Brazing, rather than welding, was chosen to join the manifold and the delicate tube parts of the washer, a rake-like component with twelve prong-like cannulas tubes that extend approximately 150 mm (6 in.) from a cylindrical manifold

The manifold assembly was brazed using BNi-7 nickel brazing filler metal in a hydrogen atmosphere at 1040 °C (1900

°F) After assembling the tubes to the manifold, the brazing filler metal was applied to the joints and the assembly was placed in the furnace Components were wired to stainless steel fixtures to maintain uniform tube spacing

The brazing permitted the filler metal to flow completely around the thin-walled tubes, which was not possible with welding, leaving smooth fillets without voids that could trap harmful bacterial particles Additional advantages of brazing were that it: minimized the amount of assembly distortion, eliminated flux hazards, simplified inspection procedures, and prevented the oxidation

Brazing of Stainless Steels

Revised by Matthew J Lucas, Jr., General Electric Aircraft Engines

Furnace Brazing in Dissociated Ammonia

When ammonia is free of moisture and is 100% dissociated, it becomes a suitable atmosphere for the brazing of stainless steel using selected brazing filler metals without requiring a flux Although dissociated ammonia is strongly reducing, it is less so than pure, dry hydrogen Consequently, it will promote wetting action by reducing chromium oxide on the surface

of stainless steel, but it may not be sufficiently reducing to promote the flow of some brazing filler metals, such as copper oxide powders Because of its high (75%) hydrogen content, dissociated ammonia forms explosive mixtures with air and must be handled with the same precautions as those required for the handling of hydrogen

A dissociated-ammonia atmosphere is prepared by heating anhydrous liquid ammonia in the presence of an iron or nickel catalyst The decomposition of ammonia to form hydrogen and nitrogen begins at 315 °C (600 °F), and the rate of decomposition increases with temperature Unless the atmosphere used in brazing stainless steel is completely decomposed, that is, 100% dissociated, even minute amounts of raw ammonia (NH3) in the atmosphere will cause the nitriding of stainless steel, especially steels containing little or no nickel In addition, because of the solubility of ammonia

in water, the atmosphere that comes from the dissociator must be extremely dry (preferably having a dew point of -60 °C,

or -80 °F, or lower) To ensure a very low dew point, the atmosphere that comes from the dissociator is commonly processed by being passed through a molecular-sieve dryer To avoid the oxidation of base metal and brazing filler metal, the atmosphere must be kept pure and dry while it is inside the furnace In the following examples of production practices, dissociated ammonia was used successfully in the furnace brazing of austenitic and precipitation-hardening (PH) stainless steels

Example 9: Brazing in Dissociated Ammonia Without Flux

The pressure gage subassembly shown in Fig 9 comprises five diaphragms of 17-7 PH stainless steel, a deep-drawn cup and a top fitting of type 304 stainless steel, and a connector of copper alloy 145 (tellurium-bearing copper) Originally, these subassemblies were furnace brazed with a silver brazing filler metal that required a flux Because applying flux and assembling the fluxed components with gloved hands was time consuming, the decision was made to change to fluxless brazing in an atmosphere of dissociated ammonia Although this necessitated using a more-expensive brazing filler metal (BAg-19), the higher cost was offset by the greater productivity of each operator In addition, subassemblies brazed with BAg-19 in dissociated ammonia exhibited fewer leaks and had improved corrosion resistance and a better appearance than those brazed with the original filler metal and a flux

FURNACE BRAZING IN DISSOCIATED AMMONIA

PRECIPITATION-HARDENING TEMPERATURE(G), °C (°F) 510 (950)

(A) ELECTRICALLY HEATED, WITH ELEVATED HIGH-BEAT ZONE

(B) FOR BRAZING THE SUBASSEMBLY AND SIMULTANEOUSLY SOLUTION HEAT TREATING THE 17-7 PH DIAPHRAGMS

(C) ACHIEVED BY RUNNING THE DISSOCIATED AMMONIA THROUGH A MOLECULAR-SIEVE DRYER AFTER CRACKING

(D) CROSS-SECTIONAL DIMENSIONS (AND PRODUCT FORMS) OF PREPLACED RINGS WERE: FOR JOINT BETWEEN DIAPHRAGM AND TOP FITTING (DETAIL A), 1.3 MM (0.050 IN.) WIDE

BY O.10 TO 0.13 MM (0.004 TO 0.005 IN.) THICK STAMPING; FOR OUTSIDE JOINTS BETWEEN DIAPHRAGM SEGMENTS (DETAIL D) AND JOINT BETWEEN DIAPHRAGM AND CUP (DETAIL B), THICK (RIBBON); FOR INSIDE JOINTS BETWEEN DIAPHRAGM SEGMENTS (DETAIL C), 0.76 MM (0.030 IN.) WIDE BY 0.13 MM (0.005 IN.) THICK (RIBBON); AND FOR JOINT BETWEEN CUP AND CONNECTOR (DETAIL E), 1.52 BY 0.254 MM (0.060 BY 0.010 IN.) (WIRE)

(E) USE OF BAG-19 ELIMINATED THE NEED FOR FLUX, WHICH HAD BEEN REQUIRED WITH THE SILVER FILLER METAL ORIGINALLY USED

(F) TO COOL RAPIDLY FROM 980 °C (1800 °F) AND ENSURE SOLUTION TREATMENT OF THE 17-7 PH DIAPHRAGMS

(G) IN DRY DISSOCIATED AMMONIA, AFTER SUBASSEMBLY HAD BEEN COOLED TO -40 °C

(-FIG 9 PRESSURE-GAGE SUBASSEMBLY THAT COMBINED FURNACE BRAZING WITH SOLUTION HEAT

TREATMENT

Prior to brazing, the deep-drawn type 304 cups were fully annealed at 1090 °C (2000 °F) Annealing served to avoid the erosive penetration of brazing filler metal in zones of high residual stress All components were chemically cleaned and then assembled by hand, along with seven preplaced rings of brazing filler metal The assemblies were placed on holding fixtures, which were loaded on the belt of a conveyor furnace heated to 980 °C (1800 °F) The cooling chamber of the furnace was cooled to below 15 °C (60 °F) to ensure rapid cooling of the 17-7 PH diaphragms from the solution-treating temperature, thereby combining solution treating with the brazing operation

After brazing, the assemblies were cooled to -40 °C (-40 °F), dried, and then heated to 510 °C (950 °F) in dry, dissociated ammonia to harden the diaphragms to 44 to 48 HRC Brazed assemblies were pressure tested in a bellows halogen leak detector by applying Freon at 520 kPa (75 psi) and then adding compressed air to bring the total pressure up to 2.1 MPa (300 psi) Leakage of Freon in the gas-air mixture would have been detected by the halogen leak detector The requirement was for no leakage at the most sensitive setting of the leak detector

The rejection rate for leakage, based on the 750,000 bellows that were produced, dropped from 2.8%, with the original silver brazing filler metal, to 1.0%, with the BAg-19 brazing filler metal Field corrosion returns dropped 96% By eliminating the stains caused by flux, it was no longer necessary to paint the assemblies

Brazing of Stainless Steels

Revised by Matthew J Lucas, Jr., General Electric Aircraft Engines

Furnace Brazing in Argon

Argon is occasionally used as a furnace atmosphere when brazing stainless steels to other stainless steels or to reactive metals such as titanium (Example 10) Argon has the advantage of being chemically inert in relation to all metals Therefore, it is a useful protective atmosphere for metals that can combine with or absorb reactive atmospheres such as hydrogen Because an argon atmosphere has the disadvantage of being unable to reduce oxides, the surface of stainless steel components must be exceptionally clean and oxide-free when brazed in argon

Example 10: Brazing in an Argon Atmosphere

A manufacturer of jet engines designed a gear-reduction box of commercially pure titanium This complicated fabrication was made from assemblies of stampings and machined forgings, most of which were joined by gas-tungsten arc welding (GTAW) in argon-filled welding chambers However, brazing was more appropriate for the joining of some assemblies

A typical assembly that was furnace brazed in argon is shown in Fig 10 This assembly consisted of a machined forging

of commercially pure titanium (per Aerospace Material Specification 4921) and a length of seamless type 347 stainless steel tubing that was flared or expanded for a distance of approximately 8 mm ( 5

16 in.) to accept the titanium forging

FURNACE BRAZING IN ARGON

TEMPERATURE

(A) GAS FIRED, 1.8 M (72 IN.) DIAMETER, 1.8 M (72 IN.) DEEP

(B) 610 MM (24 IN.) DIAMETER AND LENGTH

(C) HOLDING FIXTURE, TO KEEP ASSEMBLY UPRIGHT (FORGING DOWN) DURING BRAZING (D) INCOMING

(E) EXHAUST

(F) 1 MM (0.040 IN.) DIAMETER WIRE

FIG 10 STAINLESS STEEL AND TITANIUM ASSEMBLY THAT WAS FURNACE BRAZED IN AN ARGON

ATMOSPHERE

The outside diameter of the titanium forging was held to 12.70 mm (0.500 in.), +0.000 and -0.025 mm (-0.001 in.) The inside diameter of the stainless steel tube was held to 12.73 mm (0.501 in.), +0.025 mm (+0.001 in.) and -0.000 mm or in This allowed for a diametral clearance of 0.025 to 0.076 (0.001 to 0.003 in.) between components at room temperature From 0 to 900 °C (32 to 1650 °F), the mean coefficient of thermal expansion (CTE) of commercially pure titanium is 10.3-6/K From 0 to 870 °C (32 to 1600 °F), the mean CTE of type 347 stainless steel is 20 × 10-6/K Calculation of the expansion that would occur when both components were heated to 900 °C (1650 °F) indicated a 0.102 mm (0.004 in.) diametral clearance between the titanium and the stainless steel Adding the diametral clearance at room temperature (0.025 to 0.076 mm, or 0.001 to 0.003 in.) to the 0.102 mm (0.004 in.) clearance gave a total diametral clearance at brazing temperature of 0.127 to 0.178 mm (0.005 to 0.007 in.), which is within a range that will result in a successful joint

The selected brazing filler metal was the BAg-19 silver, because it has high fluidity in an argon atmosphere and a brazing temperature that is lower than that of pure silver Most alloy elements in silver brazing filler metals form brittle intermetallic compounds with titanium, which result in unreliable joints With the exception of a minute amount of lithium, the only alloying element contained in BAg-19 is 7.5% Cu By limiting the time at brazing temperature, sound ductile joints were made, and the formation of the titanium copper intermetallic phase was minimized

Prior to assembly, the titanium forging was degreased and cleaned in a solution that contained 40% nitric acid plus 2% hydrofluoric acid The stainless steel tubing and the brazing filler metal (preformed rings of 0.102 mm, or 0.004 in., diameter BAg-19 wire) were cleaned by washing in acetone Operators wore clean, lint-free, white cotton gloves for all subsequent handling of the components during assembly, and kept them on until after brazing was completed

The titanium forging was inserted into the expanded end of the stainless steel tube until it was completely seated A brazing filler metal ring was placed around the outside diameter of the titanium tube at the joint intersection The

assembly was placed upright (forging down) in a titanium sheet-metal-holding fixture and was loaded into an Inconel retort The retort was designed for displacement purging with an inlet and exit manifold for the argon atmosphere After loading, the retort cover was seal welded to its base, using the GTAW process

The retort was purged for 30 min with pure, dry argon at 4.2 m3/h (150 ft3/h) and then placed in a gas-fired pit-type furnace The retort was heated to 315 °C (600 °F), and it was held at that temperature for an additional 30 min, or until the dew point of the exiting argon was -60 °C (-70 °F), as recorded on an electrolytic water analyzer The furnace temperature was then raised until the assembly temperature reached 900 °C (1650 °F), as indicated by a Chromel-Alumel thermocouple attached to the titanium holding fixture within the retort As soon as this temperature was reached, the retort was removed from the furnace and fan-cooled to room temperature

The retort cover was opened by grinding away the seal weld, and the assemblies were removed The titanium and stainless steel components emerged bright and clean, with evidence of excellent brazing filler metal flow Radiographic inspection showed over 95% joint coverage All joints were visually inspected on both sides

Brazing of Stainless Steels

Revised by Matthew J Lucas, Jr., General Electric Aircraft Engines

Furnace Brazing in an Air Atmosphere

The principal advantages of furnace brazing are high production rates and a means for using controlled protective atmospheres at controlled dew points, which often precludes the use of a flux to obtain satisfactory wetting action In most furnace-brazing applications, both of these advantages are exploited Occasionally, however, furnace brazing is selected solely on the basis of production rate, and brazing is performed without a protective atmosphere, but with a suitable flux Under these conditions, the lower-melting-point brazing filler metals are generally selected, as in the following application

Example 11: Substitution of Furnace Brazing in Air Atmosphere for Torch Brazing

The gas-valve bobbin assembly shown in Fig 11 was satisfactorily brazed by both the torch and furnace brazing processes The choice of process primarily depended on the required production rate Cost data proved that furnace brazing increased the production rate per hour, reduced the direct labor rate per hour, and reduced the direct labor cost per assembly

FURNACE BRAZING IN AIR ATMOSPHERE

(A) AIR ATMOSPHERE

(B) PREFORMED RINGS, PREPLACED AS SHOWN IN ILLUSTRATION, 0.8 MM (0.031 IN.)

FIG 11 STAINLESS STEEL AND COPPER GAS-VALVE BOBBIN THAT WAS FURNACE BRAZED IN AN AIR

ATMOSPHERE

As Fig 11 shows, the bobbin assembly consisted of four parts: a screw made of type 446 stainless steel, a base made of type 303 stainless steel, a tube made of a copper alloy closely related to nickel-silver (74Cu-22Ni-4Zn), and a plug made

of copper alloy 187 (99Cu-1Pb), which held the screw in place and blocked gas passage through the tube

All components were thoroughly vapor degreased before brazing They were assembled with two preformed rings of 0.787 mm (0.031 in.) diameter BAg-3 brazing filler metal wire The diametral clearance on the joints was 0.076 to 0.127

mm (0.003 to 0.005 in.) One preform, with a 9.5 mm (3

8 in.) internal diameter, was placed over the neck of the plug The other, with a 12.7 mm (0.5 in.) internal diameter, was placed over the tube adjacent to the base joint The joint areas were coated with type FB3-A brazing flux, and the assemblies were brazed in a continuous-belt conveyor furnace Brazing filler metal BAg-3 was chosen, in preference to BAg-1 or BAg-1a, in order to avoid the risk of interface corrosion

Brazing of Stainless Steels

Revised by Matthew J Lucas, Jr., General Electric Aircraft Engines

Furnace Brazing in a Vacuum Atmosphere

The majority of vacuum brazing is performed in two types of equipment utilizing either hot- or cold-walled furnaces The hot-walled structure utilizes a retort that is evacuated and placed into a furnace, which provides the heat source The retort can be single-pumped, providing a vacuum to temperatures up to 980 °C (1800 °F), or double-pumped for temperatures above 980 °C (1800 °F) to prevent the retort from collapsing Limitations of the hot-walled furnace (retort) include longer cycle times, because the retort is heated externally; a 1200 °C (2200 °F) temperature limit; and slower cooling rates Some advantages of hot-walled furnaces are lower initial capital expenditures, reduced contamination from the retort, and easy upkeep and maintenance

The cold-walled vacuum furnace is typically designed with a water-cooled outer jacket that is protected by radiation shielding adjacent to the inner wall The heating elements are exposed directly to the workload A braze cycle may have temperatures that exceed 2200 °C (4000 °F), depending on the heating element material and the load-support structure Heating and cooling rates for cold-walled vacuum furnaces are substantially less than they are for the hot-walled vacuum retort Higher braze temperatures, in excess of 1260 °C (2300 °F), are obtainable in vacuum furnaces and are frequently employed

Effect of Filler-Metal Composition Vacuum brazing of many structural configurations made of austenitic stainless steels offers excellent heat and corrosion resistance for high-temperature service applications Brazing filler metals, such

as gold, gold-palladium, and nickel, offer greater high-temperature strength and oxidation resistance Problems can occur when brazing the 300 series of stainless steels, because of the carbide precipitation and loss of corrosion resistance that results when brazing in the temperature range from 480 to 815 °C (900 to 1500 °F) Brazing at temperatures in excess of

815 °C (1500 °F), using brazing filler metals with melting points that are higher than this temperature, followed by rapid cooling, will reduce carbide precipitation and improve corrosion properties

Occasionally, wetting does not occur with a particular lot of stainless steel Generally, alloying elements such as titanium and aluminum contribute to poor wettability

Martensitic stainless steels, such as the 400 series, can be brazed successfully in vacuum The use of a suitable brazing filler metal can result in austenitizing and brazing at the same time, followed by rapid cooling to harden the stainless steel

Care must be exercised when selecting brazing filler metals for use in vacuum Silver brazing filler metals, such as

BAg-1, BAg-1a, and BAg-3, contain alloying elements of zinc and cadmium, which have very high vapor pressures These elements vaporize if the furnace pressure is too low or the brazing temperature is excessive, or if a combination of these conditions exists Copper and silver also vaporize under low-pressure conditions at higher brazing temperatures Therefore, if brazing alloys containing copper or silver are to be used in vacuum, the furnace chamber must be backfilled with an appropriate atmosphere to 300 to 500 μm (12 to 20 mils) until melting occurs to prevent the loss of these elements Caution must be exercised with the furnace exit gases because these gases are extremely toxic Two materials that should never be put in a vacuum furnace under any condition are zinc and cadmium It is recommended that specific alloys (for example, BAg-8 or BAg-13a) or other alloys free of zinc or cadmium be used when brazing in a vacuum furnace A partial pressure may still be required for many of the alloys to prevent the vaporization of the silver

Example 12: Vacuum Brazing to Improve Beverage Can Filling Nozzles

Because of a continuous problem in obtaining uniform and void-free fillets when using silver brazing filler metals and induction brazing methods, a switch was made to vacuum furnace brazing using a nickel brazing filler metal The beverage filling nozzle shown in Fig 12 is an assembly consisting of 16 short tubes brazed to a cast body section that connects to the supply piping The tubes are pressed into the holes that are drilled at angles through the body, and then brazed to form a tight joint Both the body and tubes are made from type 304 stainless steel

FIG 12 STAINLESS STEEL TYPE 304 BEVERAGE CAN FILLING NOZZLE TUBES ARE VACUUM BRAZED WITH A

NICKEL BRAZING FILLER METAL AT 1120 °C (2050 °F) LEFT, LOCATION WHERE PASTE ALLOY IS PLACED AROUND TUBE RIGHT, COMPLETED NOZZLE, SHOWING SMOOTH, VOID-FREE FILLETS COURTESY OF WALL COLMONOY CORPORATION

The nozzles require a smooth surface that will not harbor bacteria This means that the brazed joints must have uniform, smooth fillets and full-length void-free filler metal penetration In addition, the entire assembly must be resistant to chemical attack by the beverages being handled, by steam, or by cleaning compounds

The prior method of brazing using silver-base filler metals was not very successful in obtaining void-free joints or consistent results In addition, many of the beverages being handled did attack the silver brazing filler metal, causing corrosion and discoloration in the joint area

The nickel brazing filler metal selected for this application was BNi-9 (81.5Ni-15Cr-3.5B), because of its excellent capillary flow characteristics, low base-metal erosion, and self-fluxing properties In addition, the alloy would be unaffected by subsequent processing operations, which include passivation and electropolishing

The stainless parts were degreased and a small bead of the nickel brazing filler metal was placed around each tube The brazing was done in a vacuum furnace at 1135 °C (2075 °F), where the part was heated and cooled at a controlled rate to ensure thermal stability Careful control of the brazing cycle is the key element in obtaining the uniform fillets and void-free joints required for the nozzles

Example 13: Cryogenic Valve Nickel Brazed in Vacuum

Cryogenic valves that are used to handle liquid nitrogen and liquid hydrogen require construction that provides high strength and impact resistance at extremely low temperatures, as well as light weight and good corrosion resistance (Fig 13) Valves that are part of the propellant loading systems for ballistic missiles were used at temperatures of -250 °C (-

425 °F), and at pressures of nearly 41.4 MPa (6 ksi) The body, flanges, seat, and bonnet were brazed with BNi-2 brazing filler metal in a vacuum furnace

FIG 13 CRYOGENIC VALVE THAT WAS VACUUM BRAZED WITH BNI-2 BRAZING FILLER METAL ALL JOINTS ON

THE BODY, FLANGES, SEAT, AND BONNETS WERE BRAZED SIMULTANEOUSLY COURTESY OF WALL

COLMONOY CORPORATION

The valve seat and disc were cast from 17-7 or 17-4 PH steel or from stellite They were brazed in place at the same time the assembly was brazed Brazing replaced the welding of the seat, which in this case had trouble in terms of weld deposit cracking All parts, except the valve seat, were finish-machined before brazing By eliminating the need for machining after brazing, costs were reduced enough to pay for the brazing operation

Brazing of Stainless Steels

Revised by Matthew J Lucas, Jr., General Electric Aircraft Engines

in specific heat and electrical conductivity markedly affect the response to heating by induction

Ferritic and martensitic (SAE 400 series) stainless steels are ferromagnetic at all temperatures, up to the Curie temperature Thus, given the same power input, these steels generally heat faster than austenitic stainless steels, which are nonmagnetic in the annealed condition Although cold working may induce slight magnetism in the austenitic chromium steels, the 400 series of chromium-nickel steels are strongly magnetic The rate of heating to the temperature at which the filler metal flows usually affects induction-coil design and coupling It may also influence the selection of power output frequency and other processing variables

Stainless steels can be induction brazed in an air atmosphere, using a suitable flux However, for critical applications, induction brazing is sometimes done in a protective or a vacuum atmosphere (refer to Example 14) In other applications,

an inert gas, such as argon, can be used as a protective atmosphere to either minimize or prevent oxidation

Example 14: Brazing a Tube to an End Blank

The assembly shown in Fig 14, which is part of a solenoid, consisted of a type 321 stainless steel brazed to a type 416 end blank The former material is a nonmagnetic austenitic steel, whereas the latter is martensitic and ferromagnetic Consequently, although both metals were easily brazed, the achievement of a proper joint clearance between the two components was complicated by the marked differences in the CTEs of the two steels Thus, calculations were needed to determine the room-temperature clearance required to provide a suitable clearance at the brazing temperature

INDUCTION BRAZING

BRAZING TEMPERATURE, °C (°F) 650 (1200)

TIME AT BRAZING TEMPERATURE, S 10

(A) VACUUM TUBE

FIG 14 INDUCTION-BRAZED ASSEMBLY

Because the assembly was not intended for high-temperature service, the selection of the low-melting-point silver brazing filler metal (BAg-1) and a relatively low brazing temperature of 650 °C (1200 °F) were utilized to minimize heating and oxidation of the stainless steel components

For brazing that is conducted at this temperature, calculations based on CTE values showed that the following dimensions and tolerances in the joining area would be satisfactory: for the tube diameter, 12.4 mm (0.494 in.) +0.000, -0.025 mm (-

0.001 in); and for the inside diameter of the end blank, 12.7 mm (0.500 in.) +0.000, -0.025 mm (-0.001 in.) Thus, the diametral clearance at room temperature ranged from 0.127 to 0.178 mm (0.005 to 0.007 in.)

The shape of the assembly and the low melting point and brazing temperatures favored brazing by induction The end blank was in the hardened and tempered condition prior to brazing, and the short induction heating cycle (10 s) did not reduce the hardness to less than the required minimum

Prior to brazing, the components were vapor degreased The end of the tube was dipped in flux and inserted in the end blank A preformed ring of filler-metal wire was slipped over the tube and positioned at the top of the joint Then, the end blank was placed on a holding fixture, positioned in a single-turn inductor (Fig 14), and heated for 10 s After brazing, the assembly was cooled in air for 10 s before being removed from the holding fixture The assembly was then washed in hot water to remove the flux residue

Example 15: Induction Brazing in a Vacuum Atmosphere

A distinctive advantage of induction brazing, as applied to stainless steel, is its suitability for simple setups that permit brazing in vacuum Closed, nonmetallic containers with reasonably good strength and dielectric properties can provide an enclosure for the assembly to be brazed and can be evacuated prior to brazing Because the inductor can be placed outside the container, it can heat the assembly efficiently without being part of the vacuum system

Stainless steel collar-and-tube assemblies (Fig 15) were brazed in a simple setup that combined induction heating and the protection afforded by heating in vacuum The vacuum container consisted of a high-silica, low-expansion glass tube with copper end fittings connected to a vacuum system The collar-and-tube components, with preformed BNi-7 filler-metal rings pressed into place on the shoulder of each collar, were positioned and held inside the glass tube by means of a simple holding fixture The tube was sealed and evacuated with a multiple-turn inductor, outside the tube, in position to heat one of the collars When the vacuum reached 0.133 Pa (10-3 torr), heating was started The collar was heated slowly

to 970 °C (1775 °F) After 4 min, the power was shut off The tube was then repositioned to bring the second collar into the field of the inductor, and the heating sequence was repeated

FIG 15 COLLAR AND TUBE ASSEMBLY THAT WAS INDUCTION BRAZED IN VACUUM

When the second collar had cooled to the point at which no glow was visible in normal light, the tube was backfilled for 5 min with argon Brazed joints were inspected visually and metallographically They were found to be sound and acceptable in all respects The induction heating source was an 8 kVA spark-gap converter with an operating frequency of

175 to 200 kHz The water-cooled external inductor coil was made of 6.4 mm (1

4 in.) diameter copper tubing The production rate was 22 assemblies per day

Brazing of Stainless Steels

Revised by Matthew J Lucas, Jr., General Electric Aircraft Engines

Dip Brazing in a Salt Bath

The brazing of stainless steel by immersing either all or a portion of the assembly in molten salt offers essentially the same advantages and limitations that would apply to the brazing of similar assemblies made of carbon steel

Example 16: Change From Torch or Induction Brazing to Dip Brazing

The television wave-guide assembly shown in Fig 16 consisted of a type 304 stainless steel flange brazed to a tube of copper alloy 230 (red brass, 85Cu-15Zn) Satisfactory end use depended on minimal distortion When the assembly was

brazed by torch or induction brazing, the rejection rate sometimes reached 70%, because of distortion caused by uneven heating When dip brazing was adopted, the rejection rate dropped to nearly zero

FIG 16 TELEVISION WAVE-GUIDE ASSEMBLY JOINED BY SALT-BATH DIP BRAZING

Prior to brazing, the stainless steel flange was degreased and pickled, and the brass tube was degreased and bright dipped Then, the flange was placed on the tube, the tube end was flared outward slightly, a preform of the BAg-3 filler metal was placed over the tube adjacent to the flange, and FB3-A flux (AMS 3410D) flux was applied to the joint The assembly was suspended flange-down over an electrically heated salt bath to preheat the flange and dry the flux Next, the assembly was lowered slowly into the molten bath, which was maintained at 730 °C (1350 °F) for a distance of approximately 25

mm (1 in.) above the flange After being held in the bath for 0.5 min, the assembly was removed and air cooled The flux residue was removed by rinsing the assembly in 60 °C (140 °F) water The production rate was 30 assemblies per hour

Brazing of Stainless Steels

Revised by Matthew J Lucas, Jr., General Electric Aircraft Engines

High-Energy-Beam Brazing

High-energy-beam brazing techniques, such as those based on electron or laser beams, have been used to a limited extent Both electron- and laser-beam brazing are performed in a manner similar to electron- and laser-beam welding, except that the beam is defocused to provide a larger beam and to reduce the power density to prevent the base metal from melting Generally, the speed at which the beam is swept is increased so that a larger area of the part is heated and more uniform heating of the part occurs

In electron-beam brazing, the high vacuum used in the work chamber (0.0133 to 0.00133 Pa, or 10-4 to 10-5 torr) permits the adequate flow of brazing filler metal on properly cleaned joints without the use of a reducing atmosphere or flux Thus, flux entrapment does not occur, and the work does not require cleaning after brazing The high-vacuum atmosphere and the absence of flux provide a brazing environment that avoids the problems associated with prepared atmospheres, which are encountered when brazing some stainless steels, as well as the more-reactive metals (such as titanium)

In laser-beam brazing, the paris are normally protected with a shielding gas to prevent the occurrence of oxidation If necessary, the beam spot diameter can be enlarged substantially, depending on the type of equipment, while providing an adequate amount of heat input for brazing A work movement technique can be used if an area substantially larger than the beam spot size is to be heated, and the work can be rotated or indexed under the beam for uniform heating In high-energy brazing, the brazing temperatures are quickly attained, and heat can be localized to minimize grain growth, the softening of cold-worked metal, and, in austenitic stainless steels, the sensitizing of the material by carbide precipitation

Applications Electron-beam brazing is a convenient method for brazing small assemblies, such as instrument packages

It combines the versatility and close controllability of electron-beam heating with the advantages of vacuum brazing Packaged devices can be encapsulated with an internal vacuum without damaging the basic package

Tube-to-header joints in small heat-transfer equipment made of heat-resistant alloys and refractory metals are sometimes brazed using an electron beam In one technique, the tube-to-header joint is electron-beam welded on the top side of the header The heat of the beam causes the brazing filler metal that is preplaced on the reverse side of the header at the joint

to melt and flow Small-diameter, thin-walled stainless steel tubes are readily joined by electron-beam brazing, as in the following example

Example 17: Use of Defocused Beam for the Electron-Beam Brazing of Small Tubes

Capillary and other small-diameter tubes used in instrument packages required that leak-tight joints be made without overheating the other portions of the assembly The avoidance of flux was also necessary, because entrapped flux would

be either difficult or impossible to remove These conditions were satisfied by electron-beam brazing

Figure 17 shows a typical joint in type 304 tubing with a 2.55 mm (0.100 in.) outside diameter and a 0.254 mm (0.010 in.) thick wall that was brazed by the electron-beam process The joint design was based on the use of a 19 mm (3

4 in.) long socket coupling that was counterbored with a diametral clearance of 0.076 to 0.127 mm (0.003 to 0.005 in.) over the tube diameter and to a depth of 6.4 mm (1

4 in.) The average joint clearance (per side) was therefore 0.050 mm (0.002 in.) Tubes and socket couplings were deburred and solvent cleaned They were then assembled with two wire-ring preforms

of BCu-1a brazing filler metal, as shown in Fig 17 The tubes were held in position with a small clamping fixture, and the assembly was mounted in a fixed position on a table in the vacuum chamber

ELECTRON-BEAM BRAZING

VACUUM CHAMBER DIAMETER, MM (IN.) 610 (24)

PUMPDOWN TIME, MIN 30

FIG 17 JOINT BETWEEN TWO CAPILLARY TUBES OF AN INSTRUMENT PACKAGE, MADE BY ELECTRON-BEAM

BRAZING USING A LOW-POWER DEFOCUSED BEAM IN A HIGH-VACUUM ATMOSPHERE

After pumpdown, the joint was brought to the brazing temperature by moving the table back and forth under the defocused electron beam, which caused the heat of the 4.76 mm ( 3

16 in.) diameter beam spot to be applied primarily to the central portion of the coupling After being heated by conduction at a relatively low beam power, the brazing filler metal melted and flowed through the joint in approximately 30 s

About ten assemblies were brazed, at a rate of one per pumpdown, using the machine settings and other brazing conditions specified in Fig 17 Sensitizing the austenitic stainless steel was not a problem in this application, because the service environment was not significantly corrosive The relatively short-time brazing cycle minimized grain growth and the dilution of the thin-walled tubing with copper brazing filler metal

Brazing of Heat-Resistant Alloys, Low-Alloy Steels, and Tool Steels

Revised by Darrell Manente, Vac-Aero International Inc

Introduction

PROCEDURES for brazing heat-resistant alloys, low-alloy steels, and tool steels share much in common This article focuses primarily on brazing of heat-resistant alloys; some information about particulars involved in the brazing of low-alloy steels and tool steels is also provided

Brazing of Heat-Resistant Alloys, Low-Alloy Steels, and Tool Steels

Revised by Darrell Manente, Vac-Aero International Inc

Brazing of Heat-Resistant Alloys

Heat-resistant alloys are frequently referred to as superalloys, because of their strength, oxidation resistance, and corrosion resistance at elevated service temperatures (650 to 1205 °C, or 1200 to 2200 °F) This article discusses primarily nickel- and cobalt-base alloys Superalloys can also be subdivided (according to manufacturing technology) into two categories: conventional cast and wrought alloys and powder metallurgy (P/M) products Powder metallurgy products may be produced in conventional alloy compositions and as oxide-dispersion-strengthened (ODS) alloys Almost any metal, as well as nonmetallics, can be brazed to these heat-resistant alloys, if it can withstand the heat of brazing For

additional information on these alloys, see Volume 1 of the ASM Handbook

Brazing Filler Metals

The American Welding Society (AWS) has classified several gold-, nickel-, and cobalt-base brazing filler metals which can be used for elevated-temperature service (Table 1) These brazing filler metals are suitable for high-temperature service; however, if the application is for temperatures above 980 °C (1800 °F) or in severe environments, the required brazing filler metal may not be in Table 1 It should be noted that for lower service temperatures, copper (BCu) (below

480 °C, or 900 °F) and silver (BAg) (below 425 and 200 °C, or 800 and 400 °F) brazing filler metals have been used for many successful applications

TABLE 1 AWS BRAZING ALLOYS FOR ELEVATED-TEMPERATURE SERVICE

2.75-5.0

5.0

4.0-0.9

2.75-5.0

5.0

2.75-5.0

4.0-3.5

8.0

0.10 0.02 0.02 0.05 0.05

21.5-24.5

5.0

°C °F °C °F °C °F

COBALT-BASE ALLOY FILLER METALS

BCO-1

18.0-20.0

18.0

16.0- 8.5

7.5- 4.5

3.5-1.0 0.9

0.7- 0.45

Many nickel-palladium-base brazing filler metals exhibiting good wetting and flow are not classified by AWS but are also available These filler metals have been developed to replace gold-containing (BAu) brazing filler metals, which are more expensive

Another group of brazing filler metals not classified by AWS are used for repair and overhaul of nickel- and cobalt-base superalloy aerospace and industrial turbine engine components Many of these brazing filler metals are proprietary compositions of the engine manufacturer for which they were developed However, a good variety of these filler metals are commercially available These "diffusion braze" filler metals do not contain silicon However, they do contain boron, which acts as a melting point suppressant Both boron and silicon react with metals (such as chromium and molybdenum)

to form intermetallic phases These intermetallic phases are very hard and brittle and can seriously weaken a brazed joint

It is possible to perform annealing heat treatments on diffusion brazed joints that result in the diffusion of the boron out of the brazed joint and into the base metal The result is a brazed joint in which intermetallic phases are partially dissolved or eliminated completely (Fig 1) Diffusion heat treatment increases the strength and ductility of the brazed joint

FIG 1 PHOTOMICROGRAPH OF THE CRACK REPAIR OF A DIFFUSION BRAZED JOINT BY HEAT TREATING OF

THE IN-792 BASE METAL SAMPLE WAS ETCHED WITH KALLING'S REAGENT 76×

Silicon atoms do not diffuse as easily as boron atoms (because silicon atoms are larger and sit on a regular lattice site, whereas boron atoms sit on interstitial sites, providing much faster diffusion paths) Therefore, similar heat treatments do not tend to dissolve or eliminate silicon intermetallic phases

These boron-containing filler metals (used on their own or mixed with base metal powder to act as a gap filler) are used to fill cracks and eroded areas on turbine engine components Some type of cleaning operation (such as hydrogen fluoride cleaning or mechanical cleaning) is employed before the brazing operation in order to remove oxides on the components and thus facilitate brazing filler metal wetting

A second group of filler metals not classified by AWS are used for wide gaps (1.52 mm, or 0.060 in., and greater) and eroded areas These filler metals are generally nickel- or cobalt-base superalloys that have additions of a melting point suppressant These filler metals do not tend to flow away from the area in which they are placed The microstructure of the joint formed using these filler metals is homogeneous and more closely approximates the base metal microstructure (Fig 2)

FIG 2 MICROSTRUCTURE OF A WIDE GAP FILLER METAL BRAZE REPAIR SAMPLE WAS ETCHED WITH

KALLING'S REAGENT 40×

Generally, heat-resistant alloys are brazed with nickel- or cobalt-base alloys containing boron and/or silicon, which serve

as melting-point depressants In many commercial brazing filler metals, the levels are 2 to 3.5% B and 3 to 10% Si Phosphorus is another effective melting-point depressant for nickel and is used in filler metals from 0.02 to 10% It is also used where good flow is important in applications of low stress, where service temperatures do not exceed 760 °C (1400

°F)

In addition to boron, silicon, and phosphorus, chromium is often present to provide oxidation and corrosion resistance The amount may be as high as 20%, depending on the service conditions Higher amounts, however, tend to lower brazement strength

Cobalt-base filler metals are used mainly for brazing cobalt-base components, such as first-stage turbine vanes for jet engines Most cobalt-base filler metals are proprietary In addition to containing boron and silicon, these alloys usually contain chromium, nickel, and tungsten to provide corrosion and oxidation resistance and to improve strength

Product Forms Available forms of AWS classified and proprietary brazing filler metals include wire, foil, tape, paste, and powder The form used can be dictated by the application or by the composition of the filler metal If the filler metal required for a specific application is only available as a dry powder, then brazing aids such as cements and pastes are available to help position the brazing filler metal

Brazing filler metal powders usually are gas atomized and sold in a range of specified particle sizes, which ensures uniform heating and melting of the brazing filler metal during the brazing cycle These powders can be mixed with plasticizers or organic cements to facilitate positioning If the mixture must support its own weight until the brazing cycle begins, an organic binder or cement is required These binders burn off in atmosphere brazing and little or no residue results When the brazing filler metal is supplied as a paste, it is simply a premixed powder and binder

Brazing Tapes and Foils Brazing filler metals in the form of tapes and amorphous foils appear similar, but the foils are usually made by rapid solidification during melt spinning operations and tend to exhibit homogeneous amorphous structures Brazing tapes are made of powder that is held together by a binder and formed into a rather fragile sheet Most amorphous foils have a high metalloid (phosphorus, silicon, boron) content, while tapes can be made from brazing filler metals that have no metalloid content The metalloids usually are melting-point depressants and frequently form brittle phases In some cases, where the composition is workable, such as BAu, foils can be made by cold rolling Foil products can also be produced by rolling an alloy of suitable composition into a foil before adding the metalloids Tapes and foils are best suited for applications requiring a large area joint, good fit-up, or where brazing flow and wetting may be a problem

Brazing wires of nonfabricable alloys usually are made by P/M processes from gas atomized powder, which is held together by a binder or by extruding powder into wire and sintering This form of brazing filler metal is better able to support itself than are pastes and powders of filler metal, but is not used to replace tapes or amorphous foils where precision is needed in preplacing the filler metal, such as joint gaps less than 0.13 mm (0.005 in.)

Surface Cleaning and Preparation

Cleaning of all surfaces that are involved in the formation of the desired brazed joint is necessary to achieve successful and repeatable brazed joints All obstruction to wetting, flow, and diffusion of the molten brazing filler metal must be removed from both surfaces to be brazed prior to assembly The presence of contaminants on one or both surfaces to be brazed may result in void formation, restricted or misdirected filler-metal flow, and contaminants included within the solidified brazed area, which reduces the mechanical properties of the resulting brazed joint Common contaminants are oils, greases, residual liquid penetrant fluids, pigmented markings, residual casting or coring materials, and oxides formed either through previous thermal exposures or by exposures to contaminating environments or engine service

Chemical cleaning methods are most widely used As part of any chemical cleaning procedure for preprocessing assemblies for brazing, a degreasing solvent should be used to remove all oils and greases and to ensure wettability of the chemicals used for cleaning Oils and greases form a very thin film on metals, which prevents wettability by both the

subsequent chemical cleaning and/or the molten filler metals Oils and grease removers that are widely used include degreasing solutions such as stabilized perchloroethylene or stabilized trichloroethylenes These may be used as simple manual soaks, sprays, or by suspending the parts in a hot vapor of the aforementioned chemicals, commonly referred to as

a vapor degreasing process In conjunction with these processes, anodic and cathodic electrolytic cleaning can be used

Other cleaning methods include alkaline-based cleaners and emulsion-type cleaners Alkaline-based cleaners use mixtures

of phosphates, carbonates, silicates, soaps, detergents, and hydroxides Emulsion-type cleaners contain wetting agents, surface actuators, fatty acids, and hydrocarbons

A chemical cleaning procedure can be a simple single-step process or may involve multistep operations If the surfaces of the brazed joint are in the machined condition, vapor degreasing may be sufficient to remove machining oils, handling oils, and liquid penetrant fluids to yield a sound, clean surface for brazing If, on the other hand, one or both of the surfaces to be brazed is not a machined surface, then additional chemical cleanings should be employed Once vapor degreasing is accomplished, care must be taken to maintain the surface cleanliness of the brazed components by handling

in an environmentally clean atmosphere Additional methods of chemical cleaning to remove oxides and other adherent metallic contaminants include immersion in phosphate acid cleaners or acid pickling solutions, which are comprised of nitric, hydrochloric, or sulfuric acids or combinations of these

Care must be taken in time of exposure for both acid cleaning and acid pickling of heat-resistant base metals Overexposure during chemical cleaning can lead to excessive metal loss, grain-boundary attack, and selective attack on microstructural phases As the last step in chemical cleaning, an ultrasonic cleaning in alcohol or clean hot water is recommended to ensure removal of all traces of previous cleaning solutions

If no subsequent mechanical cleaning is used, the components to be brazed should be stored and transported to the braze preparation areas in dry, clean containers, such as plastic bags The time between cleaning and braze application to the assembled joints should be kept as short as manufacturing processes allow

Mechanical cleaning usually is confined to those metals with heavy tenacious oxide films or to repair brazing on components exposed to service (Nickel- or cobalt-base superalloy components that require repair brazing can be processed in hydrogen fluoride atmospheres in order to remove oxide films.) Mechanical methods are standard machining processes abrasive grinding, grit blasting, filing, or wire brushing (stainless steel bristles must be used) These are used not only to remove surface contaminants, but to slightly roughen or fray the surfaces to be brazed

Care must be taken that the surfaces are not burnished and that mechanical cleaning materials are not embedded in the metals to be brazed In grit blasting, choice of the blasting medium is critical Wet and dry grit blasting commonly are used, but wet mediums are subject to additional cleaning requirements to remove the embedded grit The mediums used are iron grits, silicon carbide grits, and grits composed of brazing filler metals Grit sizes as coarse as No 30 (0.589 mm,

or 0.0232 in.) are recommended for cleaning forgings and castings Finer grits (No 90 and No 100, 0.165 and 0.150 mm [0.0065 and 0.0059 in.], respectively) are used for general blasting All grit mediums should be changed frequently, as extensive reuse of the same medium results in loss of sharp angles or facets Once these configurations are lost or markedly reduced in the grit medium, burnishing rather than cleaning occurs Overused medium results in the entrapment

of oxides in the metal If possible, the angle of grit blasting should be less than 90° to the surfaces to be cleaned This also reduces the chances of embedding the oxides or medium in the surfaces to be brazed

Care must be taken to remove all blasting medium from the surface after mechanical cleaning, as these mediums will contaminate the braze Iron grit may impart an iron film which oxidizes as a rust Aluminum oxides, if not removed, prevent wetting and flow of the brazing filler metal; thus, use of aluminum oxides are not recommended Silicon carbide

is extremely hard and has sharp facets Consequently, it becomes embedded if an improper blasting angle is used Blasting with a nickel-base brazing filler metal or similar alloy gives the best results; stainless steel blasting medium is also acceptable

After mechanical cleaning, air blasting or ultrasonic cleaning should be used to remove all traces of loosened oxides or cleaning medium Care must be taken to ensure maintenance of the clean surfaces and components; once cleaned, they should be assembled and brazed as soon as possible

Hydrogen Fluoride Cleaning The successful repair of superalloy components requires that oxides formed during service, on external surfaces and within cracks, be removed prior to brazing to ensure proper wetting and flow of the braze material For cobalt-base superalloy parts, heating in a hydrogen-rich atmosphere (hydrogen reduction) is generally

successful in this regard However, hydrogen reduction is not effective in reducing oxides of aluminum and titanium that are formed on nickel-base superalloy parts in service because these oxides exhibit a high degree of thermodynamic stability Hydrogen fluoride reduction, on the other hand, has been shown capable of reducing these oxides from the surfaces of nickel-base parts

Oxides of aluminum and titanium are changed to their respective metallic fluorides by the fluoride ions created in the hydrogen fluoride process Oxides of chromium are also changed to chromium fluoride by the fluoride ions In addition, workpiece surfaces are depleted of aluminum and titanium so that oxides of these metals do not tend to reform during subsequent operations, such as braze repair (Fig 3)

FIG 3 SEM PHOTOMICROGRAPH OF A RENÉ 125 AEROSPACE ENGINE COMPONENT THAT WAS CLEANED WITH

HYDROGEN FLUORIDE TO DEPLETE THE ELEMENTS AT THE SURFACE

Nickel Flashing Certain heat-resistant alloys that are used as base metals in brazed assemblies particularly nickel-base alloys containing high percentages of aluminum and titanium (such as Inconel 718) may require a surface pretreatment

to ensure maintenance of the cleaned surfaces This surface pretreatment after cleaning is generally an electroplate of nickel, commonly referred to as nickel flashing Thickness of the plate flashing is kept under 0.015 mm (0.0006 in.) for alloys with less than 4% (Ti + Al) and 0.020 to 0.030 mm (0.0008 to 0.0012 in.) for alloys with greater than 4% (Ti + Al) This promotes wettability in the braze joint without seriously affecting the braze strength and other mechanical properties

of the braze The thickness of nickel plating may have to be increased as the brazing temperature is increased and as the time above 980 °C (1800 °F) is increased Titanium and aluminum will diffuse to the surface of the nickel plating upon heating

Hot fixtures (fixtures used in the furnace for brazing) must have good stability at elevated temperatures and the ability to cool rapidly Metals can be used but generally are not stable enough to maintain tolerances during the brazing cycle Therefore, ceramic, carbon, or graphite assemblies often are used for hot fixturing Ceramics, due to their high processing cost, are used for small fixtures and for spacer blocks to maintain gaps during brazing of small components Graphite has

been found to be the most suitable material for maintaining flatness in a high vacuum or argon atmosphere, and it provides faster cooling because of its high thermal conductivity, which is partially due to its porosity Graphite should be coated with an Al2O3 slurry to prevent carburization of parts during the brazing cycle It should not be used in a pure dry hydrogen atmosphere as it will cause carburization of the base metals by gaseous transfer Molybdenum and tungsten may

be used, but they are generally avoided because of their cost

Controlled Atmospheres

Controlled atmospheres (including vacuum) are used to prevent the formation of oxides during brazing and to reduce the oxides present so that the brazing filler metal can wet and flow on clean base metal Controlled-atmosphere brazing is widely used for the production of high-quality joints Large tonnages of assemblies of a wide variety of base metals are mass produced by this process

Controlled atmospheres are not intended to perform the primary cleaning operation for the removal of oxides, coatings, grease, oil, dirt, or other foreign materials from the parts to be brazed All parts for brazing must be subjected to appropriate prebraze cleaning operations as dictated by the particular metals Controlled atmospheres commonly are employed in furnace brazing; however, they may also be used with brazing processes that utilize induction, resistance, infrared, laser, and electron-beam heat sources In applications where a controlled atmosphere is used, postbraze cleaning

is generally not necessary In special cases, flux may be used with a controlled atmosphere (1) to prevent the formation of oxides of titanium and aluminum when brazing in a gaseous atmosphere, (2) to extend the useful life of the flux, and (3)

to minimize postbraze cleaning Fluxes should not be used in a vacuum environment Some types of equipment, such as metallic muffle furnaces and vacuum systems, may be damaged or contaminated by the use of flux

The use of controlled atmospheres inhibits the formation of oxides and scale over the whole part and permits finish machining to be done before brazing in many applications In some applications, such as the manufacture of electronic tubes, eliminating flux is tremendously important

Pure dry hydrogen is used as a protective atmosphere because it dissociates the oxides of many elements Hydrogen with a dew point of -51 °C (-60 °F) dissociates the oxides of most elements found in heat-resistant alloys, with the notable exception of aluminum and titanium, which are found in most of the high-strength heat-resistant base metals

Inert gases, such as helium and argon, do not form compounds with metals In equipment designed for brazing at ambient pressure, inert gases reduce the evaporation rate of volatile elements, in contrast to brazing in a vacuum Inert gases permit the use of weaker retorts than required for vacuum brazing Elements such as zinc and cadmium, however, vaporize in pure dry inert atmospheres

Vacuum An increasing amount of brazing of heat-resistant alloys, particularly precipitation-hardenable alloys that contain titanium and aluminum, is done in a vacuum Vacuum brazing in the range of 13 mPa (10-4 torr) has proved adequate for brazing most of the nickel-base superalloys By removal of gases to a suitably low pressure, including gases that are evolved during heating to brazing temperature, very clean surfaces are obtainable A vacuum is particularly useful

in the manufacture of parts for the aerospace, electronic tube, and nuclear fields, and where metals that react chemically with a hydrogen atmosphere are used or where entrapped fluxes or gases are intolerable The maximum tolerable pressure for successful brazing depends on a number of factors that are primarily determined by the composition of the base metals, the brazing filler metal, and the gas that remains in the evacuated chamber

Vacuum brazing is economical for fluxless brazing of many similar and dissimilar basemetal combinations Vacuums are especially suited for brazing very large, continuous areas where (1) solid or liquid fluxes cannot be removed adequately from the interfaces after brazing, and (2) gaseous atmospheres are not completely efficient because of their inability to purge occluded gases evolved from close-fitting brazing interfaces It is interesting to note that a vacuum system evacuated to 1.3 mPa (10-5 torr) contains only 0.00000132% residual gases based on a starting pressure of 100 kPa (760 torr)

Vacuum brazing has the following advantages and disadvantages compared with brazing that is carried out under other high-purity brazing atmospheres:

ELIMINATING THE NECESSITY FOR PURIFYING THE SUPPLIED ATMOSPHERE

COMMERCIAL VACUUM BRAZING GENERALLY IS DONE AT PRESSURE VARYING FROM

0.0013 TO 13 PA (10-5 TO 10-1 TORR), DEPENDING ON THE MATERIALS BRAZED, THE

FILLER METALS BEING USED, THE AREA OF THE BRAZING INTERFACES, AND THE

DEGREE TO WHICH GASES ARE EXPELLED BY THE BASE METALS DURING THE

BRAZING CYCLE

METALS) DISSOCIATE IN VACUUM AT BRAZING TEMPERATURES

INTERFACES, DUE TO EXPULSION OF GASES FROM THE PARTS TO BE BRAZED, ARE NEGLIGIBLE IN VACUUM BRAZING

TEMPERATURES REMOVES VOLATILE IMPURITIES AND GASES FROM THE METALS FREQUENTLY, THE PROPERTIES OF BASE METALS ARE IMPROVED THIS

CHARACTERISTIC IS ALSO A DISADVANTAGE WHEN ELEMENTS IN THE FILLER METAL

OR BASE METALS VOLATILIZE AT BRAZING TEMPERATURES, THUS CHANGING THE MELTING POINT OF THE FILLER METAL OR PROPERTIES OF THE BASE METAL THIS TENDENCY MAY, HOWEVER, BE CORRECTED BY EMPLOYING PARTIAL-PRESSURE

VACUUM BRAZING TECHNIQUES

Nickel-Base Alloys

In the selection of a brazing process for a nickel-base alloy, the characteristics of the alloy must be carefully considered The nickel-base alloy family includes alloys that differ significantly in physical metallurgy (such as the mechanism of strengthening) and in process history (cast versus wrought) These characteristics can have a profound effect on their brazeability

Precipitation-hardening alloys present several difficulties not normally encountered with solid-solution alloys Precipitation-hardening alloys often contain appreciable (greater than 1%) quantities of aluminum and titanium for improved strength and corrosion resistance The oxides of these elements are almost impossible to reduce in a controlled (vacuum or hydrogen) atmosphere Therefore, nickel plating or the use of a flux is necessary to obtain a surface that allows wetting by the filler metal

Because these alloys are hardened at temperatures of 540 to 815 °C (1000 to 1500 °F), brazing at or above these temperatures may alter the alloy properties This frequently occurs when using silver-copper filler metals, which occasionally are used on heat-resistant alloys

Liquid metal embrittlement is another difficulty encountered in brazing of precipitation-hardening alloys Many nickel-, iron-, and cobalt-base alloys crack when stressed parts are exposed to molten metals This is usually confined to the silver-copper filler metals If precipitation-hardening alloys are brazed in the hardened condition, residual stresses am often high enough to initiate cracking

Cleanliness, as in all metallurgical joining operations, is important when brazing nickel and nickel-base alloys Cleanliness of base metal, filler metal, flux (when used for torch or induction brazing), and purity of atmosphere should

be as high as practical to achieve the required joint integrity Elements that cause surface contamination or interfere with braze wetting or flow should be avoided in prebraze processing All forms of surface contamination such as oils, chemical residues, scale, or other oxide products should be removed by using suitable cleaning procedures The use of nickel-base filler metals can offer some cost effectiveness in this regard because certain nickel-base brazing filler metals containing boron and/or silicon are known to be self-fluxing and thus more forgiving to slight imperfections in cleanliness

Attempting to braze over the refractory oxides of titanium and aluminum that may be present on precipitation-hardenable nickel-base alloys must be avoided Procedures to prevent or inhibit the formation of these oxides before and/or during brazing include special treatments of the surfaces to be joined or brazing in a highly controlled atmosphere Surface treatments include electrolytic nickel plating and reducing the oxides to metallic form As stated earlier in this section, a typical practice is to nickel plate the joint surface of any alloy that contains aluminum and/or titanium For vacuum brazing of alloys containing aluminum and titanium in trace amounts, use of 0.0025 to 0.0075 mm (0.0001 to 0.0003 in.) thick plate is considered optimal Alloys with up to 4% AI and/or titanium require 0.010 to 0.015 mm (0.0004 to 0.0006 in.) thick plate, and alloys with aluminum and/or titanium contents greater than 4% require 0.020 to 0.030 mm (0.0008 to

0.0012 in.) thick plate When brazing in a pure dry hydrogen atmosphere, thicker plating (0.025 to 0.038 mm, or 0.001 to 0.0015 in.) is desirable for alloys with high (>4%) aluminum and/or titanium contents

Atmospheres Dry, oxygen-free atmospheres that are frequently used include inert gases, reducing gases, and vacuum The brazing atmosphere, whether gaseous or vacuum, should be free from harmful constituents such as sulfur, oxygen, and water vapor When brazing in a gaseous atmosphere, monitoring of the water vapor content of the atmosphere (that is, the dew point) is the common practice A dew point of -50 °C (-60 °F) results in average brazeability; average; -60 °C (-

80 °F) or below produces a better quality braze

Stresses During brazing, residual or applied tensile stress should be eliminated or minimized as much as possible Also, inherent stresses present in the precipitation-hardening alloys may lead to stress-corrosion cracking Therefore, stress relieving or annealing prior to brazing is also recommended for all furnace, induction, or torch brazing operations Brazing filler metals that melt below the annealing temperature are likely to cause stress-corrosion cracking of the base metal

Thermal Cycles Consideration must be given to the effect of the brazing thermal cycle on the base metal Filler metals that are suitable for brazing nickel-base alloys may require relatively high temperatures This is particularly true for the filler-metal alloy systems most frequently used in brazing of nickel-base alloys the nickel-chromium-silicon or nickel-chromium-boron systems

Solid-solution-strengthened nickel-base alloys such as Inconel 600 may not be adversely affected by brazing temperatures

of 1010 to 1230 °C (1850 to 2250 °F) Precipitation-strengthened alloys such as Inconel 718 may, however, display adverse property effects when exposed to brazing cycles with maximum temperatures that are higher than their normal solution heat treatment temperatures Inconel 718, for example, is solution heat treated at 955 °C (1750 °F) for optimum stress-rupture life and ductility Braze temperatures of 1010 °C (1850 °F) or above result in grain growth and a corresponding decrease in stress-rupture properties, which cannot be recovered by subsequent heat treatment

Consideration of base-metal property requirements for service enables selection of an appropriate braze alloy Lower melting temperature (below 1040 °C, or 1900 °F) braze filler metals are available within the nickel-base alloy family and within other brazing filler-metal systems (see Table 1)

Inconel 718 is often used in the fabrication of air diffusers for aerospace turbine engines One manufacturer found that vacuum brazing of diffuser components at 10-4 torr in a cold-walled vacuum furnace provided the best results Prior to brazing, all joint surfaces were nickel plated to 0.005 to 0.015 mm (0.0002 to 0.0006 in.) thicknesses Plating was done in accordance with specification AMS 2424 or equivalent Prior to assembly, application of BNi-2 braze filler metal tape (approximately 0.11 mm, or 0.0045 in., thick) was preplaced between all joint surfaces After assembly, an additional braze slurry of BNi-2 filler metal was applied to all joints to ensure joint soundness

Brazing of ODS Alloys

Oxide dispersion-strengthened alloys are P/-M alloys that contain stable oxides evenly distributed throughout the matrix The oxide does not go into solution in the alloy even at the liquidus temperature of the matrix However, the oxide is usually rejected from the matrix upon melting of the matrix, which occurs during fusion welding, and cannot be redistributed in the matrix on solidification; therefore, these alloys are usually joined by brazing There are two commercial classes of ODS alloys: the dispersion-strengthened nickel, and mechanically alloyed Inconel MA 754, Inconel MA 6000, and Incoloy MA 956

Inconel MA 754, dispersion-strengthened nickel alloys, and dispersion-strengthened nickel-chromium alloys are the easiest to braze of the family of ODS alloys Vacuum, hydrogen, or inert atmospheres can be used for brazing Prebraze cleaning consists of grinding or machining the faying surfaces and washing with a solvent that evaporates without leaving a residue Generally, brazing temperatures should not exceed 1315 °C (2400 °F) unless demanded by a specific application that has been well examined and tested The brazing filler metals for use with these ODS alloys usually are not classified by AWS In most cases, the brazing filler metals used with these alloys have brazing temperatures in excess of 1230 °C (2250 °F) These include proprietary alloys that are based on nickel, cobalt, gold, or palladium

Brazements made of ODS alloys to be used at elevated temperature must be tested at elevated temperature to prove fitness-for-purpose In the case of stress-rupture testing, AWS specification C3.2 may be used as a guide because it gives

the actual joint configuration The configuration shown in Fig 4 is preferred by some as a test model, although any test configuration without stress raisers is adequate The elevated-temperature brazement properties for Inconel alloy MA 754 should meet the following requirements:

SHEAR STRESS TEMPERATURE

Inconel MA 6000 is used for its high-temperature strength and corrosion resistance; unfortunately, the passive oxide scale that provides good corrosion resistance also prevents wetting and flow of brazing filler metal Therefore, correct cleaning procedures are very important Surfaces to be brazed should be mechanically cleaned with a water-cooled, low-speed belt

or wheel of approximately 320-grit and stored in a solvent, such as methanol, until immediately before the beginning of the brazing cycle

Cobalt-Base Alloys

The brazing of cobalt-base alloys is readily accomplished with the same techniques used for nickel-base alloys Because most of the popular cobalt-base alloys do not contain substantial amounts of aluminum or titanium, brazing atmosphere requirements are less stringent Table 2 gives typical compositions of several cobalt-base alloys These materials can be brazed in either a hydrogen atmosphere or a vacuum Filler metals are usually nickel- or cobalt-base alloys or gold-palladium compositions Silver- or copper-base brazing filler metals may not have sufficient strength and oxidation resistance in many high-temperature applications Although cobalt-base alloys do not contain appreciable amounts of aluminum or titanium, an electroplate or flash of nickel is often used to promote better wetting of the brazing filler metal

TABLE 2 TYPICAL COMPOSITIONS OF SELECTED COBALT-BASE ALLOYS

NOMINAL COMPOSITION, WT%

ALLOY(A)

CHARACTERISTICS AND TYPICAL

ELGILOY 0.15 2.0 20.0 15.0 40.0 7.0 BAL 0.04 BE SPRINGS;

CORROSION RESISTANT, HIGH STRENGTH

FSX-414(B) 0.25 1.0(C) 1.0(C) 29.5 10.5 BAL 7.0 0.012 2.0(C) GAS TURBINE VANES

FSX-418(B) 0.25 1.0(C) 1.0(C) 29.5 10.5 BAL 7.0 0.012 2.0(C) 0.15 Y GAS TURBINE

VANES; IMPROVED OXIDATION

RESISTANCE

FSX-430(B) 0.40 29.5 10.0 BAL 7.5 0.027 0.9 0.5 Y GAS TURBINE

VANES; IMPROVED STRENGTH AND DUCTILITY

X-40(B) 0.50 0.50 0.50 25 10 BAL 7.5 1.5 GAS TURBINE PARTS,

MP35N 20.0 35.0 35.0 10.0 STRESS-CORROSION

RESISTANT, STRENGTH

APPLICATIONS

S-816 0.38 1.20 0.40 20 20 BAL 4.0 4.0 4.0 4 GAS TURBINE

BLADES, BOLTS, SPRINGS

V-36 0.27 1.00 0.40 25 20 BAL 4.0 2.0 2.0 3

HIGH-TEMPERATURE SHEET

WI-52(B) 0.45 0.50(C) 0.50(C) 21 1.0(C) BAL 11 2.0 2.0 GAS TURBINE PARTS,

NOZZLE VANES X-45(B) 0.25 1.0(C) 25.5 10.5 BAL 7.0 0.010 2.0(C) NOZZLE VANES

(A) SOME SUPERALLOYS ARE MADE BY MORE THAN ONE MANUFACTURER THE PROPRIETARY DESIGNATIONS FOR SUCH ALLOYS HAS BEEN USED IN THIS COMPILATION

(B) CAST ALLOY

(C) MAXIMUM COMPOSITION

Nickel-base brazing alloys such as AWS BNi-3 have been used successfully on Haynes 25 for honeycomb structures After brazing, a diffusion cycle was reportedly used to raise the braze joint remelt temperature to 1260 to 1315 °C (2300

to 2400 °F) Table 3 presents the effects of a high-temperature braze (1225 °C, or 2240 °F, for 15 rain) on the mechanical properties of Haynes 25 One cobalt-base brazing filler metal (AWS BCo-1, see Table 1) appears to offer a good combination of strength, oxidation resistance, and remelt temperature for use on Haynes 25 foil

TABLE 3 EFFECT OF BRAZING ON MECHANICAL PROPERTIES OF HAYNES 25

TEST TEMPERATURE(A)

ULTIMATE STRENGTH

0.2% OFFSET YIELD

STRENGTH CONDITION

STRESS CONDITION

Cobalt-base alloys, much like nickel-base alloys, can be subject to liquid metal embrittlement or stress-corrosion cracking when brazed under residual or dynamic stresses This frequently is observed when using silver- or silver-copper-base (BAg) filler metals Liquid metal embrittlement of cobalt-base alloys by copper-base (BCu) filler metals occurs with or without the application of stress; therefore, BCu filler metals should be avoided when brazing cobaltbase alloys

Brazing of Heat-Resistant Alloys, Low-Alloy Steels, and Tool Steels

Revised by Darrell Manente, Vac-Aero International Inc

Brazing of Low-Alloy Steels and Tool Steels

Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of such alloying elements as manganese, nickel, chromium, and molybdenum Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr For many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment In some cases, however, alloy additions are used to reduce environmental degradation under certain specific service conditions For compositions of specific low-alloy steels, see the

article "Classification and Designation of Carbon and Low-Alloy Steels" in Volume 1 of the ASM Handbook

Tool steels are any steels used to make tools for cutting, forming, or otherwise shaping a material into a part or component adapted to a definite use Tool steels could be classified as low-alloy steels, but they are generally considered

to be a separate group Table 4 lists selected tool steels and their typical uses

TABLE 4 PRINCIPAL TYPES OF TOOL STEELS

CHISELS, HAMMERS, RIVET SETS

COLD-WORK TOOL STEELS

SHORT-RUN COLD-FORMING DIES, CUTTING TOOLS

MEDIUM-ALLOY COLD-WORK TOOL STEELS

THREAD ROLLING AND SLITTING DIES, INTRICATE DIE SHAPES

HIGH-CHROMIUM COLD-WORK STEELS

USES UNDER 480 °C (900 °F), GAGES, LONG-RUN FORMING AND BLAKING DIES

STEELS

ALUMINUM OR MAGNESIUM EXTRUSION DIES, DIE-CASTING DIES, MANDRELS, HOT SHEARS, FORGING DIES

STEELS

HOT EXTRUSION DIES FOR BRASS, NICKEL AND STEEL; HOT-FORGING DIES

HIGH-85% OF ALL CUTTING TOOLS IN UNITED STATES MADE FROM THIS GROUP

Brazing Filler Metals

The AWS BAg, BCu, and BNi brazing filler metals can be used effectively to braze low-alloy steels or tool steels The specific type of brazing filler metal used depends on the application, the available brazing equipment, and the strength requirements of the braze joint Many BAg brazing filler metals are used for torch or induction brazing BCu and BNi brazing filler metals are used for furnace brazing High-solidus-point brazing filler metals can be chosen so that a component can be brazed and then heat treated For example, a plate assembly made from 4340 material can be brazed with copper filler metal at 1120 °C (2050 °F) for 10 min The plates are then kept at a temperature of 845 °C (1550 °F) for 1 h in order to austenitize the base metal and subsequently quenched in oil The plates are tempered at 510 °C (950 °F) for 2 h in order to produce a hardness of about 40 HRC The copper braze filler is not affected by the heat treatment due

to its relatively high melting point

Precleaning

The surface condition of the material will determine the cleaning method that is chosen For example, a newly machined component would normally require only a vapor degreasing operation or a solvent wipe in order to remove machining lubricants A component with a light oxide layer on the surface might be grit blasted with Al2O3 and then vapor degreased This component could also be cleaned with emery paper or a stainless steel wire brush and wiped with a solvent

A component with a heavy oxide layer on the surface would require a combination of mechanical and chemical cleaning

in order to ensure that the oxide layer was removed Another option available for removing oxides is to furnace clean

components in an atmosphere of vacuum or hydrogen While this method will reduce oxides of iron and nickel, other oxides may not be reduced (Fig 4)

Fixturing

Fixturing methods for heat-resistant alloys are also suitable for low-alloy steels and tool steels However, greater use can

be made of metal fixturing because many of these brazements are made at lower temperatures

Atmospheres and Fluxes

Low-alloy steels and tool steels can be brazed in air, vacuum, hydrogen, nitrogen or endothermic or exothermic atmospheres Components can also be torch or induction brazed in air using fluxes containing fluorides, borates, or chlorides

Components brazed with AWS BNi brazing filler metals are usually brazed in a vacuum Vacuum nitrogen, or hydrogen atmospheres are generally used for brazing components with BCu or BAg brazing filler metals Care must be taken to ensure that the chosen atmosphere does not cause carburization or decarburization of the component

Brazing of Copper, Copper Alloys, and Precious Metals

Roy E Beal, Amalgamated Technologies, Inc Rodger E Cook, The Wilkinson Company

Introduction

COPPER, COPPER ALLOYS, AND PRECIOUS METALS are probably the most easily brazed metals available A wide range of brazing filler metals is used to join the many different coppers, copper alloys, and precious metals that are manufactured The selection of the brazing process and the filler metal depends on the alloy or material composition, the shape and dimension of the parts to be joined, and the intended application Gold, silver, and the platinum group metals are well known for their resistance to oxidation at high temperatures Not only does this characteristic make the precious metals desirable in industrial and medical applications, it helps enhance the brazeability of components made from them

Brazing of Copper, Copper Alloys, and Precious Metals

Roy E Beal, Amalgamated Technologies, Inc Rodger E Cook, The Wilkinson Company

Copper and Copper Alloys

Because the brazeability of most copper alloys is very good, the material considerations are generally not as difficult as those of some other metals However, the specific metallurgy of the individual copper or copper alloy is an important factor and should be considered when selecting a manufacturing method Most brazing operations will result in recrystallization of the copper alloy being joined Fine grain sizes can be eliminated and the cold-working step removed, which may or may not be desirable Often, electrical conductivity is an important factor in a copper alloy brazed joint when overall resistance across the joint must be controlled Therefore, the finished metallurgical structures, grain sizes, and mechanical properties of the specific copper alloy must be considered when brazing is utilized

The metallurgical structures of brazed joints are largely those of the brazing filler metal, enriched with copper from the parent material Because some surface melting of the copper alloys being joined occurs, the interfaces usually show primary dendrites of a copper-rich phase growing from the copper alloy and braze metal interface The brazing filler metal becomes more copper-rich, producing additional dendritic growth with increased brazing temperature and time

The widely used oxygen-containing coppers must be carefully handled when heated Brazing temperatures are normally above 480 °C (900 °F), which allows the dispersed copper oxides in the material to react with a hydrogen atmosphere, creating a high-pressure steam within the solid metal Tough-pitch coppers cannot be satisfactorily brazed by furnace