Brazing, Soldering and Joining

The application of high-frequency induction heat for the joining of metal parts with solders and brazing alloys offers a wide variety of applications.  In many ways this process of joining metals parts enables the adoption of new manufacturing techniques, such as the design of fabricated parts to supersede those made from one piece, especially where silver brazing alloys are used as the bonding medium.  The outstanding features of induction joining are the speed with which heat can be applied; the uniformity of heat transfer, once a cycle has been established; and economical heating cost when only a local surface is treated in contrast to an entire components or assembly.

Low-temperature solders, melting at temperatures of 250 to 500 ˚F, for the joining of metal parts can be bonded at exceptional speeds, because of the capacity of high-frequency current to cause metals to reach this relatively low temperature so quickly.  With little difference in time, however, the harder solders, having a melting range of 500 to 700˚F, also are quickly melted into joints more effectively than by any other means.  Soldering operations that ordinarily might require a half minute by irons or gas flames can be performed by induction heating equipment in a second or two.

High frequency induction brazing with silver alloys provides an excellent means of joining together ferrous, nonferrous, and dissimilar metals.  An outstanding advantage of this is that the fluidity of silver alloys provides penetration into restricted openings, which other alloys might not reach.  Capillary attraction results in their spreading evenly over the surfaces to be joined, thus forming a solid bond.

To some extent silver brazing is a form of hard soldering.  However, the joint usually becomes stronger than the alloying material thus making exceedingly strong bonds possible.  In many cases silver brazing, using high-frequency induction heat for the melting of the alloy and the heating of the parts to be joined, will produce joints with strengths equal to those obtained by copper brazing, and when considered as a substitute for copper brazing, the need of a controlled-atmosphere furnace can be eliminated.

Silver alloys melt at temperatures ranging from 1100 to 1500˚F, according to their composition.  They have tensile strengths of 40,000 to 70,000 lb. per sq. in., but with some designs can provide joints with strengths in excess of 100,000 lb. per sq. in.  To obtain the maximum strength, the spacing between the parts to be jointed must be closely held.  The better the fit, the stronger the joint.

In the long run induction brazing with silver alloys is more economical than other methods.  It is quick ,produces a uniform flow of the alloy, uses a relatively small amount of it, and produces a joint that requires practically no finishing.  Briefly, a sliver brazed joint combines strength, smoothness, and ductility; it assures leak proof assemblies, and withstands temperatures that ordinarily would melt solder.  High frequency induction heat for silver brazed joints is suitable for more uses than other means of heating.  The acetylene torch, for example, gives excessive heat which might have a tendency to distort parts adjacent to the brazed surfaces.  The virtue of high frequency induction heat lies in the closer control of heat to restricted surfaces which it provides.

Miscellaneous Brazed Joints.  In Fig. 93 are represented various types of silver-brazed  joints which are adaptable to the inductive-heating process.  In the example at A, a preformed silver-alloy rings is brazed at the joint as indicated and when heat is applied around the bottom surface of the shaft the alloy flows in such a way that it fills the surface between the two parts.

At B shown a joint in which the brazing material is placed at the edge of the flange, so that when it melts it runs down through the jointed area.  In the example at C the silver-alloy washer, as indicated at D, is placed between the shoulder of the shaft and the edge of the flange.  With this type of brazing  application, it is necessary to apply pressure on the shafts so that when the silver rung metals the excess metal will go down into the joint.  At E is a similar application, although in this case the insert is made in the form of a bushing, which likewise requires a pressure applied to it, so that when the silver ring melts the flow of the material will go into the joint, and thus provide a metal-to-metal contact.

In applying bushings of this type, it also is possible to use a preformed ring of silver material placed under the shoulder, as shown at F.  In the example at G, a ring is placed under the head of the bushing which, when melted, will flow into the joint.  If an insert of this type has sufficient weight, it is likely that the part will settle into position as soon as the alloy is heated to its flowing point, and thus form a metal-to-metal contact.  However, with light pieces it is preferable to apply a suitable pressure to insure a tight metal-to-metal joint.

Fig. 93 - A variety of brazed joints that lend themselves favorably to induction heating.

Another group of brazed joints is illustrated in Fig. 94.  The example at A represents a shouldered flange brazed to an upset sheet-metal opening.  For this application, a washer of silver alloy is inserted prior to the assembly of the two parts.  At B is illustrated a method for mounting a tube to a flange, in which  the silver-alloy disk is placed under the edge of the tube and then squeezed down into position after it is melted.  The alloy then runs up through the joint and forms a solid bond.

Another way to assembly a tube to a flange is shown at C.  Here the performed silver-alloy ring is placed at the upper portion of the joint.  When heat is applied the alloy will flow down the sides of the tubing to the bottom edge,  thus covering the entire surface.  Another way to perform this operation is to provide a counterbore at the upper portion of the flange and set the silver ring into it, as shown at D.  One of the  most satisfactory methods, however, is to machine a groove either into the flange or the member to be inserted into it, and to place a preformed ring of silver alloy into the groove prior to the assembly of the two parts.  The example shown at E has a groove cut into the flange and, when the alloy reaches melting temperature, it will tend to flow up and down, so that the entire joining surface are covered.  Still another way to braze such a joint is indicated at F, in which a preformed ring of silver alloy is placed within the tube.  When the alloy melts it will run out along the edge of the tube and up the sides of the joint.

Fig. 94 - A miscellany of induction-brazed joints showing various methods for applying silver alloy.

In joining together sheet-metal assemblies, such as tubing and covers, or drawn shells and caps, often required for containers and the like, there are many varieties of brazed joints well adapted to the application of induction heat.  In Fig. 95 are shown a variety of these joints.  In the example at A both edges of the assembly are formed out, so that a preformed silver-alloy ring can be inserted between them, at assembly, as shown.  For the joint at B, however, the silver-alloy ring is placed underneath the cover and, when heat is applied, the alloy will flow up through the joint.  One of the better type of  joint, however, is illustrated at C.  Here the inner body is preformed with a groove, into which a silver-alloy ring is placed before assembly of the cap.  After heat has been applied to melt the alloy, it will flow upward toward the top of the joint and also downward, so that both contacting surfaces become completely covered, thus forming an effective bond.  For the joint at D, the preformed wire alloy is placed on the inside surface and, when it melts, flows downward through the joint.

Fig. 95 - Various ways in which silver alloy can be used in connection with the induction brazing of sheet-metal parts.

The reverse of this is shown at E, where the alloy ring is placed on the outside edge and, when melted, flows through the inside joint.  In the example at F the cover is inserted on the inside of the sleeve and, when melted, flows through the inside joint.  In the example F the cover is inserted on the inside of the sleeve and a preformed ring of alloy is placed on the inside which, likewise, flows through the joint. The example at G also is arranged so that the cover has a groove rolled into it.  Into  this he alloy rings is placed prior to assembly, so that upon  being heated the metal will flow up and down.  Another form of braze, in a similar assembly, can be seen at H, where the ring is place on the inside and, when melted, will flow upward.  The example at J is similar, though here the cover is placed over the outside of the body.

Another type of assembly, compris8ing a cap to a sleeve, is shown at K, in which the preformed alloy ring is placed inside, as shown.  As soon as the surrounding metal is heated sufficiently to flow the alloy, it will run down through the joint.  IN the example at L, however, the ring is placed on the inside and must flow up into the joint.  With this type of brazing it is important that the ring remain in position, and usually it would be better to turn the part upside down, to ensure a perfect bond, although because of the characteristics of silver alloys they will flow upward practically as well as downward.

The five examples shown at the lower position of the illustration are representative of the method used for applying shallow covers to tubular bodies.  The preformed rings are mounted in various ways somewhat along the lines already discussed.  With the joints at M and N, however, it is necessary to apply pressure to the outer sleeve in order to form a metal-to-metal contact, as soon as the preformed silver alloy ring begins to flow.

Production Brazing Setups.  In Fig. 96 are illustrated a variety of brazing operations for miscellaneous types of metal parts, which are representative of some of the joints that can be made by high frequency induction heating equipment generators.  The assembly at A includes a tube to which a solid type insert is brazed.  The silver brazing rings is placed within a groove, cut

Fig. 96 - Six representative types of joining operations performed by means of high frequency induction heat.

into the plug, prior to assembly, as shown.  The tube is then placed over the plug, and induction heat is generated to the surface by the single turn inductor as shown. For example two stations are used, and one at the right of the fixture during the brazing operations.  The fixture at the left however, is open and ready for loading.  There are eight pieces brazed simultaneously, and each assembly is held in place firmly by means of individual spring plungers, located on the upper cross plate of the fixture.  After the proper heat has been applied to the joint, the silver brazing ally flow both upward and downward, so that a uniform brazing action results.  Pressure on the tube against the shoulder of the insert assures correct alignment and eliminates the possibility of the tube's moving out of place while being heated.

An example of brazing setup for joining a nose cup and a spacer.  Here six pieces are handled simultaneously.  For this an internal type coil is used, details of which may be seen in Fig. 96 at B.  The silver alloy ring is placed underneath the cup section and when heat is applied to the inside surface of the spacer, the molten alloy is drawn up through the joint, completely surfacing the contacting areas of both parts.

The operation illustrated at C in Fig. 96 represents the brazing of a tube to a body, in which the preformed silver brazing alloy rings is placed at the joint on top.  This table is arranged with two fixtures and, four pieces are br5azed at one time.  The coil is arranged so that heat is generated from the outside surface.  When both parts attain a temperature slightly above the melting point of the alloy, the silver runs down through the joint, thus completing the braze.

For brazing an insert to a drawn cup, in which an internal type coil is used.  Details of this assembly are illustrated at D in Fig. 96.  Each insert is held firmly in place during the brazing operation, so that after the ring of alloy has melted, the excess material will be squeezed out and distributed uniformly, thus assuring a perfect metal to metal joint.

For soldering eight Bourdon tubes, used in the manufacture of pressure gauges, to the body component.  For this operation two parallel type  inductors are used, arranged so that the current comes in on one bar and goes out on the other.  At E in Fig. 96 may be seen details of the operations, showing the relation of the inductor to the parts to be brazed.  The inductors when energized, generate heat to both sides of the body from where it conducts to the tube joints.

For brazing tubing to a flange and in this operation the fixture is arranged to handle four pieces at one time.  Internal type coils are used, as may be seen at F in Fig. 96.  The silver alloy rings is located in the chamfered section of the flange and, upon reaching melting temperature, flows down throughout the entire joint.  The outer sleeve is used as a guide to centralize the work over the coil.

Preparation of Surfaces.  In any form of induction joining, whether a solder or a silver alloy is used, clean surfaces are essential.  Even though a flux will have a tendency to dissolve oxide films on the surface of metal parts, it is better to clean the surfaces thoroughly before applying heat.  This will assure a stronger bond.  Also surfaces to be joined should be as smooth as can be commercially produced.  This procedure assures a better flow of the alloy and a more equal distribution of it than would result with irregular surface contact.

In connection with silver alloy brazing, it is important that a suitable amount of flux be applied around the areas to be jointed, as a means of protection against oxidation.   This flux, when heated, also provides a free flowing surface for the alloy, so that uniform bonding is made possible.  Usually fluxes having a borax base are used and are applied in liquid form, which results in a bubbling action when subjected to heat.  The fluidity of the flux must be sufficient for it to flow at a temperature below that required to melt the silver alloy.  With a fluidity temperature differential of 50 to 100˚F, the flux will serve as a good temperature indicator during the brazing operation.

Induction heating equipment can be used effectively when a small temperature differential exists between the metal parts to be joined and the solder used for bonding.  This is due to the fact that the heating coil can be placed far enough away to bring the heat up gradually to the melting temperature.  Then, through the automatic timber, the heat can be cut off at the exact moment when the solder flows.

Multiple Internal Brazing.  For joining a steel ring to the open end of a  sheet metal windshield.  The ring is provided with a groove into which the brazing alloy is inserted prior to assembly.  The parts are then assembled and placed in the fixture arranged to join six pieces at one time.  The fixture is provided with a hinged bar at the top, arranged with plunger and individual springs plungers, which hold the windshields in position.

Jumper connections are used between the coils to provided for a series connection, whereas copper tubing is jointed over the ends of the coil connections to provide for the continuous passage of cooling water.  The coils themselves are made from a section of large copper tubing, formed into an oval shape.

A sheet metal body onto which three inserts are to be soldered, as well as the two different heating coils which are used for heating.  The upper coil is arranged so that two inserts are soldered simultaneously.  The lower coil, which also is of the single turn type is arranged to solder the side insert.

Continuous Soldering.   A turntable fixture for soft-soldering small parts is placed on an asbestos disk, which is mounted on a slowly revolving spindle.  As the parts pass to the rear over a pancake-type induction coil located under the disk, they become heated to the temperature needed to make the preplaced solder flow uniformly around the joint.  After the work has passed the heating zone, it is ejected by means of a plate and falls through the chute into a tote box.  A fixture of this design will find many uses in plants where much soldering is encountered.  The drive to the spindle should be provided with a variable control to make possible correct heating for parts varying in size and shape.  Such a design is also feasible for coils of different styles, so that the length of the heating zone can be made shorter or smaller as may be desired.  The asbestos disk offers no resistance to the passing of high frequency current, although is desired or if the part being soldered so requires the induction coil can be arranged on top, permitting to work to pass under it.

Another type of continuous soldering and brazing fixture for small condenser can cover is being soft soldered.  A preformed ring of solder is place on the cover and the assembly laid on the asbestos belt, which is power driven by a small motor and variable feed drive.  A section of asbestos board is placed under the belt to support it between pulleys.  The pulleys are made of hard wood, so that there are no metallic parts close enough to the induction coil to absorb any of the high frequency magnetic flux.

This type of fixture is suitable for a variety of soldering operations through the application of different coils and changing their location as needed.

Multi-brazing Operations.  A simplified form of fixture and a series-type coil used for silver brazing a sheet-steel dome to a steel base plate in which six pieces are jointed together simultaneously.  The parts are prepared for brazing by assembling them with a ring of silver alloy at the joint, to which is applied a suitable quantity of brazing flux.  Details of the work assembly and its location in relation to the heating inductor which is of the solid type made of a copper plate, bored to provide a coupling about 1/8 in. from the work piece.  The plate is provided with saw cuts so that the high frequency current follows the path of the arrows, coming in at one lead and going out at the other.  A copper tube cooling coils is brazed around the outside edge of the inductor plate, the ends of which are attached to hose connection providing the water supply.  The six assemblies are brazed in 30 sec.

A tandem type coil used for brazing two assemblies at one time is made of a flat copper plate, bored out with two openings large enough to provide correct coupling from the work.  This coupling is made to 1/8 in. The plate is then slotted by a 1/32 in. cutter, so that the  high frequency current will pass continuously around the openings.  A single copper tube is then brazed on the outside to provide for cooling.

In operating the change over switch used with two station brazing or hardening tables it is possible to actuate it by means of a solenoid, which in turn is controlled by the master timer.  In operation the solenoid is actuated immediately after the completion of the heating operation on one side of the table, so that the setup on the opposite side is automatically engaged, thus eliminating any manual work.

Vertically operated Fixture.  A two-station brazing setup equipped with fixtures having air oeprat3ed elevating platforms the two parts are brazed simultaneously by a series type coil of solid inductor design.  On the table may be seen a flange and a sleeve which are brazed together, as well as the silver alloy rings which are used as the bonding agent.  Two assemblies are placed on the fixture platform while it is at its lowest position.  Them, through operation of the air valve, the work pieces are elevated into the coil to the correct brazing position.  While one pair of parts is being brazed, the other fixture, is being loaded.  With this type of setup a generator is in almost constant service, its only down time being the required to throw the changeover switch and engage the start pushbutton.

Strength of Joints.  There is definite relation between the strength of the joint and the thickness of the alloying agent.  Usually the closer the fit between the surfaces to be joined, the higher the tensile strength of the joint.  This relationship is illustrated graphically in the chart shown in Fig. 97.   The lower curve represents alloy soldered joints, having clearances of from 0.001 to 0.0016 in., for which the theoretical tensile strengths in pounds per square inch are listed in the left-hand column.  Tensile up to about 8,000 lb. can be obtained when the clearance or fit is held to 0.002 in.  When the thickness of the joint increases, the tensile strength falls off until it reaches the strength of the solder.

Fig. 97 - Theoretical strength of soldered and brazed joints in relation to the thickness of brazing alloy.

The upper curve represents a silver brazing alloy, for which the tensile-strength calculations are shown in the right hand column.  Silver alloys make it possible to obtain strengths above 100,000 lb. per sq. in., where the fit or clearance between surfaces is held to 0.001 to 0.003 in.  As in the case of solder, the strength drops off as the thickness of joint increases to a point where it equals the tensile of the alloy.  Thin films of silver alloy offer  much more ductility tan heavier sections and, of course, are stronger and more economical.  Clearances below 0.001 in. often results in bare spots, inasmuch as the silver alloy is unable to flow and, as shown on the curve, the strength of the joint falls off sharply at this point.

The figures represented in this chart are for tubular sections having uniform circumferential spacing, as well as for flat parts, where parallel surfaces are maintained.  Usually with flat surface soldering and brazing it is best to apply a slight amount of pressure to the joint as it heats, in order to force out excess alloy and thus assure a thin film, which offers the strongest joint.

Copper Brazing.  Small gun sights brazing operation is carried out within a quartz tube, which is provided with a hydrogen atmosphere to prevent oxidation of the surface being joined.  The parts to be brazed are fed over the rod.  Seven pieces are inserted into the chamber at one time.  The coil is located within the quartz tube.  Each group of parts requires a separate time cycle for brazing.  After one group has been brazed, the door at the right is opened, and the next  charge is inserted into the correct brazing position.  This causes the previously brazed group to advance into a cooling chamber, which is located at the left.  The operation is practically continuous and produces a total of three thousand pieces per day.

These parts have been previously held together by a pin.  The coil used is on of the hairpin type and straddles the part eat each side, so that heat is induced to the joints only.  One of the advantages of brazing in a quartz tube of this type lies in the fact that the part can be observed during the brazing operation.  The heating cycle, however, is automatic and is controlled by the timer shown at the upper portion of the panel.

In joining a small tube and screw machine flange twelve pieces are brazed simultaneously.  the parts are located and centralized over a stud, mounted in the base of the fixture.  Since the generator is used almost continuously, the only lost time being that required to throw the change over switch from one station to the other, a setup of this type provides for maximum output.

Conveyor Soldering.   A continuous type setup  for soldering covers on condenser cans are completely assembled when soldered and comprise alternate layers of paper and tinfoil would together.  preparation includes fluxing the bottom edge of the can body by dipping in a solution.  A preformed ring of solder is then placed within the cover, and finally the cover is assembled to the condenser body.  The fit between both parts is important in order the proper joint may be obtained.

With the assembly ready for soldering, it is placed on the conveyor and fed between inductors at a speed of approximately 10 ft. per min.  As the work passes the inductor, the high frequency current flows around the entire edge of the condenser assembly, causing the solder to melt and flow within the joint.  The heat is inducted to the joint area only, and therefore the condensers cool rapidly after they pass the heating coils.

Because of the shape of the part, it is sometimes preferable to join one piece at a time and the coil used for the brazing of a Bourdon tube to a flanged casting.  For this operation a focus type inductor with a connected coil surrounding the entire surface to be heated is used.  The heating coil is made from a flat tubular shape of copper, so that water cooling can be maintained. 

A fixture used for soldering a series of plates to a shaft of a small variable radio trimmer condenser, by means of a single turn inductor coil.  The assembly is first placed within a fixture, and then a strip of solder material is placed on top.  With the assembly in place, power is applied to the coil, so that the high frequency current is induced into the shaft and plates, heating them sufficiently to melt the solder, and to solder all joints at one time.

For brazing four lugs to a cylindrical receiver body.  The fixture is arranged to hold two pieces, so that eight joints are completed at one setting.  The parts are assembled on to the fixture body when the locating plate is at its lowest position.  By raising the crosshandle the work pieces are elevated so that the lugs are surrounded by the heating coil.  The coil in this case is of the solid series type, 3/8 in., thick and formed out on the underneath side to clear the shape of the work piece.  A preformed ring of silver alloy is placed over each lug before the parts are placed on the fixture.

Continuous Feed for Brazing.  The assemblies to be brazed are placed on a continuous conveyor that is arranged with a series of work holding stations.  The parts enter the compartment in the center, where the induction heating coil is located.  The parts are heated as they pass the coil and them make a complete circuit around the conveyor.  This process provides sufficient time for cooling, so that they can be removed at the same position in which parts requiring brazing are loaded.

Induction heating equipment is widely used in progressive feeding operations for soldering, brazing, and hardening, in which various types of conveyors can be used.  As a rule such operations should be arranged so that a variable speed is available, as well as a certain amount of adjustment to the heating coil.  With this combination it is possible to make adjustments to compensate for incorrect heating temperatures.  For example, if during their travel the parts should become a little too hot in relation to the heating coil, the conveyor system can be speeded up slightly.  On the other hand, if loading difficulties should make this procedure undesirable, the coupling of the coil might be increased slightly to reduce the generation of heat proportionately.

Brazing alloy steel inserts to cast-iron valve bodies.  When the bushing is of sufficient size to permit the use of an internal coil, the setup can be made in the center.  The two leads coming from the coil should be kept very close together, however, so that there will be a minimum of heat losses along the flanged surface, where the leads enter the work.

Sometimes the insert of these parts is of such proportions that an internal type coil cannot be used.  In this case it is possible to use a single loop cylindrical coil.  On the other hand, when the flange of the insert is wider, thus requiring a broader distribution of heat, it is possible to use the multiturn pancake coil.

The induction brazing of valve inserts is more economical than the former method, which required heating of the entire valve body.  In operations of this kind, however, there are limitations and restrictions usually because of the shape and size of the part.  However, with slight modifications in the design of valve, induction brazing is possible.

For brazing fins to a sleeve and is arranged so that six pieces are brazed at one time.  Several of the fin assemblies may be seen at the front of the left hand fixture a cross section through the coil and the work piece.  The coil is of the internal hairpin type, over which the sleeve assembly is placed.

Each work holding station is proved with a locating sleeve for alignment of the assembly, to assure concentricity with the heating coil.  The fins are spot welded to the sleeve, as a preceding operation, to assure a firm metal-to-metal assembly.  For brazing, a preformed silver-alloy ring is placed over the sleeve so that when the assembly is properly heated the alloy flows throughout the joining surfaces.

Spot welding and induction brazing provide an excellent mans of joining a large variety of assemblies, especially those that do not hold together by virtue of their construction.  Many flat assembles can be joined by this combination  method.  Usually a small piece of silver wire, placed on top of or adjacent to the joint, will flow into the contacting surfaces by capillary action, thus producing an exceptionally strong bond.

Brazing of Tools.  The application of high frequency heat provides for a fast and dependable method for the silver alloy brazing of tungsten carbide cutting tools.  The process usually is carried out by forming a heating coil according to the shape of the tool and is so arranged that heat will be applied only to the area surrounding the joining surfaces.  The coils used can be shaped in many ways and cal be of the single turn or multiturn type, depending upon the size of tool.  With single shank tools the carbide tip and recess should be provided with a good fit; then, a piece of silver brazing material may be placed either on top of or directly under the tip. 

Another way to braze tools is by means of a multiturn coil which surrounds the entire end of the shank to be brazed.  In this method the tool is inserted into the coil and withdrawn after heating has been completed, so that as the alloy sets the tip can be pressed firmly into place.

A straight shank tool located within the heating coil this form of brazing, the heating coil can be more loosely coupled around the tool, because of the high intensity of the flux generated within the coil.  As soon as the tool has become heated beyond the melting temperature of the silver alloy, it is withdrawn to one side, so that the tip can be pressed down into position to form a firm metal-to-metal contact while squeezing out any excess brazing material. 

For the brazing of tungsten carbide inserts to milling cutters a copper tube heating coil of the double hairpin type can be used, so that heat is generated around the entire area of the tip and the adjacent area of the cutter body.  With this type of setup each tooth is brazed separately.

A variety of tungsten carbide tipped tools, some of the single shank type and other of the mutli-blade type.  One of these is the reamer within the coil at the upper section, which can be fabricated by induction brazing.  With multi-brazing, such as reamers would require, it is necessary to hold the tips in place by wire or some other means, so that the inserts will be held firmly against the recesses of the cutter body during the induction heating equipment operations.

An example of the application of induction heat in tool brazing in which an inert atmosphere is used.  This setup is arranged for the brazing of tungsten carbide tipped reamers.  The brazing operations is carried out by means of copper strip, which requires a melting temperature of about 2100˚F.  The heating coil is located around the outside of a cylindrical quartz chamber into which hydrogen is brought form the underneath side and burns off through the tube.  The use of the quartz chamber permits observation of the reamer during the heating cycle

In some instances it is possible to copper braze by means of induction heat without the use of a controlled atmosphere, tub usually such operations are limited to very small parts that heat up at exceptionally fast rates.  Usually, however, the steel part being brazed develops a scale at temperatures exceeding 1800˚F, with the result that the brazed surfaces become contaminated and will not produce a satisfactory bond.

A high frequency inductor, comprising two parallel bars, like those used for the soldering of condenser cans having an inserted type cover.  A preformed ring of solder is placed around the inside joint, and when the inductor is energized heat is concentrated to the open portion of the assembly, causing the solder to flow into the joint and to provide a smooth, tight joint.  For the soldering of parts, such as condensers, the work can be fed on a conveyor under the coil or a series of condensers can be placed in a fixture having a vertical travel and then elevated to proper relation to a long heating inductor, comprising two parallel bars.

For brazing six tubes and flanges simultaneously the heating coil is located at the uppermost section of the panel.  The parts to be brazed are inserted through the coil and are located by means of a plate just below the coil.  Alignment of the tubes at the bottom end is accomplished by cylindrical bushings, into which the tubes fit.  The upper portion of the tube is flared out or beaded, so that it fits into the counterbore of the flange.  A ring of silver brazing alloy is placed at the joint and when the proper temperature has been reached flows freely through the area requiring joining.