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Brazing Technology

Welding, soldering and brazing are the metal joining process. Each type of joining process has its own significance. Type of joining process to be applied for joining two parts depends on many factors. Below tabular comparison tells us the differences between the joining processes welding, soldering and brazing in aspects like strength comparison, temperature requirement, change in properties after joining, cost involved, heat treatment, preheating, etc.

S.No	Welding	Soldering	Brazing
1	Welding joints are strongest joints used to bear the load. Strength of the welded portion of joint is usually more than the strength of base metal.	Soldering joints are weakest joints out of three. Not meant to bear the load. Use to make electrical contacts generally.	Brazing joints are weaker than welding joints but stronger than soldering joints. This can be used to bear the load up to some extent.
2	Temperature required is 3800°C in welding joints.	Temperature requirement is up to 450°C in soldering joints.	Temperature may go to 600°C in brazing joints.
3	To join work pieces need to be heated till their melting point.	Heating of the work pieces is not required.	Work pieces are heated but below their melting point.
4	Mechanical properties of base metal may change at the joint due to heating and cooling.	No change in mechanical properties after joining.	May change in mechanical properties of joint but it is almost negligible.
5	Heat cost is involved and high skill level is required.	Cost involved and skill requirements are very low.	Cost involved and sill required are in between other two.
6	Heat treatment is generally required to eliminate undesirable effects of welding.	No heat treatment is required.	No heat treatment is required after brazing.
7	No preheating of workpiece is required before welding as it is carried out at high temperature.	Preheating of workpiece before soldering is good for making good quality joint.	Preheating is desirable to make strong joint as brazing is carried out at relatively low temperature.

Brazing Technology& Procedure

Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal.

Brazing differs from welding in that it does not involve melting the work pieces and from soldering in using higher temperatures for a similar process, while also requiring much more closely fitted parts than when soldering. The filler metal flows into the gap between close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal (known as wetting) and is then cooled to join the work pieces together. It is similar to soldering, except for the use of higher temperatures. A major advantage of brazing is the ability to join the same or different metals with considerable strength.

Brazing Procedure

1.) Cut Pipe Square

Cut to the exact length required using a tube cutter or hacksaw. If a hacksaw is used, a sawing fixture should also be used to ensure square cuts. Remove all inside and outside burrs with a reamer, file or other sharp edge scraping tool. If tube is out of round, it should be brought to true dimension and roundness with a sizing tool.

2.) Clean Tube and Inside Surface of Fitting

The joint surface areas should be clean and free from oil, grease, or oxide contamination. Surfaces may be properly cleaned for brazing by brushing with a stainless steel wire brush, or by a stiff rubbing with emery cloth. If oil or grease is present, clean with a commercial solvent. Remember to remove small foreign particles, such as emery dust, by wiping with a clean dry cloth. The joint surfaces must be clean.

3.) Select Brazing Alloy

When brazing Copper to Copper, low cost JA 15% may be used. These alloys contain phosphorus and are self-fluxing on copper. When brazing iron, steel or other ferrous metals, select a cadmium-free brazing alloy.
Do not use phosphorus bearing alloys as the joint may be brittle. To estimate the amount of brazing alloys needed, see Filler Metal Chart.
Proper Fluxing is important because the flux absorbs oxides formed during heating and promotes the flow of filler metal. To prevent excess flux residue inside refrigeration lines, apply a thin layer of flux to only the male tubing and, if possible, revolve the fitting once or twice on the tube to ensure uniform coverage.

4.) Assemble Tube and Fitting

Insert the fluxed tube end into the fitting. Maintain support to ensure the proper alignment until the brazing alloy solidifies. Maintain for a few seconds (or more) depending on the size of the joint area. The assembly is now ready to braze, using brazing alloy in rod, wire or coil form manually fed into the joint.
Adjust Torch Flames
For most brazing jobs using oxygen-acetylene gases, a 'neutral' flame should be used. The neutral flame has a well-defined inner cone. Avoid an oxidizing flame.
Neutral Flame - Inner cone bluish white, no acetylene feather, tip Bluish to Orange.
Oxidizing Flame - Sharp inner cone bluish white, inner cone two-tenths shorter than cone of neutral flame, tip nearly colorless.
Carburizing (Excess Acetylene) Flame - Inner cone bluish white, acetylene feather bluish white with feathery edge, tip light orange.

6.) Making Vertical 'Alloy-Up' Joints

Heat the tube first, then apply heat to the fitting. It is important to bring both pieces up to temperature evenly. If the tube is overheated the brazing alloy may run down the tube rather than into the joint.

Making Horizontal Joints

Heat the circumference of the tube first, then apply heat to the fitting. Deciding where to start feeding the alloy will depend on the size of the pipe and operator preference. On large diameter pipe, however, sometimes the best approach is to start at the bottom of the pipe. Apply brazing alloy at the bottom and work around the pipe.

As the alloy solidifies, it will create a 'dam' and help prevent the brazing alloy from running out of the joint as the remainder of the connection is filled. When adding alloy, make sure both the pipe and the fitting are up to temperature.

7.) Clean After Brazing

All flux residues must be removed for inspection and pressure testing. Immediately after the brazing alloy has set, quench or apply a wet brush or swab to crack and remove the flux residues. Use emery cloth or a wire brush if necessary.