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Beginners' Guide to Welding, Part 3

Flames and brazing

by Julian Edgar

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At a glance...

  • Setting the type of flame
  • Brazing
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This article was first published in 2007.

Last week in Part 2 of this series we covered the basics of oxy acetylene gear, including setting the gear up and checking that there are no leaks. Now it’s time to light the flame and do some brazing, the easiest of the different oxy welding techniques.

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Before lighting the flame you should be wearing welding gloves and welding goggles, the latter with the tinted lenses flipped up. You should also have available a flint lighter designed for lighting oxy acetylene flames.

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Open the acetylene blowpipe valve a little: in a quiet workplace you should just be able to hear the gas flowing. Hold the nozzle pointing away from you and light the acetylene with the flint lighter. When the gas has ignited, open the acetylene blowpipe valve further until black smoke and soot cease being produced. Next, flip down the tinted lenses of the goggles and slowly open the oxygen valve. The colour and shape of the flame will immediately alter. (If the flame goes out, you’ve probably opened the oxygen valve too quickly and too far. Turn off the oxygen, turn down the acetylene and start the process again.)

Types of Flame

When you adjust the ratio of oxygen to acetylene you’ll soon see that three distinctively different flames can be produced.

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A carburising (or reducing) flame occurs when there is excess acetylene. In appearance the carburising flame has three distinctive parts – the outer flame envelope and two inner cones, where the innermost flame cone is surrounded by a luminous feather.

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Increasing the amount of oxygen will cause one of the inner cones to disappear. This flame is said to be a neutral flame. The remaining inner cone is long and sharply defined.

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A flame with an excess of oxygen (an oxidising flame) has a shortened innermost cone.

In most work a neutral flame is used. However, an oxidising flame can be used on brass alloys, as the loss of zinc is reduced. A carburising flame is used on steels being hard-faced as the carbon in the excess acetylene is absorbed into the surface of the steel. Always check during use that the flame remains as you have set it.

Brazing

Brazing is the easiest of the metal joining techniques achievable with oxy acetylene gear. It can be used to join steel, copper, brass and some types of aluminium. Additionally, dissimilar metals can be joined – for example, copper tube to steel plate.

Brazing Rods

In this type of welding, the brazing rod melts and becomes the glue that sticks the surfaces together. Therefore, the rod must be matched to the application – there’s no such thing as a universal rod that will work with all metals. Brazing rods vary in three characteristics:

1.   Thickness

2.   Material

3.   Flux-coated or bare

Let’s look at them in turn.

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As with electric arc welding, the diameter of the road should be proportionally matched to the thickness of the material being welded. That is, the thicker the material, the thicker the rod. If the rod is too thick for the application, it will take too long to melt and as a result, the materials being welded may melt rather than just getting dull red. A rod that is too thin will melt off before the metals being welded are hot enough and so the braze won’t ‘take’.

The material from which the brazing rod is made depends on the application – that is, the materials being brazed together, the required strength and appearance. The following table shows the different brazing rods available from one manufacturer. As can be seen, there are rods to suit different base metals, different working temperatures and giving different strengths.

BrazeTec Brazing alloy for brazing of

BrazeTec

brazing alloy

Working

temperature in °C

BrazeTec

Flux

Tensile strength of brazed joint (MPa)

Any steels

Copper /

Copper alloys

Nickel /

Nickel alloys

BrazeTec 5600

BrazeTec 4576

BrazeTec 4076

BrazeTec 3476

BrazeTec 4404

BrazeTec 3076

650

670

690

710

730

740

BrazeTec h

350

350

350

360

400

360

Stainless steels

BrazeTec 6009

720

BrazeTec Special h

400

Copper to Copper

BrazeTec Silfos 15

BrazeTec Silfos 5

BrazeTec Silfos 2

BrazeTec Silfos 94

700

710

740

760

n/a

250

BrazeTec brazing alloy for brazing of

BrazeTec

brazing alloy

Working

temperature in °C

BrazeTec

Flux

Shear strength of the brazed joint (MPa)

Tungsten carbide to steel

BrazeTec 4900

BrazeTec 49/Cu

BrazeTec 49/Cuplus

690

690

690

BrazeTec spezial h

BrazeTec spezial h

BrazeTec spezial h

250 – 300

150 – 300

200 – 300

Bare rods require the addition of a flux. The flux, which cleans the base materials of surface oxides, is applied to the work either directly by means of a brush or by heating the rod and then dipping it in the flux, so causing the flex to adhere to the rod. Flux-coated rods come with the flux already on the rod.

Step by Step - courtesy www.BrazeTec.com

Step 1 – Cleaning

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Oxide layers and foreign matter such as rust and scales must be removed from the brazing joint either mechanically or chemically before brazing. In the case of sensitive workpieces, thick layers of grease or oil can be wiped off or removed with solvents (e.g. acetone). Polished workshop pieces do not require any cleaning. Any oxide remaining on the workpiece after precleaning will be dissolved by the flux.

Step 2 - Applying flux

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The flux paste is applied to the cold workpiece using a brush. Most fluxes are slightly corrosive and skin contact, particularly with wounds, should be avoided.

Step 3 - Fixing the workpieces

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The pieces to be joined must be fixed in the correct position until the brazing alloy sets. A narrow brazing gap of between 0.05 mm (0.002") and 0.2 mm (0.008") is to be set if possible.

Step 4 - Heating the brazing joint evenly

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The brazing gap must be heated evenly to working temperature so that the brazing alloy can fill the gap. The brazing alloy selected should reach working temperature within 3 minutes at most. Overheating will damage the braze and the workpiece.

Step 5 - Placing the brazing alloy on the brazing gap

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The brazing alloy can be placed on the brazing gap when the flux has melted to an even glass flow and the working temperature has been reached. The brazing alloy fills the narrow brazing gap and rises upwards against gravitational force.

Step 6 - Cooling

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When the brazing alloy has filled the brazing gap, the workpiece must be left to cool until the brazing alloy returns to its solid state. The workpiece can then be removed from the clamp and then rinsed in water.

Step 7 - Removing flux residue

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Residual flux must be removed after brazing to prevent corrosion. Where possible, use water or a brush to remove any flux residue.

Technique

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As with all welding, best results come from practice.

Use a neutral to slightly oxidising flame. Hold the blowpipe so that the inner cone of the flame is just above the workpiece and heat the two surfaces until they are a dull red (in the case of steel). Introduce the flux-coated rod (or having previously applied flux to the join) and the brazing rod should melt on application to the metals. The brazing material should flow into and along the join, following the heat.

Where the gap is very small, just a tiny amount of rod is needed. In this situation, apply the rod and then withdraw it, then using the flame to heat the metal ahead of the braze and so make it flow forward into the joint. Where the gap is larger, or a fillet is to be built up, make sure that the brazing material has first taken to the surfaces before applying more rod and building the fillet.

If you get the join too hot, the brazing material will sizzle and spark; too cold and the braze will sit in blobs and not flow into and along the surface.

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Control is obtained by removing and applying the flame as necessary to maintain the correct heat, and removing and applying the rod as required to add the correct amount of filler material.

Joins with very small gap clearances will not need cleaning up but where there was a larger gap or a fillet has been created, mechanical sanding and grinding can be used to shape the join.

Conclusion

The benefits of brazing include reduced heat when compared with welding, so resulting in less distortion of the workpiece. The fact that the parent material is not melted means it’s far easier to weld very thin gauges, and – where tight-fitting joins are being brazed – the result is very neat without further work being needed. Brazing is also very easy to do, even by a beginner. The downsides are that the ultimate strength is usually lower than achieved by fusion, arc, MIG or TIG welding (and that’s especially the case at very high temperatures), and the brazing process is usually slower than electric welding. Finally, some brazing rods are quite expensive.

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