Adjusting Oxy-Acetylene Flames

In the last issue we looked at Safety when Brazing. This month, we move onto Adjusting Oxy-Acetylene Flames.

NOTE: Adhere to and apply all Safety Precautions!

Lighting the Torch

  • Connect the welding equipment and purge all the impure gas.
  • Set the working pressure for the work to be welded.
  • Point the blowpipe (torch) away from any persons or material, open the acetylene valve and light it with the flint lighter.

NOTE: If correct amount of gas is used, there will be a clean flame, free from smoke, and it will be burning close to the tip of the nozzle.

If there is a gap between the flame and the nozzle tip when the acetylene is lit, there is too much acetylene. A dirty and smoky flame will indicate insufficient oxygen.

Oxy-acetylene-Flame Blog Post 

Image: Oxy-Acetylene Flame

Adjusting a Carburising Flame

Open the oxygen control valve. As the pressure increases, a small white cone will appear centrally at the nozzle tip. The carburising flame will be indicated by a ragged bluish white feather surrounding the central white cone.

Carburising (or Reducing) Flame

The carburising (or reducing) flame, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. A carburising flame has an approximate temperature of 5500°F (3038°C).

A reducing flame can be recognised by acetylene feather which exists between the inner cone and the outer envelope. The outer flame envelope is longer than that of the neutral flame and is usually much brighter in colour. As shown in the image above.

Use of Carburising Flame

With iron and steel it produces very hard, brittle substance known as iron carbide. This chemical change makes the metal unfit for many applications in which the weld may need to be bent or stretched. Metals that tend to absorb carbon should not be welded with reducing flame.

Adjusting a Neutral Flame

As the oxygen control valve is opened further, a neutral flame is obtained when it is possible to observe two distinct zones, the central white cone and an outer envelope. The white cone should be sharply defined.

Neutral Flame

The neutral flame as shown in the image above is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to-one. It’s termed “neutral” because it will usually have no chemical effect on the metal being welded. It will not oxidise the weld metal; it will not cause an increase in the carbon content of the weld metal. As shown in the image.

The neutral flame is commonly used for the welding of:

  • Mild steel
  • Stainless steel
  • Cast iron
  • Copper
  • Aluminium

Adjusting an Oxidising Flame

If the oxygen control valve is opened further, a shorter flame is produced having an excess of oxygen. It will be possible to observe two distinct zones, the central white cone and an outer envelope. The white cone should be sharply defined.

Oxidising Flame

The oxidising flame results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidise or” burn” some of the metal being welded. The outer flame envelope is much shorter and tends to fan out at the end; on the other hand the neutral and carburising envelopes tend to come to a sharp point. An oxidising flame tends to be hotter than the neutral flame. This is because of excess oxygen, which causes the temperature to rise as high as 6300°F (refer to the image above).

The Oxidising Flame is commonly used for the welding of:

  • Copper base metals
  • Zinc base metals
  • A few types of ferrous metals, such as manganese steel and cast iron

When stopping work temporarily, close the acetylene control valve on the welding torch before closing the oxygen control valve. This will avoid depositing soot in the welding torch. When work is finished, close the cylinder valves and keep them closed until the equipment is required again.

The Brazing Process

Brazing is a joining process used extensively in refrigeration. A joint using one of the brazing alloys is the best method for insuring leak free equipment. It involves the use of filler metal with a melting temperature above 425°C, but below the melting point of the metal or alloy to be joined. The filler material (usually in rods) is a nonferrous (does not contain iron) metal or alloy.

Using an oxy-acetylene flame as a source of heat and a filler material of a copper and zinc alloy does the brazing in this element. When brazing, the strength of the joint depends upon the molecular penetration of the bronze filler metal into the surface of the base metal. Molecular penetration means that the smallest possible particles of each metal mix and form a bond.

In order to obtain the greatest joint strength, it is necessary for the metal parts to be closely fitted and for a thin uniform layer of filler metal to flow between the surfaces. The strength of a brazed joint is only as strong as the bond between the filler metal and the base metal.

Brazing is a very important industrial process. It is particularly valuable in jointing or repairing such metals as mild steel, cast iron and brass. It is used extensively in joining tubing and a variety of surface build-up applications.

Fluxes for Brazing

Without the use of a flux, the molten bronze rod would form beads, which roll across a metal surface without sticking. It would be like drops of water slipping over a dirty window pane. It is therefore necessary that the base metal not only be clean but also free from oxides. Since oxides form so quickly on a clean metal surface, they must be removed by using a high quality flux so that the filler alloy can make a good bond.

Introduction to Silver Brazing

The term brazing and silver soldering are commonly given to the same process. However, they do not mean the same thing. In silver, brazing the filler material melts and flows at temperatures lower than that of other brazing processes, and as with other brazing processes the base metal does not melt.

Silver brazing is widely used for joining both ferrous and nonferrous metals. It is an ideal process for joining small precision parts, tubing, copper and brass fittings – another important use is for mounting carbide tips on cutting tools. It is still one of the major processes used in the fabrication of jewellery, air conditioning and refrigeration units, bicycles, aircraft parts, electrical and household appliances.

The main difference between silver brazing operations and silver soldering processes, previously described, are:

  1. The type of filler alloy used
  2. The thickness of the alloy in the joint
  3. Use of much lower working temperatures in silver brazing

The flow temperature range of filler alloys for silver brazing

Silver brazing filler alloys usually contain a number of different metals. Among them are copper, cadmium, gold, phosphorus and zinc. A typical alloy for joining brass, bronze and copper parts contains 15% silver, 80% copper and 5% phosphorus.

It melts at about 635°C and flows into the joint by capillary action at 705°C. Capillary action means it is drawn into the joint by the attraction of molecules of the alloy for the molecules of the solid base metals.

Joint design is similar to those used in other brazing processes except that all joints should have a large bonding area. They must also be closely fitted. Since the filler alloy is fairly weak, joint strength depends upon the close bond between the mating surfaces and the paper thin layer of alloy.

The best capillary action and joint strength is obtained when the alloy film is about 0,05mm thick. Further build-up of a bead does not add strength and lap joint designs prove to be the best for silver brazing.

The parts to be joined MUST be cleaned before a flux is applied. Fluxes will not spread evenly over heated unclean metal. This uneven spreading leaves bare spots over which the filler alloy will not flow.


Most silver brazing requires a good flux. During brazing, the flux begins to melt and dissolves surface oxides at about 425°C. Further heating to around 595°C causes the flux to become a clear liquid. Just above this point, the flux begins to flow and will allow the molten filler alloy to from a complete bond between the close fitted surfaces. Flux is usually applied by dipping the heated filler rod into the powdered flux. The flux will stick to the rod and transfer to the joint when the torch applies enough heat as brazing begins. Paste fluxes may be applied to the filler rod with a small brush.

REMEMBER: Adhere to and apply all safety precautions


  1. Make the necessary pipe joint.
  2. Clean the surfaces of the pipes with abrasive paper until a shiny surface is obtained.
  3. Select a No. 3 welding nozzle and fit it into the welding torch.
  4. Light the torch and adjust the control valves to obtain a neutral flame.
  5. Apply the flux to both overlapping surfaces with a small brush or by using the torch method described previously.
  6. Position the pipes to be joined.
  7. Heat the joint carefully. Keep the inner cone of the flame at least 25mm away from the metal being joined.
  8. Watch the flux as the heat increases; it will turn a milky colour and then begin to bubble at about 320°C. Upon further heating, the flux will turn into a clear liquid and will soon begin to flow.
  9. Add some silver solder at the appropriate end and hold the flame over the joint at all times to keep the oxygen out. Continue until a thin film of silver solder has spread between the surfaces being joined.
  10. When the joint is completed, it is necessary to thoroughly clean the surfaces. Any flux left on the metal will cause corrosion.
  11. Examine the finished work for areas where the filler metal did not adhere.

Copper to Copper Joints

The same method is used for joining two copper pipes as in silver soldering with the exception that a copper-tech / copper-alloy rod is used. These rods are inexpensive in comparison to silver solder rods. They also do not require the use of any flux. The copper rods can only be used to join copper and no other material.

Joining Materials



Repairs can be made in aluminium components using special solders. These are low temperature solders and are used at temperatures far below that of brazing or silver soldering. Many may be applied using a soldering iron or a handy gas torch. These special solder have a melting temperatures in the 200°C – 400°C range. When using these solders special fluxes are used and the manufacturer’s instructions should be adhered to.

Until next time.

Grant K Laidlaw

Grant K Laidlaw F.S.A.I.R.A.C.



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