Correctly adjusting an oxy-acetylene flame is essential for safe, controlled welding and brazing. The balance between oxygen and acetylene affects the shape, temperature and chemical behaviour of the flame.
The three main oxy-acetylene flame types are:
Carburising, also known as reducing
Neutral
Oxidising
Each flame has distinctive visual characteristics and is suited to different materials and applications.
For refrigeration and HVAC technicians, understanding these flame types is particularly important when brazing copper tubing, fittings and other components.
Safety notice: Oxy-acetylene equipment must only be used by trained and competent people. Follow all applicable safety procedures, equipment instructions and workplace requirements. Wear the correct personal protective equipment and keep the torch directed away from people and combustible materials.
Understanding Oxy-Acetylene Flame Types
The flame type is controlled by changing the proportion of oxygen and acetylene leaving the torch.
| Flame type | Gas balance | Visual characteristics | Typical applications |
|---|---|---|---|
| Carburising or reducing | More acetylene than required for a neutral flame | A ragged bluish-white feather surrounds the central white cone | Selected applications where additional carbon will not damage the material |
| Neutral | Oxygen and acetylene are almost evenly balanced | A sharply defined central white cone and a separate outer envelope | General welding and many brazing applications |
| Oxidising | More oxygen than required for a neutral flame | A shorter flame with a sharply defined cone and an outer envelope that tends to fan out | Certain copper-base, zinc-base and selected ferrous metals |
The correct flame must be selected according to the base material, filler alloy, equipment instructions and approved work procedure.
Preparing and Lighting the Torch
Before lighting the torch, inspect and connect the equipment according to the manufacturer’s instructions and your workplace safety procedure.
Prepare the Equipment
Complete the following checks before introducing a flame:
Confirm that the cylinders, regulators, hoses, flashback arrestors and torch are suitable for the work.
Check the equipment for visible damage, leaks or loose connections.
Fit the correct nozzle for the application.
Set the required working pressures.
Purge the gas lines according to the approved procedure.
Remove combustible materials from the work area.
Position suitable fire-control equipment nearby.
The nozzle size and working pressure must suit the torch, material thickness and work being completed.
Light the Acetylene
Point the torch away from people, equipment and combustible materials.
Open the acetylene control valve and ignite the gas using an approved flint lighter.
The acetylene flame should burn cleanly and close to the nozzle tip.
A visible gap between the flame and the nozzle may indicate excessive gas flow. A dirty or smoky flame indicates that the gas mixture still requires adjustment.
Never use matches or a cigarette lighter to ignite an oxy-acetylene torch.
Adjusting a Carburising Flame
A carburising flame is created when the gas mixture contains more acetylene than is required for a neutral flame. It is also called a reducing flame.
The approximate flame temperature is 3,038°C.
How to Identify a Carburising Flame
Gradually open the oxygen control valve after lighting the acetylene.
As oxygen is introduced, a small white cone will appear at the nozzle tip. A carburising flame can be identified by:
A central white cone
A ragged bluish-white acetylene feather
A visible feather between the inner cone and outer envelope
A longer outer flame envelope
A brighter flame than a neutral flame
The length of the acetylene feather indicates how much excess acetylene is present.
Effects on Iron and Steel
A carburising flame can introduce additional carbon into some metals.
When used on iron or steel, it may produce iron carbide. This can leave the material hard and brittle, making it unsuitable for applications where the completed weld must bend, stretch or remain ductile.
Metals that readily absorb carbon should not be welded with a carburising flame unless the procedure specifically requires it.
Adjusting a Neutral Flame
A neutral flame is produced when the oxygen and acetylene are almost evenly balanced.
Continue opening the oxygen control valve until the acetylene feather disappears.
How to Identify a Neutral Flame
A correctly adjusted neutral flame has two distinct zones:
A sharply defined central white cone
A separate outer flame envelope
The inner cone should be clear and stable, with no visible acetylene feather.
The oxygen-to-acetylene ratio in the mixture leaving the torch is approximately one-to-one.
Why It Is Called a Neutral Flame
A neutral flame is described as neutral because it generally has no significant chemical effect on the metal being heated.
It should not:
Oxidise the weld metal
Increase the carbon content of the weld metal
Produce an excessively reducing atmosphere
A neutral flame is commonly used when working with:
Mild steel
Stainless steel
Cast iron
Copper
Aluminium
It is also widely used for brazing operations where controlled, even heating is required.
Adjusting an Oxidising Flame
An oxidising flame is created by opening the oxygen control valve beyond the neutral setting.
This produces a gas mixture containing more oxygen than is required for neutral combustion.
How to Identify an Oxidising Flame
An oxidising flame has:
A short, sharply defined central white cone
A shorter outer flame envelope
An outer envelope that tends to fan out
A more intense flame appearance
The neutral and carburising flame envelopes usually narrow to a point. The oxidising flame is shorter and tends to spread at the end.
An oxidising flame may reach a temperature of approximately 3,482°C.
Effects on the Base Metal
The excess oxygen can oxidise or burn certain metals during welding.
An oxidising flame is used for selected applications involving:
Copper-base metals
Zinc-base metals
Manganese steel
Certain cast-iron applications
Only use an oxidising flame when it is specified for the material and joining procedure.
Shutting Down the Torch
When temporarily stopping work, close the acetylene control valve on the torch before closing the oxygen control valve. This helps reduce soot deposits in the torch.
When the work is finished:
Shut down the torch according to the equipment manufacturer’s procedure.
Close both cylinder valves.
Release pressure from the system as required.
Close the torch control valves.
Back off the regulator adjusting screws.
Store the equipment safely.
Keep the cylinder valves closed until the equipment is needed again.
The exact shutdown sequence must follow the instructions supplied with the torch and regulators.
What Is Brazing?
Brazing is a joining process used extensively in refrigeration and air-conditioning work.
The process uses a filler metal with a melting temperature above 425°C but below the melting point of the base metals being joined.
The base metals do not melt. Instead, the filler alloy melts and flows between the closely fitted surfaces.
How a Brazed Joint Forms
The strength of a brazed joint depends on the bond between the filler metal and the base metals.
For a strong joint:
The parts must fit closely together.
The surfaces must be clean.
Oxides and contaminants must be removed.
The joint must be heated evenly.
A thin, uniform layer of filler metal must flow between the surfaces.
The filler material is usually supplied in rod form and is commonly made from a non-ferrous metal or alloy.
Brazing is used to join tubing and to repair or join metals such as:
Mild steel
Cast iron
Brass
Copper
Why Flux Is Used for Brazing
Metal surfaces form oxides quickly when exposed to air and heat.
Without suitable flux, molten filler material may form beads and move across the surface without bonding correctly.
Flux helps to:
Dissolve surface oxides
Protect the metal during heating
Improve the flow of the filler alloy
Support bonding between the filler and base metal
Flux does not replace proper surface preparation. The parts must still be thoroughly cleaned before heating.
What Is Silver Brazing?
Silver brazing is a joining process that uses a silver-containing filler alloy.
The filler alloy melts and flows at a lower temperature than the base metals, which remain solid throughout the process.
Silver brazing is commonly used for joining:
Ferrous metals
Non-ferrous metals
Copper tubing
Brass fittings
Small precision parts
Refrigeration components
Air-conditioning components
Electrical components
Cutting-tool tips
Silver Brazing and Silver Soldering
The terms silver brazing and silver soldering are sometimes used interchangeably, but they are not always technically identical.
The processes may differ according to:
The filler alloy used
The working temperature
The flow temperature of the alloy
The thickness of the filler layer
The intended application
Always select the filler alloy according to the metals being joined and the approved joining procedure.
How Capillary Action Supports the Joint
Capillary action draws molten filler alloy into the narrow space between closely fitted surfaces.
For capillary action to work effectively:
The joint surfaces must be clean.
The parts must fit closely.
The correct joint clearance must be maintained.
The joint must be heated evenly.
The filler alloy must be applied at the correct temperature.
A large filler bead does not necessarily increase joint strength.
Silver-brazed joints normally rely on a thin layer of filler alloy distributed across a sufficiently large bonding area. Lap joints are often effective because they provide a larger contact area.
Preparing a Joint for Silver Brazing
Good preparation is essential for a reliable brazed joint.
Clean the Surfaces
Use suitable abrasive material to clean the surfaces until they appear bright and free from:
Dirt
Oil
Grease
Paint
Corrosion
Scale
Surface oxides
Do not handle the cleaned surfaces unnecessarily, as oils from the skin may contaminate them.
Check the Joint Fit
Position the components so that the mating surfaces are closely and evenly fitted.
Excessive gaps may prevent effective capillary action. A joint that is too tight may also restrict filler flow.
The required clearance must be confirmed from the filler-alloy and equipment instructions.
Apply the Flux
Apply the correct flux to the surfaces that will overlap.
Flux may be applied with a clean brush or according to the filler and flux manufacturer’s instructions.
During heating, the flux may:
Dry on the surface.
Become milky in appearance.
Begin to bubble.
Turn into a clear liquid.
Flow across the joint.
These changes can help indicate the joint temperature, but the exact behaviour depends on the flux being used.
Silver Brazing Procedure
Only trained and competent personnel should perform silver brazing.
Prepare the pipe or component joint.
Clean all mating surfaces thoroughly.
Select and fit the correct welding nozzle.
Position and secure the components.
Apply the appropriate flux.
Light the torch safely.
Adjust the torch to produce a neutral flame.
Heat the joint carefully and evenly.
Move the flame around the joint to prevent localised overheating.
Observe the flux as the temperature increases.
Apply the filler material when the joint reaches the correct temperature.
Allow the molten filler to flow through the joint by capillary action.
Remove the heat once the filler has flowed sufficiently.
Allow the joint to cool according to the approved procedure.
Remove all remaining flux.
Inspect the completed joint.
Do not melt the filler rod directly with the flame. The heated joint should melt the filler material and draw it into the joint.
Controlling the Heat
The inner cone of the flame should not be held directly against the surface unless the approved procedure requires it.
Keep the torch moving to distribute the heat evenly.
Directing excessive heat at one point can:
Overheat the base metal
Burn the flux
Oxidise the surface
Damage the filler alloy
Distort the tubing
Weaken the completed joint
Heat the thicker or heavier component first, as it will normally require more energy to reach brazing temperature.
Applying the Filler Material
Apply the filler material at the joint opening while maintaining even heat around the connection.
The filler should melt against the heated metal and flow through the joint.
Continue adding only enough filler material to create a complete, thin bond between the surfaces.
A large external bead does not necessarily indicate a stronger joint.
Cleaning and Inspecting the Joint
Allow the joint to cool before removing flux residue.
Remaining flux may contribute to corrosion and must be cleaned away according to the flux manufacturer’s instructions.
Inspect the joint for:
Complete filler flow
Gaps or missed areas
Cracks
Excessive oxidation
Burnt flux
Distortion
Overheating
Excessive filler build-up
The joint should be smooth and show evidence that the filler material has flowed around and through the connection.
Complete any required pressure or leak testing according to the applicable refrigeration procedure.
Brazing Copper-to-Copper Joints
Copper-to-copper joints may use a copper-phosphorus filler alloy, depending on the application and system requirements.
Certain copper-phosphorus alloys can be used on copper-to-copper joints without additional flux because the phosphorus acts as a fluxing agent.
This does not apply to every filler alloy or material combination.
Copper-phosphorus filler materials must not automatically be used for joining copper to brass, steel or other metals. Confirm compatibility with the filler manufacturer and the approved procedure before beginning work.
Selecting the Correct Filler Material
The filler material must be compatible with both base metals and the operating conditions of the completed system.
Common combinations include:
| Filler material | Typical base materials |
|---|---|
| Brazing bronze rod | Mild steel, brass and cast iron |
| Silver brazing alloy | Mild steel, copper and brass |
| Copper-phosphorus alloy | Copper-to-copper joints |
The exact filler composition must be selected according to:
The metals being joined
The required joint strength
The system operating temperature
The system pressure
Vibration conditions
Corrosion risk
Applicable industry requirements
Repairing Aluminium Components
Aluminium components may require specialised low-temperature soldering or brazing products.
These products can operate at temperatures below those normally associated with conventional brazing or silver brazing.
Specialised fluxes may also be required because aluminium forms a strong oxide layer very quickly.
When working with aluminium:
Identify the aluminium alloy where possible.
Select a compatible filler material.
Use the specified flux.
Avoid excessive heat.
Follow the product manufacturer’s instructions.
Confirm whether repair is permitted for the component.
Not every aluminium refrigeration component is suitable for repair.
Common Brazing Problems
The Filler Forms Beads
Filler material that forms beads instead of flowing may indicate:
Dirty surfaces
Remaining oxides
Insufficient heat
Incorrect flux
Burnt flux
Incompatible filler material
Stop and allow the joint to cool before cleaning and preparing it again.
The Filler Does Not Enter the Joint
Poor capillary flow may be caused by:
Incorrect joint clearance
Uneven heating
Insufficient temperature
Excessive temperature
Poor joint alignment
Contamination
Incorrect filler selection
The Flux Turns Dark or Burns
Dark or burnt flux may indicate overheating.
Reduce the heat, keep the torch moving and avoid directing the inner cone at one point for too long.
The Tube Becomes Distorted
Distortion can occur when thin tubing is overheated.
Use controlled heat, select the correct nozzle and distribute the flame evenly around the joint.
Key Safety Precautions
Before welding or brazing:
Inspect the torch, hoses, regulators and connections.
Check for gas leaks using an approved method.
Keep cylinders upright and secured.
Use flashback arrestors and non-return valves where required.
Remove flammable materials from the work area.
Ensure adequate ventilation.
Wear suitable eye, hand and body protection.
Keep fire-control equipment nearby.
Never use oil or grease on oxygen equipment.
Never direct the flame towards a gas cylinder.
Never leave a lit torch unattended.
Follow site-specific hot-work permit requirements.
Refrigeration systems must be safely isolated, depressurised and prepared before heat is applied.
Never apply a flame to a sealed, pressurised or improperly prepared system.
Key Points to Remember
Learn to identify carburising, neutral and oxidising flames by their shape.
Select the flame type according to the material and approved procedure.
Use a neutral flame where specified for silver brazing.
Clean all joint surfaces thoroughly.
Choose a compatible filler material and flux.
Maintain the correct joint clearance.
Heat the joint evenly.
Allow the joint to melt the filler material.
Remove all remaining flux.
Inspect and test the completed joint.
Follow all safety and equipment instructions.
Technical Support for Refrigeration and HVAC Work
Correct flame adjustment and proper brazing technique help technicians create clean, reliable joints while reducing the risk of overheating, oxidation and incomplete filler flow.
MACS Cool supports refrigeration and HVAC professionals across South Africa with refrigeration products, tools, spares, accessories and practical technical resources.
Explore the MACS Cool product range, visit a branch or contact the team for assistance with selecting suitable products for your refrigeration or air-conditioning work.
On Spec. On Site. On Time.
What are the three main oxy-acetylene flame types?
The three main oxy-acetylene flame types are carburising, neutral and oxidising. Each is identified by the balance of acetylene and oxygen and by the shape of the inner cone and outer envelope.
How do you identify a neutral oxy-acetylene flame?
A neutral flame has two distinct zones: a sharply defined central white cone and an outer envelope. The supplied guide describes the oxygen-to-acetylene ratio as almost one-to-one.
Which oxy-acetylene flame is used in the supplied silver brazing procedure?
The supplied silver brazing procedure uses a neutral flame. The joint must be cleaned, fluxed where required, heated carefully and inspected after the filler metal has flowed.
Why is flux used during brazing?
Flux helps remove surface oxides and allows the molten filler alloy to spread and bond more evenly to clean base metal.
