What is Oxy-Acetylene Welding? | Oxy-Fuel Welding (2024)

What Is Oxy-Acetylene Welding?

Oxyacetylene welding is a process that uses heat from a gas flame to melt base materials and create fusion at the welding joint. This type of welding is autogenous, joining metal without the addition of filler metal. The more common approach is to use filler metal.

You might hear this type of welding referred to as “gas welding.” The flame is created by the combustion of oxygen and acetylene in a precise mixture or volume. Other fuel gases can be used, including propane, hydrogen, natural gas, and methyl acetylene-propadiene (MPS, formerly MAPP gas), but the most common for welding is acetylene.

It is the chosen fuel because oxygen and acetylene burn in a neutral flame at a temperature between 5,600 and 6,300 degrees Fahrenheit, the hottest of any gas flame and capable of melting most metals.

Other oxygen-fuel gas mixtures are not capable of producing the necessary BTUs for welding, but they can be used for soldering or brazing. The oxyacetylene process is versatile, as it can be used for both welding and cutting metals and also for heating, soldering, and brazing.

It can be much less expensive than arc welding and is very portable because it needs no electrical power source. It can also be used to weld a variety of thicknesses, though sections over 1⁄4 inch are difficult.

Unfortunately, the techniques of oxyacetylene welding can be very hard to master, and there are serious safety concerns with the extreme flammability of acetylene. Oxyacetylene welding equipment consists of an oxygen cylinder, an acetylene cylinder, pressure regulators, hoses, a torch, and a welding tip.

Related: What is Gas Welding?

Oxy-Acetylene Welding Equipment

1. Cylinders.

Oxygen is stored in a high-pressure cylinder (sometimes called a bottle or tank) at 2,200 pounds per square inch (psi) when full. The cylinder valve should always be covered with the provided safety cap when stored or during transport. Never lift the cylinder by the safety cap.

Always store and transport the cylinders in the upright position. Acetylene is an extremely unstable gas. It should never be pressurized above 15 psi in its free state. To safely pressurize the acetylene to 225 psi in a cylinder, the acetylene cylinder is filled with a porous material and acetone.

When pressurized, the acetylene is absorbed by the acetone, which provides a stable medium for the gas. When the pressure is released (the valve is opened), the acetylene bubbles out of the solution. Acetylene cylinders have fusible safety plugs that will melt in case of a fire and allow the gas to slowly release, not explode.

Acetylene cylinders should always be stored in an upright position. Using an acetylene cylinder at an angle will allow acetone through the regulator and hose, which can damage them.

Cylinders should always be kept upright and secured with a chain or strap to prevent them from falling over. Never use a cylinder as a roller for moving heavy objects or for any purpose other than its intended use.

2. Cylinder Valves.

Each type of cylinder has a valve that controls the flow of gas from the cylinder, operated by a hand wheel or valve wrench.

High-pressure cylinders, such as oxygen and argon, should always be opened fully to backseat the valve—the valve is leak-proof when completely closed or completely opened, but not otherwise.

Cylinder valves are designed for specific gases. Each cylinder valve is threaded for a specific regulator—this is a safety feature to ensure that the proper regulator is used for each gas. The acetylene valve should never be opened more than 3⁄4 to 1¹⁄₂ turns.

Any less may lead to insufficient gas flow and a backflash; more makes it difficult to turn the cylinder valve off quickly in case of an emergency. If the acetylene valve is opened all the way or opened too quickly, the acetone may escape from the cylinder and may destroy the hoses and regulator diaphragms.

3. Pressure Regulators and gauges.

Pressure regulators reduce the pressure of the gas leaving the cylinder. A single-stage regulator reduces the cylinder pressure to a working pressure in one step, while a two-stage regulator reduces the pressure in two steps.

A two-stage regulator can provide a more precise gas flow and is considerably more expensive than a single-stage regulator, though a single-stage is sufficient for most welding conditions. To adjust, turn the regulator screw clockwise to increase pressure and counterclockwise to decrease.

The regulator is completely off when the adjustment screw is loose or “backed out.” Always back out the adjusting screws as part of the post-welding routine.

WARNING: Never use oil or any other type of lubricant on regulators or other welding equipment. The combination of oil and oxygen in a spark-filled environment can lead to a fire in the regulator, which could cause a cylinder explosion.

4. Hoses.

The flexible rubber hoses that move the gas from the regulator to the torch are designed to be leak-proof and to withstand high pressure. The oxygen hose is green, and the acetylene or fuel hose is red.

Hoses are available as a single hose or dual hose (twin), in which the fuel and oxygen hoses are paired together. Twin houses are the most common and most convenient.

Always protect hoses from damage by moving them off of the floor when not in use, keeping them behind the welding area so they are not exposed to sparks, and keeping them out of traffic lanes or areas so they are not stepped on or run over by vehicles.

Hoses should not be allowed to come in contact with oily surfaces and should be protected from sunlight and chemical fumes to keep them in good condition. Always drain hoses when you have finished the welding session to remove the pressure, as this will extend the life of the hoses.

5. Fittings.

Fittings connect the hoses to the regulator at one end and the torch at the other. For safety, the nut for the fuel hose (red) uses left-handed threads and has a groove machined around the surface of the nut to help identify that it is for fuel use only.

The oxygen (green) hose nut has right-hand threads and no groove. Never interchange the oxygen and acetylene hoses or fittings— they are color-coordinated and have specific threads to help maintain consistency and safety.

Never force a fitting: brass is a very soft metal, and it is easy to damage the fittings. Always hand-thread and hand-tighten fittings before using a wrench and be careful not to overtighten them. Never use pliers to tighten fittings, as they will damage the brass fittings.

6. Check Valves and flashback Arrestors.

Check valves and flashback arrestors are two safety features designed to prevent reverse gas flow or flashbacks. The check valve allows gas to flow from the cylinder to the torch and in that direction only.

If the gas pressure within the torch exceeds the hose pressure, a spring closes the valve to prevent backflow. In the event of a flashback, the check valve needs to be replaced. A flashback arrestor is installed between the torch and the hose, and it offers more protection than a check valve.

The flashback arrestor prevents burning oxygen from flashing back into the hoses and regulator, which can cause an explosion. The flashback arrestor consists of a check valve, pressure-sensitive valve, stainless steel filter, and heat-sensitive check valve.

In the event of a flashback, the flashback arrestor does not need to be replaced. When these valves are used, a restriction is created, and gas supply pressure may have to be increased to reduce the chance of fuel starvation—a condition that may cause the torch to overheat.

7. Torch.

The torch mixes and controls the flow of fuel gas and oxygen. The torch consists of valves, a torch body, a mixing chamber, and a welding tip. In a combination torch, the mixing chamber and adjustment valve will be attached to the cutting attachment.

8. Welding Tips.

The welding tips attach to the torch body and come in many sizes to create different-sized flames. The tip size refers to the orifice opening: as the tip size increases, the amount of fuel gas mixture increases and produces a hotter flame.

It is important to match the correct size tip to the correct thickness of the metal being welded. Matching the tip size, the welding material, and the gas pressure is critical to creating quality welds.

How To Set Up An Oxyacetylene Welding Outfit

1. Secure the cylinders in an upright position, chained to a cart or strapped to a wall or post. Remove the protective cylinder caps. Wipe off the cylinder valve seats, regulator connections, and hose connections with a clean cloth.

Crack open each cylinder valve briefly to expel any trapped dirt particles. “Flat top” acetylene cylinders (inset) may have antifreeze in the recessed valve seat. Use a clean rag to remove the liquid and dry the valve seat. This style of acetylene cylinder requires a cylinder wrench to open the valve.

2. Attach the regulators to the cylinders. (The acetylene connectors have left-hand threads.) Always hand tighten, then use a fixed wrench, not pliers or an adjustable wrench, to tighten. Do not overtighten—firm seating is all that is necessary.

Attach the hoses to the regulators. The acetylene hose is red and left-hand threaded. The oxygen hose is green. Never use grease, oil, or pipe dope to lubricate fittings. Grease and oil can ignite spontaneously when they come in contact with oxygen—even without a spark or flame present.

3. Turn the regulator adjustment screws on the oxygen and acetylene regulators counterclockwise until they are loose. (Some regulators have a knob.)

4. Open the oxygen valve slowly all the way while standing to the side in case the regulator gauge glass shatters. Turn the regulator adjustment screw until oxygen begins to flow through the hose, then loosen the regulator adjustment screw to stop the oxygen flow. Slowly turn the acetylene cylinder valve 3⁄4 to 1¹⁄₂ turns.

5. Adjust the acetylene regulator valve until the gas begins to flow, then loosen the regulator adjustment screw to stop the flow. Attach the torch to the hoses. Be sure to pressurize the system and check for leaks before lighting.

How To Pressurize (Turn On) An Oxyacetylene Welding Outfit

Make sure both torch valves are off. Turn both regulator adjustment screws counterclockwise until loose. Slowly turn the oxygen cylinder valve on. Once open, turn the valve all the way open to ensure proper seating without leaks.

Turn the oxygen regulator adjustment screw clockwise until the gauge reads the desired pressure. (Refer to the manufacturer’s specific instructions for operating pressures.) Open the oxygen valve on the torch to check the flowing oxygen pressure.

Adjust if necessary and close the torch valve. Slowly open the acetylene cylinder valve 3⁄4 to 1¹⁄₂ turns. (Leave the wrench on the valve if it is a wrench-style valve.) Turn the acetylene regulator pressure adjustment screw until the desired pressure reads on the regulator gauge.

Open the acetylene valve on the torch briefly to make sure the flowing pressure matches the desired working pressure. If not, adjust the regulator until the proper pressure is reached.

Checking For Leaks

Apply leak-detecting solution to all connections with a small brush. (You can use soap and water so long as the soap is not petroleum-based.) If any connections cause bubbles in the solution, tighten the connections and check again.

WARNING: Backfire and flashback are two hazardous situations that can be caused by improper gas pressures. Backfire is the pre-ignition of the acetylene and oxygen inside the tip that causes a popping sound.

This may damage the tip or spray molten metal from the weld area. Flashback is the flame burning backward into the torch or hoses, causing a popping or squealing noise. Flashback can cause an explosion in the hoses.

Avoid both hazards by matching the tip size to the material being welded and by using the proper pressure settings. Using lower pressures than recommended can cause backfire and possibly flashback.

Lighting The Torch

Prelighting Checklist:

• Make sure the torch valves are closed.
• Turn both regulator adjustment screws counterclockwise until loose.
• Slowly turn on the oxygen cylinder valve all the way. Turn the regulator adjustment to the proper pressure.
• Slowly turn the acetylene cylinder valve 3⁄4 to 1¹⁄₂ turns. Turn the regulator adjustment valve to the proper pressure.

Some welders have been taught to open both the acetylene and oxygen valves before lighting the torch to avoid the smoky acetylene flame. This practice is no longer recommended. Never light a torch with a match or butane lighter.

Lighting The Torch:

1. Hold the torch in one hand with the thumb and forefinger on the acetylene torch valve. Hold the striker in front of the torch about 3 to 6″ away at a slight angle. Turn on the acetylene torch valve 1⁄4 to 1⁄2 turn. Immediately use the spark lighter to light the flame.
2. The flame will be yellow and smoky.
3. Put the striker down and adjust the acetylene torch valve with your right hand so the flame is burning without producing soot. The flame should not be separated from the torch (inset). Open the oxygen torch valve slowly. Adjust the oxygen to get a bright white inner flame and a bluish outer flame. Turn down the acetylene to eliminate the excess acetylene feather if present. When you have finished welding, turn off the oxygen first, then the acetylene.

Flame & Flame States

The flame of an oxyacetylene torch has two parts—the inner or primary flame and the outer or secondary flame. The flame has different temperatures at different locations. The outside edges are cooler because they are burning with the ambient air, which is only 21% oxygen.

The torch tip is cooler because complete combustion hasn’t been reached. The hottest area is the tip of the primary or inner flame cone. Here the gases are completely combusted and are insulated by the secondary flame.

There are three flame states for the oxyacetylene flame: carburizing, neutral, and oxidizing.

• Carburizing or reducing flame has an excess of fuel. This is a useful flame as it will break down metal oxides to get into the oxygen, thus cleaning the weld area to a small degree. This process adds carbon to welds, which makes them harder. The carburizing flame has a bright white primary flame, an acetylene “feather” around the primary flame, and a bluish-white secondary flame with orange edging.
• The neutral flame is the exact point where the feather and the inner cone come together. In this flame, there is exactly enough oxygen present to provide total combustion of the fuel gas. Most welding and cutting operations use a neutral flame. The neutral flame has a bright white primary flame and a colorless to bluish secondary flame.
• The oxidizing flame has an excess of oxygen. The white cone of this flame is small and pointed and somewhat paler than the neutral flame. A hissing sound often accompanies this flame. This flame is not particularly useful as it hastens oxidizing, which is not desirable in welding. It can, however, be used for removing carbon from molten metal, thus softening the metal.

How To Weld with Oxyacetylene?

Pre-weld Checklist:

• Check hoses for damage before pressurizing the system.
• Prepare metal for welding by wire brushing or sanding off mill scale and rust. Use acetone or denatured alcohol to remove oil or other chemical residues.
• Use fire bricks to avoid unnecessary heat loss and prevent welding to the welding table.
• See the manufacturer’s recommendations for appropriate tip sizes and gas pressures.
• Set up materials and clamp if necessary.

NOTE: The directions for oxyacetylene welding are for righthanded welding. Reverse the directions for left-handed welding, or if you find it easier to manipulate the filler rod with your right hand.

Step by Step:

1. Select an appropriate filler rod and lay it on the table next to the bricks. Light the torch and adjust to a neutral flame. Pull down your face shield. Place small fusion tack welds at each end of the joint and in the middle if it is a long joint. (A fusion weld uses no filler rod.)

Turn off the torch, oxygen first then acetylene, and check that your tacked piece is still in the desired position. If not, use a hammer to move it into position or break the tack weld and reposition.

2. With the torch at a 45° angle to the right and oscillating the torch in a 1⁄4 to 1⁄2″ circle over both metal pieces, create a weld puddle at the right end of your workpiece.

3. When a molten puddle has formed, move the filler rod close to the puddle and flame, but not in it. Begin moving slowly to the left while oscillating and maintaining the molten puddle. Dip the filler rod into the middle of the molten puddle and remove it, but keep it within the heat zone.

4. Continue dipping, oscillating, and moving to the left. As you reach the end of the weld, the cumulative heat buildup may make it necessary to adjust to a shallower angle to deflect heat away from the puddle and prevent burn-through. When finished, turn off the oxygen torch valve first, then the acetylene torch valve. The weld should penetrate to the back without burning through.

Depressurizing An Oxyacetylene Welding Outfit

When you have finished welding, or if you are going to stop welding for more than 10 or 15 minutes, depressurize your setup.

The hoses and regulators are designed to leak small amounts of fuel and oxygen if they are pressurized and not being used, so it is important for safety and economy to depressurize them.

You will soon be so comfortable with the pressurizing and depressurizing steps that it won’t seem like an inconvenience at all. It is important to do the fuel gas and oxygen in separate steps, to prevent having mixed, unburned oxyacetylene in the torch.

1. Close the fuel gas cylinder valve.
2. Open the fuel gas torch valve until both gauges read zero.
3. Loosen (counterclockwise) the fuel gas regulator adjustment screw.
4. Close the fuel gas torch valve.
5. Close the oxygen cylinder valve.
6. Open the oxygen torch valve until both gauges read zero.
7. Loosen (counterclockwise) the oxygen regulator adjustment screw.
8. Close the oxygen torch valve.

Why Use Acetylene For Welding?

Welding is an important fabrication process in various manufacturing industries. There are different types of welding procedures. One important type is gas welding. In this type of welding, the heat necessary for the process is produced by the combustion of oxygen and acetylene. Sometimes, hydrogen, butane, and propane are also used as replacements for acetylene.

The combination of oxygen and acetylene to cut metals has been in use since approximately 1906. Over the years, acetylene has earned recognition as one of the safest, yet strongest welding gas. But, there is a rising misconception that propane is a better welding gas than acetylene. This post shares the differences between these gases and discusses the benefits offered by acetylene gas in detail.

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