Oxy-Acetylene Welding

Unlike in MIG MAG welding and TIG welding, in the oxygen welding family, the heat necessary for melting the base metal and the filler material is provided by the combustion of a gas (fuel) with oxygen (oxidizing). The fuel and oxygen are contained in high-pressure cylinders, and they are conveyed separately to the device, called a "torch", which takes care of mixing them in the appropriate ratio; from this, they exit through a special nozzle burning at a very high temperature.

The combustible gases that can be used are:

• Hydrogen (oxyhydrogen flame), which reaches 2500 °C
• Acetylene (oxyacetylene flame), which reaches 3200 °C
• Other gases including methane, propane and butane, with temperatures around 2750 °C

How oxyacetylene welding works

In oxyacetylene welding, the flame reaches a temperature suitable for welding. Contrary to other oxygen welding which reaches lower temperatures, they are used to heat the material or for oxyfuel cutting, but not for welding.

The parameters that influence oxyacetylene welding are as follows:

• Type of base material to be welded
• Thickness of the pieces
• Joint type
• Position of the welding seam according to the position operator

To these must be added the welding parameters, including the inclination of the torch, the direction and the speed of advancement of the torch and the rod.

Working Principle

1. Gas welding is a fusion welding process in which welding is done by heating the workpiece with flames obtained from oxy-fuel gases.
2. In these processes, a mixture in the proper proportion of such as acetylene, liquefied petroleum, methyl acetylene propadiene, natural gas and hydrogen with oxygen are burnt to get flame.
3. This flame can be used to melt metal and the flame is directed by a welding torch.
4. A filler metal is sometimes added, which is available as a rod or wire with or without flux.

Heat and Temperature of fuel Gases

Requirement for Oxy-Acetylene Flame

Figure 1 - Combustion Triangle

Materials suitable for Oxy-Acetylene Welding

• Plain carbon steel and low-alloy steel.
• Cast iron (best results)
• Stainless steel
• Aluminium and magnesium
• Copper and copper alloys
• Mild steelhead
• Mild steel

Typical  Oxy-Acetylene Welding (OAW) Station

Figure 2 - Oxy-Acetylene Welding Station

Oxy-Acetylene Welding Equipment

1. Oxygen gas cylinder
2. Acetylene gas cylinder
3. Oxygen pressure regulator
4. Acetylene pressure regulator
5. Oxygen gas hose (blue)
6. Acetylene gas hose (red)
7. Welding torch
8. Filler rods
9. Protective clothing for welders (apron, gloves, goggles)
10. Trolleys for cylinder transportation

Main valve of Oxygen Cylinder

Oxygen cylinders incorporate a thin metal “ pressure safety disk” made of stainless steel and are designed to rupture prior to the cylinder to becoming damaged by pressure

The cylinder valves should always be handled carefully. For more details see Figure 3 below

Figure 3 - Main valve oxygen cylinders

Oxygen Cylinder

Cylinders are regularly re-tested using hydrostatic (NDE) while in test.

Cylinders are regularly chemically cleaned and annealed to relieve “job site” stresses created by handling. Depending on the geographical location these cylinders should be inspected periodically (mandatory) Figure 4 below shows all the details.

 Figure 4 - Oxygen Cylinder

1. Type of cylinder ‑ ICC3A or DOT3AA indicates medical gases
2. Tank serial number
3. Identification symbol of the purchaser and/or manufacturer of cylinder.
4. Manufacturing data
5. Manufacturer's symbol
6. Date of first hydrostatic test and most recent testing
7. Owner's identification

Cylinder Pressure Regulator

The pressure regulators are used to reduce high storage cylinder pressure to lower working pressure. Most regulators have gauges for cylinder working and working pressure. Normally, the Regulator diaphragms are made of stainless steel. Figure 5 below shows all the details.

Figure 5 - Pressure Regulator

There are two basic types of regulator mechanisms: the nozzle-type and the stem-type. Figure 6 shows the construction of a nozzle-type regulator. Figure 7 shows a cross-section of a stem-type regulator. Figure 8 shows a typical oxyfuel gas welding outfit with regulators in place on each cylinder.

 Figure 8 below shows a typical oxyfuel gas welding outfit with regulators in place on each cylinder.

Figure 8 - Typical Oxyfuel Gas Welding

Most regulators have two gauges. The high-pressure gauge shows the cylinder pressure. The low-pressure gauge shows the working pressure or the pressure of the gas being delivered to the torch. Two varieties of regulators are available:

• Single-stage regulators, which reduce the cylinder pressure to a working pressure in one step. Example—2000 psig to 5 psig (13790kPa to 34.5kPa).
• Two-stage regulators, which reduce the cylinder pressure to working pressure in two steps or stages. Example—2000 psig to 200 psig (13790kPa to 1379kPa), then 200 psig to 5 psig (1379kPa to 34.5kPa).

Two-stage regulators provide more accurate pressure control and readings. A single-stage regulator must drop the pressure in one large step from 2000 psi to a working pressure of 3–20 psi so it is not as well-regulated. A two-stage regulator is more accurate and better regulated. By reducing the pressure to a lower level in two steps, a more precise reduction of pressure results. Regulator bodies are generally forged or cast and are made of brass, aluminium, or stainless steel. The regulator's pressure gauges are threaded into the regulator body. A pressure relief valve is also threaded into the regulator body. If the pressure within the regulator becomes too high, the relief valve will vent the pressure so that the regulator does not rupture.

Nozzle-Type Pressure Regulators

See Figure 6 for an illustration of a nozzle-type pressure regulator. A fitting to attach the regulator to the cylinder is threaded into the body. The regulator body generally has threaded openings for a high-pressure gauge and a low-pressure gauge. Line regulators, used with manifold gas supplies, may have only one opening for a low-pressure gauge. An additional opening is used to connect the torch hose to the regulator's low-pressure outlet.

A diaphragm or flexible disc separates and seals the body from the bonnet. The diaphragm spring is mounted between the diaphragm and the adjusting screw. Force on the diaphragm is adjusted by means of an adjusting screw. A cage or carrier is attached to the body side of the diaphragm. This cage curves down into the regulator body chamber. The seat is attached to the cage near the bottom of the body chamber. This seat is constantly pushed toward the nozzle by the body spring.

The gap between the nozzle and seat is controlled by the position of the diaphragm. The line that leads to the nozzle comes from the cylinder and has a port for attaching a high-pressure gauge. A fine mesh screen or ceramic filter is commonly located in this line. This filter keeps dirt from entering and damaging the regulator. The screen also serves as a flame arrestor and should always be left in place.

If the adjusting screw in the body is turned “in” (clockwise), the diaphragm spring on the bonnet side of the diaphragm will overcome the force of the body spring. The diaphragm spring will then move the seat away from the nozzle, allowing some gas to pass from the cylinder into the regulator body. As this gas enters the regulator body, it builds up pressure in the body. The force created by this pressure pushes the diaphragm up against the diaphragm spring. The upward movement of the diaphragm moves the cage and the seat up. This closes off the nozzle opening. The pressure falls as the gas is released from the regulator and flows through the hose to the torch. This action allows the diaphragm spring to move the diaphragm down slightly. The nozzle and seat valve open and allow more gas to come into the regulator body. The balance of the diaphragm spring pushing the diaphragm and seat open, and the pressure under the diaphragm pushing the seat closed, keeps the working pressure flowing through the regulator relatively constant.

​Note: The flow of gas from the cylinder is stopped completely when the adjusting screw on the regulator is turned all the way out.

These regulators come in various gas flow capacities and nozzle orifice sizes. The diaphragm, seat orifice, and spring sizes are designed to provide the volume of gas desired. Master regulators are designed to allow a large amount of gas to flow. Line regulators allow the flow of relatively small amounts of gas. The springs are made of a good grade of spring steel, while the diaphragm may be made of brass, phosphor bronze, sheet spring steel, or stainless steel.

The diaphragm is sealed at the joint between the diaphragm and the regulator body by means of suitable gaskets and the clamping action between the body and the bonnet. The nozzle is usually made of bronze, while the seat may be made of various materials such as nylon, Teflon®, neoprene, or some proprietary materials.

Stem-Type Pressure Regulators

A stem-type regulator works on the same principle as a nozzle-type, but instead of using a nozzle and seat, it employs a poppet valve and seat. Figure 7 shows a cutaway view of a typical stem-type regulator.

High-pressure gas enters the chamber below the seat when the cylinder valve is opened. The construction is such that the high pressure and the body spring force the valve against its seat. The regulator's pressure-adjusting screw is turned in to force the diaphragm spring and diaphragm down. The diaphragm pushes the valve stem down. This downward movement forces the valve away from the seat.

As pressure builds below the diaphragm, the diaphragm moves up, closing the valve. Gas from the low-pressure chamber is fed to the torch.

As the pressure in the low-pressure chamber drops, the diaphragm spring forces the diaphragm, stem, and valve down again. This allows more high-pressure gas to enter the low-pressure chamber and shut off the valve again. This constant opening and closing of the valve as the diaphragm moves up and down controls the working (outlet) pressure within close preset limits.

Stem-type regulators are used where a high rate of flow is required. The high rate of flow they provide makes these regulators ideal for manifolds and flame-cutting machines. ​ The materials in a stem-type pressure regulator are similar to those used in the nozzle-type regulator. The seat may be constructed of neoprene, Teflon®, or special proprietary materials. The stem (pin) is usually made of stainless steel. The stem and seat may be designed to enable the complete assembly to be removed as a cartridge, permitting easy servicing. Figure 9 shows a stem-type valve built as a cartridge.

Figure 9 - Stem-type regulator valve in a capsule or cartridge.

Acetylene Cylinders

Acetylene is an extremely volatile gas which has a high risk of a dangerous chemical reaction called decomposition. Decomposition is a spontaneous reaction that can cause high-energy explosions. To stabilise the gas against decomposition, acetylene is never stored in a pure state. Instead, it is mixed with a liquid solvent (usually acetone) and stored in a special cylinder which contains a porous filler material (also known as a porous mass). For more details see Figure 10 below

Figure 10 - Structure of Cylinder

Figure 11 below shows the Fuse Plug location

Figure 11 - Fuse Plug location

The liquid acetone is absorbed by the porous filler material which also serves to slow down or halt any decomposition reactions. Decomposition can be caused by a flashback, acetylene cylinders being dropped, or exposure to heat/fire. These specifically designed cylinders must be stored and handled with extreme care. The most important things are explained below

• Cylinders are filled with a very porous substance “monolithic filler” to help prevent large pockets of pure acetylene form forming.
• Cylinders have safety plugs in the top and bottom designed to melt at 100°c.
• Acetylene is extremely unstable in its pure form at a pressure above 15psi.
• Acetone is also present within cylinder to stabilize the acetylene.

Recommendation:

• Acetylene cylinders contain acetylene gas, dissolved in acetone, that is absorbed into a calcium silicate porous mass within the cylinder. The porous mass is designed to inhibit decomposition, which, when left to itself, could create an extremely hazardous and volatile condition. Acetylene cylinders should always be stored in an upright position to avoid any leaks or unwanted reactions.
• The acetylene cylinders must be fully inspected and leak-tested before delivery to ensure reliability.

• There must always be a label which shows acetylene in the cylinder as shown in Figure 12 below

Figure 12 - Acetylene Cylinder

Acetylene Valve

Figure 13 - Acetylene Valve

Acetylene cylinder shut-off valves should be only opened ¼ to ½ turn. This will allow the cylinder to be closed quickly in case of fire. Cylinder valve wrenches should be left in place on cylinders that do not have a hand wheel.

Oxygen and Acetylene Regulator Setting

Regulator pressure may vary with different torch style and tip sizes. Below the of Common gauge setting for cutting

• 1/4” material =  Oxy 30-35psi -  Acet 3-9 psi
• ½” material = Oxy 55-85psi - Acet 6-12 psi
• 1” material = Oxy 110-160 psi - Acet 7-15 psi

The maximum safe working pressure for acetylene is 15psi. Oxygen regulators usually painted Green and acetylene are Red

Filler

• The rod which provides additional metal in completing the welding is known as Filler
• The composition of filler metal should be the same as that of the metal to be welded

Methods of welding

Figure 14 - Filler System

• Forehand technique is relatively used in thin The torch point in the same direction that weld is being done so that the heat is not flowing into the metal as much as it could. The tip of torch is held 45° angle, which direct some of the heat away form metal.
• Backhand technique is used on heavier or thicker base Torch point in the direction opposite to that in which the weld is being done.

Type of Flame used in OAW

CARBURIZING FLAME: 

• These flame contain excess of
• It is suitable for application of requiring low
• Carburizing flame is used in welding of monel metal, nickel, high-carbon steel and many of non-ferrous.
• Steel is not welded by using this

Figure 15 - Carburizing Flame

NEUTRAL FLAME:

• If acetylene and oxygen are present in equal proportions(1:1),then neutral flame is produced.
• For most welding operations, neutral flame is used, since it has least chemical effect on heated metal.

Figure 16 - Neutral Flame

OXIDIZING FLAME:

• If flame has high oxygen content (ratio of oxygen & acetylene is 1.5:1), then oxidizing flame is produced.
• Such flame are used in welding of copper and copper alloys(brass and bronze).
• This flame is harmful for steel because it oxidizes the Steel.

Figure 17 - Oxidizing Flame

Flame Adjustment for OAW

Welding Parameter for Welding Carbon Steel

Advantages

• The equipment cost is low and requires little maintenance.
• It is portable and can be used anywhere.
• The gas flame is generally more easily controlled
• The process can also be used for cutting
• Filler metal can be properly
• Can be used in all weld

Disadvantages

• The process is very slow. Therefore, it has been largely replaced by arc welding.
• Proper operator training and skills are also essential.
• In some cases, there is a loss of corrosion resistance.
• Less efficiency, since the heat transfer rate is poor when compared to arc welding.
• The heat source is not concentrated, a large area of the metal is heated an distortion is likely to occur.

Recommendation:

• Never Transport cylinders without the safety cap in place.
• Never Transport with the regulator in place.
• Never allow the bottle to stand freely. Always chain them to a secure cart or some other object that cannot be toppled easily

MAIN HAZARDS

• Fire is caused by heat, sparks, molten metal or direct contact with the flame.
• Explosion when cutting up or repairing tanks or drums which contain or may have contained flammable materials.
• Fire/explosion caused by gas leaks backfires and flashbacks.
• Fire/burns resulting from misuse of oxygen.
• Crushing or impact injuries when handling and transporting cylinders.