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Oxy-fuel (Oxyacetylene) welding and cutting
Oxy-fuel welding of metal is commonly called oxyacetylene welding since acetylene is the predominant choice for a fuel, or often simply gas welding. In gas welding and cutting, the heat needed to melt the metal comes from a fuel gas burning with oxygen in a torch.
Oxy-fuel cutting of metal is a similar process, using a different type of gas torch, called a blowtorch. (But, colloquially, many people also call a welding torch a blowtorch.) Here the metal is heated until it glows, and then a long lever on the torch is pressed to blow an excess of oxygen into the gas mixture, to blow and melt the metal with the resulting extra heat, much of which comes from the metal burning rather than from the gas burning. Sometimes a metal-cutting blowtorch is colloquially called a gas-axe.
Torches that do not mix pure oxygen with the fuel inside the torch, but burn it with atmospheric air, are not oxy-fuel torches and can be identified by their single tank. (Oxy-fuel welding needs two tanks, fuel and oxygen.) Most metals cannot be melted with such single-tank torches, so they can only be used for soldering and brazing, not welding. See also Blowlamp.
The apparatus used in gas welding consists basically of an oxygen source and a fuel gas source (usually cylinders), two pressure regulators and two flexible hoses (one of each for each cylinder), and a torch. This sort of torch can also be used for soldering and brazing.
The cylinders are often carried in a special wheeled trolley.
The torch is the part that the welder holds and manipulates to make the weld. It has two valves and two connections, one each for the fuel gas and the oxygen, a handle for the welder to grasp, a mixing chamber (set at an angle) where the fuel gas and oxygen mix, with a tip where the flame comes out.
A cutting torch is used to cut metal. It is similar to a welding oxy-gas torch. The most common fuel used for cutting torches is acetylene. Oxygen is combined with the acetylene in the torch, which produces a high temperature flame. The differences from an oxy-gas welding torch are:-
The regulators are attached one each to the fuel source and to the oxygen source. The oxygen regulator is attached to the oxygen tank and drops the pressure from about 21000 kPa (3000 lbf/in² = 200 atmospheres) to a lower pressure for the torch. This pressure can be adjusted to suit the job at hand by turning a knob on the regulator, and can be set from 0 to about 700 to 1400 kPa (100 to 200 pounds/in²). Likewise the fuel regulator is attached to the fuel source and drops the pressure to a level for the torch to use. For acetylene this is 0 to 100 kPa (15 lbf/in²).
Most regulators have two gauges: one indicates the cylinder pressure when the valve is opened; the other indicates the pressure of the gas coming out of the regulator when the regulator is opened. This is the delivery pressure of the gas, which must be set for the current job. The gauges are calibrated to read correctly at 70°F.
(Note that when a cylinder's pressure gauge says that the cylider is empty, it actually contains gas equal to the surrounding atmosphere pressure.)
These regulators can be single-stage or double-stage. See Diving regulator#Constant flow for more information.
A single-stage regulator mechanism consists of a nozzle that the gas passes through, a valve seat to close off the nozzle, a diaphragm, and balancing springs. These mechanisms are all enclosed in a suitable housing. Fuel-gas regulators and oxygen regulators are basically the same design, but fuel gas regulators (except for hydrogen) are not made to withstand the high pressures that oxygen regulators are subjected to.
In the oxygen regulator, the oxygen enters through the high-pressure inlet connection and passes through a glass wool filter that removes dust and dirt. Turning the adjusting screw in (clockwise) increases the oxygen flow.
The major disadvantage of single-stage regulators is that the working gas pressure you set will decrease as the cylinder pressure decreases; therefore, you must constantly monitor and reset the regulator if you need a fixed pressure and flow rate
The double-stage regulator is similar in principle to the one-stage regulator. The main difference being that the total pressure drop takes place in two stages instead of one. In the high-pressure stage, the cylinder pressure is reduced to an intermediate pressure that was predetermined by the manufacturer. In the low-pressure stage, the pressure is again reduced from the intermediate pressure to the working pressure you have chosen.
Safety of regulators
Regulators are precise and complicated pieces of equipment. Carelessness can do more to ruin a regulator than any other gas-using equipment. One can easily damage a regulator by simply forgetting to wipe clean the cylinder, regulator, or hose connections. When you open a high-pressure cylinder, the gas can rush into the regulator at the speed of sound. If there is any dirt in the connections, it will be blasted into the precision-fitted valve seats, making them leak. This results in a condition known as creep, which occurs when the regulator is shut off but not the cylinder and gas pressure is still being delivered to the low-pressure side.
Oil or other petroleum products, or fat or oil of biological origin, must never be used around oxygen regulators because these products will either cause a regulator explosion or fire.
The flexible hoses connect from the regulators to the torch and carry the fuel gas and the oxygen must be strong, nonporous, light, and flexible enough to make torch movements easy. They must be made to withstand internal pressures that can reach as high as 100 psig. The rubber used in hose manufacture is specially treated to remove the sulfur that could cause spontaneous combustion.
Welding hose is available in single-hose and double-hose lengths. Size is determined by the inside diameter, and the proper size to use depends on the type of work for which it is intended. Hose used for light work has a 3/16 or 1/4 inch inside diameter and one or two plies of fabric. For heavy-duty welding and cutting operations, use a hose with an inside diameter of 5/16 inch and three to five plies of fabric. Single hose is available in the standard sizes as well as 1/2-inch, 3/4-inch, and 1-inch sizes. These larger sizes are for heavy-duty heating and for use on large cutting machines. The most common type of cutting and welding hose is the twin or double hose which is the fuel hose and the oxygen hose joined together side by side by either a special rib or by clamps. Because they are joined, the hoses are less likely to become tangled and are easier to move from place to place.
The length of hose used is important. The delivery pressure at the torch varies with the length of the hose. A 20-foot 3/16-inch hose may be adequate for a job, but if the same hose was 50 feet long, the pressure drop would result in too little gas flow to the torch. Longer hoses musy be wider inside to ensure the correct flow of gas to the torch.
The hoses used for fuel gas and oxygen are identical in construction, but they differ in color. The oxygen hose cover is green, and the fuel-gas hose cover is red. This color coding aids in the prevention of mishaps that could lead to dangerous accidents. The fuel gas connections have left-hand threads and the oxygen connectors have right-hand threads so that the two cannot be interchanged, to help to prevent accidents. The basic hose connection is a nut and gland. The nut has threads on the inside that match up with the male inlet and outlet on the torch and regulator. The gland slides inside the hose and is held in place by a crimped ferrule. The nut is loose and can be turned by hand or a wrench to tighten the threaded nut onto the equipment.
All connections for welding and cutting hoses have been standardized by the Compressed Gas Association. Letter grades A, B, C, D, and E plus the type of gas used correspond directly with the connections on the regulators. A, B, and C are the most common size connections. A-size is for low-flow rates; B-size for medium-flow rates; and C-size is for heavy-flow rates. D and E sizes are for large cutting and heating torches
Between the regulator and hose, and ideally between hose and torch on both oxygen and fuel lines, a flashback arrestor and/or non-return valve should be installed to prevent flame or oxygen-fuel mixture being pushed back into either cylinder and damaging the equipment or making a cylinder explode.
The flashback arrestor (not to be confused with a check valve) prevents shockwaves from downstream coming back up the hoses and entering the cylinder (possibly rupturing it), as there are quantities of fuel/oxygen mixtures inside parts of the equipment (specifically within the mixer and blowpipe/nozzle) that may explode if the equipment is incorrectly shut down; and acetylene decomposes at excessive pressures or temperatures. The flashback arrestor will remain switched off until someone resets it, in case the pressure wave created a leak downstream of the arrestor.
A check valve lets gas flow in one direction only. Do not confuse it with a flashback arrestor, as check valves are not designed to block a shockwave: the pressure wave could occur while the ball is so far enough from the inlet that the pressure wave gets past before the ball reaches its off position. A check valve is usually a chamber containing a ball that is pressed against one end by a spring: gas flow one way pushes the ball out the way, and no flow or flow the other way lets the spring push the ball into the inlet, blocking it.
Acetylene and gases that liquefy under cylinder pressure should only be used where it can be relied on that the gas cylinder in use will always be vertical with its valve on top.
It should be noted that there is not a single gas called "oxyacetylene"; that misconception is sometimes found among the unknowledgeable.
Acetylene is the fuel first used for oxy-fuel welding and remains the fuel of choice for repair work and general cutting and welding. Acetylene gas is shipped in special cylinders designed to keep the gas dissolved. The cylinders are packed with various porous materials (e.g. kapok fibre, diatomaceous earth, or, formerly, asbestos), then filled about half way with acetone. The acetylene dissolves into the acetone. This method is necessary because above 207 kPa (30 lbf/in²) acetylene is unstable and may explode. There is about 1700 kPa (250 lbf/in²) of pressure in the tank when full. Acetylene when burned with oxygen gives a temperature of 3200 °C to 3500 °C (5800 °F to 6300 °F), which is the highest temperature of any of the commonly used gaseous fuels. Its main disadvantage is its comparatively high cost.
How acetylene is made is described at Acetylene#Preparation.
As acetylene is unstable at a pressure equivalent to being roughly 33 feet = 10 meters underwater, underwater cutting and welding must use hydrogen instead of acetylene.
Hydrogen has a clean flame and is good for use on aluminium. It can be used at a higher pressure than acetylene and is therefore useful for underwater welding. For small torches, hydrogen is often produced, along with oxygen, by electrolysis of water in an apparatus which is connected directly to the torch.
Propane does not burn as hot as acetylene, and so cannot be used for welding. Propane is cheaper than acetylene and so is often used for cutting.
Propylene is used in production welding.
MAPP gas is a registered product of the Dow Chemical Company. It is liquefied petroleum gas mixed with methylacetylene-propadiene. It has the storage and shipping characteristics of LPG and has a heat value a little less than acetylene. Because it can be shipped in small containers for sale at retail stores, it is used by hobbyists. Other welding gasses that develop comparable temperatures require special procedures for safe shipping and handling. +
The role of oxygen
Oxygen is not the fuel: It is what chemically combines with the fuel to produce the heat for welding. This is called 'oxidation', but the more general and more commonly used term is 'combustion'. In the case of hydrogen, the product of combustion is simply water. For the other hydrocarbon fuels, water and carbon dioxide are produced. The heat is released because the molecules of the products of combustion have a lower energy state than the molecules of the fuel and oxygen.
The word "oxygen" is often shorted to 'oxy', as in the term 'oxy-acetylene torch'.
Oxygen is usually produced elsewhere by distillation of liquified air and shipped to the welding site in high pressure vessels (commonly called "tanks" or "cylinders") at a pressure of about 21000 kPa (3000 lbf/in² = 200 atmospheres). It is also shipped as a liquid in Dewar type vessels (like a large Thermos jar) to places that use large amounts of oxygen.
It is also possible to separate oxygen from air by passing the air, while under pressure, through a zeolite sieve which selectively absorbs the nitrogen and lets the oxygen (and argon) pass. This gives a purity of oxygen of about 93%. This works well for brazing.
Learning to operate an oxyacetylene torch
If you have not been trained in oxyacetylene welding or brazing, get help and advice from a knowledgeable person or enroll in a community college course. Oxyacetylene welding is not difficult, but there are a good number of subtle points that should be learned such as: never fully empty an acetylene cylinder or use more than 1/7 the capacity of the cylinder per hour. The material below highlights many of the important points, but should be supplemented by personal instruction and a good welding textbook.
Know how to contact emergency services at whatever site you are working. Welding is a practical and satisfying art; however, oxyacetylene welding can be dangerous. Severe and fatal burns and violent building-destroying explosions can result from inattention and carelessness. Familiarize yourself with safety procedures and weld safely!
Before using an oxyacetylene set, ensure that flammable materials such as grease, oil, paint, sawdust, etc are cleared from the area and workpieces. Oil and grease can spontaneously ignite and burn violently in the presence of pure oxygen. For this reason it is important to wear clean clothing (preferably a boilersuit) free from oil and grease.
Wear shaded goggles with enclosed sides, or a shield with a shaded lens to protect your eyes from glare and flying sparks and splatter. Sunglasses are not adequate. Wear leather gloves to help protect your hands from burns. Wear clothes and shoes/boots appropriate for welding; the boots should have steel toecaps because of the risk from dropped objects.
The fluxes and filler rods and base metals heated during welding and brazing all release toxic fumes, and acetylene gas is highly explosive, so ensure adequate ventilation before welding. Acetylene is often described as having a sweet garlic-like smell. If you can smell it in the air and do not know where it is coming from, evacuate the area immediately. Call the fire department from a phone outside of the area, as a phone can start a fire or explosion in vapours.
Detecting no smell does not mean that no gas is escaping. The gas may not be reaching your nose, or your sense of smell may have been dulled by a nose infection or smoking.
Other gas hazards
In confined spaces, respirator masks designed for welding can be used. For the home welder, an outdoor area is probably the most practical, though the weld site should be protected from winds as wind will disperse the heat of the flame. Never weld on containers that have previously contained toxic or flammable substances. Do not weld inside enclosed spaces or in tanks where the only ventilation comes from above - you might suffocate. Some fuel gases have an anaesthetic-type effect when breathed.
Hazard from hot flying droplets
Leather gloves with long cuffs, and heavy work boots (preferably with steel toecaps) are recommended. Do not wear trainers / sneakers, as falling or flying droplets and bits of hot metal will instantly burn through running shoes and continue burning through the operator's foot. Long trousers and shirt sleeves should be worn, or a boilersuit. Do not roll up trouser legs, as the cuffs could catch sparks. Leather aprons and flame proof jackets can be used to protect the operator. Many operators use a baseball cap put on backwards to stop sparks from lodging in hair or going down the neck of the shirt.
Hazard from flame glare
Eye protection is necessary. Gas welding does not generate the same ultraviolet light as arc welding, but sparks and infra-red radiation are still very dangerous. Safety gear such as shaded enclosed protective goggles should be adjusted before lighting. For brazing and light welding, "shade 5" goggles can be used. Note, gas welding goggles and arc welding masks are not the same! Sunglasses are not suitable.
Before using an oxyacetylene setup, ensure that a fire extinguisher in good working order is present. Water does not work on grease fires, however a bucket of water can be handy for putting out small wood fires and quenching parts. A wet cloth can also be used to extinguish a flame on a hose or fitting. Sand can also be used to put out fires. It is also generally handy to have at least one pair of heavy pliers around for moving hot things if necessary. Be sure your workspace is well organized before starting. Have tools laid out where they can be easily reached and make sure there ias nothing that you can trip up on. A fireproof surface should be used for welding. Steel table tops and firebricks are commonly used.
Do not use pressurized oxygen for blowing dirt out or cleaning clothing. The oxygen can saturate the material making it eight times more flammable!
Safety with cylinders
When using fuel and oxygen tanks they should be fastened securely upright to a wall or a post or a portable cart. An oxygen tank is especially dangerous for the reason that the oxygen is at a pressure of 21 MPa (3000 lbf/in² = 200 atmospheres) when full, and if the tank falls over and its valve strikes something and is knocked off, the tank will become an unguided unpredictable rocket missile powered by the compressed oxygen, able to smash through walls. For this reason, never move an oxygen tank around without its valve cap screwed in place.
On your oxyacetylene torch system there will be three types of valves, the tank valve, the regulator valve, and the torch valve. There will be one of them for each gas. The gas in the tanks or cylinders is at high pressure. Oxygen cylinders are generally filled to something like 2200 psi. The regulator converts the high pressure gas to a low pressure stream suitable for welding. Never attempt to directly use high-pressure gas.
Never lay an acetylene tank on its side while being used, as it contains acetone which the acetylene is dissolved in, and this acetone would start to come out through the valve. If it was laid down while being transported, it must be set upright, valve on top.
Never lay a propane or other liquified gas tank on its side while being used, as in the tank the gas is liquiified by pressure, and this liquid would start to come out through the valve. If it was laid down while being transported, it must be set upright, valve on top.
If an oxygen or hydrogen cylinder is laid on its side, secure it so that it cannot roll about.
Setting up the equipment
Read the safety considerations.
Remove the oxygen cylinder's valve cap. With the valve opening pointed away from the welder, open the valve slightly for just a moment and then close it. This serves two purposes:-
Attach the oxygen regulator and tighten the nut. Never use pliers, as the pliers will soon damage the brass nut; always use a wrench/spanner. Also, welders tend to overtighten the nut. If the connection is not leaking, then the nut is tight enough. If a great amount of torque is needed to stop it leaking, or if it will not stop leaking despite any amount of tightening, then there is something wrong with the nut or the gasket or the valve.
Attach the fuel regulator to the fuel tank in the same manner. The nut on the fuel regulator usually has left-hand threads.
Attach the flexible hoses from the regulators to the torch. The oxygen hose is usually colored blue (green in the USA) and the fuel hose red. The fuel hose has left-hand threaded connectors at both ends and the oxygen hose has right-hand threaded connectors.
Ensure that the hoses are untangled and kept well clear of the flame. It is a good practice to quickly check the hoses for cracks and abrasion before use. Generally, tanks are set up behind the welding area and the hoses run backwards away from the weld site to minimize chances of sparks hitting hoses.
Make sure that the regulator valves are turned all the way out and that all the valves on the torch are closed.
Screw out the knobs on the regulators until loose (0 setting).
Make sure the valves are easily accessible in case emergency shutdown is necessary.
Open the valves on the fuel and oxygen tanks, slowly to minimize the impact of unleashing highly compressed gases on the regulators. Do not stand directly in front of the regulator gauges, as malfunctioning gauges can blow glass and bits outwards. Open the oxygen valve slightly and then wait while the high-pressure gauge on the regulator stops rising. Then open the valve fully, until it stops turning. This is a backstop valve. Turning the valve all the way out prevents leakage through the packing of the valve.
Open the fuel valve also.
If the setup has not been used in a while, you may wish to check for leaks. With the torch valves closed and the regulators open, shut off the tank valves and observe the line pressure for several minutes. If the pressure falls, there is a leak in the system. Bubble solution can be used to test connections for leaks. Never use a setup with a leak! If there are any leaks in the connections or regulators or torch, or any other faults with the equipment, there is a safety hazard and the equipment should not be used.
Never oil an oxygen regulator. It will cause a fire or explosion: solid brass regulators can be blown apart from the force. Keep oxygen away from all combustibles.
It is recommended that you purge the gas hoses before use to ensure that no oxygen is in the acetylene line and vice versa. Ensure that you have adequate ventilation. For a 20-foot hose, open the torch's acetylene valve for 5 seconds to let any oxygen be blown out of the acetylene hose. For a longer hose, consult a welding reference. Close the torch valve and repeat for the other hose.
Turn both regulator valves inwards until you reach the desired pressure.
Once the flame is lit, open the acetylene valve just until the flame stops smoking. With a normal-sized torch you should get a flame about 8 inches long with a toothy splintering end.
Now open the oxygen torch valve until the flame loses its feather around its inner core, but no farther than this amount. This is called a "neutral" flame, where the oxygen and the fuel gas completely consume each other and the burnt gases do not chemically alter the metal to be welded or brazed. An oxidising flame has an excess of oxygen. A reducing flame has an excess of fuel (carbon). An oxidising flame is used for cutting. A reducing flame is used for annealing e.g. to soften steel sheet metal.
Sorts of flame
An acetylene flame (as is characteristic of most fuel/oxygen flames) has two parts; the light blue to white colored inner cone and the blue colored outer cone. The inner cone is where the acetylene and the oxygen combine. The tip of this inner cone is the hottest part of the flame. The outer cone is where hydrogen and carbon monoxide from the acetylene breaking down and partly burning in the inner cone, combine with more oxygen in the surrounding air and burn.
A neutral flame has a well-defined inner cone. A reducing flame has a feathery inner cone. An oxidising flame has a smaller inner cone that is sharply defined and is pale blue. The welder observes this while adjusting the fuel and oxygen valves on the torch to get the correct balance for the job at hand. There is also a difference in the noise the flame makes. Adjusting the flame is not a hard thing to do after a little experience and practice.
The size of the flame can be adjusted to a limited extent by the valves on the torch and by the regulator settings, but in the main it depends on the size of the orifice in the tip. In fact, the tip should be chosen first according to the job at hand, and then the regulators set accordingly.
The flame is applied to the base metal and held until a small puddle of molten metal is formed. The puddle is moved along the path where the weld bead is desired. Usually, more metal is added to the puddle as it is moved along by means of dripping metal from a wire ("welding rod" or "filler rod") into the molten metal puddle. The force of the jet of flame issuing from the torch tip helps to manipulate the puddle. The amount of heat can be controlled by the distance of the flame from the metal, as well as the gas flow rate and nozzle size selected. There should be a bright incandescent spot on the molten puddle. When the puddle is correctly maintained, a sound weld will result.
For cutting, the set-up is a little different. A cutting torch has a 90-degree angled head with six orifices placed around a central jet. The six outer jets are for oxygen and acetylene (oxy-propane devices use an array of many jets) and the central jet carries only oxygen.
The flame intended not to melt the metal, but to bring it to its ignition temperature.
The torch's trigger blows extra oxygen at high pressure down the torch's third tube out of the central jet into the workpiece, causing the metal to burn and blowing the resulting molten oxide through to the other side and off the workpiece completely. The ideal cut is a narrow gap with a sharp edge on either side of the workpiece; overheating the workpiece and thus melting through it causes a rounded edge.
Cutting is initiated by heating the edge of the steel to melting point using the six pre-heat jets only, then using the separate cutting oxygen valve to release the oxygen from the central jet. The steel is instantly oxidized into molten iron oxide, producing the cut. It is worth noting several things at this point:-
For a basic oxy-acetylene rig, the cutting speed in light steel section will usually be nearly twice as fast as a petrol-driven cut-off grinder. The advantages when cutting large sections are obvious - an oxy-fuel torch is light, small and quiet and needs very little effort to use, whereas a cut-off grinder is heavy and noisy and needs considerable operator exertion and may vibrate severely, leading to stiff hands and possible long-term repetitive strain injury.
Robotic oxy-fuel cutters sometimes use a high-speed divergent nozzle. This uses an oxygen jet that opens slightly along its passage. This allows the compressed oxygen to expand as it leaves, forming a high-velocity jet that spreads less than a parallel-bore nozzle, allowing a cleaner cut. These are not used for cutting by hand since they need very accurate positioning above the work. Their ability to produce almost any shape from large steel plates gives them a secure future in shipbuilding and in many other industries.
Oxy-propane torches are usually used for cutting up scrap for reasons of economy, as LPG is far cheaper joule-for-joule than acetylene, although propane does not produce acetylene's very neat cut profile. Propane also finds a place in production, for cutting very large sections.
To shut down the torch, close the oxygen torch valve first, then the acetylene. (A 'pop' may occur if you reverse the order. The pop throws carbon soot back into the torch, which may partially plug the gas passages.)
Next, close both the tank valves. Turn on the torch valves again, one at a time, to bleed any remaining pressure, then close them again. Lastly, turn the regulator valves until there is no pressure on the adjusting spring and the screw turns freely.