Oxy-fuel welding and cutting
Oxy-fuel welding is a welding process commonly called oxyacetylene welding since acetylene is the predominant choice for a fuel, or often simply gas welding. A virtually identical procedure, with a different type of gas torch, is used for cutting metal and called oxy-fuel cutting. In gas welding and cutting, the heat energy and high temperature needed to melt the metal is obtained by the combustion of a fuel gas with oxygen in a torch. This sort of torch is often called a blowtorch, see below.
Fuels
The most commonly used fuel gas is acetylene. Other gases used are liquified petroleum gas (LPG), natural gas, hydrogen, and MAPP gas.
Acetylene can be made near where the welding is being done in an acetylene generator. More often it is made elsewhere and shipped to the welding site in special containers. These containers are packed with various porous materials (e.g. kapok fibre), 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.
Hydrogen has a clean flame and is good for use on aluminum. 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.
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.
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.
Oxygen is usually shorted to 'oxy' for use 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.
Apparatus
The apparatus used in gas welding consists basically of a torch, two pressure regulators and twin flexible hoses.
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 where the fuel gas and oxygen mix, and a tip where the flame issues from.
The regulators are attached to the fuel and to the oxygen sources. The oxygen regulator is attached to the oxygen tank and drops the pressure from about 21000 kPa (3000 lbf/in² = 200 amospheres) 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 700–1400 kPa (100–200 lbf/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).
The flexible hoses connect from the regulators to the torch and carry the fuel gas and the oxygen. The fuel gas connections have left hand threads and the oxygen connectors have right hand threads so that the two cannot be interchanged, so as to help prevent accidents.
The welder wears goggles or a shield with a shaded lens to protect his eyes from glare and flying sparks and splatter, and wears leather gloves to help protect his hands from burns. He should also wear clothes and shoes appropriate for welding. Sunglasses are not adequate.
Note that the procedures and equipment used for gas welding are essentially the same as for gas brazing.
Setting up the equipment
When using fuel and oxygen tanks they should be fastened securely to a wall, a post or a portable cart in an upright position. 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 the valve strikes something and is knocked off, the tank will become an unguided and unpredictable missile powered by the compressed oxygen. It is for this reason that an oxygen tank should never be moved around without the valve cap screwed in place.
Never lay the acetylene tank down while being used, as the 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.
After the oxygen tank is securely fastened, remove the 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. For one, it blows out any dirt or dust that may have settled in the valve. This dirt would otherwise end up in the regulator and shorten its life and accuracy. For another, when a tank is filled, the worker has a tendency to tighten the valve securely to make certain it is closed completely. It is better to break it loose now than when the regulator is in place. Attach the oxygen regulator and tighten the nut. Never use pliers, as the pliers will soon damage the brass nut; always use a wrench. Also, there is a tendency of welders to overtighten the nut. If it is not leaking, then it is tight enough. If a great amount of torque is needed to stop it leaking, or if it will not stop leaking in spite of any amount of tightening, then there is something wrong with the nut, 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 green and the fuel hose red. The fuel hose has left hand threaded connectors at both ends and the oxygen has right hand threaded connectors.
With the valves on the torch closed, and the knobs on the regulators screwed out until loose (0 setting), open the valves on the fuel and oxygen tanks. 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 back stop valve. Turning the valve all of the way out prevents leakage through the packing of the valve.
Open the fuel valve also. Only open an acetylene valve one quarter turn. This helps prevent the acetylene from being drawn off too quickly. If acetylene 'bubbles' too rapidly from the acetone, it might become unstable. Open the valve on a LPG tank out completely as on an oxygen tank and for the same reasons.
If there are any leaks in the connections, regulators or torch, or any other faults with the equipment, a safety hazard exists. 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.
The flame
After this preparation, set the regulators at the desired pressure. For acetylene, this should never be more than 103 kPa (15 lbf/in²). To prevent a large yellow, sooty flame when first lighting the torch, open both the fuel and the oxygen valves (more fuel than oxygen), and light a flame with a 'striker' or by some other means. After the flame is adjusted to the proper size, open the oxygen valve and adjust it to give the desired balance of fuel and oxygen. Usually a neutral flame is used: this is a flame where the fuel and oxygen supplied to the torch tip are both completely combined with each other. An oxidizing flame has an excess of oxygen and a reducing flame has an excess of fuel (carbon). An oxidising flame is used for cutting and a reducing flame is used for annealing e.g. to soften steel sheet metal.
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 breakdown of the acetylene and partial combustion of the inner cone combine with the oxygen in the surrounding air and burns.
A neutral flame has a well defined inner cone. A reducing flame has a feathery inner cone. An oxidizing 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 flowrate 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 purposes, 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. Cutting is initiated by heating the edge of the steel to melting point using the six pre-heat jets only, then using the seperate cutting oxygen valve to release the oxygen from the central jet. The steel is instantly oxidised into molten iron oxide, producing the cut. It is worth noting several things at this point. Firstly, that the oxygen flowrate is critical - too little will result in a slow, ragged cut, too much will waste oxygen and pruduce a wide, concave cut. Many torches don't have a seperate control for cutting oxygen so this must be controlled using the regulator. Typically for cutting the oxygen pressure from the regulator will be set higher than for welding. Secondly, the oxidation of iron by this method is highly exothermic. Once started, steel can be cut at a surprising rate, far faster than if it was simply melted through. At this point, the pre-heat jets are there purely for assistance. The rise in temperature will be obvious by the intense glare from the ejected material, even through proper goggles (typically, these should be darker than those used for welding). A Thermal lance is a tool which also uses the rapid oxidation of iron to cut through almost any material.
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, quiet and requires very little effort to use, whereas a cut-off grinder is heavy, noisy, requires considerable operator exertion and may vibrate severely, leading to stiff hands and possible long-term 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, and allowing a cleaner cut. These are not used for hand-cutting since they require very accurate positioning above the work. Their ability to produce almost any shape from large steel plates gives them a secure future in shipbuilding, as well as many other industries.
Oxy-propane torches are usuallly used for cutting up scrap for reasons of economy, LPG being far cheaper joule-for-joule than acetylene at the expense of acetylene's very neat cut profile. It also finds a place in production, for cutting very large sections.
The blowtorch
A Blowtorch is a simple heating torch, sometimes referred to as a blowlamp, which uses fuel mixed with atmospheric air. It will typically run on propane or butane cartridges, or be fed from a liquid petroleum gas cylinder via a hose. They produce a much larger, softer flame than an oxy-fuel torch and are used for lower temperature applications - soldering, brazing, melting roof tar, or pre-heating large castings before welding (such as for repairing cast-iron cylinder heads). They cannot be used for welding, but find many other uses, not least because in their simplest form of a disposable cannister feeding a hand-held torch they are very cheap and highly portable, and because the LPG fuel is very cheap in comparison to acetylene and oxygen.
Older blowlamps used liquid fuel such as kerosene (paraffin) or gasoline (petrol). these are largely redundant, and may be difficult to start, requiring pre-heating with methylated spirit. If any doubts exist as to the integrity of the pressurised fuel tank or any of the seals in the torch, it should be treated strictly as an antique - if the tank bursts there is a very real risk of explosion or fire.
References
- Modern Welding by Althouse, Turnquist, and Bowditch. The Goodheart-Willcox Co. 1970
- The Welding Encyclopedia, The Welding Engineer staff, ninth ed. 1938