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Oxy-fuel welding and cutting
There are a few differences between the two. In Oxy-fuel welding, a welding torch is used to weld metals. In Oxy-fuel cutting, a cutting torch is used to heat up ferrous metal to kindling temperature (about 980°C). A stream of pure oxygen is trained on the hot metal which chemically combines with the iron which then flows out of the cut, or kerf, as an iron-oxide slag .
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/cutting 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.
Note: Sometimes a metal-cutting torch is colloquially called a "gas-axe", "smoke wrench", "hot wrench", "blue wrench" or "hot blue spanner" (in Britain). Colloquially, many people mistakenly call a welding torch a blowtorch. In the USA the word blowtorch is also used for what in Britain is called a blowlamp.
Oxy-gas torches are or have been used for:
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.
There have been examples of oxyhydrogen cutting sets with small (scuba-sized) gas cylinders worn on the user's back in a backpack harness, for rescue work and similar.
There are also examples of pressurized liquid fuel cutting torches, usually using gasoline. These are used for their increased portability.
The regulator is used to control pressure from the tanks by reducing pressure and regulating flow rate. Oxy-gas regulators usually have two stages: The first stage of the regulator releases the gas at a constant rate from the cylinder despite the pressure in the cylinder becoming less as the gas in the cylinder is used, as in the first stage of a scuba-diving regulator. The second stage of the regulator controls the pressure reduction from the intermediate pressure to low pressure. It is constant flow. The valve assembly has two pressure gauges, one showing cylinder pressure, one showing intermediate pressure.
Some oxy-gas regulators only have one stage, and one pressure gauge. With those the gas flow gets less as the cylinder pressure drops.
The hoses used are specifically designed for welding and cutting. The hose is usually a double-hose design, meaning that there are two hoses joined together. The oxygen hose is black (in Britain) or green and the fuel hose is red. The type of gas the hose will be carrying is important because the connections will have different threads for different types of gas. Fuel gases (red) will use left-hand threads and a groove cut into the nut, while the oxygen (green) will use right-hand threads. This is a safety precaution to prevent hoses from being hooked up the wrong way. 
There are basically two types of connections that can be used. The first is using a jubilee clip. The second option is using a crimped connector. The second option is probably safer as it is harder for this type of connection to come loose. The hoses should also be clipped together at intervals approximately 1 metre apart.
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 shock waves 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. Not to be confused with a flashback arrestor, a check valves is not designed to block a shock wave. The pressure wave could occur while the ball is so far 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 of the way, and no flow or flow the other way lets the spring push the ball into the inlet, blocking it.
The torch is the part that the welder holds and manipulates to make the weld. It has a connection and valve for the fuel gas and a connection and valve for 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 forms.
A welding torch head is used to weld metals. It can be identified by having only two pipes running to the nozzle and no oxygen-blast trigger.
A cutting torch head is used to cut metal. It is similar to a welding torch. Oxygen is combined with the acetylene in the torch, which produces a high temperature flame. It can be identified by having three pipes that go to a 90 degree nozzle and by the oxygen-blast trigger that provides oxygen to blast away material while cutting.
A rose-bud torch is used to heat metals for bending, straightening, etc. where a large area needs to be heated. It is called as such because the flame at the end looks like a rose-bud. A welding torch can also be used to heat small area such as rusted nuts and bolts. In this case, no filler rod is used with the torch.
In most torches the two gases merely mix: this is an equal-pressure torch equal-pressure torch. But in some torches (called injector torches), inside the torch head the oxygen comes out of a small nozzle under pressure so it drags the fuel gas along with it by the venturi effect.
Oxy-fuel processes may use a variety of fuel gases, the most common being acetylene. Other gases that may be used are propylene, liquified petroleum gas (LPG), propane, natural gas, hydrogen, and MAPP gas.
Note: there is not a single gas called "oxyacetylene".
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) (absolute pressure) 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.
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.
Oxy-gasoline (= oxy-petrol) torches have been found to perform very well, especially where bottled gas fuel is not available or difficult to transport to the worksite. Tests showed that an oxy-gasoline torch cut steel plate up to 0.5 inch thick as well as oxyacetylene; and 0.5 to 4 inches thick better: 3 times better at 4 inches thick.
The gasoline is fed from a pressure tank whose pressure can be hand-pumped or fed from a gas cylinder.
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 and cutting. It is a good type of flame to use when heating much material. The flame temperature is high, about 2,000°C at atmospheric pressure.
For some oxyhydrogen torches the oxygen and hydrogen are produced by electrolysis of water in an apparatus which is connected directly to the torch. Types of this sort of torch:
According to Julius Thomsen, when oxygen and hydrogen burn, 34,116 calories of heat is produced for each gram of hydrogen burned: that is 286 kJ/mol. How the process is conducted affects the temperature of the flame: it obviously is highest in burning a pure stoichiometric mixture (a mixture of hydrogen with exactly half its volume of oxygen, the quantity it combines with in becoming water, German Knallgas). It becomes less when the "oxyhydrogen" is mixed with excess of oxygen or hydrogen, or an inert gas such as nitrogen, because there the same amount of heat spreads over a larger quantity of matter.
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, and large industrial companies and shipyards because it is only as volatile as water while stored in cylinders, and is therefore much less dangerous than acetylene. MAPP gas can be used at much higher pressures than acetylene, sometimes up to 40 or 50 psi in high-volume oxy-fuel cutting torches which can cut up to 12 inch thick steel. Other welding gases that develop comparable temperatures need special procedures for safe shipping and handling. A MAPP gas leak is easy to identify because of its particularly terrible odor.
Propane does not burn as hot as acetylene in its inner cone, and so cannot be used for welding. Propane, however, has a very high number of BTUs per cubic feet in its outer cone, and so with the right torch (injector style) can make a faster and cleaner cut than acetylene, and is much more useful for heating and bending than acetylene.
Propane is cheaper than acetylene and easier to transport.
Like propylene, most propane tips are of a two piece design. Propane often gets unfair criticism because it really needs changing your torch (from an equal pressure torch to an injector torch) and not just changing your tip to get the best performance. Most torches are equal pressure and designed for gases such as acetylene which are lighter than oxygen. Propane is a great deal heavier and runs much better through a low-pressure injector torch with a setting from a few ounces to about two pounds per square inch when cutting.
Propylene is used in production welding and cutting. It cuts similarly to propane. When propylene is used, the torch rarely needs tip cleaning. There is often a substantial advantage to cutting with an injector torch (see #propane) rather than an equal-pressure torch when using propylene.
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.
Sorts of flame
The welder can adjust the oxy-acetylene flame to be either carbonizing (aka reducing), neutral, or oxidizing. Adjustment is made by adding more or less oxygen to the acetylene flame. The neutral flame is the flame most generally used when welding or cutting. The welder uses the neutral flame as the starting point for all other flame adjustments because it is so easily defined. This flame is attained when welders, as they slowly open the oxygen valve on the torch body, first see only two flame zones. At that point, the acetylene is being completely burned in the welding oxygen and surrounding air . The flame is chemically neutral. The two parts of this flame are the light blue inner cone and the darker blue to colorless 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. It is approximately 6000 degrees F and provides enough heat to easily melt steel . In the inner cone the acetylene breaks down and partly burns to hydrogen and carbon monoxide, which in the outer cone combine with more oxygen from the surrounding air and burn.
An excess of acetylene creates a carbonizing flame. This flame is characterized by three flame zones; the hot inner cone, a white-hot "acetylene feather", and the blue-colored outer cone. This is the type of flame observed when oxygen is first added to the burning acetylene. The feather is adjusted and made ever smaller by adding increasing amounts of oxygen to the flame. A welding feather is measured as 2X or 3X, with X being the length of the inner flame cone. The unburned carbon insulates the flame and drops the temperature to approximately 5000 degrees F. The reducing flame is typically used for hardfacing operations or backhand pipe welding techniques. The feather is caused by incomplete combustion of the acetylene to cause an excess of carbon in the flame. Some of this carbon is dissolved by the molten metal to carbonize it. The carbonizing flame will tend to remove the oxygen from iron oxides which may be present, a fact which has caused the flame to be know as a "reducing flame" .
The oxidizing flame is the third possible flame adjustment. It occurs when the ratio of oxygen to acetylene required for a neutral flame has been changed to give an excess of oxygen. This flame type is observed when welders add more oxygen to the neutral flame. This flame is hotter than the other two flames because the combustible gases will not have to search so far to find the necessary amount of oxygen, nor heat up as much thermally inert carbon.  It is called an oxidizing flame because of its effect on metal. This flame adjustment is generally not preferred. The oxidizing flame creates undesirable oxides to the structural and mechanical detriment of most metals. In an oxidizing flame, the inner cone acquires a purplish tinge, gets pinched and smaller at the tip, and the sound of the flame gets harsh. A slightly oxidizing flame is used in braze-welding and bronze-surfacing while a more strongly oxidizing flame is used in fusion welding certain brasses and bronzes 
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 dipping metal from a welding rod or filler rod into the molten metal puddle. The metal puddle will travel towards where the metal is the hottest. This is accomplished through torch manipulation by the welder.
The amount of heat applied to the metal is a function of the welding tip size, the speed of travel, and the welding position. The flame size is determined by the welding tip size. The proper tip size is determined by the metal thickness and the joint design.
Welding gas pressures using oxy-acetylene are set in accordance with the manufacturer's recommendations. The welder will modify the speed of welding travel to maintain a uniform bead width. Uniformity is a quality attribute indicating good workmanship. Trained welders are taught to keep the bead the same size at the beginning of the weld as at the end. If the bead gets too wide, the welder increases the speed of welding travel. If the bead gets too narrow or if the weld puddle is lost, the welder slows down the speed of travel. Welding in the vertical or overhead positions is typically slower than welding in the flat or horizontal positions.
The welder must add the filler rod to the molten puddle. The welder must also keep the filler metal in the hot outer flame zone when not adding it to the puddle to protect filler metal from oxidation. Do not let the welding flame burn off the filler metal. The metal will not wet into the base metal and will look like a series of cold dots on the base metal. There is very little strength in a cold weld. When the filler metal is properly added to the molten puddle, the resulting weld will be stronger than the original base metal.
For cutting, the set-up is a little different. A cutting torch has a 60 or 90-degree angled head with orifices placed around a central jet. The outer jets are for preheat flames of oxygen and acetylene. The central jet carries only oxygen for cutting. The use of a number of preheating flames, rather than a single flame makes it possible to change the direction of the cut as desired without changing the position of the nozzle or the angle which the torch makes with the direction of the cut, as well as giving a better preheat balance . Manufacturers have developed custom tips for Mapp, propane, and polypropylene gases to optimize the flames from these alternate fuel gases.
The flame is not intended to melt the metal, but to bring it to its ignition temperature.
The torch's trigger blows extra oxygen at higher pressures 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. The ideal kerf 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 the ignition temperature ( approximately bright cherry red heat) using the pre-heat jets only, then using the separate cutting oxygen valve to release the oxygen from the central jet . The oxygen chemically combines with the iron in the ferrous material to instantly oxidize the iron 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. Oxy-acetylene torches can easily cut through ferrous materials in excess of 50 mm (2 inches). Oxygen Lances are used in scrapping operations and cut sections thicker than 200 mm (8 inches). Cut-off grinders are useless for these kinds of application.
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 to save money, 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.
Oxyacetylene welding/cutting is not difficult, but there are a good number of subtle points that should be learned such as
Fuel gases heavier than air (such as Propane, Propylene, MAPP, and Butane), can pool in low areas if allowed to escape, so special care should be taken to not use these gases over areas where they can collect (such as over basements, sinks, storm drains, etc.).
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 extremely deadly flying missile propelled by the compressed oxygen. For this reason, never move an oxygen tank around without its valve cap screwed in place.
On 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Oxy-fuel_welding_and_cutting". A list of authors is available in Wikipedia.|