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.
Fuels
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Compressed gas cylinders containing oxygen oxygen and MAPP gas |
The most commonly used fuel gas is acetylene. Other gases used are liquefied 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 methylacelylene-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 liquefied 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.
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Two types of oxy-gas torch head |
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 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 700-1400 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.
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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
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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 over tighten 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. There should be a bright, incandescent spot on the molten puddle. When the puddle is correctly maintained, a sound weld will result.
The blowtorch
A blowtorch is a special type of gas welder. It has an additional lever on the handle that, when squeezed, supplies an extra flow of oxygen out of a center hole in the mouthpiece. When the steel is heated to its melting point, the extra flow of oxygen will burn (oxidise) the molten iron and blow it away, effectively cutting the steel. The word "blowtorch" is often used loosely to mean any sort of oxy-gas torch.
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1. acetylene |
A. A flammable fuel gas composed of carbon and hydrogen having the chemical formula C2H2.When burned with oxygen, it produces a hot flame, having a temperature between 5700°F and 6300°F. It is a colorless gas, having a disagree-able odor that is readily detected even when the gas is highly diluted with air. When a portable welding outfit, it is obtained directly from the cylinder. In the case of stationary equipment, it can be piped to a number of individual cutting stations. |
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2.oxygen |
B. A chemical element with the chemical symbol O and atomic number 8. It is the second most common element on Earth, composing around 49% of the mass of Earth's crust and 28% of the mass of Earth as a whole, and is the third most common element in the universe. On Earth, it is usually covalently or ionically bonded to other elements. |
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3. hydrogen |
C. A chemical element in the periodic table that has the symbol H and atomic number 1. At standard temperature and pressure it is a colorless, odorless, nonmetallic, univalent, tasteless, highly flammable diatomic gas (H2). With an atomic mass of 1.00794 g/mol, it is the lightest element. It is also the most abundant, constituting roughly 75% of the universe's elemental matter |
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4. acetone |
D. In chemistry, (also known as propanone, dimethyl ketone, 2-propanone, propan-2-one and в-ketopropane) it is the simplest representative of the ketones. It is a colorless mobile flammable liquid with melting point at -95.4 °C and boiling point at 56.53 °C. It has a relative density of 0.819 (at 0 °C). It is readily soluble in water, ethanol, ether, etc., and itself serves as an important solvent. The most familiar household use of it is as the active ingredient in nail polish remover. It is also used to make plastic, fibers, drugs, and other chemicals. |
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5. aluminum |
E. A silvery and ductile member of the poor metal group of chemical elements. In the periodic table it has the symbol Al and atomic number 13. It is found primarily in the bauxite ore and is remarkable for its resistance to corrosion (due to the phenomenon of passivation) and its light weight. It is used in many industries to manufacture a large variety of products and is very important to the world economy. Structural components made from this metal and its alloys are vital to the aerospace industry and very important in other areas of transportation and building. |
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6. carbon |
F. A chemical element in the periodic table that has the symbol C and atomic number 6. An abundant nonmetallic, tetravalent element, it has several allotropic forms. It occurs in all organic life and is the basis of organic chemistry. This nonmetal also has the interesting chemical property of being able to bond with itself and a wide variety of other elements, forming nearly ten million known compounds. When united with oxygen it forms carbon dioxide, which is vital to plant growth. When united with hydrogen, it forms various compounds called hydrocarbons which are essential to industry in the form of fossil fuels. |
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7. oxy-gas torch |
G. A blowtorch has several meanings, which deal with tools that consume fuel. It may mean a cutting torch, a handheld torch used for cutting metal. |
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8. natural gas |
H. Commonly referred to as gas, is a gaseous fossil fuel consisting primarily of methane. It is found in oil fields and natural gas fields, and in coal beds. When methane-rich gases are produced by the anaerobic decay of non-fossil organic material, these are referred to as biogas. Sources of biogas include swamps, marshes, and landfills (see landfill gas), as well as sewage sludge and manure by way of anaerobic digesters, in addition to enteric fermentation particularly in |
cattle.
True or false?
1. The only used fuel gas in oxy-fuel welding is acetylene.
2. 'Oxidation' and 'combustion' are two terms which mean one and the same process.
3. Oxygen is the best fuel in oxy-fuel welding.
4. The blowtorch has two valves and one connection.
5. The welder wears goggles to protect his hands from burns.
6. The procedures and equipment used for gas welding and gas brazing are different.
7. When setting up the equipment, the valve cap is removed before the oxygen tank is fastened.
8. Both pliers and a wrench are used to tighten the nut of the regulator.
9. Sunglasses can be used instead of goggles to protect eyes from sparks and splatter.
10. The size of the flame depends on the size of the orifice in the tip and can be adjusted by the valves on the torch and by the regulator settings.
Answer the following questions:
1. What are the other two names of the oxy-fuel welding process?
2. What does the high temperature needed to melt the metal obtained by?
3. Is acetylene always shipped to the welding site in containers?
4. Why is hydrogen useful for underwater welding?
5. How can hydrogen be produced for small torches?
6. How is oxygen delivered to the welding site?
7. What does the apparatus used in gas welding consist of?
8. What colour are the oxygen and fuel hoses?
9. What is a neutral flame?
10. How many parts does an acetylene flame have?
Use the plan to make a report about gas welding:
1. Difference between Oxy-fuel welding and oxy-fuel cutting.
2. Fuel gases used for gas welding.
3. Parts of the apparatus used in gas welding.
4. Rules of setting up the equipment
5. Regulating the flame.
Translate the following sentences from Russian into English:
1. Контактная сварка осуществляется нагревом или расплавлением металлов при прохождении электрического тока в месте контакта сдавливаемых изделий. Выполняется сжатием листовых заготовок между стержневыми электродами (точечная контактная сварка) или вращающимися роликами (шовная контактная сварка) либо прижатием торцов изделий (стыковая контактная сварка).
2. Дуговая сварка (электродуговая сварка) - вид сварки, при которой кромки свариваемых металлических частей расплавляют дуговым разрядом между электродом и металлом в месте соединения.
3. Электронно-лучевая сварка - сварка расплавлением материалов в месте их соединения пучком электронов с энергией до 100 кэВ. Выполняется в вакууме. Применяется для прецизионной сварки, сварки изделий из особо чистых, разнородных или тугоплавких металлов (например, в микроэлектронике).
4. Газовая сварка - способ сварки металлических изделий с помощью газового пламени, образованного при сгорании смеси горючего газа (ацетилена, водорода, паров бензина и др.) с кислородом. Применяют для сварки тонкостенных изделий из стали, цветных металлов и сплавов, для наплавки твёрдых сплавов при ремонтных работах.
5. Электрошлаковая сварка - сварка плавлением металлов в месте соединения. Источником нагрева металла служит теплота, которая выделяется при прохождении электрического тока через расплавленный шлак, находящийся в зазоре между соединяемыми деталями.
6. Печная сварка (кузнечная, горновая) - соединение пластическим деформированием (например, с помощью молота) металлических изделий, нагретых в печах или горнах.
7. Диффузионная сварка - производится сдавливанием предварительно нагретых (без расплавления материалов) соединяемых деталей в вакууме, в результате чего происходит диффузия атомов материалов контактирующих деталей. Используют для сварки изделий из трудносвариваемых металлов, неметаллов, пластмасс.
8. Термитная сварка - технологический процесс, при котором зазор между соединяемыми деталями, предварительно нагретыми до 400 — 700°C, заполняется металлическим расплавом, полученным при сгорании термита. Используется для сварки проводов, труб, рельсовых стыков.
9. Газопрессовая сварка, соединение встык стержней, труб, фасонных профилей и т. д. нагревом мест сварки газовым пламенем до оплавления или пластического состояния металла и последующим сжиманием (осаживанием) соединяемых частей.
10. Высокочастотная сварка, сварка, при которой кромки свариваемых деталей нагревают токами ВЧ до их размягчения или оплавления и сжимают. Ток в изделии наводится индукционным или контактным способом. Используют, например, для сварки труб из ленты.
