Presentation on theme: "UNIT 5: Welding process:"— Presentation transcript:
1 UNIT 5: Welding process: Hareesha N GowdaLecturer, Dept of Aeronautical EnggDSCE, Bengaluru-78
2 Introduction: Arc Welding: Gas Welding: Definition Principles ClassificationApplicationAdvantages & limitations of welding.Arc Welding:PrincipleMetal Arc welding (MAW)Flux Shielded Metal Arc Welding (FSMAW)Inert Gas Welding (TIG & MIG)Submerged Arc Welding (SAW)Atomic Hydrogen Welding processes. (AHW)Gas Welding:Oxy – Acetylene weldingReaction in Gas weldingFlame characteristicsGas torch construction & workingForward and backward welding.
3 INTRODUCTIONWelding is a process for joining different materials.The large bulk of materials that are welded are metals and their alloys, although the term welding is also applied to the joining of other materials such as thermo plastics.Welding joins different metals/alloys with the help of a number of processes in which heat is supplied either electrically or by means of a gas torch.In order to join two or more pieces of metals together by one of the welding processes, the most essential requirement is Heat. Pressure may also be employed.Since a slight gap usually exists between the edges of the work pieces, a 'filler metal’ is used to supply additional material to fill the gap. But, welding can also be carried out without the use of filler metal.The filler metal is melted in the gap, combines with the molten metal of the work piece and upon solidification forms an integral part of the weld.
5 PRINCIPLE OF WELDINGAn ideal joint between two pieces of metal or plastic can be made by heating the workpieces to a suitable temperature. In other words, on heating, the materials soften sufficiently so that the surfaces fuse together.The bonding force holds the atoms, ions or molecules together in a solid. This 'bonding on contact' is achieved only when:the contaminated surface layers on the workpiece are removed,recontamination is avoided, andthe two surfaces are made smooth, flat and fit each other exactly.In highly deformable materials, the above aims can be achieved by rapidly forcing the two surfaces of the workpiece to come closer together so that plastic deformation makes their shape conform to each another; at the same time, the surface layers are broken up, allowing the intimate contact needed to fuse the materials.This was the principle of the first way known to weld metals; by hammering the pieces together while they are in hot condition.
6 CLASSIFICATION OF WELDING PROCESSES There are about 35 different welding and brazing processes and several soldering methods in use by industry today.There are various ways of classifying the welding and allied processes. For example, they may be classified on the basis of:Source of heat, i.e., flame, arc, etcType of interaction i.e. liquid/liquid (fusion welding) or solid/solid (solid state welding).In general, various welding and allied processes are classified as follows:1. Gas WeldingAir Acetylene WeldingOxyacetylene WeldingOxy hydrogen WeldingPressure gas Welding
8 ADVANTAGES OF WELDINGA good weld is as strong as the base metal.General welding equipment is not very costly.Portable welding equipments are available.Welding permits considerable freedom in design.A large number of metals/alloys both similar and dissimilar can be joined by welding.Welding can join workpieces through spots, as continuous pressure tight seams, end-to-end and in a number of other configurations.Welding can be mechanized.DISADVANTAGES 0F WELDINGWelding gives out harmful radiations (light), fumes and spatter.Welding results in residual stresses and distortion of the work-pieces.Edge preparation of the workpieces is generally required before welding them.A skilled welder is a must to produce a good welding job.Welding heat produces metallurgical changes. The structure of the welded joint is not same as that of the parent metal.A welded joint, for many reasons, needs stress-relief heat-treatment.
9 PRACTICAL APPLICATIONS OF WELDING Welding has been employed in Industry as a tool for:Regular fabrication of automobile cars, air-crafts, refrigerators, etc.Repair and maintenance work, e.g., joining broken parts, rebuilding worn out components, etc.A few important applications of welding are listed below:1. Aircraft ConstructionWelded engine mounts.Turbine frame for jet engine.Rocket motor fuel and oxidizer tanks.Ducts, fittings, cowling components, etc.2. Automobile ConstructionArc welded car wheelsSteel rear axle housing.Frame side rails.Automobile frame, brackets, etc.
10 3. BridgesSection lengths.Shop and field assembly of lengths, etc.4. BuildingsColumn base platesTrussesformation of structure, etc.5. Pressure Vessels and TanksClad and lined steel platesShell constructionJoining of nozzles to the shell, etc.6. Storage TanksOil, gas and water storage tanks.7. Rail Road Equipment LocomotiveUnder frameAir receiverEngineFront and rear hoods, etc.
11 8. Pipings and PipelinesRolled plate pipingOpen pipe joints,Oil gas and gasoline pipe lines, etc.9. ShipsShell frames.Deck beams and bulkhead stiffeners.Girders to shellsBulkhead webs to plating, etc.10. Trucks and trailers.11. Machine tool frames, cutting tools and dies.12. Household and office furniture.13. Earth moving machinery and cranes.In addition, arc welding finds following applications in repair and maintenance work:14. Repair of broken and damaged components and machinery such as tools, punches, dies, gears, shears, press and machine tools frames.15. Hard-facing and rebuilding of worn out or undersized (costly) parts rejected during inspection.16. Fabrication of jigs, fixtures, clamps and other work holding devices.
12 ARC WELDING PROCESSArc welding process is fusion method of welding that utilizes the high intensity of the arc generated by the flow of current to melt the workpieces.A solid continuous joint is formed upon cooling.
13 PRINCIPLEThe source of heat for arc welding process is an 'electric arc' generated between two electrically conducting materials.One of the workpiece material called 'electrode' is connected to one pole of the electric circuit, while the other workpiece which forms the second conducting material is connected to the other pole of the circuit.When the tip of the electrode material is brought in contact with the workpiece material and momentarily separated by small distance of 2-4 mm, an arc can be generated.The electrical energy is thus converted to heat energy.The high heat of the arc melts the edges of the workpieces.Coalescence takes place where the molten metal of the one workpiece combines with the molten metal of the other workpiece.When the coalesced liquid solidifies, the two workpieces join together to form a single component.The electrode material can be either a non-consumable material or a Consumable material.The non-consumable electrode made of tungsten, graphite etc., serve only to strike the arc and is not consumed during the welding process.Whereas, the consumable electrode which is made of the same material as that of the workpiece metal helps to strike the arc and at the same time melt (gets consumed) and combines with the molten metal of the workpiece to form a weld.
14 1. METALLIC ARC WELDING (MAW) In metallic arc welding an arc is established between work and the filler metal electrode.The intense heat of the arc forms a molten pool in the metal being welded, and at the same time melts the tip of the electrode.As the arc is maintained, molten filler metal from the electrode tip is transferred across the arc, where it fuses with the molten base metal.Arc may be formed with direct or alternating current.Petrol or diesel driven generators are widely used for welding in open, where a normal electricity supply may not be available.
15 METALLIC ARC WELDING (MAW) ( continued…….) A simple transformer however widely employed for A.C. arc welding.The transformer sets are cheaper and simple having no maintenance cost as there are no moving parts.With AC system, the covered or coated electrodes are used, whereas with D.C. system for cast iron and non-ferrous metals, bare electrodes can be used.In order to strike the arc an open circuit voltage of between 60 to 70 volts is required.For maintaining the short arc 17 to 25 volts are necessary.The current required for welding, however, varies from 10 amp. to 500 amp. depending upon the class of work to be welded.
16 2. CARBON ARC WELDINGHere the work is connected to negative and the carbon rod or electrode connected to the positive of the electric circuit.Arc is formed in the gap, filling metal is supplied by fusing a rod or wire into the arc by allowing the current to jump over it and it produces a porous and brittle weld because of inclusion of carbon particles in the molten metal.The voltage required for striking an arc with carbon electrodes is about 30 volts (A.C.) and 40 volts (D.C).A disadvantage of carbon arc welding is that approximately twice the current is required to raise the work to welding temperature as compared with a metal electrode, while a carbon electrode can only be used economically on D.C. supply.
17 3. FLUX SHIELDED METAL ARC WELDING (MMAW OR SMAW) a. Definition:It is an arc welding process wherein coalescence is produced by heating the workpiece with an electric arc set up between a flux coated electrode and the workpiece.The flux covering decomposes due to arc heat and performs many functions, like arc stability, weld metal protection, etc.,The electrode itself melts and supplies the necessary filler metal.
18 b. Principle of the process: Heat required for welding is obtained from the arc struck between a coated electrode and the workpiece.The arc temperature and thus the arc heat can be increased or decreased by employing higher or lower arc currents.A high current arc with a smaller arc length produces very intense heat.The arc melts the electrode end and the job.Material droplets are transferred from the electrode to the job, through the arc, and are deposited along the joint to be welded.The flux coating melts, produces a gaseous shield and slag to prevent atmospheric contamination of the molten weld metal.c. Striking the arc:In manual metal arc welding (MMAW), arc between the electrode and the workpiece is generally struck either by momentarily touching the electrode with the workpiece and taking it (electrode) a predetermined distance away from the workpiece by the wrist motion or by scratching the electrode on the job in the arc of a circle.d. Electrode holder:It can hold the electrode at various angles and energizes it at the same time.
19 e. Welding the jointOnce the arc has been established and the arc length adjusted, the electrode is inclined to an, angle of approximately 20 degrees with the vertical.To achieve comparatively deeper penetration, electrode angle with the vertical is further reduced.The electrode is progressed along the joint at a constant speed, it is lowered, at the same time, at a rate at which it is melting.f. Welding Equipment:AC or DC welding supply, electrode holder and welding cables.Welding electrodes.AC transformers and DC generators or rectifiers can be employed for welding with covered electrodes.Both AC and DC power sources produce good quality welds, but depending upon welding situation one may be preferred over the other.The most commonly used power source for AC welding is a transformer.A transformer may be operated from the mains on single phase, two phases or three phases.A typical specification for the transformer is as follows:Current range up to Amps.Open circuit voltage to 100 volts.
20 Advantages of Shielded Metal Arc Welding (SMAW) SMAW is the simplest of all the arc welding processes.The equipment can be portable and the cost is fairly low.This process finds innumerable applications, because of the availability of a wide variety of electrodes.A big range of metals and their alloys can be welded.Welding can be carried out in any position with highest weld quality.LimitationsBecause of the limited length of each electrode and brittle flux coating on it, mechanization is difficult.In welding long joints (e.g., in pressure vessels), as one electrode finishes, the weld is to be progressed with the next electrode. Unless properly cared, a defect (like slag inclusion or insufficient penetration) may occur at the place where welding is restarted with new electrode.The process uses stick electrodes and thus it is slower as compared to MIG welding.Because of flux coated electrodes, the chances of slag entrapment and other related-defects are more as compared to MIG or TIG welding.
21 ApplicationsToday, almost all the commonly employed metals and their alloys can be welded by this process.Shielded metal arc welding is used both as a fabrication process and for maintenance and repair jobs.The process finds applications inAir receiver, tank, boiler and pressure vessel fabrications;Shipbuilding;Pipes and Penstock joining;Building and Bridge construction;Automotive and Aircraft industry, etc.
22 A.C. WeldingAt higher currents AC gives a smoother arc.Once established the arc can be easily maintained and controlled.It is suitable for welding thicker sections.AC is easily available.AC welding power source has no rotating parts.It does not produce noise.It occupies less spaceIt is less costly to purchase and maintain.It possesses high efficiency (0.8).It consumes less energy per unit weight of deposited metal.Melting rate of electrode cannot be controlled in AC as equal heat generates at electrode and job.An AC welding power source is TransformerD.C. WeldingDC arc is more stable.DC is preferred for welding certain non-ferrous metals and alloys.It has lower open circuit voltage and therefore is safer.ARC heat can be regulated (i.e., through DCRP and DCSP)A DC welding equipment is a self contained unit. It can be operated in fields where power supply is not availableDC welding power source is a transformer-rectifier unit or a DC generator (motor or engine driven)
23 TUNGSTEN INERT GAS WELDING (TIG) Tungsten inert gas welding or gas tungsten arc welding (GTAW) is a group of welding process in which the workpieces are joined by the heat obtained from an electric arc struck between a non-consumable tungsten electrode and the workpiece in the presence of an inert gas atmosphere.A filler metal may be added if required, during the welding process.Figure shows the TIG process.
24 DescriptionTIG equipment consists of a welding torch in which a non-consumable tungsten alloy electrode is held rigidly in the collet.The diameter of the electrode varies from mm.TIG welding makes use of a shielding gas like argon or helium to protect the welding area from atmospheric gases such as oxygen and nitrogen, otherwise which may cause fusion defects and porosity in the weld metal.The shielding gas flow from the cylinder, through the passage in the electrode holder and then impinges on the workpiece.Pressure regulator and flow meters are used to regulate the pressure and flow of gas from the cylinder.Either AC or DC can be used to supply the required current.
25 OperationThe workpieces to be joined are cleaned to remove dirt, grease and other oxides chemically or mechanically to obtain a sound weld.The welding current and inert gas supply are turned ON.An arc is struck by touching the tip of the tungsten electrode with the workpiece and instantaneously the electrode is separated from the workpiece by a small distance of mm such that the arc still remains between the electrode and the workpiece.The high intensity of the arc melts the workpiece metal forming a small molten metal pool.Filler metal in the form of a rod is added manually to the front end of the weld pool.The deposited filler metal fills and bonds the joint to form a single piece of metalThe shielding gas is allowed to impinge on the solidifying weld pool for a few seconds even after the arc is extinguished (shut off)This will avoid atmospheric contamination of the solidifying metal thereby increasing the strength of the joint.
26 AdvantagesSuitable for thin metals.Clear visibility of the arc provides the operator to have a greater control over the weld.Strong and high quality joints are obtained.No flux is used. Hence, no slag formation. This results in clean weld joints.DisadvantagesTIG is the most difficult process compared to all the other welding processes. The welder must maintain short arc length, avoid contact between electrode and the workpiece and manually feed the filler metal with one hand while manipulating the torch with the other hand.Tungsten material when gets transferred into the molten metal contaminates the same leading to a hard and brittle joint.Skilled operator is required.Process is slower.Not suitable for thick metals.
27 METAL INERT GAS (MIG) WELDING Metal inert gas welding or gas metal arc welding (GMAW) is a group of arc welding process in which the workpieces are joined by the heat obtained from an electric arc struck between a bare (uncoated) consumable electrode and the workpiece in the presence of an inert gas atmosphere.The consumable electrode acts as a filler metal to fill the gap between the two workpieces.Figure shows the MIG welding process.
28 DescriptionThe equipment consists of a welding torch in which a bare consumable electrode in the form of a wire is held and guided by a guide tube.The electrode material used in MIG welding is of the same material or nearly the same chemical composition as that of the base metal.Its diameter varies from mm.The electrode is fed continuously at a constant rate through feed rollers driven by an electric motor.MIG makes use of shielding gas to prevent atmospheric contamination of the molten weld pool.Mixture of argon and carbon dioxide in a order of 75% to 25% or 80% to 20% is commonly used.The shielding gas flow from the cylinder, through the passage in the electrode holder and then impinges on the workpiece.AC is rarely used with MIG welding; instead DC is employed and the electrode is positively charged. This results in faster melting of the electrode which increases weld penetration and welding speed.
29 OperationThe workpieces to be joined are cleaned to remove dust, grease and other oxides chemically or mechanically to obtain a sound weld. The tip of the electrode is also cleaned with a wire brush.The control switch provided in the welding torch is switched ON to initiate the electric power, shielding gas and the wire (electrode) feed.An arc is struck by touching the tip of the electrode with the workpiece and instantaneously the electrode is separated from the workpiece by a small distance of mm such that the arc still remains between the electrode and the workpiece.The high intensity of the arc melts the workpiece metal forming a small molten pool.At the same time, the tip of the electrode also melts and combines with the molten metal of the workpieces thereby filling the gap between the two workpieces.The deposited metal upon solidification bonds the joint to form a single piece of metal.
30 AdvantagesMIG welding is fast and economical.The electrode and inert gas are automatically fed, and this makes the operator easy and to concentrate on the arc.Weld deposition rate is high due to the continuous wire feedNo flux is used. Hence, no slag formation. This results in clean welds.Thin and thick metals can be welded.Process can be automated.DisadvantagesEquipment is costlierPorosity (gas entrapment in weld pool) is the most common quality problem in this process. However, extensive edge preparation can eliminate this defect.
31 SUBMERGED ARC WELDING (SAW) Submerged arc welding is a group of arc welding process in which the workpieces are joined by the heat obtained from an electric arc struck between a bare consumable electrode and workpiece.The arc is struck beneath a covering layer of granulated flux.Thus, the arc zone and the molten weld pool are protected from atmospheric contamination by being 'submerged under a blanket of granular flux.This gives the name 'submerged arc welding' to the process.Figure shows the submerged arc welding process.
32 DescriptionThe equipment consists of a welding head carrying a bare consumable electrode and a flux tube.The flux tube remains ahead of the electrode, stores the granulated or powdered flux, and drops the same on the joint to be welded.The flux shields and protects the molten weld metal from atmospheric contamination.The electrode which is bare (uncoated) and in the form of wire is fed continuously through feed rollers.It is usually copper plated to prevent rusting and to increase its electrical conductivity (since it is submerged under flux).The diameter of the electrode ranges from mm and the electrode material depends on the type of the work piece metal being welded.The process makes use of either AC or DC for supplying the required current.
33 OperationEdge preparation is carried out to obtain a sound weld.Flux is deposited at the joint to be weldedWelding current is witched ON.An arc is struck between the electrode and the workpiece under the layer of flux.The flux covers the arc thereby increasing the heat near the weld zone.This heat melts the filler metal and the workpiece metal forming a molten weld pool.At the same time, a portion of the flux melts and reacts with the molten weld pool to form a slag.The slag floats on the surface providing thermal insulation to the molten metal thereby allowing it to cool slowly.The welding head is moved along the surface to be welded and the continuously fed electrode completes the weld.The un-melted flux is collected by a suction pipe and reused.The layer of slag on the surface of the weld portion is chipped out and the weld is finished.Since the weld pool is covered by flux, solidification of molten metal is slow. Hence, a backing plate made from copper or steel is used at the bottom of the joint to support the molten metal until solidification is complete.
34 AdvantagesHigh productivity process, due to high heat concentration.Weld deposition rate is high due to continuous wire feed. Hence, single pass welds can be made in thick plates.Deep weld penetration.Less smoke, as the flux hides the arc. Hence, improved working conditions.Can be automatedProcess is best suitable for outdoor works and in areas with relatively high winds.There is no chance of spatter of molten metal, as the arc is beneath the flux.DisadvantagesThe invisible arc and the weld zone make the operator difficult to judge the progress of welding.Use of powdered flux restricts the process to be carried only in flat positions.Slow cooling rates may lead to hot cracking defects.Need for extensive flux handling.
35 ATOMIC HYDROGEN WELDING (AHW) Atomic hydrogen welding is a thermo-chemical welding process in which the workpieces are joined by the heat obtained on passing a stream of hydrogen through an electric arc struck between two tungsten electrodes.The arc supplies the energy for a chemical reaction to take place.Filler rod may or may not be used during the process.Figure shows the arrangement for atomic welding process.
36 (endothermic reaction) DescriptionThe equipment consists of a welding torch with two tungsten electrodes inclined and adjusted to maintain a stable arc.Annular nozzles around the tungsten electrodes carry the hydrogen gas supplied from the gas cylinders.AC power source is suitable compared to DC, because equal amount of heat will be available at both the electrodes.A transformer with an open circuit voltage of 300 volts is required to strike and maintain the arc.OperationThe workpieces are cleaned to remove dirt, oxides and other impurities to obtain a sound weld. Hydrogen gas supply and welding current are switched ON.An arc is struck by bringing the two tungsten electrodes in contact with each other and, instantaneously separated by a small distance, say 1.5 mm, such that the arc still remains between the two electrodes.As the jet of hydrogen gas passes through the electric arc, it dissociates into atomic hydrogen by absorbing large amounts of heat supplied by the electric arc.(endothermic reaction)The heat thus absorbed can be released by recombination of the hydrogen atoms into hydrogen molecule (H2 ).
37 H + H H2 + 422kJ (exothermic reaction) Recombination takes place as the atomic hydrogen touches the cold workpiece liberating a large amount of heat.H + H H kJ (exothermic reaction)Note: The hydrogen can be thought of as simply a transport mechanism to extract energy from the arc, and transfer it to the work.A arc is produced due to the heat liberated during the chemical reaction. A feature of the arc is the speed by which it can deliver heat to the workpiece surface.The welding torch is moved along the surface to be welded with the arc tip touching the surface.The heat of the arc melts and fuses the workpiece and the filler metal to form a joint.The operator can control the heat by varying the distance of the arc stream between the two electrodes and the distance between the workpiece.
38 AdvantagesIntense flame is obtained which can be concentrated at the joint. Hence, less distortion.Welding is faster.Workpiece do not form a part of the electric circuit. Hence, problems like striking the arc and maintaining the arc column are eliminated.Separate flux/shielding gas is not required. The hydrogen envelope itself prevents oxidation of the metal and the tungsten electrode. It also reduces the risk of nitrogen pick-up.DisadvantageCost of welding by this process is slightly higher than with the other processes.Welding is limited to flat positions only.
39 GAS WELDINGDefinitionGas welding is a fusion-welding process.It joins metals, using the heat of combustion of an oxygen/air and fuel gas (i.e. acetylene, hydrogen, propane or butane) mixture.The intense heat (flame) thus produced melts and fuses together the edges of the parts to be welded, generally with the addition of a filler metal.Principle of gas weldingWhen the fuel gas and oxygen are mixed in suitable proportions in a welding torch and ignited the flame resulting at the tip of the torch is sufficient enough to melt the edges of the workpiece metals.A solid continuous joint is formed upon cooling.The two familiar fuel gases used in gas welding are:Mixture of oxygen and acetylene gas -called oxy-acetylene welding process.Mixture of oxygen and hydrogen gas - called oxy-hydrogen welding process.Oxy-acetylene welding is the most versatile and widely used gas welding process due to its high flame temperature (up to 3500oC) when compared to that of oxy hydrogen process (up to 2500oC)Note: Oxygen is not a fuel: It is what chemically combines with the fuel gas to produce the heat for welding. This is called 'oxidation', but the more general and commonly used term is 'combustion'.
40 OXY ACETYLENE WELDINGPrinciple of OperationWhen acetylene is mixed with oxygen in correct proportions in the welding torch and ignited, the flame resulting at the tip of the torch is sufficiently hot to melt and join the parent metal.The oxy-acetylene flame reaches a temperature of about 3200°C and thus can melt all commercial metals which, during welding, actually flow together to form a complete bond.A filler metal rod is generally added to the molten metal pool to build up the seam slightly for greater strength.
41 Description and Operation The equipment consists of two large cylinders: one containing oxygen at high pressure and the other containing acetylene gas.Two pressure regulators fitted on the respective cylinders regulates or controls the pressure of the gas flowing from the cylinders to the welding torch as per the requirements.The welding torch is used to mix both oxygen and acetylene gas in proper proportions and burn the mixture at its tip.A match stick or a spark lighter may be used to ignite the mixture at the torch tip.The resulting flame at the tip has a temperature ranging from 3200°C °C and this heat is sufficient enough to melt the workpiece metal.Since a slight gap usually exists between the two workpieces, a filler metal is used to supply the additional material to fill the gap.The filler metal must be of the same material or nearly the same chemical composition as that of the workpiece material.The molten metal of the filler metal combines with the molten metal of the workpiece and upon solidification form a single piece of metal.Flux, if required, may be used during the process. It can be directly applied to the surface of the workpiece or, the heated end of the filler metal may be dipped in a flux material and used.
42 Advantages of Gas Welding It is probably the most versatile process. It can be applied to a wide variety of manufacturing and maintenance situations.Welder has considerable control over the temperature of the metal in the weld zone.The rate of heating and cooling is relatively slow. In some cases, this is an advantage.Since the sources of heat and of filler metal are separate, the welder has control over filler-metal deposition rates.The equipment is versatile, low cost, and usually portable.The cost and maintenance of the gas welding equipment is low when compared to that of some other welding processes.Heavy sections cannot be joined economically.Flame temperature is less than the temperature of the arc.Fluxes used in certain welding and brazing operations produce fumes that are irritating to the eyes, nose, throat and lungs.Gas flame takes a long time to heat up the metal than an arc.More safety problems are associated with the handling and storing of gases.Acetylene and oxygen gases are rather expensive.Flux shielding in gas welding is not so effective as an inert gas shielding in TIG or MIG welding.
43 (1/3 of total heat generation) REACTIONS IN GAS WELDINGWhen suitable proportions of oxygen and acetylene are mixed and ignited at the torch tip, a flame with a temperature of about 3200°C is produced.For complete combustion to take place, two volumes of acetylene is combined with five volumes of oxygen. The reaction is given below:2C2H2+502->4C02 + 2H20Complete combustion takes place in two stages.1) First stage combustionAt the beginning of the process, when the gas torch is ignited, equal volumes of oxygen and acetylene are issued from the torch to burn in the atmosphere.The reaction occurs due to which the inner cone is visible at the torch tip.For example, consider one volume of each oxygen and acetylene.C2H > 2CO + H2 + heat(1/3 of total heat generation)This is an exothermic reaction that produces CO and H2 as products of the first stage of combustion.
44 2/3 of total heat generation 2) Second stage combustionThe second stage combustion involves the combustion of CO and H2 which are the products of combustion of first stage.Both these products are capable of supporting combustion and hence, utilize 02 from the surrounding atmosphere for combustion.The reactions are as follows:2CO > 2C02andH O2->H2O2/3 of total heat generationCarbon monoxide burns and forms carbon dioxide, while hydrogen combines with oxygen to form water.The combustion is therefore complete and carbon dioxide and water (turned to steam) are the chief products of combustion.
46 a) Neutral FlameA neutral flame is produced when approximately equal volumes of oxygen and acetylene are mixed in the welding torch and burnt at the torch tip. (More accurately the oxygen-to-acetylene ratio is 1.1 to 1).The temperature of the neutral flame is of the order of about 3260°CThe flame has a nicely defined inner cone which is light blue in color. It is surrounded by an outer flame envelope, produced by the combination of oxygen in the air and superheated carbon monoxide and hydrogen gases from the inner cone. This envelope is usually a much darker blue than the inner cone.A neutral flame is named so because it affects no chemical change on the molten metal and, therefore, will not oxidize or carburize the metal.The neutral flame is commonly used for the welding of:Mild steel Stainless steelCast Iron CopperAluminium
47 b) Oxidizing Flame (O2 : C2H2 = 1.5 : 1) If, after the neutral flame has been established, the supply of oxygen is further increased, the result will be an oxidizing flame.An oxidizing flame can be recognized by the small cone which is shorter, much bluer in color and more pointed than that of the neutral flame.The outer flame envelope is much shorter and tends to fan out (disperse) at the end.An oxidizing flame tends to be hotter than the neutral flame. This is because of excess oxygen and which causes the temperature to rise as high.The excess oxygen, tends to combine with many metals to form hard, brittle, low strength oxides.Moreover, an excess of oxygen causes the weld bead and the surrounding area to have a scummy or dirty appearance.For these reasons, an oxidizing flame is of limited use in welding. It is not used in the welding of steel.A slightly oxidizing flame is helpful when welding mostCopper-base metalsZinc-base metals
48 c) Reducing FlameIf the volume of oxygen supplied to the neutral flame is reduced, the resulting flame will be a carburizing or reducing flame, i.e., rich in acetylene.A reducing flame can be recognized by acetylene feather which exists between the inner cone and the outer envelope.The outer flame envelope is longer than that of the neutral flame and is usually much brighter in color.A reducing flame does not completely consume the available carbon; therefore, its burning temperature is lower and the leftover carbon is forced into the molten metal. With iron and steel it produces very hard, brittle substance known as iron carbide.This chemical change makes the metal unfit for many applications in which the weld may need to be bent or stretched.Metals that tend to absorb carbon should not be welded with reducing flame.A reducing flame has an approximate temperature of 3038°C.
49 d) Carburizing flameA reducing flame may be distinguished from carburizing flame by the fact that a carburizing flame contains more acetylene than a reducing flame.A carburizing flame is used in the welding of lead and for carburizing (surface hardening) purposes.A reducing flame, on the other hand, does not carburize the metal, rather it ensures the absence of the oxidizing condition. It is used for welding with low alloy steel rods and for welding those metals, (e.g. non-ferrous) that do not tend to absorb carbon.This flame is very well used for welding high carbon steel.To conclude, for most welding operations, the Neutral Flame is correct, but the other types of flames are sometimes needed for special welds, e.g., non-ferrous alloys and high carbon steels may require a reducing flame, whilst zinc-bearing alloys may need an oxidizing flame for welding purposes.
50 Welding TechniquesDepending upon the ways in which welding rod and the welding torch may be used, there are two usual techniques in gas welding, namely:Leftward technique or Forehand welding method.Rightward technique or Back hand welding method.
51 Leftward TechniqueThe welder holds welding torch in his right hand and filler rod in the left hand.The welding flame is directed away from the finished weld, i.e., towards the un-welded part of the joint. Filler rod, when used, is directed towards the welded part of the joint (Fig.).The weld is commenced on the right-hand side of the seam, working towards the left-hand side.The blowpipe or welding torch is given small sideways movements, while the filler rod is moved steadily across the seam.The filler rod is added using a backward and forward movement of the rod, allowing the flame to melt the bottom edges of the plate just ahead of the weld pool.
52 Leftward Technique (Continued…..) Since the flame is pointed in the direction of the Welding, it preheats the edges of the joint.Good control and a neat appearance are characteristics of the leftward method. Leftward technique is usually used on relatively thin metals, i.e., having thicknesses less than 5 mm.When workpiece thickness is over 3 mm, it is necessary to bevel the plate edges to produce a V-joint so that good root fusion may be achieved.The included angle of V-joint is 80-90°.When welding materials over 6.5 mm thick, it is difficult to obtain even penetration at the bottom of the V and, therefore, the quality of the weld decreases as plate thickness increases.The leftward technique requires careful manipulation to guard against excessive melting of the base metal, which results in considerable mixing of base metal and filler metal.
53 Rightward TechniqueHere again the welding torch is held in the right hand of the welder and the filler rod in the left hand.Welding begins at the left-hand end of the joint and proceeds towards the right, hence the name rightward technique.The direction of welding is opposite to that when employing the leftward technique.The torch flame in rightward technique is directed towards the completed weld and the filler rod remains between the flame and the completed weld section (Fig.).Since the flame is constantly directed on the edges of the V ahead of the weld puddle (Molten metal pool) , no sidewise motion of the welding torch is necessary. As a result, a narrower V-groove (30° bevel or 60° included angle) can be utilized than in leftward welding. This provides a greater control and reduced welding costs.During welding, the filler rod may be moved in circles (within the puddle) or semicircles (back and forth around the puddle).The rightward technique is one used on heavier or thicker (above 5 mm) base metals, because in this technique the heat is concentrated into the metal. Welds with penetrations of approximately 12 mm can be achieved in a single pass.
54 Rightward technique has got certain advantages over the leftward one, as listed below: Up to 8.2 mm plate thickness, no bevel is necessary. This saves the cost of preparation and reduces the consumption of filler rod.For welding bigger thicknesses, where beveling of plate edges becomes necessary, the included angle of V need be only 60°, which requires less filler metal against 80°V preparation used in leftward welding technique.The welder's view of the weld pool and the sides and bottom of the V groove is unobstructed. This results in better control and higher welding speeds.The smaller total volume of deposited metal, as compared to leftward welding, reduces shrinkage and distortion.The weld quality is better than that obtained with the leftward technique.Owing to less consumption of the filler metal, the rightward technique involves lower cost of welding than leftward technique.
55 Welding Torch or Blow Pipe Oxygen and the fuel gas having been reduced in pressure by the gas regulators are fed through suitable hoses to a welding torch which mixes and controls the flow of gases to the welding nozzle or tip where the gas mixture is burnt to produce a flame for carrying out gas welding operation.There are two types of welding torches, namely:High pressure (or equal pressure) type.Low pressure (or injector) type.High pressure blow-pipes or torches are used with (dissolved) acetylene stored in cylinders at a pressure of 8 bar.Low pressure blow-pipes are used with acetylene obtained from an acetylene generator at a pressure of 200 mm head of water (approximately 0.02 bar).
56 Working of a low pressure blow-pipe It is termed as a low pressure blow-pipe because it can be operated at low acetylene pressure; it is frequently used with acetylene generators.As acetylene is of low pressure, it is necessary to use oxygen at a high pressure (2.5 bar).As shown in fig., the oxygen enters the mixing chamber through a passage located in the centre of the torch. The oxygen passage is surrounded by the acetylene. The high pressure oxygen passes through a small opening in the injector nozzle, enters the mixing chamber and pulls (or draws) the acetylene in after it.An advantage of low pressure torch is that small fluctuations in the oxygen supplied to it will produce a corresponding change in the amount of acetylene drawn, thereby making the proportions of the two gases constant while the torch is in operation.
57 Working of a high pressure blow-pipe In this type of blow-pipe, both the oxygen and acetylene are fed to the blow pipe at equal pressures and the gases are mixed in a mixing chamber prior to being fed to the nozzle tip.The equal pressure or high pressure type of blow-pipe is the one most generally used becauseIt is lighter and simpler.It does not need an injector.In operation, it is less troublesome since it does not suffer from backfires to the same extent.To change the power of the welding torch, it is only necessary to change the nozzle tip (size) and increase or decrease the gas pressures appropriately.