Welding Fundamentals

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. Practical applications of welding a) Aircraft Construction i) Welded engine mounts ii) Turbine frame for jet engine iii) Rocket motor fuel and oxidizer tanks iv) Ducts and other fittings b) Automobile construction i) Arc welded car wheels ii) Steel rear axle housing iii) Frame side rails iv) Automobile frame, brackets, etc. c) Bridges i) Pier construction ii) Section lengths d) Buildings i) Column base plates ii) Trusses iii) Erection of structure e) Pressure vessels and tanks i) Clad and lined steel plates ii) Shell construction iii) Joining of nozzle to the shell iv) Oil, gas and water storage tanks f) Rail road equipment i) Locomotive under frame ii) Locomotive air receiver iii) Locomotive engine iv) Locomotive front and rear hoods, etc. g) Pipings and pipelines i) Rolled plate piping ii) Open pipe joints iii) Oil, gas and gasoline pipe lines, etc. h) Ships i) Shell frames ii) Deck beams and bulkhead stiffeners iii) Girders to shells iv) Bulkhead webs to plating i) Repair and maintenance work i) Repair of broken and damaged components and machinery such as tools, punches, dies, gears, shears, press and machine tools frames. ii) Hard facing and rebuilding of worn out or undersized parts rejected during inspection.

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Classification of welding processes Welding processes are generally classified as 1. Gas welding Air-acetylene welding Oxy-acetylene welding Oxy-hydrogen welding Pressure gas welding 2. Arc welding Flash welding Stud arc welding Bare metal arc welding Carbon arc welding Flux cored arc welding Submerged arc welding Shielded metal arc welding Electro gas welding Plasma arc welding Gas metal arc welding Gas tungsten arc welding 3. Resistance welding Spot welding Seam welding Projection welding Resistance Butt welding Flash Butt welding High frequency resistance welding Percussion building 4. Solid state welding Cold welding Diffusion welding Explosive welding Forge welding Friction welding Hot pressure welding Roll welding Ultrasonic welding 5. Thermo-chemical welding processes Thermit welding Atomic hydrogen welding 6. Radiant Energy welding processes Electron beam welding Laser beam welding Advantages of welding A good weld is as strong as the base metal. General welding equipment is not very costly.

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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, endto-end and in a number of other configurations. Welding can be mechanized. Limitations of welding Welding gives out harmful radiations, fumes and spatter. Welding results in residual stresses and distortion of the workpieces. Jigs and fixtures are generally required to hold and position the parts to be welded. 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 needs stress-relief heat treatment.

GAS WELDING is a fusion welding process in which the metal surfaces to be joined are melted progressively by heat from a gas flame, with or without filler metal, and are caused to flow together and solidify without the application of pressure to the parts being joined. The commonly employed fuel gases are acetylene, hydrogen, propane or butane mixture. Either air or oxygen is provided for facilitating combustion.

The simplest and most frequently used gas welding system consists of compressed gas cylinders, gas pressure regulators, hoses, and a welding torch. Oxygen and fuel are stored in separate cylinders. The gas regulator attached to each cylinder, whether fuel gas or oxygen, controls the pressure at which the gas flows to the welding torch. At the torch, the gas passes through an inlet control valve and into the torch body, through a tube or tubes within the handle, through the torch head, and into the mixing chamber of the welding nozzle or other device attached to the welding torch. The mixed gases then pass through the welding tip and produce the flame at the exit end of the tip. This equipment can be mounted on and operated from a cylinder cart, or it can be a stationary installation. Filler metal, when needed, is provided by a welding rod that is melted progressively along with the surfaces to be joined. OXY-ACETYLENE WELDING Oxygen and acetylene are the principal gases used in gas welding. Oxygen supports combustion of the fuel gases. Acetylene supplies both the heat intensity and the atmosphere needed to weld steel.

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The high heat transfer intensity required in gas welding can be obtained only by burning selected fuel gases with high-purity oxygen in a high-velocity flame. Oxygen is supplied for oxyfuel gas welding and cutting at a purity of 99.5% and higher, because small percentages of contaminants have a noticeable effect on combustion efficiency. When the consumption requirement is relatively small, the oxygen is supplied and stored as a compressed gas in a standard steel cylinder under an initial pressure of up to 180 kPa (26 ksi). The most frequently used cylinder has a capacity of 6.91 m 3. The gas is distributed for use under reduced pressure. Oxygen cylinders are painted black and the valve outlets are screwed right handed. When consumption of oxygen is somewhat greater, banks of cylinders are joined through a manifold to permanent pipeline systems that terminate at various stations of use.

Acetylene is a hydrocarbon gas with the chemical formula C2H2. When under pressure of 203 kPa (29.4 psi) and above, acetylene is unstable, and a slight shock can cause it to explode, even in the absence of oxygen or air. Safety rules for the use of acetylene and the

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handling of acetylene equipment are extremely important. An acetylene cylinder is painted maroon and the valves are screwed left handed to make this easily recognizable they are chamfered or grooved. Acetylene should not be used at pressure greater than 105 kPa (15 psi). Commercially supplied portable cylinders are specially constructed to store acetylene under high pressure. Acetylene cylinders must not be subjected to sudden shock and should be stored well away from any source of heat or sparks. The cylinders must be stored in an upright position to keep the acetone from escaping during use. Under normal sustained use, withdrawal rate from an acetylene cylinder should not exceed one-seventh of the cylinder capacity per hour. Commercial Production of Oxygen and Acetylene Oxygen is commercially produced by the fractional distillation of liquefied air. Before air is liquefied, water vapor and carbon dioxide are removed, because these substances solidify when cooled and would clog the pipes of the air liquefaction plant. The dry, CO 2-free air is compressed to about 200 atmospheres. This compression causes the air to become warm, and the heat is removed by passing the compressed air through radiators. The cooled, compressed air is then allowed to expand rapidly. The rapid expansion causes the air to become cold, so cold that some of it condenses. By the alternate compressing and expanding of air, most of it can be liquefied. Oxygen is obtained from liquid air by distillation at -183˚C. The common processes employed for the commercial production of acetylene are By the action of water on calcium carbide CaC2 + 2H2O = Ca(OH)2 + C2H2 By the partial oxidation of methane or natural gas 5CH4 + 3O2 = C2H2 + 3CO +6H2 + 3 H2O If large quantities of acetylene gas are being consumed, it is much cheaper to generate the gas at the place of use with the help of acetylene gas generators. Acetylene generators for on-site gas production are constructed so that the gas is not given off at pressures much greater than 105 kPa (15 psi). Acetylene gas is generated by carbide-to-water method. The generator unit feeds controlled amounts of calcium carbide into the water. When these ingredients are mixed, acetylene gas is produced.

Advantages of gas welding Welder has considerable control over the temperature of the metal in the weld zone. The relatively lower rate of heating and cooling is an advantage in some cases. Since the sources of heat and filler metal are separate, the welder has control over filler metal deposition rates. Heat can be applied preferentially to the base metal or filler metal. JO/VJCET

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The equipment is versatile, low cost, self-sufficient and usually portable. Besides gas welding, the equipment can be used for preheating, post heating, torch brazing and oxygen cutting. The cost and maintenance of welding equipment is low when compared to that of some other welding processes. Limitations of gas welding Heavy sections cannot be joined economically. Flame temperature is less than the temperature of the arc. Fluxes used in some gas welding operations produce fumes Refractory metals and reactive metals cannot be gas welded. Gas flame takes a long time to heat up the metal than an arc. Prolonged heating of the joint in gas welding results in a larger heat affected area. This often leads to increased grain growth, more distortion and loss of corrosion resistance. More safety problems are associated with the handling and storage 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. Applications of gas welding For joining thin materials. For joining materials in whose case extremely high temperatures or rapid heating and cooling of the job would produce unwanted or harmful changes in the metal. For joining materials in whose case extremely high temperatures would cause certain elements in the metal to escape into the atmosphere. For joining most ferrous and non-ferrous metals eg., carbon steels, alloy steels, Cast Iron, aluminium, copper, nickel, magnesium and its alloys etc. In automotive and aircraft industries. In sheet metal fabrication plants. CHEMISTRY OF OXY-ACETYLENE WELDING Combustion of gas mixture takes place in two stages. Stage1: Oxygen and acetylene in equal proportions by volume burn in the inner white cone. The oxygen combines with carbon of the acetylene and forms carbon monoxide, while hydrogen is liberated. 2C2H2 + 2O2 → 4CO + 2H2 ……... (1) Stage 2: The carbon monoxide uses the oxygen supplied from the air surrounding the flame for burning into carbon dioxide. The hydrogen also burns with oxygen and forms water vapour. 4CO + 2H2 + 3O2 → 4CO2 + 2H2O ………. (2) Combining equations (1) and (2), 2C2H2 + 5O2 → 4CO2 + 2H2O ………. (3) TYPES OF WELDING FLAMES By varying the relative amounts of acetylene and oxygen in the gas mixture in the torch, a welder can produce different flame atmospheres and temperatures. There are three distinct types of flames.

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Neutral flame: A neutral flame is produced when approximately equal volumes of oxygen and acetylene (oxygen to acetylene ratio of 1.1 to 1) are mixed in the welding torch and burnt at the torch tip. The temperature of the neutral flame is of the order of about 3260˚ C. The flame has a nicely defined inner cone which is light blue in colour. It is surrounded by an outer flame envelope, produced by the combination of atmospheric oxygen and superheated carbon monoxide. This envelope is usually a much darker blue than the inner cone. The flame is named neutral because it effects no chemical change in the molten metal. Neutral flame is commonly employed for welding of mild steel, stainless steel, cast iron, copper and aluminium. Oxidising flame: An oxidizing flame is obtained by increasing the oxygen supply after a neutral flame is established. The oxygen to acetylene ratio will be around 1.5. An oxidizing flame can be recognized by the small inner cone which is shorter, much bluer in colour and more pointed than that of the neutral flame. The outer flame envelope is much shorter and tends to fan out at the end. The flame burns out with a loud roar. Because of excess oxygen, the temperature is higher and reaches around 3500˚ C. The excess oxygen tends to combine with many metals to form hard, brittle, low strength oxides. An excess of oxygen causes the weld bead and surrounding area to have a scummy or dirty appearance. A slightly oxidizing flame is helpful in welding copper base metals, Zinc base metals and a few types of ferrous metals such as manganese steel and cast iron. The oxidizing atmosphere, in these cases, creates a base metal oxide that protects the base metal. For example, in welding brass, the zinc has a tendency to separate and fume away. The formation of covering copper oxide prevents the zinc from disappearing. Reducing flame / Carburising flame: If the volume of oxygen supplied to the neutral flame is reduced, the resulting flame will be carburizing or reducing flame, 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 colour. A reducing flame does not completely consume the available carbon, its burning temperature is lower and the left over carbon is forced into the molten metal. A reducing flame has an approximate temperature of 3038˚ C. A carburizing flame contains more acetylene than a reducing flame. A carburizing flame is used in the welding of lead and for surface hardening by carburizing. A reducing flame does not carburize the metal; rather it ensures absence of the oxidizing condition. It is used for welding metals that do not tend to absorb carbon. This flame is used for welding high carbon steel.

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Filler Metals and fluxes used for gas welding Filler metal is the material added to the weld pool to assist in filling the gap or groove. Filler metal forms an integral part of the weld. Filler rods have the same or nearly the same chemical composition as the base metal. A flux is a material used to prevent, dissolve or facilitate removal of oxides and other undesirable substances. A flux prevents the oxidation of molten metal. The flux is fusible and nonmetallic. During welding, flux chemically reacts with the oxides and a slag is formed that floats to and covers the top of the molten puddle of metal and thus helps keep out atmospheric oxygen and other gases. Fluxes are available as powders, pastes or liquids. Over fluxing must be avoided as it embrittles the weld. It is advisable to wash off the flux thoroughly from the part after welding.

Oxy-Fuel Gas(Flame) Cutting

Oxy-acetylene flame preheats the metal to the ignition point at the place to be cut. It also provides a protective shield between the cutting oxygen stream and the atmosphere. High purity cutting oxygen combines with iron to form iron oxide. Fe + O  FeO + heat 3Fe + 2O2  Fe3O4 + heat 4Fe + 3O2  2Fe2O3 + heat

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Cutting oxygen jet blows away molten iron and iron oxide thereby cutting a narrow slit or kerf in the metal object. The maximum thickness that can be cut by OFC depends mainly on the gases used. With oxyacetylene gas, the maximum thickness is about 350 mm and with oxyhydrogen it is 600 mm. Kerf width ranges from 1.5 mm to 10 mm with reasonably good control of tolerances. The flame leaves drag lines on the cut surface.

Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or nonconsumable electrodes. The welding region is usually protected by some type of shielding gas, vapor, or slag. Arc Column theory

The arc column is generated between an anode, which is the positive pole of the dc power supply, and the cathode, the negative pole. Electrons are easily dissociated from the metal at the cathode. These electrons are accelerated away from the cathode to the anode, striking it at a highly accelerated velocity. This path of the negatively charged mass is generally in the interior of the arc column which is the hotter portion of the arc column. The electrons carry an electrostatic charge. This electrostatic or small current carrying capacity is multiplied thousands of times, causing part of the heat of the arc column. Also the kinetic energy of electrons gets converted into heat energy on striking the anode. Intermingling with electrons, ions are returning from anode to cathode producing the ionized gas layer which further protects the electrons and the electrostatic unit within the electron. The electrostatic unit is induced into the anode causing an emf in the anode , which is directly transferred into heat energy.

There are three areas of heat liberation in the arc column – cathode area, plasma area and anode area. Of the three areas, the anode area is the high heat area where approximately 10,000 to 11,000˚F of heat is liberated. The liberation of heat results from the combination of the impingement of the electrons upon the anode anvil and the current carrying capacity of the electrons. The plasma area is heated mainly as a result of the atomic collision of the few electrons and the many ions that are passing through the ionized gas column. The cathode is mainly subjected to ionic bombardment, which produces the state of medium heat in the arc column. Approximately two thirds of the energy released in the arc column system is always at the anode.

Methods of arc initiation The method of initiating a welding arc depends upon the process used. In general these methods can be grouped into two categories.

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In one category, the ionization of gases b/w electrode to work gap is achieved by the application of high voltage across it. This high frequency high voltage is applied with the help of a spark gap oscillator. This helps in ionizing the gases in the gap b/w the electrode and work piece and the arc is initiated in few milli seconds. This method of arc initiation is used in TIG welding and Carbon Arc Welding process so as to avoid, contamination of tungsten electrode or to eliminate the chance of carbon pick up from the carbon electrode if touch method is used. In the other category, the arc initiation is done by touching the electrode and work piece and withdrawing it. Upon touching a heavy short circuit current flows in the circuit, causing melting of minute points of contacts. When the electrode is withdrawn it results in sparking and ionization of the gap b/w the electrode and the work piece. This method is used for arc initiation in manual metal arc welding and SMAW processes. ARC WELDING POWER SOURCES An arc welding power source is designed to change high voltage low amperage current into a safe voltage b/w (50-100V) and heavy current supply (200-600 A) suitable for arc welding. Arc welding power sources can be divided into 3 categories. 1. Those that supply direct current (DC) Eg: Motor generator Sets Diesel Engine driven generator and Transformer Rectifier sets 2. Those that supply Alternating current Eg: Transformers and AC generators 3. AC or DC arc welding combination supplying either AC or DC. Such power sources are AC transformers with DC rectifiers. Factors Affecting the Selection of Power Sources Available power (AC or DC), single phase etc Available Floor Space Initial and Running cost Location of operation Personnel available for maintenance Flexibility of the equipment Required output Type of electrode and metals used Type of work - heavy or light Duty cycle Efficiency Electrode polarity in arc welding

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In straight polarity, electrode is connected to the negative terminal and work piece to the positive terminal. In reverse polarity, electrode is connected to the positive terminal and work piece to the negative terminal. Two third of the heat is developed near the positive pole.

In all processes using non consumable electrodes, it is better to connect the electrode to the negative terminal to keep the heat losses to the minimum. When consumable electrode is used, the metal transfer from the wire electrode to the work piece is more uniform, frequent and better directed if the electrode is made positive. DCEP or reverse polarity is therefore popular with GMAW which also provides necessary cleaning action on metals with tenacious oxide layer such as Aluminium. Shielded metal arc welding SHIELDED METAL ARC WELDING (SMAW) is a manual welding process whereby an arc is generated between a flux-covered consumable electrode and the work piece. The process uses the decomposition of the flux covering to generate a shielding gas and to provide fluxing elements to protect the molten weld-metal droplets and the weld pool.

Heat required for welding is obtained from the arc struck between the coated electrode and the work piece. The arc is initiated by momentarily touching or "scratching" the electrode on the base metal. The resulting arc melts both the base metal and the tip of the welding electrode. The molten electrode metal/flux is transferred across the arc (by arc forces) to the base-metal pool, where it becomes the weld deposit covered by the protective, less-dense slag from the electrode covering. The arc temperature and thus the arc heat can be increased or decreased by employing higher or lower arc currents.

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Advantages It is the simplest of all the arc welding processes. The equipment is portable and cost is fairly low. A big range of metals and their alloys can be welded. Welding can be carried out in any position with good weld quality. The process can be employed for hard facing and metal deposition to reclaim parts. Limitations Mechanization is difficult because of the limited length of each electrode and brittle flux coating on it. Unless properly cared, defects like slag inclusion or insufficient penetration may occur at the places of electrode change in long welding joints. 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 related defects are more as compared to TIG or MIG welding. Because of fumes and particles of slag, the arc and metal transfer is not very clear and thus welding control in this process is a bit difficult as compared to MIG welding. Applications It is used both as a fabrication process and for maintenance and repair jobs. Heavy construction, such as shipbuilding, and welding "in the field," away from many support services that would provide shielding gas, cooling water etc. It is primarily used to join steels. This family of materials includes low-carbon or mild steels, alloy steels, stainless steels, and many of the cast irons. In addition to joining metals, the SMAW process is frequently used for the protective surfacing of base metals. The surfacing deposit can be applied for the purpose of corrosion control or wear resistance. Gas metal arc welding (GMAW) or Metal inert gas welding (MIG welding) MIG welding is an arc welding process that joins metals together by heating them with an electric arc that is established between a consumable electrode (wire) and the work piece. An externally supplied gas or gas mixture acts to shield the arc and molten weld pool. Helium and argon or their mixtures are the commonly employed shielding gases.

The arc is established between a continuously fed electrode of filler metal and the work piece. After proper settings are made by the operator, the arc length is maintained automatically at the set value. This automatic arc regulation can be achieved in

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two ways. The most common method is to utilize a constant-speed (but adjustable) electrode feed unit with a variable-current (constant-voltage) power source. As the gun-towork relationship changes, which instantaneously alters the arc length, the power source delivers either more current (if the arc length is decreased) or less current (if the arc length is increased). This change in current will cause a corresponding change in the electrode melt-off rate, thus maintaining the desired arc length. The second method of arc regulation utilizes a constant-current power source and a variable-speed, voltage-sensing electrode feeder. In this case, as the arc length changes, there is a corresponding change in the voltage across the arc. As this voltage change is detected, the speed of the electrode feed unit will change to provide either more or less electrode per unit of time. Advantages The process can be easily mechanized. Electrode length does not face the restrictions encountered with SMAW process. Welding speeds are higher than those of the SMAW process. Deposition rates are significantly higher than those obtained by the SMAW process. Continuous wire feed enables long welds to be deposited without stops and starts. Penetration that is deeper than that of the SMAW process is possible, which may permit the use of smaller-sized fillet welds for equivalent strengths. Less operator skill is required than for other conventional processes. Minimal post weld cleaning is required because of the absence of a heavy slag. Limitations The welding equipment is more complex, usually more costly, and less portable than SMAW equipment. The process is more difficult to apply in hard-to-reach places because the welding gun is larger than a SMAW holder and must be held close to the joint to ensure that the weld metal is properly shielded. The welding arc must be protected against air drafts that can disperse the shielding gas, which limits outdoor applications unless protective shields are placed around the welding area. Relatively high levels of radiated heat. Applications GMAW finds extensive use in fabrication of structures, ship building, pressure vessels, tanks, pipes, domestic equipment, general and heavy electric engineering and the aircraft engine manufacturing industries. It is also used successfully for the fabrication of railway coaches and in the automobile industry where long, high speed welds of fairly heavy sections are used. The welding of lorry frames is an example. It is used for welding tool steels and dies. It is used for the manufacture of refrigerator parts. CO2 welding or MAG (Metal Active Gas) welding or MIG-CO2 welding CO2 process is a variant of GMAW process in which CO2 is used as the shielding gas. CO2 being an active gas, the process is known as Metal Active Gas (MAG) welding. This process is used for the welding of carbon and low alloy steels. It produces deeper penetration than argon or argon mixtures. During welding operation, CO2 exposed to the high temperature of the welding arc, changes into carbon monoxide and oxygen. The molecular oxygen changes to its atomic form.

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2CO2→2 CO + O2 O2→2O The nascent oxygen could be damaging, if the wrong type of electrode wire is used. The electrode wire for CO2 welding must contain deoxidizers such as manganese and silicon that readily combine with the oxygen and prevent it from combining with the weld metal. The oxides formed – SiO2 and MnO pass into the slag. Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas welding (TIG welding) TIG welding is an arc welding process wherein coalescence is produced by heating the job with an electric arc struck between a tungsten electrode and the job. A shielding gas (argon, helium and their mixtures) is used to avoid atmospheric contamination of the molten weld pool. A filler metal may be added, if required.

The arc is struck either by touch method or with the help of a high frequency unit. Sometimes an arc is struck initially between the tungsten electrode and a scrap metal piece or a tungsten piece. This heats the tip of the tungsten electrode and then it will be easy for striking the arc with pre cleaned work piece. The electrode material may be tungsten or tungsten alloy – thoriated tungsten or zirconiated tungsten. Alloy tungsten electrodes possess higher current carrying capacity, high resistance to contamination and produce a steadier arc, as compared to pure tungsten electrodes. Advantages Produces high-quality, low-distortion welds. Free of the spatter associated with other methods. Can be used with or without filler wire. Can be used with a range of power supplies. Welds almost all metals, including dissimilar ones. Gives precise control of welding heat. Limitations Produces lower deposition rates than consumable electrode arc welding processes. Less economical than consumable electrode arc welding for thick sections. Problematic in drafty environments because of difficulty in shielding the weld zone properly.

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Filler rod end if it by chance comes out of the inert gas shield can cause weld metal contamination. Tungsten if it transfers to molten weld pool can contaminate the same. Tungsten inclusion is hard and brittle. Applications It is a very good process for welding nonferrous metals, their alloys and stainless steel. Welding of expansion bellows, transistor cases, instrument diaphragms and can sealing joints. Precision welding in atomic energy, aircraft, chemical and instrument industries. Rocket motor chamber fabrications in launch vehicles. Submerged Arc Welding SUBMERGED ARC WELDING (SAW) is an arc welding process in which the arc is concealed by a blanket of granular and fusible flux. Heat for SAW is generated by an arc between a bare, solid-metal consumable wire or strip electrode and the work piece. The arc is maintained in a cavity of molten flux or slag, which refines the weld metal and protects it from atmospheric contamination. Alloy ingredients in the flux may be present to enhance the mechanical properties and crack resistance of the weld deposit.

A continuous electrode is being fed into the joint by mechanically powered drive rolls. A layer of granular flux, just deep enough to prevent flash through, is being deposited in front of the arc. Electrical current, which produces the arc, is supplied to the electrode through the contact tube. The current can be direct current (with reverse polarity or straight polarity), or alternating current. After welding is completed and the weld metal has solidified, the unfused flux and slag are removed. The unfused flux may be screened and reused. The solidified slag may be collected, crushed, resized, and blended back into new flux. Advantages The arc is under a blanket of flux, which virtually eliminates arc flash, spatter, and fume.

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High current densities increase penetration and decrease the need for edge preparation. High deposition rates and welding speeds are possible. Cost per unit length of joint is relatively low. The flux acts as a scavenger and deoxidizer to remove contaminants such as oxygen, nitrogen, and sulfur from the molten weld pool. This helps to produce sound welds with excellent mechanical properties. Low-hydrogen weld deposits can be produced. The shielding provided by the flux is substantial and is not sensitive to wind as in shielded metal arc welding and gas metal arc welding. The slag can be collected, reground, and sized for mixing back into new flux as prescribed by manufacturers and qualified procedures. Limitations The initial cost of wire feeder, power supply, controls, and flux handling equipment is high. The weld joint needs to be placed in the flat or horizontal position to keep the flux positioned in the joint. The slag must be removed before subsequent passes can be deposited. Because of the high heat input, saw is most commonly used to join steels more than 6.5 mm thick. Applications Submerged arc welding is most commonly used to join plain carbon steels. Alloy steels can be readily welded with SAW if care is taken to limit the heat input as required to prevent damage to the heat-affected zone (HAZ). Because SAW is used to join thick steel sections, it is primarily used for shipbuilding, pipe fabrication, pressure vessels, and structural components for bridges and buildings. Flux Cored Arc Welding FLUX-CORED ARC WELDING (FCAW) is an arc welding process in which the heat for welding is produced by an electric arc between a continuous filler metal electrode and the work piece. A tubular, flux-cored electrode is used in this process.

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Flux-cored arc welding has two major variations. The gas-shielded FCAW process uses an externally supplied gas to assist in shielding the arc from nitrogen and oxygen in the atmosphere. Generally, the core ingredients in gas shielded electrodes are slag formers, deoxidizers, arc stabilizers, and alloying elements. In the self-shielded FCAW process, the core ingredients protect the weld metal from the atmosphere without external shielding. Some self-shielded electrodes provide their own shielding gas through the decomposition of core ingredients. Others rely on slag shielding, where the metal drops being transferred across the arc and the molten weld pool are protected from the atmosphere by a slag covering. Many self-shielded electrodes also contain substantial amounts of deoxidizing and denitrifying ingredients to help achieve sound weld metal. Self-shielded electrodes can also contain arc stabilizers and alloying elements. Advantages High deposition rates, especially for out-of-position welding Less operator skill required than for GMAW. Simpler and more adaptable than SAW. Deeper penetration than SMAW. More tolerant of rust and scale than GMAW. Good weld appearance. Can be easily mechanized. Economical engineering joint designs. Limitations Slag must be removed from the weld and disposed of. More smoke and fume are produced in FCAW than in the GMAW and SAW processes. Fume extraction is generally required. Equipment is more complex and much less portable than SMAW equipment. Used only to weld ferrous metals, primarily steels. Electrode wire is more expensive. Applications Both the gas-shielded and self-shielded FCAW processes are used to fabricate structures from carbon and low-alloy steels. Both process variants are used for shop fabrication, but the self-shielded FCAW process is preferred for field use. Gas-shielded flux-cored electrodes are commonly used to weld carbon, low-alloy steel, and stainless steels in the construction of pressure vessels and piping for the chemical processing, petroleum refining, and power-generation industries. Flux-cored electrodes are also used in the automotive and heavy-equipment industries in the fabrication of frame members, axle housings, wheel rims, suspension components, and other parts. Small-diameter flux-cored electrodes are used for automotive body repair.

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Classification of welding electrodes

Electro Slag Welding Electro slag welding is a welding process wherein coalescence is produced by molten slag which melts the filler metal and the surfaces of the work to be welded.

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Electro slag welding is initiated by starting an arc between the filler metal/electrode and the work. This arc heats the flux and melts it to form the slag. The arc is then extinguished and the (conductive) slag is maintained in molten condition by its resistance to the flow of electric current between the electrode and work. Molten slag remains between the electrode and the work. The molten metal pool remains shielded by the molten slag which moves along the full cross-section of the joint as the welding progresses. Advantages Joint preparation is often much simpler than for other welding processes. Much thicker steels can be welded in single pass and more economically. Electroslag welding gives extremely high deposition rates. Residual stresses and distortion produced are low. Flux consumption as compared to that in submerged arc welding is very low. During the electroslag process, since no arc exists, no spattering or intense arc flashing occurs. Disadvantages Submerged arc welding is more economical than electroslag welding for joints below 60mm. There is some tendency toward hot cracking and notch sensitivity in the HAZ. It is difficult to close cylindrical welds. Electroslag welding tends to produce rather large grain size. Welding is carried out in vertical uphill position. Applications Heavy plates, forgings and castings can be butt welded. Where plates or castings of uniform thickness are involved or if they taper at a uniform rate, electroslag welding has virtually replaced thermit welding, being much simpler. Following alloys can be welded: Low carbon and medium carbon steels. Plasma Arc Welding Plasma Arc Welding (PAW) is an arc welding process similar to TIG welding . The electric arc is formed between an electrode and the work piece. The key difference from TIG is that in PAW, by positioning the electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. The plasma is then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 20,000 °C.

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Plasma arc welding process can be divided into two basic types: Non-transferred arc process – The arc is formed between the electrode (-) and the water cooled constricting nozzle (+). Arc plasma comes out of the nozzle as a flame. The arc is independent of the work piece and the work piece does not form a part of the electrical circuit. Just as an arc flame, it can be moved from one place to another and can be better controlled. A non-transferred arc is initiated by using a high frequency unit in the circuit. Transferred arc process – For initiating a transferred arc, a current limiting resistor is put in the circuit which permits a flow of about 50amps between the nozzle and the electrode and a pilot arc is established between the electrode and the nozzle. As the pilot arc touches the job, main current starts flowing between electrode and job, thus igniting the transferred arc. The pilot arc initiating unit gets disconnected and pilot arc extinguishes as soon as the arc between the electrode and the job is started.

Advantages Stability of arc Uniform penetration Simplified fixtures Rewelding of the root of the joint saved. It is possible to produce fully penetrated keyhole welds on pieces upto and about 6mm thick with square butt joint. Excellent weld quality. Plasma arc welding can produce radiographic quality welds at high speeds. It can weld steel pieces up to about one half inch thick, square butt joint in single run with no filler metal addition. Limitations Infra-red and ultraviolet radiations necessitate special protection devices. Welders need ear plugs because of unpleasant, disturbing and damaging noise. More chances of electrical hazards are associated with this process. The process is limited to metal thickness of 25mm and lower for butt welds. Plasma arc welding process and equipments are more complicated and require greater knowledge on the part of the welder as compared to TIG welding. Inert gas consumption is high.

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Electron Beam Welding Electron beam welding is a fusion welding process wherein coalescence is produced by the heat obtained from a concentrated beam of high velocity electrons. As the high velocity electrons strike the surfaces to be joined, their kinetic energy changes to thermal energy thereby causing the work piece metal to melt and fuse.

The EBW equipment includes the following subsystems. - An electron beam gun with a high voltage power supply and controls. - A vacuum pumping system. - Mechanical tooling – fixtures, drives and motor controls. - A beam alignment system including optics, scanner, tape control and tracker. The tungsten filament in the electron beam gun is electrically heated in vacuum to approximately 2000˚C and it emits electrons. The electrons emitted from the heated filament carry a negative charge, are repelled by the cathode and are made to pass through the central hole of the anode. The electrons are greatly accelerated by the tremendous difference of potential, voltage between the cathode and anode. The electron beam is then focused by means of an electromagnetic focusing coil (lens). The focusing coil concentrates or spreads the electron beam to the user’s needs. Kinetic energy of the electrons is converted to heat energy when striking the work piece. Advantages - High quality welds can be made at high speeds. - The fusion zone and HAZ are extremely narrow. - As the energy input is in a narrow concentrated beam, distortion is almost eliminated. - The hard vacuum makes it possible to weld such highly reactive vacuum melted materials as titanium, zirconium etc. with the same control of purity as in the original material. - Welded joint surfaces are clean and bright having no oxides, scales or flux slags and thus no need of cleaning up after the weld is completed.

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Electron beam welds have a highly desirable depth to width ratio. Much deeper penetration can be obtained in a single pass. - Power requirement is small when compared to the power requirements of other electrical welding devices. - Precise control is possible. Limitations - Initial cost of equipment is high and portable equipment is rare. - Work is to be manipulated through vacuum seals. - Time and equipment is required to create vacuum every time a new job is to be welded. - Obstructed joints cannot be welded. - Good welding skill is required. - Work piece size is limited by the work chamber dimensions. Applications - For welding reactive and refractory metals used in the atomic energy and rocketry fields. - For welding automobile, air plane, aerospace, farm and other types of equipment where especially low distortion is required. - EBW is very suitable where high quality, large scale automatic welding operations are required. - EBW machines can be modified to make two parallel beads simultaneously with one gun and find applications in the mass production of type writer carriages. -

Laser Beam Welding Laser beam welding is defined as a welding process wherein coalescence is produced by heat obtained from a laser beam impinging upon the surfaces to be joined. The laser beam is highly directional, strong monochromatic and coherent. The laser beam can be focused to a very small spot giving a very high energy density which may reach 10 9W/mm2. There are three basic types of lasers – solid state laser, gas laser and semi-conductor laser.

The laser welding system consists of a ruby crystal, flash tube, capacitor tank, mirror, optical focusing lens and cooling system. A flash tube containing inert gas Xenon is placed around the outer side of the ruby crystal. The flash tube operates at a rate of thousands of flashes per second. Flash tube converts electrical energy to light energy. The capacitor bank charged by a high voltage power supply stores electrical energy and energises the flash tube by appropriate triggering systems. When subjected to electrical discharge from capacitors, JO/VJCET

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Xenon transforms a high proportion of the electrical energy into white light flashes of about 1/1000 second duration. As the ruby is exposed to intense light flashes, a laser beam is emitted from it. The laser welding machine has an optical reflective cavity which reflects and focuses high intensity radiation from the flash tubes onto the ruby rod. This narrow laser beam is focused by an optical focusing lens to produce a small intense spot of laser on the job. Optical energy as it impacts the work piece is converted into heat energy and the temperature generated is sufficient to melt the work pieces to be welded. Since most of the power output of a laser source is lost as heat, cooling system is used to carry away the same. There are two techniques for laser welding. One is to move the work piece so fast that a complete joint passes by and is welded by one burst. The other, and the more common method, is to fuse a series of spots, commonly overlapping. In laser welding, a minute puddle is melted and frozen in a matter of microseconds. Since this time is very short, no chemical reaction between the molten metal and atmosphere takes place and hence in laser welding no protection is needed against atmospheric contamination. Advantages As no electrode is used, electrode contamination or high electric current effects are eliminated. Areas not readily accessible can also be welded. It permits welding of small, closely spaced components with welds as small as few microns in diameter. Unlike EBW, it operates in air. No vacuum is required. Laser beam being highly concentrated and narrowly defined produces narrow HAZ. It is possible to weld heat treated alloys without affecting their heat treated condition. Mechanical contact of any kind with the job is not required; moreover, the material being welded need not be a conductor of electricity. Laser can be focused to microscopic dimensions and directed with great accuracy. Laser welding is clean – no vaporized metal or electrodes dirty up the delicate assemblies. Limitations Laser welding is limited to depths of approximately 1.5mm and additional energy only tends to create gas voids and undercuts in the work. Materials such as magnesium tend to vapourise during laser welding and produce severe surface voids. Slow welding speeds. Welding machine should be designed to preclude exposure of the operator’s eyes to the direct or reflected laser beam. Applications For connecting leads on small electronic equipments and in integrated circuitry in the electronic industry. To weld lead wires having polyurethane insulation without removing insulation. The laser evaporates the insulation and completes the weld. In space and aircraft industry for welding light gauge materials. Laser beam is used for microwelding purposes. It is suitable for the welding of miniaturized and micro miniaturized components.

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Explosive Welding Explosive welding is a solid phase welding process wherein coalescence is effected by high velocity movement produced by a controlled detonation. The process involves a high velocity impact between a plate propelled by an explosive charge and a stationary plate. Some of the commonly used explosives for the purpose are PETN, TNT, RDX, Metabel, Tetryl and Datasheet.

Figure above shows the two arrangements for explosion welding – parallel arrangement (direct stand off method / contact explosion welding) and angular arrangement (angular stand off method / impact explosion welding). The flyer plate is to be joined with the parent plate. There is a buffer above the flyer plate which may be of rubber, cardboard or similar material to protect its top surface from damage from the detonation. Above the buffer is a layer of explosive which is detonated from the lower edge. The parent plate rests on an anvil to limit distortion of the final product. As the explosive is ignited, the detonation wave front progresses across the surface of the flyer plate. The explosive impulse provides both extremely high normal pressure and a slight, relatively shear pressure between the flyer plate and the parent plate. At the point of impact, a high instantaneous pressure is generated which is large compared with the shear strength of the materials. A thin high velocity jet is formed from the surfaces of both plates. This creates fresh virgin surfaces which are brought together and adhere. Advantages Simplicity of the process Extremely large surface can be bonded. Welds can be produced on heat treated metals without affecting their microstructures. Thin foils can be bonded to heavy plates. Wide range of thicknesses can be explosively clad together. Explosive bonds have a solid state joint that is free from HAZ. Lack of porosity, phase changes and structural changes impart better mechanical properties to the joints. Limitations In industrial areas, the use of explosives will be severely restricted by the noise and ground vibrations caused by explosion. Regulations and Government norms for handling the explosives have to be taken care of. Metals to be bonded by this process should possess some ductility and some impact resistance. Metals harder than 50 RC are difficult to weld.

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Because of the complexity of the welding mechanisms and the need to maintain controlled collision conditions, EXW is generally confined to welding simple geometries such as flat plates, cylinders and cones. The thickness of the cladding or flyer plate is limited. Cladding plate of thicknesses upto 5 mm are explosive welded successfully. Applications The following metals can be clad to carbon steels and low alloy steels, and some may be clad to stainless steels – Zirconium, Titanium, SS, Cu and Ni alloys Pipes and tubes upto 1.5 m length have been clad with this process. Heat exchanger tube sheets and pressure vessels are major areas of use of explosively clad products. Explosive welding has been used for plugging of nuclear heat exchangers. Thermit Welding It is a process in which a mixture of aluminium powder and a metal oxide called Thermit is ignited to produce the required quantity of molten metal by an exothermic nonviolent reaction. The superheated metal so produced is poured at the desired place which on solidification results in a weld joint. It is thus a casting cum welding process.

The thermochemical reaction that takes place on the ignition of thermit is based on the following basic equation. Metal oxide + (Al powder) → Metal + Al oxide + heat This reaction can be started only if the mixture is ignited with a special ignition powder or an ignition rod. Though the metal oxide used in Thermit welding is usually iron oxide, however oxides of Cu, Ni and Cr can also be employed to give the following reactions and the corresponding theoretical temperatures attained.

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Apart from the high purity of the thermit material, the presence of aluminium strongly promotes rapid nucleation and small grain size. Advantages Broken parts can be welded at the site itself. The remoteness of location is not a problem since no costly power supply is required. Limitations Thermit welding is applicable only to ferrous metal parts of heavy sections. The process is uneconomical if used to weld cheap metals or light parts. Applications For repairing fractured rails. For butt welding pipes end to end. For welding large fractured crankshafts. For welding broken frames of machines. For replacing broken teeth on large gears. For welding cables for electrical conductors. For end welding of reinforcing bars to be used in concrete construction. ELECTRIC RESISTANCE WELDING (ERW) ERW refers to a group of welding processes such as spot , seam & projection welding that produce coalescence of faying surfaces where heat to form the weld is generated by the electrical resistance of material vs. the time and the force used to hold the materials together during welding. Some factors influencing heat or welding temperatures are the proportions of the work pieces, the metal coating or the lack of coating, the electrode materials, electrode geometry, electrode pressing force, electrical current and length of welding time. Small pools of molten metal are formed at the point of most electrical resistance (the connecting or "faying" surfaces) as an electrical current (100–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high. Resistance Spot Welding (RSW) is a process in which faying surfaces are joined in one or more spots by the heat generated by resistance to the flow of electric current through work pieces that are held together under force by electrodes. The contacting surfaces in the region of current concentration are heated by a short-time pulse of low-voltage, high amperage current to form a fused nugget of weld metal. When the flow of current ceases, the electrode force is maintained while the weld metal rapidly cools and solidifies. Spot welding machines are composed of three principal elements: Electrical circuit, which consists of a welding transformer, tap switch, and a secondary circuit. Control circuit, which initiates and times the duration of current flow and regulates the welding current. Mechanical system, which consists of the frame, fixtures, and other devices that hold and clamp the work piece and apply the welding force.

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Advantages Low cost High speed of welding Less operator skill requirement High uniformity of products No edge preparation neded Operation may be made automatic or semiautomatic Applications Spot welding is the most widely used joining technique for the assembly of sheet metal products such as automotive body assemblies, domestic appliances, furniture, building products, enclosures and aircraft components. Many assemblies of two or more sheet metal stampings that do not require gas-tight or liquid-tight joints can be more economically joined by high-speed RSW than by mechanical methods. Containers frequently are spot welded. The attachment of braces, brackets, pads, or clips to formed sheet-metal parts such as cases, covers, bases, or trays is another common application of RSW. Resistance Seam Welding (RSEW) is a process in which heat generated by resistance to the flow of electric current in the work metal is combined with pressure to produce a welded seam. The resulting seam consists of a series of spot welds.

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Roll spot welding (relatively large unwedded gaps between nuggets) Reinforced roll spot welding (small gaps between nuggets) Leak-tight seam welding (nugget overlap) Two rotating circular electrode wheels are often used to apply current, force, and cooling to the work metal. A variety of work piece/wheel configurations are possible. When two electrode wheels are used, one or both wheels are driven, either by a direct drive of the wheel axles or by a knurl drive that contacts the peripheral surface of the electrode wheel. For some applications, the electrode wheels idle while the work piece is driven. Advantages Gas-tight or liquid-tight joints can be produced. Seam width may be less than the diameter of spot welds, because the electrode contour can be continuously dressed and is therefore of a stable shape. High-speed welding (especially on thin stock) is possible. Tooling cost is generally favorable per inch of flange welded. Coated steels are generally more weldable using seam welding than spot welding, because coating residue can be continuously removed from the electrode wheels if special provisions are made. Limitations Welds must ordinarily proceed in a single plane or on a uniformly curved surface. Obstructions along the path of the electrode wheel must be avoided or compensated for in the design of the wheel. Material handling must not induce extraneous forces into the fragile, molten weld zone during welding. Components using multiple crossing seam welds can be quality-sensitive at the weld intersections. External water cooling of the electrodes and the weld zone may be required for highspeed welding. External cooling may add tooling cost for water containment and water removal from the parts after welding. Applications Girth welds can be made in round, square or rectangular parts. Lap seams are popular in automotive applications, such as automotive fuel tanks, catalytic converters, mufflers, and roof joints, as well as in nonautomotive applications, such as furnace heat exchangers, water tanks, and certain types of can making. Typical applications of mash seam weld include drums, buckets, vacuum-jacketed bottles, aerosol cans, water tanks, and steel mill coil joining. Butt seam welding is employed for tube welding and for sheet metals in rail road cars. Projection Welding (PW) is a variation of resistance welding in which current flow is concentrated at the point of contact with a local geometric extension of one (or both) of the parts being welded. These extensions, or projections, are used to concentrate heat generation at the point of contact. The process typically uses lower currents, lower forces, and shorter welding times than does a similar application without the projections. Projection welding is often used in the most difficult resistance-welding applications because a number of welds can be made at one time, which speeds up the manufacturing process.

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Advantages A number of welds can be made simultaneously. Projection welds can be made in metals that are too thick to be joined by spot welding. Scale, rust, oil and and work metal coatings interfere less with projection welding than with spot welding. Projection welding electrodes possess longer life than spot welding ones because of less wear and maintenance. Show sides of the jobs can be produced with no electrode marking, thus making it possible to paint or plate them without grinding. Lower current and pressure requirements reduce chances of shrinkage and distortion Limitations Limited to combinations of metal thickness and composition which can be embossed. Metals that are not strong enough to support projections cannot be projection welded satisfactorily. Forming of projection on work piece is an extra operation. For proper welding, all projections must be of same height. Applications welding of automobile components Small fasteners and nuts welded to larger components. Refrigerator condensers, crossed wire welding etc. Metallurgy of welding There are 3 distinct zones in the macro structure of a welded joint a) weld metal zone b) Heat affected zone Grain Growth region Grain refined region Transition region c) Unaffected Base Metal or parent metal Weld metal zone: Weld metal zone is formed as the weld metal solidifies from the molten state. This is a mixture of parent metal and electrode or filler metal, the ratio depending upon the welding process used, the type of joint, plate thickness etc. Micro structure of the weld metal zone reflects the cooling rate in the weld. Heat affected zone (HAZ) HAZ adjacent to the weld metal zone is composed of parent metal that did not melt but was heated to a high enough temperature for a sufficient period that grain growth occurs. The mechanical properties and microstructure is this region is altered by the heat of JO/VJCET

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welding. The width of heat affected zone varies according to the welding process and techniques. There are three metallurgically distinguished region is the heat affected zone.

Grain Growth Region: Grain growth region is immediately adjacent to the weld metal zone. In the zone, parent metal has been heated to a temperature well above the upper critical temperature. This results in grain growth or coarsening of the structure. The maximum grain size and the extent of this grain growth region increases as the cooling rate decreases. The coarse primary grain structure causes 1. Decreased zone plasticity 2. Increased susceptibility of steel to cold cracking, stress relief cracking etc. 3. Lowering of strength in metals which do not undergo polymorphic transformation. Grain Refined Region: The finest grain structure exists in this region. The peak temperature in this zone does not exceed 1150˚ C. Therefore, ferrite to austenite transformation during heating does not have time to develop properly and thus the grain size remains small. Also the carbides may not be fully dissolved. The austenite to ferrite transformation on cooling tends to produce a fine grained ferrite-pearlite structure depending on factors like heat input plate thickness etc. The large grain boundary area tends to permute ferrite nucleation and the austenite that remains at grain centre is rich in carbon and transforms to pearlite. Transition Zone: Partial allotropic recrystallization takes place in this zone. Steel is heated b/w 650˚ C - 950˚C in this zone. Eutectic pearlite begins to dissolve in the zone heated beyond 750˚ C. Pearlite to austenite transformation requires certain time to become completed but this process comes to an end at 950˚ c. If subsequent cooling rate is higher, there is no time for the reverse process to take place. I.e., Carbon fails completely to diffuse back into the former pearlite grains. The result is the formation of a chunky pearlite. At higher speeds of cooling, a former pearlite grain can be quenched to martensite.

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Spherodised cementite particles can be found in the region where temp was b/w 650˚ C and 750˚ C. Unaffected Parent Metal: This area is not heated sufficiently to change the micro structure. Weld defects I) Cracks Cracks are the most dangerous of all weld defects. Crack is a form of stress relief in the weld metal or Heat Affected zone (HAZ).

Cracks are usually caused by 1. High contraction stresses - This can be minimized by back step or block welding sequence 2. Rigidity of the joint - This can be reduced by pre-heating or relieving the residual stresses mechanically. 3. Poor ductility of Base Metal - This can be rectified by pre-heating and annealing the based metal. 4. Poor Fit cap and incorrect welding procedures - Reduced root opening and proper welding procedures reduces this type of cracks. 5. Poor edge Quality - This can be reduced by proper edge preparation before welding. 6. Electrode with high hydrogen content - Use proper electrode to avoid this. 7. High welding speed - Use appropriate welding speed. 2) Distortion Uneven heating of work piece during welding results in the development of welding stresses which often lead to distortion or warpage of the welded structure.

Various factors leading to distortion are 1) More number of passes with small diameter electrode. 2) Slow arc travel speed. 3) Type of joint - A V-joint shows more distortion compared to U-joint. 4) High residual stresses in the plates to be welded. JO/VJCET

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Distortion can be corrected by 1) Proper jigging of joints prior to welding. 2) Post weld slow cooling or stress relieving heat treatment 3) Peening the weld metal and heat affected zone if the fabrication specifications allow it. 4) Proper sequencing of welding procedure. 3) Inclusion Inclusion may be in the form of slag or any other foreign material which does not get a chance to float on the surface of the solidifying weld metal and thus gets entrapped inside the same (weld metal). Inclusions lower the strength of the joint and make it weaker. Inclusions can be continuous, intermittent or very randomly spaced.

Slag inclusion occur as a result of 1) Incomplete deslagging of a previous pass. 2) Wide weaving which permits slag to solidify at the sides of the bead. 3) Erratic progressions of travel. 4) Excessive amount of slag ahead of the arc particularly in deep groove. 5) Use of too large electrodes The preventive measures are 1) To deslag the slag deposited thoroughly before a subsequent weld bead is deposited. 2) To restrict the width of beading so that the entire width of slag immediately behind the weld metal remains molten. 3) To keep the slag behind the arc by shortening the arc, increasing the electrode angle or increasing travel speed. 4) To use a smaller electrode. 4) Porosity and Blow Holes Blow holes and porosity are voids, holes, or cavities formed by gas trapped by the solidifying weld metal. Porosity is a group of small voids where as blow holes or gas pockets are comparatively bigger isolated holes or cavities.

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The sources of trapped gases may be 1) Rust dirt, grease, paint or primer on the edges of the parent metal or on the electrode. 2) Damped SAW fluxes 3) Impurities and moisture in the shielding gas 4) Excessive welding speed 5) High welding current in SMAW burns the deoxidisers. 6) Electrode with damped and damaged coatings. Porosity can be reduced by (1) Use of perfectly cleaned dry welding equipments. (2) Proper use of electrode baking procedure. (3) Use of moisture resistant SMAW electrode with hydrophobic flux coating. (4) Purge the shielding gas lines before welding. (5) Avoid the excessive welding current and too long arc lengths. 5) Lack of fusion Lack of fusion or incomplete fusion may occur b/w the parent metal and the weld metal and also b/w the various layers in multipass welding. Lack of fusion appreciably reduces the strength of weld and makes welded structures unreliable.

The main causes of lack of fusion are 1) Low arc current 2) Faster arc travel speed 3) An offset of electrode from the axis of weld. 4) Improper weaving technique so that the edges are not melted thoroughly. 5) Incorrect joint preparation 6) Incorrect electrode manipulation. 6) Spatter Spatter are small metal particles which are thrown out of the arc during welding and get deposited on the base metal around the weld bead along its length. Spatter may be due to 1) Excessive Arc Current 2) Longer Arc 3) Damped electrode 4) Electrode being coated with improper flux ingredients. 5) Arc blow making the arc uncontrollable. 6) Bubbles of gas getting entrapped in the molten metal globule expand with great violence projecting small drops of metal outside the arc stream.

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7) Under Cutting Under cuts are grooves melted into the parent metal adjacent to the toe of the weld and left unfilled by weld metal. Groove reduces the thickness of the plate which in turn weakens the weld.

The main causes of under cutting are (1) Excessive welding current (2) Large electrode diameter (3) Wrong manipulation and inclination of electrode and excessive weaving (4) Low arc (5) Faster arc travel speed (6) Magnetic arc blow. 8) Overlapping An overlap occurs when the molten metal from the electrode flows over the parent metal surface and remains there without getting properly fused and united with the same. An overlap tends to produce mechanical notch, parallel to the weld axis, where stresses will build up and start a crack. Overlaps need to be chipped off and the weld ground to proper shape.

Overlapping may occur due to (1) Excessive weld current. (2) Wrong tilt of electrode in making filet weld (3) Longer arc (4) Improper joint geometry (5) Incorrect electrode diameter Destructive tests for welding inspection 1) Tensile test A tensile test helps in determining

Tensile properties such as tensile strength, yield strength and modulus of elasticity. Ductility of a weld measured in % elongation or % reduction of area before failure. Tensile test is carried out by gripping the ends of specimen in a tensile testing machine and applying and increasing pull on the specimen till it fractures. During the test tensile load as well as the elongation of a previous marked gauge length in the specimen is measured. These readings help plotting the stress strain curve as shown below. JO/VJCET

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After fracturing, the two pieces of broken specimen are placed as if fixed together and distance Lf between two gauge marks and the area Af at the place of fracture are noted. Yield Strength = load at yield paint /A0 Ultimate tensile Strength = Ultimate load / A0 % Reduction of Area = (A0 – Af) / A0 * 100 % elongation = (Lf – L0) / L0 * 100 Young’s modulus of Elasticity, E = (P * L0) / (A0 x Δ L) Location of fracture whether in weld, in the HAZ or in the base metal is noted. Also look for any defects on the fractured surface of the specimen. 2) Bend Test The quality of weld in terms of ductility of the weld metal and heat affected zone as well as lack of side wall fusion, root fusion and penetration of welded joint are most frequently check by the bending test. Bend test results are expressed in various terms as 1. % elongation in outer fibers 2. Minimum bend radius prior to failure 3. Go or No Go for specific tool conditions 4. Angle of bend prior to failure

The test shows the quality of the welded joint. Any cracking of the metal will indicate false fusion or defective penetration. The stretching of the metal determines ductility to some extent. Fractured surface shows crystalline structure. Large crystal usually indicates wrong procedure or poor heat treatment after welding. A good weld has small crystals. 3) Impact test Impact testing becomes essential in order to study the behavior of the welded metal under dynamic loading. It determines the relative toughness of a material. Toughness is the resistance of a metal to fracture after plastic deformation has begun. Two major tests for determining the impact toughness are the Izod test (cantilever test) and JO/VJCET

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Charpy test (beam test). Both methods use the same type of machine. Specimen machined and notched is struck and broken by a single blow in a specially designed testing machine. The quantity measured is the energy absorbed in breaking the specimen by a single blow.

The swinging pendulum weight is raised to a standard height depending upon the type of specimen to be tested. Higher the pendulum, the more potential energy it has got. As the pendulum is released, its potential energy is converted into K.E until it strikes the specimen. A portion of energy possessed by the pendulum is used to rupture the specimen and the pendulum rises on the other side of the machine to a height lower than its initial height. The energy consumed in breaking the specimen is the weight of the pendulum times the difference in two heights on either sides. Inspection of fracture is carried out for flaws and fracture surfaces for type of fracture. This test can also indicate temper brittleness. 4) Nick – Break test

This test involves breaking the weld joint to examine the fractured surfaces for internal defects such as gas pockets, slag inclusions and porosity. The test also determines weld ductility and degree of fusion. The test specimen shall be cut transversely to the welded joint and shall have the full thickness of the plate at the joint. Slots are sawed at each end of the specimen to be tested. The specimen is then placed up-right on two supports and the force on the weld is applied either by a press or by sharp blows of a

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hammer until a fracture occurs. A visual inspection of the fractured surface is carried out in order to find defects. 5) Hardness test Hardness is the ability of a metal to resist abrasive wear. Hardness measurement can provide information about the metallurgical changes caused by welding. The different hardness test includes Brinnel test, Vickers test and Knoop test which use the area of intention under load as the measure of hardness. Rockwell test relates hardness to the depth of indentation under load. Indentations are made in specific area of interest including the weld centre line, face or root regions of the deposit, heat affected zone and the base metal. Traverses covering all those areas are made with indentations spaced at regular intervals along the line of traverse as shown below. Traverses along the weld center line from root to reinforcement or weld surface may be use to determine multipass effects.

Brinnel test uses 10mm diameter hardened steel rod for making the indentation. Brinnel test is used only for large welds in heavy plates because of its large indentation. Vickers test uses a square base pyramid diamond having 136˚ b/w opposite faces. The Vickers and Knoop test make relatively small indentations and are thus well suited for hardness measurement of various regions of heat affected zone for fine scale traverses. Indenters and loads are smaller for Rockwell test when compared with Brinnel test. 6) Pillow test for seam welds This test is the most common weld test for determining the strength of seam welds.

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Figure shows seam welding two pieces of metal to enclose a cavity. An appropriate pipe fitting is either put on with a nipple or welded on to the two pieces that were seam welded together. Hydraulic fluid or air is pumped through the fitting expanding the cavity into a pillow shape. The pressure at which the pillow burst is recorded and compared to the fracture strength of the base metal. Failure should always occur in the base metal and not in the welded seam. If the weld seam fractures, then the weld will not support that particular metal. Non drestructive tests for welding Visual inspection It is the simplest, fastest, economical and most commonly used test for detecting defects on the surfaces of welded objects. Visual examination can help in detecting the following flaws on the surface of the welded structure. Porosity, blowholes, exposed inclusions, poor fusion in welds, unfilled craters etc. Surface cracks in the weld metal, HAZ or in the parent metal. Undercutting, burning or overheating of the base metal adjoining weld metal. Improper profile and dimensional inaccuracy of welds. Poor weld appearance i.e., irregular ripple marks, weaving faults, chipping and peening marks, spatter, surface roughness etc. Some of the equipment used to aid visual inspection includes: Magnifying lens Flexible or rigid bore scopes Image sensors for remote sensing Magnifying systems Dyes and fluorescent penetrants Leak test Leak refers to an actual through-wall discontinuity or passage through which a fluid flows or permeates. The fluid that has flowed through a leak is called leakage. A leak is measured by how much leakage it will pass under a given set of conditions. Because leakage will vary with conditions, it is necessary to state both the leak rate and the prevailing conditions to define a leak properly. The two most commonly used units of leakage rate with pressure systems are standard cubic centimeters per second (std cm3/s). The standard conditions are defined as standard pressure is 1 atm and standard temperature is 273.15 K (0 °C). The welded vessel, after closing all its outlets, is subjected to internal pressure using water, oil, air or gas. The internal pressure may be raised to two times the working pressure. When under pressure, the weld may be tested as follows for detecting the leak. Pressure on the gauge may be noted immediately after applying the internal pressure and after 12 to 24 hours. Any drop in pressure reading indicates a leak. After generating air pressure in vessel, soap solution may be painted on the weld seam and carefully inspected for bubbles which would indicate leak. Another method of detecting water leakage employs a strip of aluminum foil laid over a wider strip of water-soluble paper. The strips are then laid over the welded seams of a water filled vessel, as shown in figure below. If a leak exists, the water-soluble strip will dissolve, indicating the leak location and the aluminum foil strip will be in electrical contact with the vessel. A corresponding change in resistance indicates that a leak is present.

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Liquid (dye) penetrant inspection It is a nondestructive method of revealing discontinuities that are open to the surfaces of solid and essentially nonporous materials. Liquid penetrants seep into various types of minute surface openings by capillary action. Because of this, the process is well suited to the detection of all types of surface cracks, laps, porosity, cold shuts etc. Regardless of the type of penetrant used, i.e., fluorescent or visible, penetrant inspection requires at least five essential steps, as follows. Surface Preparation. All surfaces to be must be thoroughly cleaned and completely dried before being subjected to penetrant inspection. Flaws exposed to the surface must be free from oil, water, or other contaminants if they are to be detected. Penetration. After the work piece has been cleaned, penetrant is applied in a suitable manner so as to form a film of the penetrant over the surface. This film should remain on the surface long enough to allow maximum penetration of the penetrant into any surface openings that are present. Removal of Excess Penetrant. Excess penetrant must be removed from the surface. The removal method is determined by the type of penetrant used. Some penetrants can be simply washed away with water; others require the use of emulsifiers (lipophilic or hydrophilic) or solvent/remover. Development. Depending on the form of developing agent to be used, the work piece is dried either before or directly after application of the developer. The developer forms a film over the surface. It acts as a blotter to assist the natural seepage of the penetrant out of surface openings and to spread it at the edges so as to enhance the penetrant indication.

Inspection. After it is sufficiently developed, the surface is visually examined for indications of penetrant bleed back from surface openings. This examination must be performed in a suitable inspection environment. Visible penetrant inspection is performed in good white

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light. When fluorescent penetrant is used, inspection is performed in a suitably darkened area using black (ultraviolet) light, which causes the penetrant to fluoresce brilliantly. Magnetic particle inspection It is a method of locating surface and subsurface discontinuities in ferromagnetic materials. It depends on the fact that when the material or part under test is magnetized, magnetic discontinuities that lie in a direction generally transverse to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. The presence of this leakage field, and therefore the presence of the discontinuity, is detected by the use of finely divided ferromagnetic particles applied over the surface, with some of the particles being gathered and held by the leakage field. This magnetically held collection of particles forms an outline of the discontinuity and generally indicates its location, size, shape, and extent. Maximum sensitivity of indication is obtained when the discontinuity lies in a direction normal to the applied magnetic field. Magnetic particles are applied over a surface as dry particles, or as wet particles in a liquid carrier such as water or oil. The defects commonly revealed by magnetic particle inspection are quenching cracks, thermal cracks, grinding cracks, overlaps, non metallic inclusions, fatigue cracks, hot tears etc.

Radiographic inspection (X ray and γ ray) Radiographic inspection is based on the differential absorption of penetrating radiation by the part or test piece (object) being inspected. Because of differences in density and variations in thickness of the part or differences in absorption characteristics caused by variations in composition, different portions of a test piece absorb different amounts of penetrating radiation. Radiographic techniques produce two-dimensional, plane-view images from the unabsorbed radiation. Film or paper radiography: A two-dimensional latent image from the projected radiation is produced on a sheet of film or paper that has been exposed to the unabsorbed radiation passing through the test piece. This technique requires subsequent development of the exposed film or paper so that the latent image becomes visible for viewing Real-time radiography: A two-dimensional image can be immediately displayed on a viewing screen or television monitor. The unabsorbed radiation is converted into an optical

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or electronic signal, which can be viewed immediately or can be processed in near real time with electronic and video equipment

Ultrasonic inspection

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Ultrasonic inspection is a non-destructive method in which beams of high-frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material. The sound waves travel through the material with some attendant loss of energy (attenuation) and are reflected at interfaces. The reflected beam is displayed and then analyzed to define the presence and location of flaws or discontinuities. The degree of reflection depends largely on the physical state of the materials forming the interface and to a lesser extent on the specific physical properties of the material. Most ultrasonic inspection is done at frequencies between 0.1 and 25 MHz. Most ultrasonic inspection instruments detect flaws by monitoring one or more of the following: Reflection of sound from interfaces consisting of material boundaries or discontinuities within the metal itself. Time of transit of a sound wave through the test piece from the entrance point at the transducer to the exit point at the transducer. Attenuation of sound waves by absorption and scattering within the test piece. Features in the spectral response for either a transmitted or a reflected signal. Eddy current inspection Eddy current inspection is based on the principles of electromagnetic induction and is used to identify or differentiate among a wide variety of physical, structural, and metallurgical conditions in electrically conductive ferromagnetic and non ferromagnetic metals and metal parts. The part to be inspected is placed within or adjacent to an electric coil in which an alternating current is flowing. This alternating current, called the exciting current, causes eddy currents to flow in the part as a result of electromagnetic induction. The electromagnetic field in the region in the part and surrounding the part depends on both the exciting current from the coil and the eddy currents flowing in the part. The flow of eddy currents in the part depends on The electrical characteristics of the part The presence or absence of flaws or other discontinuities in the part The total electromagnetic field within the part

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The change in flow of eddy currents caused by the presence of a crack in a pipe is shown in the figure. The pipe travels along the length of the inspection coil as shown in figure. In section A-A, no crack is present and the eddy current flow is symmetrical. In section B-B, where a crack is present, the eddy current flow is impeded and changed in direction, causing significant changes in the associated electromagnetic field. From figure, it is seen that the electromagnetic field surrounding a part depends partly on the properties and characteristics of the part. Finally, the condition of the part can be monitored by observing the effect of the resulting field on the electrical characteristics of the exciting coil, such as its electrical impedance, induced voltage, or induced currents. Alternatively, the effect of the electromagnetic field can be monitored by observing the induced voltage in one or more other coils placed within the field near the part being monitored. Weld design & process selection The welding process and type of joint selected depends on a number of factors as listed below. • the configuration of the components or structure to be welded and their thickness & size. • the methods used to manufacture the components • the service requirements such as type of loading and stresses generated. • the location, accessibility and ease of welding • the effects of distortion and discolouration • the appearance • the costs involved in the edge preparation, the welding and post processing of weld. General design guidelines for welds can be summarised as follows Product design should minimize the number of welds. Weld location should be selected so as to avoid stress concentration in the welded structure. Appearance also is a factor to be considered. The method used to prepare edges (sawing, machining or shearing) can affect weld quality. Some designs can avoid or minimize edge preparation. Weld-bead size should be kept to a minimum to conserve weld metal. Types of weld joints

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Brazing comprises a group of joining processes in which coalescence is produced by heating to a suitable temperature above 450°C and by using a ferrous or nonferrous filler metal that must have a liquidus temperature above 450 °C and below the solidus temperature(s) of the base metal(s). The filler metal is distributed between the closely fitted surfaces of the joint by capillary action. Brazing proceeds through four distinct steps: The assembly or the region of the parts to be joined is heated to a temperature of at least 450°c. The assembled parts and brazing filler metal reach a temperature high enough to melt the filler metal but not the parts. The molten filler metal spreads into the joint by capillary action and wets the base metal surfaces. The parts are cooled to solidify the filler metal and anchor the parts together by metallurgical reaction and atomic bonding. Brazing procedure 1) Cleaning and preparing the surface to be brazed. 2) Fluxing both the base metal and filler metal surfaces. 3) Aligning the base metal parts to be joined. 4) Heating the joint. 5) Applying brazing filler metal onto the joint. 6) Cooling of the brazed joint. 7) Removing flux residue from the completed joint. Advantages of Brazing Strong, uniform, leak proof joints can be made rapidly, inexpensively, and even simultaneously. Inaccessible joint areas which could not be welded by other methods can be formed by brazing. Complicated assemblies comprising thick and thin sections, odd shapes, and differing wrought and cast alloys can be turned into integral components by a single trip through a brazing furnace or a dip pot. Metal as thin as 0.01 mm (0.0004 in.) and as thick as 150 mm (6 in.) can be brazed with higher joint strength. The natural shapes of brazing fillets are excellent. The meniscus surface formed by the filler metal, as it curves across corners and adjoining sections is ideally shaped to resist fatigue. Complex shapes with greatly varied sections can be brazed with little distortion, and precise joining is comparatively simple. Unlike welding, in which the application of intense heat to small areas acts to move the parts out of alignment and introduces residual stresses, brazing involves fairly even heating and thus part alignment is easier. Limitations of Brazing There is a size limitation for the parts to be brazed. Since the outer area to be brazed must be heated, large cast sections or large heavy plates cannot be easily brought up to temperature. Brazing requires tightly mating parts to ensure capillary flow of the filler metal. This involves expensive machining to attain the desired fit. Flux residues if not properly removed can cause corrosion. Brazed joints do not give satisfactory results when used at elevated temperatures.

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Brazing requires skilled labour. Brazing fluxes and filler rods may evolve toxic fumes and poisonous vapours. Applications of Brazing Brazing is applicable to cast and wrought irons, steels, Cu and Cu alloys, Al and Al alloys, Mg and Mg alloys and to so many other materials. Brazing is used in place of welding where special metallurgical characteristics of metals have to be preserved after joining. Brazing can join - cast metals to wrought metals - non metals to metals - dissimilar metals - porous metal components - fibre and dispersion strengthened composites Brazing joint design

Brazing methods • Torch brazing • Furnace brazing • Vacuum brazing • Induction brazing • Dip brazing • Resistance brazing • Exothermic brazing Torch brazing (TB) utilizes a fuel gas flame as the heat source for the brazing process. The fuel gas is mixed with either air or oxygen to produce a flame, which is applied to the work piece until the assembly reaches the proper brazing temperature. Then, preplaced filler metal will be melted or hand-fed wire can be introduced. The typical fuel gases used in torch brazing are acetylene, propane, and methane (natural gas).

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Soldering Soldering is defined as a joining process by which two substrates are bonded together using a filler metal (solder) with a liquidus temperature that does not exceed 450 °C. The substrate materials remain solid during the bonding process. The solder is usually distributed between the properly fitted surfaces of the joint by capillary attraction. A metallurgical bond is produced at the filler metal / base-metal interface. The solder reacts with a small amount of the base metal and wets the metal by intermetallic compound formation. Soldering procedure Select of proper joint design and clearance. Select of right solder and flux. Clean the joint components. Apply flux and assembling components with proper preplacement or addition of solder. Heat the joint to soldering temperature for optimum time. Cool the solder as quickly as possible. Clean and remove any undesirable flux residues on the surfaces. Soldering alloys Tin-lead solders : good corrosion resistance, employed to join most metals Tin-antimony-lead solders : antimony content improves the mechanical properties of the solder Tin-zinc solders : used for joining aluminium Lead-silver solders : more readily wet steel and copper, susceptible to humid atmosphere corrosion in storage Cadmium-zinc solders : used for soldering aluminium Zinc-aluminium solders : used for joining aluminium, develops joints with high strength and good corrosion resistance. Indium-tin solders : used for glass to metal and glass to glass soldering Soldering joint design

Soldering processes  Soldering iron method  Torch soldering  Dip soldering  Wave soldering  Condensation soldering Wave soldering is one of the primary techniques for mass assembly of printed wiring boards involving through holes, surface mount devices, or a combination of these two technologies. JO/VJCET

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A schematic of the wave soldering process is shown above. A solder "fountain" or "wave" is created by a pump located at the bottom of the solder pot; suitable baffles are mounted in the pot to direct the flow of solder into the desired configuration. The printed wiring board is placed onto a conveyor, which brings it into contact with the wave surface. The circuit boards travel along the surface of the solder; molten alloy does not flow on top of the board. As the printed circuit board passes on the wave, the solder wets the surface-mount package leads, terminations, and exposed metal surfaces in the circuit board, and also fills plated through holes. This technique can produce several thousand solder joints in a matter of minutes. The implementation of wave soldering for surface-mount technology requires that the devices be glued to the substrate prior to wave soldering. First, the substrate receives a coating of flux. Flux application methods include: A wave technique similar to that shown above. Coating the circuit board by flux foam created by passing air or nitrogen through a flux bath to generate the foam on the bath surface Directly spraying the flux onto the board surface After flux is applied, the substrate is passed through a preheating stage. Warming the board promotes activation of the flux, accelerates the evaporation of volatiles from the flux and reduces thermal shock to the substrate and devices when it passes onto the solder wave. Then, the circuit board contacts the solder wave for the formation of the joints. After passing the wave, the board cools through natural heat loss or, more quickly, by the use of forced air. Adhesive Bonding It involves the joining of components with the help of an adhesive. Adhesives are available in several forms – liquid, paste, solution, emulsion, powder, tape and film. Liquid adhesives are applied by brushes, sprayers and rollers. Surface preparation is essential for the process. To meet the requirements of a particular application, an adhesive may need one or more of the following properties • Strength • Toughness • Resistance to various fluids and chemicals • Resistance to degradation due to heat and moisture • Ability to wet the surfaces to be bonded

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Surface preparation for adhesive bonding Joint strength depends greatly on the absence of dirt, dust, oil and other contaminants. Contaminants affect the wetting ability of the adhesive and prevent spreading of the adhesive evenly over the interface. Thick, weak or loose oxide films on work piece surfaces are detrimental to the process. A porous thin strong oxide film is desirable, particularly one with surface roughness to improve adhesion. Different types of adhesives Adhesive systems may be classified on the basis of their specific chemistries. Epoxy-based systems: high strength and high temperature properties, withstand upto 200:C Acrylics: Suitable for applications with substrates that are not clean. Anaerobic systems: Curing is done under oxygen deprivation and the bond is usually hard & brittle. Urethanes: High toughness and flexibility at room temp. and hence used as sealants. Silicones: Highly resistant to moisture and solvents but curing time is high(1 to 5 days) Cyanoacrylate: Bond lines are thin and bond sets within 5 to 40 s. Adhesive Bond Joints

Advantages of Adhesive bonding • It distributes the load at an interface and thereby eliminates localized stresses. • Improved appearance. • Very thin and fragile components can be bonded without significant increase in their weight. • Porous materials and materials of very different properties and sizes can be joined. Limitations of Adhesive bonding • Limited range of service temperatures. • Long curing time. • Need for great care in surface preparation. • Difficulty in NDT. • Limited reliability during service life. Applications • Used in the aerospace industry for satellite repair & assembly, solar arrays & panels, frames & reinforcement, honeycomb parts & bonding • Used in the field of electronics in robotics, test & measurement equipment, engine control modules, telecommunication devices, mobile devices etc. JO/VJCET

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The medical industry uses adhesive bonding for medical knives and scalpels, needle bonding, membrane switches, guide wire assembly etc.

Effect of weld parameters on weld quality Welding current: Welding current is the most influential variable in arc welding process which controls the electrode burn off rate, the depth of fusion and geometry of the weldments. Welding voltage: This is the electrical potential difference between the tip of the welding wire and the surface of the molten weld pool. It determines the shape of the fusion zone and weld reinforcement. High welding voltage produces wider, flatter and less deeply penetrating welds than low welding voltages. Depth of penetration is maximum at optimum arc voltage. Welding speed: Speed of welding is defined as the rate of travel of the electrode along the seam or the rate of the travel of the work under the electrode along the seam. Increasing the speed of travel and maintaining constant arc voltage and current will reduce the width of bead and also increase penetration until an optimum speed is reached at which penetration will be maximum. Increasing the speed beyond this optimum will result in decreasing penetration. In the arc welding process increase in welding speed causes: • Decrease in the heat input per unit length of the weld. • Decrease in the electrode burn off rate. • Decrease in the weld reinforcement. If the welding speed decreases beyond a certain point, the penetration also will decrease due to the pressure of the large amount of weld pool beneath the electrode, which will cushion the arc penetrating force.

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