JO/VJCET Page 1
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 weldinga) 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.
JO/VJCET Page 2 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.
JO/VJCET Page 3 Portable welding equipments are available.
Welding permits considerable freedom in design.
A large number of metals/alloys both similar and dissimilar can be joined by welding. Welding can join workpieces through spots, as continuous pressure tight seams, end-to-end and in a number of other configurations.
Welding can be mechanized. 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.
JO/VJCET Page 4 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 m3. 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
JO/VJCET Page 5 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, CO2-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 Page 6 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.
JO/VJCET Page 7 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.
JO/VJCET Page 8 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
JO/VJCET Page 9 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 non-consumable 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.
JO/VJCET Page 10 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
JO/VJCET Page 11 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.
JO/VJCET Page 12 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
JO/VJCET Page 13 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-to-work 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.
JO/VJCET Page 14 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.
JO/VJCET Page 15 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.
JO/VJCET Page 16 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.
JO/VJCET Page 17 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.
JO/VJCET Page 18 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.
JO/VJCET Page 19 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.
JO/VJCET Page 20 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.
JO/VJCET Page 21 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.
JO/VJCET Page 22 - 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 109W/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 Page 23 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.
JO/VJCET Page 24 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.
JO/VJCET Page 25 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 non-violent 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.
JO/VJCET Page 26 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
JO/VJCET Page 27 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.
JO/VJCET Page 28
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 high-speed 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.
JO/VJCET Page 29 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 Page 30 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.
JO/VJCET Page 31 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 Page 32 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.
JO/VJCET Page 33 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.
JO/VJCET Page 34 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.
