Welding dioosm

8/14/2019 Welding dioosm

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Classification: Welding processes may be classified according to the source of energy employed for heatingthe metals and the state of metal at the place being welded depending upon the source of heat, the weldingprocesses are classified as :-

1. Chemical (oxygen + combustible fuel gas. e.g., acetylene, propane, butane, natural gashydrogen).2. Chemico - Mechanical (Thermit welding)3. Electro - chemical (Atomic hydrogen welding)4. Electro-Mechanical (Electric resistance welding)5. Electric - Arc welding.

These may be divided into two groups as follows:(a) Pressure Processes. Inthese processes, the parts to be joined are heated to a plastic state (fusion mayoccur to a limited extent) and forced together with external pressure to make the joint. Some of the morecommon processes in this group are mentioned below:

1. Forge welding

2. Thermit Pressure welding

3. Pressure Gas welding

4. Electric Resistance welding

(b) Fusion Processes. In these processes, the material at the joint is heated to the molten state and allowed tosolidify to make the joint, without the application of pressure. Here, some joints may be made without theaddition of a filler metal, but, in general, a filler metal must be added to the weld to fill the space between theparts being welded. The filler metal deposited should ordinarily be of the same composition as the base metal.

Some of the common welding processes in this group are listed below:1. Gas welding

2. Electric Arc welding

3. Thermit Fusion welding

The welding processes can also be classified as:

Autogenous, Homogeneous and Heterogeneous

In 'Autogeneous' processes, no filler metal is added to the joint interface, for example, cold and hotpressure welding processes and electric resistance welding.

In 'Homogeneous' processes, filler metal is added and is of the same type as the parent metal, for.example, welding of plain low-C steel with a low- Cwelding rod and welding of 70 - 30 brass with a 70 - 30brass welding rod etc.

In 'Heterogeneous' processes, a filler metal is used but is of a different type from the parent metal, forexample, brazing and soldering processes. Brazing and Soldering are not strictly the welding processes inview of the definition of welding process given above. However, these processes also belong to the familyof welding processes.

The two most widely used welding methods are: Gas welding and Arc welding. The temperature of anelectric arc is much higher than that of a gas flame, so the joints melt practically instantaneously in arcwelding. Gas welding involves long preheating period which raises the metal adjacent to the joint to a hightemperature. This exerts an unfavorable effect on the crystalline structure, which results in considerablestresses being set up. So, gas welding is unsuitable for relatively large cross - sections due to this troubleand the time involved in preheating. Plates above 20 mm thick are, therefore, best welded by arc welding.

Advantages of Welded Joints over Riveted Joints :1. Economy of material and lighter weight of structure owing to:(a) Better utilization of metal elements (plates, angles) since their working sections are not weakened by the

rivet holes, consequently the sections of the welded pieces can be made smaller than the sections of rivetedelements, for the same acting forces.(b) Possibility of a wide use of butt-jointed seams, requiring no additional elements, such as cover straps.(c) Lighter weight of the joining elements (rivets weight more than the welds). The weight of welds comprisesabout 1 to 1.5 per cent of work weight, while the weight of rivets is about 3.5 to 4 per cent. The use of weldinginstead of riveting saves on an average 10 to 20 per cent in weight.2. Greater strength of the joints, due to absence of holes needed for riveting.3. Less labour is required, since there is no need for marking out and drilling or punching s. Rivetingconsumes much more labour and is a much more complicated and less productive than welding, which canbe often largely automated.4. Possibility of joining curvilinear parts.5. Tightness and impermeability of the joint.6. Noiselessness (riveting is inevitably accompanied by noise).

Advantages over Casting Process: 1. Lighter weight and saving of material due to:a. Lesser machining allowances and

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b. The possibility of utilizing smaller sections, since the wall thickness of cast parts determined in manycases by the casting process, is as a rule 2 to 3 times greater, and sometimes even more than that ofwelded parts.

c. The saving of metal in welded machine components, as compared with cast ones, may amount to40%.

2. Lower Cost.3. Greater strength.4. Maximum homogeneity.

Whether machine parts should be welded, is decided in each particular case, by design and economyconsiderations.The drawbacks of welding can be: Not all the metals are satisfactorily weldable and the weldments are lessreadily machinable, as compared to castings

Arc Welding

INTRODUCTIONArc welding is widely used method of joining the metal parts. Here the source of heat is an electric arc. Arcwelding is a group of welding processes wherein heating is produced with an electric arc or arcs, mostlywithout the application of pressure and with or without the use of filler metal, depending upon the base platethickness.Various arc welding processes are:

1. T.I.G (GTAW) welding2. M.I.G (GMAW) welding3. Submerged arc welding4. Electro slag welding5. Flux shielded metal arc welding6. Carbon-arc welding7. Electro gas welding8. Plasma arc welding

ARC WELDING PRINCIPLE

In arc welding, arc is generated between the positive pole of D.C. (direct current) called anode and negativepole of D.C. called cathode. When these two poles are brought together, and separated for a small distance(1.5 to 3 mm) such that the current continues to flow through a path of ionized particles, called plasma, anelectric arc is formed. Since the resistance of this ionized gas column is high, so more ions will flow fromanode to the cathode. Heat is generated as the ions strike the cathode.Polarity: Polarity is defined as the type of potential given to the workpiece or electrode. In case of directcurrent (D.C.) power source, positive and negative terminals are fixed whereas in case of alternating current(A.C.) power source, positive and negative terminals are not fixed i.e. the terminal which is positive duringone half of cycle becomes negative in another half. So polarity principle is applicable only in case of directcurrent power source.Polarities are of following two types:1. Straight Polarity: In straight polarity, electrode is having negative terminal while workpiece is connected tothe positive terminal of the direct current power source.2. Reverse Polarity: In reverse polarity, electrode is connected to positive terminal whereas workpiece isconnected to the negative terminal of the direct current power source.About 2/3 (67%) of total heat produced during welding is generated at positive terminal while rest of total heatis generated at negative terminal. Thick jobs which requires more heat on workpiece, so workpiece isconnected to the positive terminal hence we adopt straight polarity. Similarly, if job is thin, means less heat isrequired on job so workpiece connected to the negative terminal hence reverse polarity. If our electrode isnon-consumable, means less heat is required on electrode (generally) so electrode is connected to thenegative terminal i.e. use straight polarity for doing welding.Striking the ArcArc between the electrode and workpiece is generally struck either by momentarily touching the electrode withthe workpiece or by using high frequency unit and when the arc starts, the electrode is taken away at apredetermined distance (arc length) for doing welding.Welding the JointOnce the arc has been established and the arc length adjusted, the electrode is inclined to an angle ofapprox. 20 degrees with the vertical. Arc gap should be maintained constant. The bead width can beincreased by employing higher arc current, lower arc travel speeds and by suitably weaving the electrode.

Before using the fresh electrode (new) the bead should be properly cleaned. After completing the weld, theslag is chipped off and weld bead is cleaned with a wire brush.

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ELECTRODES

The electrodes are used for providing heat input in arc welding. Electrodes can be classified in the followingbasis:

1 Consumable or Non-consumable.2 Bare or coated electrodes.1. (a) Consumable Electrodes: When the arc is obtained with consumable electrodes, the weld metal and thetip of the electrode also gets melted under the arc. The molten metal from electrode and the base metal getsmixed under the arc and provides the necessary joint. So in this process, once the arc is initiated, theelectrode is continuously consumed. Thus the electrode is serving the purpose of filler rod as well as the heatinput to the joint. Consumable electrodes are made of various materials depending on the purpose andchemical composition he metals to be welded.1.Non-Consumable electrode: Non-consumable electrodes are those electrodes, which are not consumedduring welding. Separate filler rods if necessary can be used to fill the gap along the joints. Non-consumableelectrodes are made of carbon, graphite or tungsten. The carbon and graphite electrodes are used only inD.C. welding, whereas tungsten electrodes are used for both D.C. and A.C. welding. So in this, we can havebetter control over the heat input and filler metal because the sources are separate. Non-consumable

electrodes are used in carbon arc welding or tungsten inert gas arc welding (TIG).2. Bare Electrode: If the electrode is not coated with flux, it is called the bare electrode that case flux will beadded separately if necessary. Bare electrodes are used in carbon arc welding and tungsten arc welding. Thestriking of arc is difficult with this type of electrode especially with A.C. supply.3. Coated Electrode: If the electrode is coated with flux, it is called the coated electrode. the coating on theelectrodes serves many purposes. The coated electrode also called "stick" electrodes is available in the length350 or 450-mm.One of the major concerns with the coated electrodes is the moisture pick up by the coating. This moisture,dissociate into oxygen and hydrogen with the hydrogen being absorbed by the liquid metal and subsequentlyreleased during solidification, causing porosity. So, there should be kept in a dry place.During welding if the metal is heated/ melted in air, oxygen from the air combines with the metal to formoxides which results in poor quality, low strength welds and in some cases may even make weldingimpossible. Sometimes the flux will be added separately. Covering of the coated electrode is also called flux.

Sometimes the purpose of flux is also solved by inert or active gas (TIG, MIG, and MAG).The materials used for flux coating are termed as components. These components may be sub-divided intothe following categories:Gas Forming:The gas forming components are organic matters such as starch, wood pulp etc. which formgas layer, thus isolating the welding zone from the ambient air.Slag Forming: The slag forming components are china clay, feldspar, manganese and titanium ores etc.These components enhance slag formation. This slag covering the molten metal prevents it from coming intocontact with the ambient air. The merging of weld metal with base metal in this case is considerably smooth.Reducing: The reducing components such as Ferro-silicon, Ferro-titanium, Ferro-manganese reduces theoxides, which are likely to be formed in the liquid bath of molten metal.

Al loys:The alloying components such as Ferro-silicon, Ferro-manganese, Ferro-chromium, chromium oxideare used for rendering this metal heat proof.Stabilizing:The stabilizing components also form slag. In the presence of arc, these components ionize the

zone between the electrode and the parts to be welded and thus ensuring the stable burning of the arc.Binding:The binding components will be added so that covering should have proper binding with the bareelectrode. In this arc welding process, welding heat is produced from an electric arc set up between a fluxcoated electrode and the workpiece.

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The electrode is consumable so supplies the necessary filler metal. The covering on the electrode serves thepurpose of flux. During the welding process, the metal electrode is melted by the heat of the arc and fuseswith the workpiece. The temperature produced by the heat is about 2400C to 2700C. The arc temperatureand thus the arc heat can be increased or decreased by employing higher or lower arc currents. A highcurrent arc with a smaller arc length produces a very intense heat. Both D.C. and A.C. may be used. Forcurrent over 750 amperes, A.C. equipment is preferred as it has high efficiency, negligible loss at peak loadand minimum maintenance.

ARC WELDING EQUIPMENT

The most commonly used equipments for arc welding are :1. A.C. or D.C. power supply source2. Electrode holder3. Electrode Cable, cable connectors Cable lug4. Chipping hammer5. Earthing clamps6. Wire brush7. Helmet8. Safety goggles9. Hand gloves10. Aprons,11. Sleeves etc.

Definitions of terms used in arc weldingArc Crater :Because of the penetration of electric arc into the parent metal, small depression will be formed inthe parent metal around which molten metal is piled up, known as the arc crater. Its depth depends on thethickness of the parent metal. Operator judges the arc penetration by observing this crater depth. If arc isbroken, defect will occur therefore care should be taken to prevent this defect.

Arc Length: The distance through the centre of the arc from the tip of the electrode to the bottom of the arccrater is termed as arc length. It should be 2 to 4 mm. Arc length should be proper and constant duringwelding to obtain good results.

Arc Blow:Arc blow is the phenomenon of wandering of arc and it occurs in D.C. welding. When a currentflows in any conductor, a magnetic field is formed around the conductor in direction right angle to the current.Since in the case of D.C. arc welding, there is current through the electrode, workpiece and ground clamps,so magnetic field exists around each of these components. This magnetic field tends to bend the arc from itsintended path and is called Arc blow. Magnetic field concentration is always more at the ends so chances ofarc blow are also more at the beginning and at the end of the weld. In D.C, welding, since there is fixedpolarity, so the induced magnetic fields are constant in one direction but in A.C. welding polarity is not fixed(direction of current flows goes on changing) so there is no arc blow. Chances of arc blow are also more ifwelding is done at very high speed. Movement of arc blow causes atmospheric gases to be pulled into thearc, resulting in porosity or other defects.Corrective measures:

The ground connections should be placed as far as possible from the joints to be welded. If possible, A.C.may be used instead of D.C.Arc length and current should be proper (by reducing).Arc blow can also be minimized by welding towards a heavy tack welds or an already existing weld.The ground cable may be wrapped around the workpiece such that the current following in it sets up amagnetic field in a direction, which will counteract the arc blow.

Advantages & Disadvantages of Arc Welding

Advantages1 The welding equipment is simple, less costly, and portable.2 Welding can be carried out in any position with highest weld qualities.3 Wide varieties of electrodes are available.4 Total welding cost is less.

Disadvantages1 The length of each electrode is limited and when new electrode is used, proper cleaning has to be

done which decreases the welding speed.2 Welding control is difficult as compared to MIG welding.3 If the covering absorbs moisture, this moisture causes the porosity defect.4 Because of flux coating, chances of slag entrapment are more.

Appl ications

It is used both as a fabrication process and for maintenance and repair jobs.

It is also used in ship buildings, pipe lines, buildings and bridges construction, tanks, boilers, automotive andaircraft industries etc.

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TUNGSTEN INERT GAS (TIG) OR GAS TUNGSTEN ARCWELDING (GTAW)In this arc welding process, welding heat is produced from an electric arc established between the tungsten(nonconsumable) electrode and the job. A shielding gas (argon, helium, nitrogen etc.) is used to avoidatmospheric contamination of the molten weld pool. Filler metal, if required is fed separately.Shielding Gases

Argon is normally preferred over helium because it requires a lower arc voltage, easier arc starting andprovides a smooth arc action. It is also economical and heaviest. Helium can withstand the higher arc voltageso it is used where higher heat input is required. Sometimes active gas carbon dioxide is also used as it is

more economical. It requires slightly higher current. It is normally used with only D.C. with +ve electrode.Whereas both argon and helium can be used with A.C. as well as D.C. welding power source.Operation

This process uses a non-consumable tungsten electrode, which is mounted in a special electrode holder. Thisholder is also designed to furnish a flow of inert gas around the electrode and around the arc. Weldingoperation is done by striking an arc between the workpiece and tungsten electrode in an atmosphere of inertgas. The arc is struck either by touching the electrode with a scrap metal tungsten piece or using a highfrequency unit. After striking the arc, it is allowed to impinge on the job and a molten weld pool is created. Thewelding torch and the filler metal are generally kept inclined at angles of 70-80 degree and 10-20 degreerespectively with the flat workpiece. Filler metal, if required should be added by dipping the filler rod in theweld pool. When doing so, the tungsten electrode should be taken a little away from weld pool. However theheated end of filler rod as well as the electrode should be within the inert gas shield. D.C. and A.C. powersource can be used.Equipment

1 Welding torch,2 Tungsten electrode and filler metal.3 Welding power source,4 High frequency unit,5 D.C. suppressor unit and cable.6 Inert gas cylinder,7 Pressure regulator and flow meter.8 Cooling water supply.9 Water and gas solenoid valves.

Polarities1 DCSP (Direct Current Straight Polarity): Tungsten electrode (-ve), work (+ve)

Used for welding mild steel, stainless steel, copper and titanium.2 DCRP (Direct Current Reverse Polarity): Tungsten electrode (+ve), work (-ve)

Used for welding aluminum and heavily oxidized, aluminum castings.3 ACHF (Alternating Current High Frequency): It is used for Al and Mg. High frequency also helps in

oxide cleaning actions.Advantages:

1) No flux is used so no danger of flux entrapment.2) Clear visibility of the arc so better control.3) It can weld in all positions.4) High quality welding of thin materials (as thin as 0.125 mm).5) Heat affected zone is very less.6) Unlike metals can be welded to each other like mild steel to stainless steel, brass to copper etc.

Disadvantages:

1) Tungsten, if transfers can contaminate the weld pool.2) Filler rod end if by chance comes out of the inert gas shield can cause weld me contamination.3) Equipment costs are higher than flux shielded metal arc welding.4) Electrode is non-consumable, so separate filler rod is needed, so there is decrease welding speed.

Appl ications:

Welding of carbon steel, stainless steel, nickel, aluminum, magnesium, brass, copper, bronze,titanium etc.

Welding of sheet metal and thinner sections.

Used in aircrafts, rocket motor chambers, transistor cases, and instrument industries

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MIG WELDING

In this welding process, welding heat is produced from an electric arc established between the continuouslyfed metal electrode and the job. Argon, helium, carbon dioxide or a gas mixture shields the arc and moltenmetal from atmospheric contamination.OPERATION

The typical set up for GMAW process is shown in Fig. The consumable electrode is in the form of a wire reel,which is fed at a constant rate, through the feed rollers. The welding torch is connected to the gas supplycylinder, which provides the necessary inert gas. The electrode and the work pieces are connected to thewelding power supply. The power supplies are generally of the constant voltage type only. The current fromthe welding machine is changed by changing the rate of feeding of the electrode wire.Normally DC arc welding machines are used for GMAW with electrode positive (DCRP). The DCRP increasesthe metal deposition rate and also provides a stable arc and smooth electrode metal transfer. For semi-automatic process, arc length is maintained constant by using the principles of self-adjusted arc or self-controlled arc.Self Adjusted ArcThe electrode is fed from a coil through the grooved rollers run by a constant speed motor. If arc lengthdecreases voltage decreases and arc current increases. This increased current melts the electrode at a fasterrate, so arc length increases and become normal. Reverse will occur if arc length increases. Power sourcewith flat characteristics is preferred over another having drooping characteristics because for same change inarc length, there is a bigger change in arc currentSelf Controlled ArcThe electrode is fed from a coil through the rollers run by a variable speed electric motor whose speedincreases or decreases as the arc voltage increases or decreases. If the arc length decreases, arc voltage willdecrease, which in turn reduces the speed of electric motor and electrode feed rate. This will increase the arclength and bring to the normal value. Reverse will occur if arc length increases. Power source having droopingcharacteristics is preferred because with the same change in arc length, there is a greater change in arcvoltage, which in turn increases or decreases the speed of electrode feed motor.Equipment

1 Welding power source and cables.2 Welding torch and wire electrode coiled on a spool.3 Wire feed mechanism and controls consisting of a pair of driving rolls, electric motor

4 Shielding gas cylinder, pressure regulators and flow meters.5 Controls.Advantages

1. GMAW does not require the high degree of operator skill.

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2. Continuous welding at higher speeds and in all positions with deeper penetration ispossible.

3. Thick and thin, both types of workpieces can be welded effectively. The process can beeasily mechanized.

4. Since no flux is used so more visibility, neatness, cleanliness, spatter free weld.Disadvantages

1 Welding equipment is more complex and more costly.2 The metallurgical and mechanical properties of the joint may be affected due to high cooling

rate.3 It is difficult to weld in small corners.4 Process variables are more.

Appl ications

1. It is suitable for welding variety of ferrous and non-ferrous metals.2. Metal fabrication industries, ship buildings, automobiles, pressure vessel industries etc.3. Welding tool steels and dies.

Common points of MIG and TIG are:

1. Both use same gases for shielding depending on the metals to be welded.2. Both can be used for welding metals like stainless steel, non-ferrous alloys and aluminum.3. Gas flow and its controls are same in both.4. Cleaning the weld is important in both cases.5. No flux is required for both.

6. Differences between MIG and TIG welding are:

TIG MIG (GMAW)

TIG welding uses a permanent, non-consumable tungstenelectrode.

MIG uses consumable continuous coil electrodeof same chemical composition as the materialbeing welded.

TIG welding electrode serves the purpose of producing the arconly. Filler rod is added separately.

MIG welding electrode serves both the purposesof producing the arc as well as of filler metal.

TIG is not as fast as MIG because in this, separate filler rod isadded.

MIG is fast as compared to TIG becauseelectrode and filler metal is same, which is in theform of wire.

TIG welding requires a skilled operator. No so much skill is required for an operator.

If filler metal is added, operator's both hands are engaged. So

work must be held in position with clamps or fixtures.

In MIG welding, the wire electrode and gases

come from the same run and thus can be madeeasily automatic.

TIG welding usually uses DCSP (direct current straightpolarity), but in case of welding thin sheets DCRP (directcurrent reverse polarity) is used.

MIG uses both types of polarities. GenerallyDCRP is used.

TIG welding torch is water-cooled. Generally no water-cooling is necessary.

Tungsten if by chance comes in contact with the molten metalcauses contamination of the weld pool.

No such problem is there with MIG (no tungstenelectrode).

TIG welding is not used often for welding plates thicker than 6mm.

It is best suited for thick (more than 6 mm) jobs.

Penetration is not so much deeper as with MIG. Deeper penetrations can be obtained ascompared to TIG.

In this welding process, welding heat is produced

from an electric arc or arcs set uc

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Submerged arc weldingIn submerged arc welding process arc is generated between bare metal electrode/electrodes and the job. Thearc, end of the electrode and molten pool remain completely hidden and are invisible because these aresubmerged under a granular material (flux). The continuously fed bare metal electrode melts and acts as fillerrod. The submerged arc welding is used for doing faster welding jobs. It is possible to use large weldingelectrodes (12 mm), no. of electrodes and very high current (4000 A) so that very high metal deposition ratesof the order of 20 kg/hr or more can be achieved with this process. Also, very high welding speeds (5 m/min)are possible in SAW. These machines are able to weld plates of thickness as' high as 75 mm in butt joint in a

single pass but it is more economical for larger jobs only.OperationSince the flux when cold is non-conductor of electricity and it is placed (flux) over the job. So the arc is struckeither by touching the electrode with the job or placing steel wool between electrode and job or by using a

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high frequency unit. Initially the flux is insulator but once it melts heat of the arc, it becomes highly conductiveand hence the current flow is maintained between the electrode and the job through the molten flux. Theupper portion of the flux remains solid and can be reused. In semi-automatic process, arc length is keptconstantEquipment

1 Welding head2 Flux hoper3 Flux recovery unit

4 Welding power sources (A.C. transformer or D.C. generator)5 Flux6 Electrodes7 Controls.

Advantages

1. Operator is not exposed to usual spatter arid can work without helmet and other safety equipments.2. Fewer passes are required due to deep penetrations and more deposition rates.3. It is much faster.4. No edge preparation is required.5. Less distortion.6. Very neat appearance and smooth weld shapes can be obtained.7. It is often used in automotive mode.

Disadvantages1. Since the arc, end of the electrode remains completely hidden so jigs, fixtures, accessories and

pointers are required to ensure proper welding at the joint.2. The flux needs replacing which is not always possible.3. It is not economical for smaller welds and for thin pieces.4. Welding equipment is costly.5. Flux consumption is very much.6. It is limited to welding in flat position only.7. Cast iron, Al alloys, Mg alloys cannot be welded by this process.

Appl ications

1 Welding of bridge girders, railroads, structural shapes, pressure vessels, pipes and structureof railway coaches and locomotives.

2 Ship buildings, automotive industries, nuclear power industries etc.

3 Welding metals like mild steel, low alloy steels etc.

ELECTROSLAG WELDING

The electro slag welding (ESW) process is developed essentially to weld very large plates with out any edgepreparation. This is essentially a single pass process using a consumable electrode for filling the gap betweenthe two heavy plates. In this welding process, welding heat is produced by the molten slag, which melts thefiller metal and the surfaces of the work to be welded.Operation

Electro slag welding is initiated by starting an arc between the filler metal/ electrode and the work. This archeats the flux and melts it to form the slag. The arc is then extinguished and the slag (conductive) ismaintained in molten condition by its resistance to the flow of electric current between the electrode and thework. The temperature of this molten slag pool is approx. 1650

0C at the surface and 1950C inside, under the

surface. This much heat is sufficient to weld thick sections or joints in a single pass. Several electrodes are

used for long welds so that the heat is more uniformly spread. Water-cooled shoe or copper dam platefastened to the sides of the workpiece prevents the molten metal from running off. These plates also assistthe solidification process by removing heat and are moved up as the weld progress.Flux

Combination of oxides of silicon, manganese, titanium, calcium, magnesium and fluorspar are used as flux inthis process. It shields the molten metal and clears the impurities from the molten metal.Advantages

1 Thicker plates can be welded in single pass and economically.2 Extremely high deposition rates can be achieved (10-20 kg/hr electrode).3 Flux consumption as compared to submerged arc welding is low.4 No spattering and arc flashing occurs.5 Welding speed of about 1.5 m/hr is possible.

Disadvantages

1 Submerged arc welding is more economical than electro slag welding for joints below 60 mm.

2 It is difficult to close cylindrical welds.Appl ications of this welding process are in the fabrication of high pressure vessels, frames of heavymechanical and hydraulic presses, rolling mill frames, ship hulls, locomotive frames, etc.

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Gas WeldingDefinition

Gas welding derives the heat from the combustion of a fuel gas such as acetylene in combination withoxygen. The process is a fusion welding process wherein joint is completely melted to obtain the fusion.. The useful fuel gases used for gas welding are shown in table1.

Oxy-Acetylene Welding

Oxy-acetylene welding derives heat from the combustion of fuel gas acetylene in combination with oxygen.This process is fusion welding process where in the joint is completely melted to obtain fusion. Whenacetylene is mixed with oxygen in correct proportions in the welding torch and ignited, the flame is produced

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which is sufficiently hot to melt and join the parent metal. Temperature of flame is about 3100C. A filler rod isgenerally added to build up the seam for greater strength.Chemistr y of Oxy-Acetylene Flame

Combustion of gas mixture takes place in two main stages:Stage 1Oxygen and acetylene in equal proportions by volume burn in the inner white cone and forms carbonmonoxide, while the hydrogen is liberated.

C2H2+ 02------ 2C0 +H2+ 18.75 MJ/m3 of Acetylene... (i)Stage 2The carbon monoxide produced in stage 1 (inner cone) uses the oxygen from the air and results in carbondioxide and water vapors

4C0 + H2+O2 ------ 4C02+ 2H20 + 35.77 MJ/m3of Acetylene... (ii)

Combining equations (i) and (ii)2C2H2+ 502------- 4C02+ 2H20 ... (i)

So it can be seen that about two fifth of the oxygen necessary for the complete combustion of acetylene isreceived from the cylinders and rest from the surrounding air atmosphere.Though higher amount of heat is produced in the second stage, since it distributed over a large area, thetemperature achieved is small (1200-2000 0C) in the flame which may be used for preheating the metal. Theinner white cone temperature is of the order of 3100

0C, which is directly used for directly melting the joint.

Types of Flames1. Neutral flameNeutral flame is produced when oxygen to acetylene ratio is 1.1 to 1. The temperature is of the order of about3100C. The flame has nicely defined inner cone (light blue in colour) and is surrounded by outer envelopewhich is dark blue in colour than the inner cone. It is called neutral because it will not oxidize or carburize themetal. It is used for welding of:

Mild steelStainless steelCopperCast ironAluminium

2. Oxidizing flame

After the neutral flame, if the supply of oxygen is further increased, the result will be an oxidizing flame. Itsinner cone is more pointed; outer flame envelope is much shorter. It burns with a loud roar. The temperatureis of the order of about 3300C (because of excess O2, so complete combustion takes place).This flame is harmful for steels, because it oxidizes the steels.Only in the welding of copper and copper based alloys, oxidizing flame is desirable, because in those cases athin protective layer of slag forms over the molten metal.3. Reducing flameIf the volume of oxygen supplied to the neutral flame is reduced, the resulting flame will be a carburizing orreducing flame i.e. rich in acetylene. In this flame, acetylene feather exists between the inner cone and outerenvelope. Temperature is of the order of about 2900

OC (less because it does not completely consume the

available carbon). Metals that tend to absorb carbon should not be welded with reducing flame. Carburizingflame conations more acetylene than a reducing flame. Carburizing flame is used in the welding of lead andfor carburizing (surface hardening) purposes.Reducing flame is used with low alloy steel rod for welding high carbon steel.

GASChemical formula

Heat content MJ/m3

Flame temperature0C

primary secondary Total

Acetylene C2H2 18.97 36.03 55 3100

Propylene C3H6 16.38 71.62 88 2500

Propane C3H8 9.38 83.62 93 2450

Hydrogen H2 -- -- 10 2390

Natural Gas CH4 + H2 0.41 36.59 37 2350

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Welding Operation

To ignite a flameOpen the acetylene control valve of the welding torch and after the system has been flushed clean of air, thegas is ignited. At this stage, enough of oxygen is drawn in from the atmosphere to burn acetylene partially.The acetylene control valve of the welding torch is then opened in order to adjust the proportions in whichacetylene and oxygen are required to mix and burn. Flame adjustment will be done according to our

requirement of flame.To extinguish the flame and stop weldingWhen the welding or cutting operation is finished, close the torch acetylene valve first and then turn off thetorch oxygen valve. Then close the oxygen cylinder valve. Release the pressure in the hose and regulator by

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opening the oxygen control valve on the torch. Release the pressure on the oxygen regulator by turning theregulator to the minimum pressure positions. Close the oxygen control valve on the torch. Repeat the sameprocedure for purging acetylene.Welding Techniques

Depending upon the ways in which welding rod and the welding torch can be used, there are two usualtechniques used in gas welding namely:

1 Leftward technique or fore-hand welding method.2 Rightward technique or back-hand welding method.

Leftward Technique:1 The welder starts welding at the right hand end of the joint and proceeds towards the left.2 The welding flame is directed away from the finished weld.3 Welding torch is given small sideways movements, while the filler rod is moved steadily across the

seam.Advantages

1 Good control and neat appearances are obtained.2 It preheats the joint. If workpiece thickness is more than 3 mm, bevel of plates is necessary and

included angle of V joint is 80-90 degrees, so it is not economical.3 When the weld is recommenced, the tip of the rod causes weld contamination.

Rightward Technique

1 Welding begins at the left-hand end of the joint and proceeds towards the right, hence the namerightward technique.

2 The torch flame is directed towards the completed weld.3 Since the flame is constantly directed on the edges of the V ahead of the weld puddle, no sidewise

motion of the torch is necessary, which results narrower V-groove (30 degree bevel).4 The ripples are heavier and spaced further apart.

Advantages1 Rightward technique has got certain advantages over the leftward.2 Up to 8 mm plate thickness, no bevel is necessary. This reduces the cost of preparation of edges and

consumption of filler rod.3 For bigger thickness, included angle of V need be only 60 degrees against 80 degrees in leftward

technique.4 The welder's view of the weld pool and the sides and bottom of the V groove is unobstructed so better

control is possible.5 The weld quality is better.6 Smaller total volume of deposited metal reduces shrinkage and distortion.7 Because of less consumption of filler rod, cost of welding is less.

Gas Welding Equipments

1 Oxygen Gas Cylinder: Oxygen cylinders are painted black and valve outlets are screwed righthanded.

2 Acetylene Gas Cylinder: Acetylene cylinder is painted maroon and the valves are screwed left-handed, to make this easily recognizable, they are chamfered or grooved.

3 Pressure RegulatorsOxygen and Acetylene Pressure RegulatorsThe pressure of the gases obtained from cylinders / generators is considerably higher than thegas pressure used to operate the welding torch. The purpose of using a gas pressure regulator is,therefore:

1 To reduce the high pressure of the gas in the cylinder to a suitable working pressure.2 To produce a steady flow of gas under varying cylinder pressures.

A pressure regulator is fitted with two pressure gauges one for gas pressure in the cylinder and the othershows the reduced pressure at which the gas is coming out. A pressure regulator is connected between thecylinder/generator and the hose leading to welding torch.4. Oxygen and Acetylene Gas Hoses and Hose ConnectionsHose Clamps: A metal clamp is used to attach the welding hose to a nipple. The clamp squeezes the hosearound the nipple to prevent it from working loose.Hose Couplers: These are used to join two pieces of welding hoses.Hoses: The hose for the supply of oxygen to the welding torch is coloured blue and has right handed threadconnections, whereas the acetylene hose is coloured red and has left handed connections with chamfers orgrooves on the nuts.5. Welding Torch

Oxygen and the fuel gas having been reduced in pressure by the gas regulators are fed through suitablehoses to a welding torch which mixes and controls the flow of gases to the welding nozzle or tip where the gasmixture is burnt to produce a flame for carrying out gas welding operation. There are two types of weldingtorches namely:

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High pressure (or equal pressure) type.Low pressure (or injector) type.

6. Trolleys for the transportation of oxygen and acetylene cylinders.7. Set of keys and spanners.8. Filler rods and fluxes.9. Gas lighter.10. Protective clothing for the welder (e.g. asbestos apron, gloves, goggles etc.)

Advantages o f Gas Welding

1. It can be applied to a wide variety of manufacturing and maintenance situations.2. Rate of heating and cooling of weld deposit and job is slow.3. No electric current is required.4. Equipment is having less cost.5. Operator is having better control because sources of heat and filler metals are separate.6. Cost and maintenance of the welding equipment is low.

Disadvantages of Gas Welding

1. Flame temperature is less than the temperature of the arc.2. Refractory metals (e.g. tungsten, molybdenum, tantalum etc.) and reactive metals (titanium and

zirconium) cannot be gas welded.3. Gas flame takes a long time to heat up the metal than an arc.

4. Heat affected zone is wider.5. Acetylene oxygen gases are rather expensive.6. Storage of gases is not safe. More safety is needed.7. More skilled operators are needed.

Appl ications of Gas Weld ingIt is used for welding of mild steel, stainless steel, copper, cast iron, high cl etc.

1 For joining thin materials.2 In automotive and aircraft industries.3 In sheet metal fabricating plants.

OXY-ACETYLENE CUTTING

All metals (ferrous) can be cut by means of an oxy-acetylene flame. .The metal to be cut is heated up by means of flame and then oxygen gas is impinged on this red heat metalso as to form the metal oxide and then remove the metal from there The blowpipe carries two separate

passages of oxygen, one for supplying oxygen to burn with acetylene to form the heating flame and the otheris placed centrally in the nozzle. It is necessary to maintain a proper distance (about 6 mm) between thenozzle tip and the top surface of the metal plate. For cutting cast iron, a carburizing preheating flame shouldbe adjusted and the nozzle is kept at a comparatively larger distance. It is also used for cutting steels.

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RESISTANCE WELDING

Definition

Resistance welding is a group of welding processes in which welding heat is obtained from resistance of thework to the flow of electric current and by the application of pressure. No filler metal or flux is added.Types of Resistance Welding

1. Spot welding2. Seam welding

3. Projection Welding4. Resistance butt welding5. Flash butt welding6. Percussion welding.PRINCIPLE OF RESISTANCE WELDING

In resistance welding, a low voltage (typically 1 volt) and very high current (typically 15000 A) is passedthrough the joint for a very short time (typically .25 sec.). This high amperage heats the joint because heatgenerated in resistance welding can be expressed as:

H = I2RT

Where, H = Total heat generated in the work, Joule. .I = Electric current, Amp.T = Time for which the electric current is passing through the joint, sec.R = Resistance of the joint.Resistance of the joint is composed of:

1 Resistance of the electrodes (Re).2 The contact resistance between the electrode and the workpiece (Rwe)3 The contact resistance between the two workpieces (Rww)4 The resistance of the work piece plates (Rw).

In order to obtain a sound weld and to avoid overheating of welding electrode; R eRwe, and (Rw) should bekept as low as possible as compared to RwwHEAT BALANCE

One of the very important factors in the resistance welding process is the transfer of heat. The proper fusioncan be obtained only if there is proper heat balance. If one plate is thicker than the other then use thickerelectrode for thicker sheet and thin electrode for thin sheet because more heat flows if area of contact is more.If two plates have different thermal conductivity, it is necessary to provide an electrode contact area that issmaller near the higher conductivity metal.ELECTRODES FOR RESISTANCE WELDING

Since the electrodes in resistance welding have to carry large amount of current, pressure also help toremove the heat from the weld zone thus preventing overheating and surface fusion of work so the electrodesshould have higher electrical conductivity as well as higher hardness. Hence, copper in alloyed form is usedfor making electrodes.

Copper cadmium is used for welding of Al and Mg.Copper chromium is used for welding of mild steel and low alloy steel.

Advantages o f resistance welding :

1 Very little skill is required to operate the resistance welding machine.2 High production rate so well suited for mass production.3 Heating of the workpiece is confined to a very small part, which results in less distortion.4 No filler rod and flux is needed.5 It is possible to weld dissimilar metals as well as metal plates of different thickness.6 Semi-automatic equipments are available.7 There are no consumables used in this process except for the electrical power and a relatively small

electrode wear. As a result, it is a very economical process.Disadvantages:

1 The initial cost of equipment is high.2 Certain resistance welding operations are limited only to lap joints.3 Skilled persons are needed for maintenance.4 Bigger job thicknesses cannot be welded.

Appl ications:

1 Joining sheets, bars and tubes.2 Making metal furniture.3 Making fuel tanks of cars, tractors etc.

4 Making containers.5 Welding aircraft and automobile parts.

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Spot welding

Spot welding is a resistance welding process in which overlapping sheets are joined by local fusion at one ormore spots by the heat and pressure is applied by the electrodes one above and other below the workpieces.The heat is generated because of the resistance to the flow of electric current through workpiece.Procedure

1 The job should be clean. It should be free from grease, dirt, paint, scale, oxide etc.2 Clean the electrode tip surface. Very fine emery cloth may be used for routine cleaning.3 Water is kept running through the electrodes in order to cool the weld and avoids the electrodes from

getting overheated.4 Proper welding current has been set on the current selector switch.5 Proper time has been set on weld timer.6 Electrodes are brought together against the overlapping workpieces and pressure is applied so that

the surfaces of the two workpieces come in physical contact with each other.7 Welding current is switched on for a definite period of time.8 As the current passes, a small area where the workpieces are in contact is heated and spot weld

takes place. The temperature of this weld zone is approx. 815C to 930C.9 After the welding takes place, the welding current is cut off. Extra electrode forte is then applied or the

original force is prolonged. Hold until the metal cools down and gains strength.10 After that, electrode pressure is released to remove the spot welded workpieces.

Advantages o f Spot Weld ing:1. Low cost.2. Less skilled worker can it.3. Higher productivity.4. Operation may be made automatic or semiautomatic.5. No edge preparation is needed.

Appl ication of Spot Welding :Welding of low carbon steels, high speed steels, stainless steels, Al, Cu, nickel, nickel alloys etc.

1. In automobile and aircraft industries.2. Steel household furniture.3. Containers.

SEAM WELDING

Seam welding is a resistance welding process in which overlapping sheets are joined by local fusionprogressively along a joint by rotating the circular electrodes. Fusion takes place because of heat, which isgenerated, from the resistance to electric current flow through the work parts which are held together underpressure by electrodes.Principle of Operation (Procedure)

1 The workpieces to be seam welded are cleaned,2 Copper alloy electrode wheel overlapped suitably and placed between the two circular electrodes

which clamp the workpieces together by the electrode force.3 Switch on the coolant supply (in some machines, the electrodes are cooled by external spray of water;

in others, the electrodes are cooled by refrigerant fluid that flow inside the working electrodes).4 Switch on the current supply. As the first current impulse is applied, the power driven circular nuggets

electrodes are set in rotation and the workpieces steadily move forward.5 If the current is put off and on quickly, continuous fusion zone made up of overlapping nuggets is

obtained. It is known as stitch welding. If individual spot welds are obtained by constant and regularlytimed interruption of the welding current, the process is known as roll (spot) welding.

Advantages of Seam Welding :1. It can produce gas tight or liquid tight joints.2. Overlap can be less than Spot or Projections weld.3. Several parallel seams may be produced.

Disadvantages o f Seam Welding :1. Cost of equipment is high as compared to spot welding set.2. Welding can be done only along a straight or uniformly curved line.3. It is difficult to weld thickness greater than 3 mm.

Appl ications of Seam Weld ing:

It is used for welding of stainless steels, aluminum and its alloys, nickel and its alloys, magnesium alloys etc.

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PROJECTION WELDING:

Projection welding is another advancement of spot welding, where one of the sheets to be joined is providedwith a number of projections to help localize the current at a predetermined spot. Thus, the surfaces of theworkpieces are in contact with each other only at the projections.

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As the current is switched on, it will pass through these projections. Because of he produced due to resistanceto the flow of electric current, these projections are melted and a pressed together to complete the weld, bypressing the upper electrode downward. The melt projections form the weld. Welding can be done at severalpoints simultaneously. The projections are generally very small of the order of 0.8 mm and are obtained bymeans of embossing.

Advantages o f Project ion Welding:

1. It is possible to weld more than one spot at a given time.2. The welds may be placed closer than spot welding.

3. Proper heat balance can be easily obtained. Projections are to be made in thicker plates or in theplate which is having higher electrical conductivity.

4. Life of electrode is much longer than the life of electrode in spot welding.5. The uniformity and appearance of the weld is better as compared to spot welding.

Disadvantages of Projection Welding:

1. Making of projections is an extra operation.2. All projections should be of same height.3. Metals, which cannot support projections, cannot be welded satisfactorily.

Appl ication of Pro jection Weld ing:1. Small fasteners, nuts etc. can be welded to larger components.2. It is used for welding of refrigerator condensers, joining of wires etc.3. Welding of stainless steel parts, titanium alloys, monel alloys etc.

Resistance Butt Welding

In this welding, the two pieces to be joined are held tightly together and current is applied, so that the heat isgenerated over the entire area of abutting surfaces. Pressure is applied throughout the heating period. Thispressure is later on increased when the welding temperature has been reached. Procedure

1. One workpiece to be joined is clamped in stationary clamping block and other is clamped in movingclamping block.

2. The two workpieces are brought together (butt joint) by pressure.3. A heavy current is passed through these workpieces. The heat is generated because of resistance to

the flow of electric current.4. When the welding temperature has been reached, the pressure is increased so that, proper joint takes

place.5. After that welding current is cut off.6. Force is released when the welded joint has reached the desired temperature (normal temperature).7. After that workpieces are unclamped.

Appl ication of Resistance But t Welding :

1. In wire drawing industries.2. For producing butt joints in tubes, pipes, rods etc.

Flash Butt Welding

Flash butt welding is similar to upset butt welding except that the heat required for melting is obtained bymeans of an arc rather than the simple resistance heating.Procedure

1. In this welding, one workpiece to be joined is clamped in moving platen while other is mounted onfixed platen.

2. The workpiece held in the movable platen is moved to bring towards the workpiece which held instationary platen (until light contact has been established).

3. When the welding current is switched on, flashing is established.4. This flashing produces the welding heat. When sufficient heat is produced, more pressure is applied

so that sound joint takes place.5. After that, welding current is cut off and workpieces are unclamped.

Advan tages o f Flash Butt Weld ing:It consumes less welding current than upset butt weld process.Flash welding offers strength factor upto 100%. Preparation of weld surface is not required.The process is cheap.It is a faster process.

Disadvantages of Flash Butt Welding:

1 Concentricity and straightness of workpieces during welding is difficult to maintain.2 The shapes of the workpieces should be similar which is not always possible.3 Chances of fire hazards are there Flashes may cause eye trouble.

Appl ications of Flash Butt Weld ing:It is used for the welding of bars, rods and tubes.It is also used for the welding of saw blades into continuous loops, taps and reamers to alloy steel shanks.

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BRAZING

Brazing is the coalescence of a joint with the help of a filler metal whose liquidus temperature is above 450Cand is below the solidus temperature of the base metal. The filler metal is drawn into the joint by means ofcapillary action (entering of fluid into tightly fitted surfaces).A general brazing method is to apply flux to the joint, after it has been properly prepared, heat the joint tobring it to the liquidus temperature of the filler material, and apply the filler material so that it flows into thecrevice by capillary forces. The filler metal on solidification gives the necessary joint strength.Filler materials:

Depending on the type of base metals brazed, a number of filler metals are available.1 Copper based materials are generally used for brazing ferrous materials.2 Copper alloys having high zinc (70%) content are not extensively used because of their brittleness,

but those with less zinc content are used for brazing of various steels.3 Aluminium-silicon filler materials are used for brazing aluminium.4 Silver brazing makes use of a silver based filler metal. Silver brazing is used to give high strength

(tensile strength up to 900 MPa) joints. Silver brazing is extensively used in industrial applications andfor jewellery applications.

Design considerations for brazed joints

Because the filler metal reaches the joint by capillary action, the joint should be designed properly.The following factors should be considered for designing a brazed joint

1 The clearance between the two parts to be joined should be critically controlled. If there is too much ofclearance, the capillary forces may not be sufficient to draw the filler metal into the joint, whereasinsufficient clearance may have too small an amount of filler metal to give rise to any effectivestrength.

2 The temperature at which the filler metal is entering the joint.3 The differences in the coefficients of thermal expansion of the two pieces to be joined should be

properly accounted for.4 Joints need to be extremely clean. Any grease or oil present in the joint prevents the flow of filler

metal. Hence the joint should be thoroughly cleaned using proper solvents.5 Oxides and scales present are removed by acid pickling.

Fluxes are added into the brazed joint to remove any of the oxides present or prevent the formation of theoxides so that the base metal and the filler metal remain pure during the joining.The fluxes generally used are combinations of borax, boric acid, chlorides, fluorides and tetra borates andother wetting agents. The fluxes used for ferrous materials are mixtures of borax and boric acid in a pasteform. A popular composition is 75% borax and 25% boric acid. Alkaline bifluorides are used for brazing ofstainless steel, aluminium or beryllium copper alloys. A special flux containing sodium cyanide is used inbrazing tungsten to copper. The fumes from all these fluxes are harmful and therefore a proper ventilation andcare in use are essential. Enough flux should be applied at the joint so that it lasts throughout the brazingsession. The method of application of the flux is by spraying, brushing or with the help of a pressurizedapplicator.Heat sources that are used for brazing are: a molten bath of brazing filler metal, oxyacetylene torch,controlled atmosphere furnace, electrical resistance heating and induction heating. The main points to benoted are that the joint should be maintained in proper fit with cleanliness before heating the joint for brazing.Advantages:

Brazing is a much widely used joining process in various industries because of its many advantages.1 Dissimilar metals, such as stainless steel to cast iron can be joined by brazing.2 Almost all metals can be joined by brazing except aluminium and magnesium which cannot easily be

joined by brazing.3 Because of the lower temperatures used there is less distortion in brazed joints.4 The original heat treatment of the plates being joined is not affected by the brazing heat.5 The joint can be quickly finished without much skill.6 it is an economical joining method with reasonable joint strength. The brazed joints are reasonably

stronger, depending on the strength of the filler metal used.Disadvantages:

1 The brazed joint is generally not useful for high temperature service because of the low meltingtemperature of the filler metal.

2 The colour of the filler metal in the brazed joint also, may not match with that of the base metal.

BRAZE WELDING

Braze welding is similar to brazing in that the joint is obtained by means of a filler metal, whose liquidus

temperature is above 450 "C and below the solidus temperature of the base metal. But the difference is that inbraze welding; the filler metal reaches the joint without the capillary action since the joint gap is more. As aresult, the types of joints that can be used in braze welding are much more varied. The joint design for brazewelding is similar to that used for oxy-acetylene gas welding. Thick layers of filler metals are deposited in

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braze welding since filler metals enter the joint by gravity. The cleaning and flux requirement for braze weldingis similar to that of brazing.Typical filler metals used in braze welding are brasses with zinc content up to 40%. Here again, the joint isobtained by diffusion of the filler metal into the base metal and by surface alloy formation. These areresponsible for good strength achieved by braze welding. Silver based alloys are not used in braze weldingbecause of the large amounts of filler metals required which would increase the cost of the joint.SOLDERING

Definition: Soldering is a method of joining similar or dissimilar metals by means of a filler metal whose

liquidus temperature is below 4500C.The most commonly used soldering methods are:

1 With soldering iron (flame or electrically heated)2 Dip soldering3 Wave soldering.

A soldering iron is a copper rod with a thin tip which can be used for flattening the soldering material. Thesoldering iron can be heated by keeping in a furnace or by means of an internal electrical resistance whosepower rating may range from 15 W for the electronic applications to 200 W for sheet metal joining.In dip soldering, a large amount of solder is melted in a tank which is closed. The parts that are to besoldered are first cleaned properly and dipped in a flux bath as per the requirement. These are then dippedinto the molten solder pool and lifted with the soldering complete.The wave soldering is a variant of this method wherein the part to be soldered (e.g., an electronic printedcircuit board, PCB) is not dipped into the solder tank, but a wave is generated in the tank so that the soldercomes up and makes the necessary joint. This is normally a continuous process with the PCB's beingcontinuously moving on top of the solder tank and the waves become continuously generated. It is used formass production of the electronic equipment. There are also other methods available for soldering, such astorch soldering, oven soldering, resistance soldering, induction soldering and infrared soldering.The filler metals used are normally called as solders which are essentially alloys of lead and tin. The alloys oflead and tin have lower liquidus temperatures. The eutectic alloy (62% tin + 38% lead) has the lowest at 1830C. To improve the mechanical properties and temperature resistance, solders are added to other alloying

elements such as zinc, cadmium and silver in various proportionsFluxes:To remove the oxides from the joint surfaces and to prevent the filler metal from oxidizing, fluxes aregenerally used in soldering.

1 Rosin and rosin plus alcohol based fluxes are least active type and are generally used for electricalsoldering work. Because of the content of acids, these are corrosive at soldering temperature. Theycan be easily cleaned after the soldering.

2 The organic fluxes such as zinc chloride and ammonium chloride are quick acting and produceefficient joints. But because of their corrosive nature the joint should be thoroughly cleaned. These areto be used for only non-electrical soldering work.

Fluxes are normally available in the form of powder, paste, liquid or in the form of core in the solder metal. It isnecessary that the flux should remain in the liquid form at the soldering temperature and be reactive to be ofproper use.The soldering joints also need to be cleaned meticulously to provide chemically clean surfaces to obtain aproper bond. Solvent cleaning, acid pickling and even mechanical cleaning are applied before soldering.LimitationsThough soldering obtains a good joint between the two plates, the strength of the joint is limited by thestrength of the filler metal used. Soldering is normally used for obtaining a neat leak proof joint or a lowresistance electrical joint. The soldered joints are not suitable for high temperature service because of the lowmelting temperatures of the filler metals used.

DEFECTS IN WELDING

In view of the severe thermal regime through which the welding process proceeds, the weldments are likely tobe affected and if proper care is not taken, likely to end up with certain defects. Distortions have beendiscussed in greater detail earlier, and we will see the other defects here. The defects likely are:UndercutIncomplete fusionPorositySlag inclusionHot crackingCold crackingLamellar tearing

UndercutThis appears like a small notch in the weld interface. This is generally attributed to the improper weldingtechnique or excessive welding current. This is mainly caused by the incorrect manipulation of the electrodewhile depositing the bead, particularly, in horizontal and vertical welding.

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Incomplete fusion

This will be seen as a discontinuity in the weld zone. The main causes for this defect are improper penetrationof the joint, wrong design of the joint or incorrect welding technique including the wrong choice of the weldingparameters. The main parameter that controls is the welding current, if lower than required would notsufficiently heat all the faces of the joint to promote proper fusion. Also the improper cleaning of the jointhinders the fusion of the metal in the joint.Porosity

Porosity in welding is caused by the presence of gases which get entrapped during the solidification process.

The main gases that cause porosity are: hydrogen, oxygen, and nitrogen. Though there are other gases suchas argon, helium, carbon dioxide, that are also present in the weld pool, in view of their insolubility would notcause porosity. Hydrogen is the main cause of porosity in the weld pool. The sources of hydrogen could bethe electrode coatings such as cellulose, dissociation of water which is present as moisture pick-up ofelectrodes, leftover wire drawing lubricant on the electrode wire or the dissolved hydrogen in the base metaland filler metal.Oxygen generally reaches the weld pool is oxide of base metal or filler metal, or in some compounds of fluxespresent in the electrode coatings. Nitrogen generally enters the weld pool through atmospheric nitrogen or thecontaminated shielding gases. Porosity if present in large quantity would reduce the strength of the joint.Slag inclusionSlagis formed by the reaction with the fluxes and is generally lighter. In view of its low density, it will float ontop of the weld pool and would be chipped off after solidification. However, the stirring action of the highintensity arc would force the slag to go into the weld pool and if there is not enough time for it to float, may getsolidified inside the fusion zone and ends up as slag inclusion. Also in multipass welding, the slag solidified inthe previous pass is not cleaned before depositing the next bead which may cause slag inclusion. Some ofthe factors that cause slag inclusion are:High viscosity of weld metalRapid solidificationInsufficient welding heatImproper manipulation of the electrodeUndercut on previous pass.Slag inclusion like porosity, weakens the metal by providing discontinuities.Hot cracking

It generally occurs at high temperature and the size can be very small to be visible. The crack in most parts isintergranular and its magnitude depends upon the strains involved in solidification. They are more likely toform during the root pass when the mass of the base metal is very large compared to the weld metaldeposited. It can be prevented by preheating the base metal, increase the cross-sectional area of the rootbead, or by changing the contour or composition of the weld bead.Cold cracking

Cold cracking generally occurs at room temperature after the weld is completely cooled. This can be generallyseen in the heat-affected zone. The causes are:

1 excessive restraint of the joint which induces very high residual stresses2 Martensitic transformations making the metal very hard as a result of rapid cooling.3 Stress relieving the weldment immediately would help in relieving the residual stresses and the

potential for cracking gets reduced. Also pre and post heating of the weldment helps in reducing thecooling rates and the consequent locking of the stresses.

Lamellar tearing

It is generally seen at the edge of the heat affected zone. It appears as a long and continuous visualseparation line between the base metal and the heat affected zone. This is caused by the presence of theelongated inclusions such as Mn, Fe and S in the base metal. It can also be caused by the weld configurationwhich gives rise to high residual tensile stresses in the transverse direction.

THERMIT WELDING

Thermit welding (TW) is a process which was traditionally used for the welding of very thick plates. Thoughthis was used for welding large sections such as locomotive rails, ship hulls and broken large castings, its usehas decreased to a great extent, nowadays because of the availability of other simpler methods such assubmerged arc welding.The heat source utilized for fusion in thermit welding is the exothermic reaction of the thermit mixtures. Atypical hermit mixture for welding steels is aluminium and iron oxide. When the intimately mixed Thermitpowder is brought to its ignition temperature of 1200

0C, the Thermit reaction starts. Aluminum has greater

affinity towards oxygen, and as a result, it reduces the ferric oxide to liberate iron and in the process, releases

heat. The following reaction takes place.3Fe3O4 + 8Al 9Fe +4Al2O3 + 3.01 MJ/molThe temperature reached is of the order of 3000

0C. The enormous amount of heat liberated, melts both the

iron and aluminium oxide to a very fluid state. Because of the large differential in the densities, aluminium

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oxide would be floating on the top with the molten steel settling below. Once started, the reaction continues tillall the thermit mixture in the reaction vessel or ladle is completely reduced. Though the theoreticaltemperature is 3088

0C, in practice the heat losses from the crucible makes it about 2500

0C. The addition of

fluxes or other mild steel and alloying additions reduce the temperature still below to ultimately a super heat ofthe order of 400 to 550

0C.

The heat released during the operation is of the order of 35 kj/kg of thermit mixture. The complete reactiontakes place in a total of 1 min, irrespective of the amount of thermit mixture present in the crucible. Forexample, about 1 or 2 tonnes of molten steel with a super heat of about 500

0C can be generated in less than

a minute. Though such a large amount of heat is generated, the safety of the thermit mixture is assuredbecause the reaction can only start when the mixture is brought to its ignition temperature. There are a largenumber of other thermit mixtures available for welding different materials. For example, for welding of powercables made of copper the following thermit mixture is used.

3 CuO + 2 A1 3 Cu + Al203, + 1.152 MJ/mol (48660C)

The thermit welding process is essentially a casting process, where the molten metal obtained thermitreaction is poured into the refractory cavity made around the joint. The two pieces to be are properly cleanedand the edge is prepared. Then wax is poured into the joint so that formed where the weld is to be obtained. Amoulding flask is kept around the joint and sand is carefully around the wax pattern. as shown in Fig.providing the necessary pouring basin, sprure and risers. A bottom opening is provided to runoff the moltenwax. The wax is melted through this opening which is also used to preheat the joint and make it ready forwelding.The thermit mixture which is mixed with fluxes is filled into a ladle through a bottom opening. The bottom isinitially closed. The igniting mixture which is normally barium peroxide or magnesium is placed at the top ofthe thermit mixture. The igniting mixture is lighted by means of a heated rod, whereby the complete reactiontakes place and molten metal is produced. The bottom plug of the ladle is opened and the metal is allowed toflow into the prepared mould. The weld joint is allowed to cool slowly. Besides making a fast weld, thermitwelding also provides a reasonably strong weld. The strength of thermit welded joint reaches that of a forgedmetal without any defects.

Welding dioosm

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