electricarcwelding

Module-II of Manufacturing Science-I

Lecture Notes of Chinmay Das

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2.2 ELECTRIC ARC WELDING

The electric arc welding process makes use of the heat produced by the electric arc to fusion weld metallic

pieces. This is one of the most efficient and widely used welding process because of the ease of use and

high production rates that can be achieved economically.

Principle of Arc An electric arc is formed when an electric current passes between two electrodes separated by a

short distance from each other. In arc welding one electrode is the welding rod or wire, while the other is

the metal to be welded. The electrode and the plate are connected to the supply, one to the positive pole and

one to the negative pole. The arc is started by momentarily touching the electrode on to the plate and then

withdrawing it to about 3 to 4 mm from the plate. When the electrode touches the plate, a current flows,

and as it is withdrawn from the plate the current continues to flow in the form of a spark across the very

small gap first formed. This causes the air gap to become ionized or made conducting, and as a result the

current is able to flow across the gap, even when it is quite wide, in the form an arc. The electrode must

always be touched on to the plate before the arc can be started, since the smallest air gap will not conduct a

current 9 at the voltages used in welding) unless the air gap is first ionized or made conducting.

The arc is generated by electrons flowing from the negative pole to the positive pole and the

electrical energy is changed in the arc into heat and light. Approximately two-thirds of the heat is

developed near the positive pole , which burns into the form of a crater, the temperature near the crater

being about 6000-70000C, while the remaining third is developed near to the negative pole. As a result an

electrode connected to the positive pole will burn away 50 % faster than if connected to the negative pole.

The welding current may vary from 20 to 600 A in manual metal arc welding. When alternating current is

used, heat is developed equally at the plate and rod, since the electrode and the plate are changing polarity

at the frequency of the supply.

Arc Welding Equipment The main requirement in an arc welding setup is the source of electric power. They are essentially

of two types:

a) Alternating Current Machines

1. Transformer

2. Motor or engine driven alternator

b) Direct Current Machines

1. Transformer with DC rectifier

2. Motor or engine driven generator

In AC welding normally transformer is used. It has following operational characteristics.

1. No moving parts and less noise;

2. Less maintenance;

3. Higher efficiency;

4. Cheaper power source.

In DC arc welding a rectifier or a generator can be used to supply the required DC power. At first input

voltage is stepped down to required voltage and then through silicon controlled rectifier (SCR) is converted

from AC to DC. Its characteristics are

1. Compact setup

2. Highly reliable and efficient

3. Less noise

4. Costly setup

The welding machine can be of two types.

1. Constant current welding machines or droopers

2. Constant voltage welding machines

In constant current welding machine the change in arc current magnitude due to change in voltage

across the electrodes is very small. This machine is very essential for manual arc welding processes since

the maintenance of constant arc is nearly impossible by a human welder. With the variation of electrode

distance from the base plate in manual arc welding the voltage across the arc gap changes continuously but

the magnitude of current remains almost constant due to which good quality of weld can be made.



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In constant voltage welding machines small change in voltage makes for an extremely large change in

the output currents. Theses machines are generally preferred in the automatic machines since they become

self corrective. When the electrode comes a bit closer to the work, the arc voltage drops raising the output

current to very high value. This current instantly melts the electrode and thus maintains the arc gap.

Figure 2.2.1: Constant current characteristics Figure 2.2.2: Constant voltage characteristics

Though DC arc welding is more expensive than AC welding, it is generally preferred because of

the control of the heat input offered by it. If more heat

is required at the workpiece side, such as for thicker

sheets or for the work materials which have higher

thermal conductivity such as aluminium and copper,

the workpiece can be made as anode, liberating large

heat near it. This is termed as straight polarity or

direct current electrode negative (DCEN). This gives

rise to higher penetration of weld metal. For thinner

materials where less heat input is required in the weld

zone, the polarity could be reversed by making the

workpiece as negative. This is termed as reversed

polarity or direct current electrode positive

(DCEP).In this case weld metal penetration is small. In

case of AC welding the bead obtained is somewhere in

between the above two types. DC arc welding is

preferred for difficult tasks such as overhead welding,

since it can maintain a stable arc.

Figure 2.2.3: Weld penetration

A typical Ac arc welding set up using transformer has the workpiece on a metallic table to which the

ground lead of the secondary windings of the welding transformer is connected. The other lead of the

secondary is connected to an electrode holder into which the electrode is gripped. When the electrode is

brought into contact with the work, the welding takes place. The arc welding machines are specified by

maximum rated open circuit voltage, rated current in ampere and duty cycle. The maximum rated open

circuit voltage which is the voltage between the output terminals when no welding is being done. An arc

cannot be maintained with a voltage lower than about 14 V and is not very satisfactory above 45 V. With

DC source the voltage can be varied by a switch or regulator, but with AC supply by transformer the open

circuit voltage choice is less, being 80 or 100 V on larger units, down to 50 V on small units.



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Figure 2.2.4: Arc welding setup

A voltage of the order of 40 to 50 V should be enough for starting an arc, whereas for continuous

welding 20 to 30 V is sufficient. The minimum voltage Vm can be calculated as

Vm = 20 + 0.04 I ,

where I is the load current in amperes.

The rated current specifies the maximum current in amperes that a welding machine is capable of supplying

at a given voltage. The preferred current ratings as per Indian standard are 150, 200, 300, 400, 500, 600 and

900 A. The duty cycle as defined by American Welding Society (AWS) is the percentage of time in a 10

minutes period that a welding machine can be used at its rated output without overloading. Most of the

welding machines need not have to operate the full time since a good length of time is spent in setting up,

metal chipping, cleaning and inspection. Normally a 60 % duty cycle is suggested. However Indian

standard specifies a duty cycle of 5 minutes period. Continuous automatic welding machines may require

100 % duty cycle.

Figure 2.2.5: Shielded arc



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Electrodes The electrodes used for providing heat input in arc welding are of two types.

1. Consumable electrodes

2. Non-consumable electrodes

When consumable electrode is used, both base metal and electrode tip melt due to heat produced by the arc.

The electrode will act as filler material and gets mixed intimately with molten base metal to form solid joint

after solidification. In the welding process the electrode is continuously consumed and hence it should be

moved continuously towards the workpiece to maintain constant arc length. Consumable electrodes provide

both heat input and filler material, and made of various materials depending on the purpose and chemical

composition of the metals to be welded.

It is possible to use non-consumable electrodes made of carbon, graphite or tungsten. The carbon

and graphite electrodes are used only in DC welding, where as tungsten electrode is used for both AC and

DC welding. The filler material required has to be deposited through a separate filler rod. Therefore heat

input and amount of filler material deposited can be separately controlled.

A consumable electrode can be either bare or coated. If a bare wire is used as the electrode it is

found that the arc is difficult to control, the arc stream wandering hither and thither over the molten pool.

The molten globules are being exposed to the atmosphere in their travel from the electrode tip to the weld

metal pool and absorption of oxygen and nitrogen takes place even when a short arc is held. The result is

that the weld tends to be porous and brittle. The arc can be rendered easy to control and the absorption of

atmospheric gases reduced to a minimum by shielding the arc. This is done by covering the electrode with

various types of covering and as a result gases such as hydrogen and carbon dioxide are released from the

covering as it melts and forms an envelope around the arc and molten metal pool, excluding the atmosphere

with its harmful effects on the weld metal. Under the heat of the arc chemical compounds in the covering

also react to form a slag which is liquid and lighter than the molten metal. It rises to the surface, cools and

solidifies, forming a protective covering over the hot metal while cooling and protecting it from

atmospheric effects, also slows down the cooling rate of the weld. Some slags are self removing while

others have to be lightly chipped. In addition to above mentioned functions, coated electrodes which are

also called stick electrodes provide other useful functions which are listed below.

• Some elements which are required for stabilization of the arc are also added in these coatings. The

coatings are different for AC welding and DC welding.

• Special alloying elements can be introduced through these coatings to improve the strength and

physical properties of the weld metal.

• The electrode covering usually melts at a higher temperature than the wire core so that it extends a

little beyond the core, concentrating and directing the arc stream, making the arc stable and easier

to control. Also the thermal loses to the atmosphere from the electrode tip are reduced. This would

increase the available heat for melting the electrode and thus help in improving the metal

deposition rate.

• The coating is normally insulator of electricity and thus permits the electrode to be used in narrow

grooves and other difficult locations without causing any short circuiting problems.

• It is possible to include iron powder in the coating in large amounts so that the electrode can be

kept in contact with the workpiece, which may be necessary for welding in overhead and other

positions. Also this increases the metal penetration and deposition rate.

• The coating contains materials which can control the slag to be viscous or fluid. Viscous slag

would be useful for making welds in vertical position to cover the metal puddle for longer time.

The stick electrodes are available in diameters of 3.2, 4, 5, 6, 8 and 9 mm and the length is 350 or 450 mm.

In order to prevent moisture pick up by the covering, the electrodes are kept inside the oven to make them

thoroughly dry before put into use.

MANUAL METAL ARC WELDING (MMAW) OR SHIELDED METAL ARC WELDING

(SMAW) The manual metal arc welding also called shielded metal arc welding is the most extensively used manual

welding process which is done with stick or coated electrodes. This process is highly versatile and can be

used extensively for both simple as well as sophisticated jobs. This low cost equipment can be used for

welding in any position. Workpiece having thickness below 3 mm creates problem in this welding due to

lack of rigidity and requirement of low heat input. Similarly thicker workpiece (above 20 mm) may take

longer time to weld. The shielded arc welding can be done with either AC or DC power source. The current



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may vary from 50 to 500 A and voltage requirement is in between 20 to 40 V. The electrodes for the

welding operation should be properly selected considering following factors.

• The composition of the base metal which determines the electrode composition.

• The tensile strength of the required joint.

• The thickness of the base metal. For thinner metals the current setting should be lower.

• The required metal deposition rate.

• The type of arc welding equipment used.

• The weld position. A flat position can accommodate a larger size electrode. In case of vertical and

overhead positions, it is necessary that the weld pool be smaller for better control. So a smaller

size electrode is selected.

It is generally preferable to use the largest size electrode that is permitted for a given welding situation.

Transfer of metal across the arc gap When an arc is struck between the electrode and the plate, the heat generated forms a molten pool

in the plate and the electrode begins to melt away, the metal being transferred from the electrode to the

plate. This transfer takes place whether the electrode is positive or negative and also when it has changing

polarity. Similarly it is transferred upwards against the action of gravity, as when making an overhead

weld. The forces which cause the transfer appear to be due to : (1) its own weight, (2) the electro-magnetic

( Lorentz) forces, (3) gas entrainment, (4) magneto-dynamic forces producing movement and (5) surface

tension. If the arc is observed very closely it can be seen that the metal is transferred from the electrode to

the plate in the form of drops or globules, and these globules vary in size according to the current and the

type of electrode covering. Larger globules are transferred at longer intervals than smaller globules and the

globules form, elongate with a neck connecting them to the electrode, the neck gets reduced in size until it

breaks , and the drop is projected into the molten metal pool, which is agitated by the arc stream, and this

helps to ensure a sound bond between weld and parent metal.

Figure 2.2.6: detachment of molten globule in the metal arc process

Electrode Efficiency The efficiency of an electrode is the mass of metal actually deposited compared with the mass of that

portion of the electrode consumed. It can be expressed as:

efficiency % = consumed electrode theof metal of mass

deposited metal of mass x 100

With ordinary electrodes the efficiency varies from 75 to 95 % but with electrodes containing metallic

components in the covering the efficiency can approach 200 %(e.g. electrodes containing iron powder).The

electrodes are marked with a 6 digit numeral associated by a prefix and a suffix. The meaning of these and

the various values are shown in figure 2.2.7. The high cellulose type electrode coating gives rise to a

voluminous gas shield but also burns off quickly. Thus deep penetration can be achieved but the weld

finish is somewhat coarser. When the content of titania (as rutile, titanium-white or ilmenite) is high,

smooth arc is produced giving rise to voluminous slag and as such should be used for the flat position only.

But the weld finish is smooth. Iron oxide when present in the coating gives rise to higher metal deposition

rates. The slag becomes fairly fluid when appreciable amounts of calcium carbonate and fluoride are

present in the coating. These are used for high tensile structural steels.

The welding current, category D+ refers to direct current with electrode positive and A 90 refers to

the open circuit voltage not less than 90 V for a reference electrode size of 4 or 5 mm.



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When the electrode size varies, the voltage also changes accordingly. For example for 2.5 to 8 mm

electrodes, it may vary from 100 to 70 V in case of A 90.

Figure 2.2.7: Designation of manual metal arc welding electrode for mild steel

The size of the electrode to be chosen is based on the thickness of the plate to be welded and the weld

position.

Weld position Metal thickness, mm Electrode diameter,

mm

Root electrode diameter,

mm

All 3 to 6 3.2 -

6 4 -

9 6 4

12 8 4

16 8 5

Down hand

20 to 25 9 5

6 3.2 -

9 to 12 4 3.2

16 to 20 5 4 Vertical

25 5 5

6 to 9 4 3.2

12 to 20 6 4 Horizontal

25 6 5

Table-I: Electrode sizes based on the metal thickness to be welded

After selecting the electrode size, the choice is to be made of the parameters of the welding machine. A

typical current and voltage setting is given here for a specific electrode composition (E603413). For other

electrodes, manufacturers’ or standard welding handbook should be referred.

Electrode diameter, mm Current, A Voltage, V

3.2 100 to 150 18 to 22

4 140 to 200 20 to 24

5 200 to 220 21 to 25

6 275 to 350 23 to 27

8 375 to 475 24 to 28

Table-II: Welding machine parameters



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An arc energy is usually expressed in kilojoules per millimeter length of the weld (KJ/mm).

Arc energy (KJ/mm) =1000 x mm/s) ( speed welding

current x welding voltagearc

The greater the voltage drops across the arc the greater the energy liberated in heat for a given current. The

voltage drop can be varied by altering the type of gas shield liberated by the electrode covering, hydrogen

giving a higher voltage drop than carbon dioxide. As the length of arc increases so does the voltage drop,

but since there is an increased resistance in this long arc the current is decreased. Long arcs are difficult to

control and maintain and they lower the efficiency of the gas shield because of the greater length. As a

result, absorption of oxygen and nitrogen from the atmosphere can take place, resulting in poor mechanical

properties of the weld. It is essential that the welder should keep as short an arc as possible to ensure sound

welds.

Arc Blow When current flows through a conductor, it produces a magnetic flux that circles around the conductor in

perpendicular planes. The centres of the flux circles are located at the centre of the conductor. The

magnetic flux is produced in the steel and across the arc gap. The arc column is mainly influenced by the

lines of forces crossing the arc gap. As the weld joins the pieces together, there is less and less chance that

the magnetic field will concentrate in the arc gap. As the weld is filling the gap of the joint, it pushes the

magnetic flux ahead of the arc. As long as the flux can travel, no serious arc blow will interrupt the weld.

When flux ceases to move, it piles up and a magnetic field of considerable strength develops. The buildup

of the flux causes a deflection of arc column as it pills away from this heavy concentration of magnetic

forces. Ionized gases that carry the arc from the end of electrode to the workpiece are acting as flexible

conductors. This concentration of flux that pulls the arc from its intended path is called Arc Blow. Areas

where lines of force have a tendency to concentrate are at points of starting and stopping and in such places

as the inside corners of boxes or frames. This phenomenon is observed more frequently in DC welding as

compared to AC welding.

Figure 2.2.8: Arc blow in DC arc welding

The result of an arc blow is an excessive spatter, incomplete fusion and reduced welding speed. When large

slag is produced, the arc blow melts the slag causing still more excessive spatter. The problem of arc blow

gets multiplied when welding is done on strongly magnetic materials such as nickel alloys because of the

strong magnetic fields set up by these metals. Some of the methods that are used to reduce the severity of

arc blow are



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1. Change to AC welding, because of the continuous change in the polarity, the effect of magnetic

field is nullified.

2. Reduce the current used so that the strength of the magnetic field gets reduced.

3. Use a short arc length so that the filler metal would not be deflected but carried easily to the arc

crater.

4. Put steel blocks near the end of the plate in contact with the base metal so that the magnetic flux

line would flow through them and reduce the arc blow.

5. Place more than one ground lead from the base metal (preferably one each from the ends of the

base metal plate).

Figure 2.2.9: Appearance of magnetic field in DC arc welding as affected by grounding

Spatter At the conclusion of a weld small particles or globules of metal may sometimes be observed

scattered around the vicinity of the weld along its length. This is known as spatter and may occur through:

1. Arc blow making the arc uncontrollable.

2. The use of too long an arc or too high an arc voltage.

3. The use of excessive current.

The latter is the most frequent cause. Spatter may also be caused

by bubbles becoming entrapped in the molten metal globules of

metal, expanding with great violence and projecting the small

drops of metal outside the arc stream, or by the magnetic pinch

effect, by the magnetic fields set up, and thus the globules of

metal getting projected outside the stream. Spatter can be

reduced by controlling the arc correctly, by varying current and

voltage, and by preventing arc blow.

Figure 2.2.10: Spatter

Eccentricity of the core wire in an

electrode If the core wire of a flux coated

electrode is displaced excessively from the

centre of the flux coating because of errors in

manufacture, the arc may not function

satisfactorily. The arc tends to be directed

towards one side as if influenced by arc blow

and accurate placing of the deposited metal is

prevented. To establish whether the core wire

is displaced outside the manufacturer’s

tolerance is to clean off the flux covering on

one side at varying points down the length of

the electrode and measure the distance L. the difference Figure 2.2.11: Eccentricity of electrode

between the maximum and minimum reading is an approximate indication of the eccentricity.



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Edge Preparation The edge preparation in case of arc welding is very important particularly for thicker material. The type of

edge required for butt welds in manual arc welding process is given herewith for better understanding.

Figure 2.2.12: Butt joint preparation

Welding Techniques The normal welding can be started after selection of electrode, setting up parameters of welding

machine and preparation of edge of the workpieces. To start the arc, first the welder has to make a contact

between the electrode and the workpiece so that current flow is established. Then electrode should be

moved away from the workpiece by a very small amount so that the arc is established. To accomplish this

generally two different methods are employed. In the first method the electrode is moved in an arc so that it

will scratch the work metal and thus establish the current flow. The normal distance by which the electrode

is separated from the work metal is same as that of the diameter of the electrode wire. The scratching

method is an easier method and is generally preferred by the beginners.

Figure 2.2.13: Arc initiation techniques



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However the electrode should be brought back immediately to the point of start where the welding should

take place, otherwise the base metal will unnecessarily get defaced by the weld metal deposit. The other

method which is generally preferred by experienced welders is called the tapping start. In this the electrode

is held vertically above the point where the welding is to start and in a swift motion it is moved down to

contact the metal and then lifted as much as the arc gap which is same as the electrode wire diameter.

Unless the motion is swift, there are chances that the electrode itself would get welded to the base metal

plate.

After establishing the correct arc length, the welder should move the electrode along the length of

the joint maintaining the arc. The intense heat generated under the arc starts melting the metal, with the

metal at the centre of the arc being at the highest temperature. When the electrode is moved in the forward

direction, the bead is formed. The electrode should be moved downwards continuously to maintain the arc

length, and at the same time it should be moved sideways in a weaving motion to maintain the bead width.

After completing a sideward weaving motion, the electrode is moved forward to form a new puddle which

is separated from the previous puddle by a small distance of order of 1.5 mm. this would be continued till

the joint is completely filled.

Figure 2.2.14: Welding motion Figure 2.2.15: Welding passes

At the end of the welding, if arc is abruptly extinguished,

the arc crater would not be completely filled and a

depression would be left in the joint. Therefore the arc

has to be slowly extinguished by the gradual decrease of

the welding current which ensures a complete filling of

the arc crater. In multipass welding the brittle slag coating

present on the bead after the root pass is made, is chipped

off and then the area is cleaned with wire brush before the

second pass is commenced. Welding in flat (down hand)

position is relatively easy, but in horizontal position, the

welding becomes somewhat difficult because the molten

metal cannot be held in position against gravity.

Generally short arc is maintained so that filler metal will

be properly deposited in the groove. The electrode is

maintained at about 20 degrees from the horizontal and

inclined about 20 degree from the vertical plane in the

direction of its travel. This slant helps in pushing the

molten metal into the shape and reducing the sagging of the metal. Figure 2.2.16: weaving motion

The major problems in the horizontal welding are the undercutting and sagging of the weld bead.

This is caused mainly by gravity acting on the molten metal during solidification. These can be corrected

by short arc length and angular positioning of electrode. There are two positions in the vertical, vertically

upward and vertically downward directions. Out of these vertically upward direction is most preferred

because of the strong weld obtained. In the downward welding the slag is likely to move down due to the

gravity and mix with the weld metal. Therefore only vertical upward welding is preferred. To fight the

effect of gravity the electrode is held at an angle of about 20 degrees from the vertical plane with a short arc

length so that the filler metal will be easily moved into the groove. Further, the electrode is moved in and

out of the puddle so that a proper bead is formed. In overhead welding to reduce the chance of weld metal

falling, a smaller arc length is maintained with a smaller size electrode. The flat position is the best since



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Figure 2.2.17: Undercut, overlap and sagging of weld bead

higher metal deposition rate as well as good bead quality can be achieved. Because of this in many

situations, special fixtures are used to rotate the jobs in such a way that the weld joint comes into the flat

position. It has been observed that adding iron powder to the coating of the electrodes causes an increase in

the metal deposition rates.

Figure 2.2.18: Horizontal welding Figure 2.2.19: Vertical welding

Figure 2.2.20: Overhead welding



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Heat Input In case of arc welding, the heat input P in watts, is by direct conversion of the electrical energy. If velocity

of weld is v, then arc energy is given as H = {P / v} J/mm, where v is in mm / s.

This is the actual heat generated at the tip of the electrode and ideally should be available for melting the

joint. But the actual heat utilized by the joint depends upon how this heat is transferred from tip of the

electrode to the joint. Hence a factor of heat transfer efficiency, f1, is defined to take into account any

losses.

H = { f1 V I / v } J / mm

When heat source is concentrated such as an electric arc, the value of f1 would be greater than 0.8 and

would almost approach 1.0. The following are some expected values for the various processes:

GTAW 0.21 to 0.48

SMAW and GMAW 0.66 to 0.85

SAW 0.90 to 0.99

Though the net heat as calculated above reaches the weld joint, all of it cannot be utilized for melting since

part of it would be conducted away from the joint by the base metal as reflected in the heat affected zone.

The actual heat distributed into the surrounding metal would depend upon the welding process as well as

the process parameters including the joint design. The net heat that is actually utilized for melting can be

obtained by assuming another efficiency factor, f2 as melting efficiency as follows:

f2 = suppliedheat Net

joint melt the torequiredHeat

Advantages of Electric Arc Welding 1. Low cost welding equipment.

2. Faster welding operation compared to gas welding process.

3. Low cost of welding operation.

4. Relatively simple and versatile technique.

5. It requires relatively small variety of electrodes.

6. The covering on the electrode can provide shielding gases, alloying elements and other required

materials at comparatively low cost.

7. Welding equipment is portable and can be easily maintained.

Disadvantages 1. Heat input and filler material deposition can not be separated.

2. A lot of electrode material is wasted in the form of unused end, slag and gas.

3. There are more chances of slag inclusions in the bead.

4. If proper drying of electrode is not done, then moisture may lower the quality of weld metal.

5. Arc blow and metal spatter are common problems of this process.

6. Thin workpieces (less than 3 mm) are difficult to weld.

Application This process is commonly used in general construction, in ship building, on pipelines, and for maintenance

work, because the equipment is portable and can be easily maintained. It is useful for work in remote areas,

where a portable fuel powered generator can be used as the power supply. The process is best suited for

workpiece thickness of 3 to 19 mm, although this range can be easily extended by skilled operators using

multiple pass techniques.

Reference • Welding and Welding Technology by R.L.Little, MacGraw-Hill, NewYork, page 101-152

• The Science and Practice of Welding by A.C.Davis, Cambridge University Press, page 1-7

• Manufacturing Engineering and Technology by Kalpakjian and Schmid, Pearson Education, page

780-781

• Manufacturing Technology by P.N.Rao, TMH, page377-391



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Review Questions 1. Explain the mechanism of heat generation in electric arc welding.

2. Differentiate between arc and spark.

3. Explain the characteristics of constant voltage and constant current type welding equipment.

4. Suggest the situations in which DC welding has edge over AC welding.

5. For welding of aluminium which mode of current AC or DC will give desired result?

6. What do you mean by DCEN and DCEP?

7. For wider gap joint and for deep penetration which mode of welding you will prefer?

8. Why arc welding is not suitable for thinner sheet welding?

9. Write the function of coating on an electrode.

10. Write the welding electrode specification in Indian standard.

11. How electric welding equipment is specified?

12. What do you mean by duty cycle in case of welding?

13. How you will measure the eccentricity of coating in welding electrode?

14. Why iron powder is provided in coating of electrode?

15. What do you mean by hydrogen controlled electrode?

16. For overhead welding smaller size electrode is selected. Explain.

17. What do you mean by arc blow in arc welding? And how to avoid it.

18. Write various weaving motion of electrode in arc welding.

19. Explain the method of obtaining a weld in horizontal position by SMAW.

20. How arc welding electrode is manufactured? ( beyond syllabus)

21. Explain the procedure for overhead welding in SMAW.

22. How metal gets transferred fro tip of the electrode to the weld zone in arc welding?

23. How arc is initiated in arc welding? Explain.

24. What is electrode efficiency? How you can improve it?

25. Explain the process capability of SMAW.

26. How multi-pass welding is done in SMAW?

27. Explain open circuit voltage for arc welding equipment.

28. Why electrode is dried prior to its use?

29. Why slag removal is essential in arc welding?

30. Explain the HAZ in arc welding.

31. When you will opt for 100 % duty welding equipment?

32. Write various non-consumable electrodes used in arc welding.

33. Write the advantages of non-consumable electrodes over consumable electrodes.

34. Describe various joint preparations in case of arc welding.

35. calculate the melting efficiency in the case of arc welding of steel with a potential of 20 V and

current of 250 A. the travel sped is 6 mm/s and the cross sectional area of the joint is 20 mm2.

Heat required to melt steel may be taken as 10 J / mm3 and the heat transfer efficiency is 0.85.

36. Why voltage requirement for arc maintenance is less compared to staring arc voltage?

37. Explain overlap and undercut in arc welding.

38. Why titania is added to the covering of the electrode?

39. What is spatter in arc welding?

40. What factors govern the voltage drop across the arc?

41. Explain various self removing slags formed in arc welding.

42. Explain the various forces the filler material encounter while transferred from electrode to the

weld zone.

43. Explain primary and secondary electrons in arc welding.

44. Explain five different zones in an electric arc.

45. Why maximum size of electrode is considered in flat welding position?

46. What is the function of flux in the coating of electrode?

47. Why upward welding is preferred in case of vertical welding?

48. Explain how the heat produced at the tip of the electrode is transferred to the base metal?

49. What are the functions of calcium carbonate and fluoride in the coating of electrode?

50. What is a stick electrode?

electricarcwelding

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