au pp unit 5 final

8/6/2019 AU PP Unit 5 Final

1/71

Unit-V

Welding

Gas Welding

Gas Welding Processes - Gas welding is a fusion welding process. It joins metals, using

the heat of combustion of an oxygen/air and fuel gas (i.e acetylene, hydrogen, propane or

butane) mixture. The intense heat (flame) thus produced melts and fuses together theedges of the parts to be welded, generally with the addition of a filler metal


2/71

Oxy Acetylene Welding Principle of Operation - When acetylene is mixed with

oxygen in correct proportions in the welding torch and ignited, the flame resulting at the

tip of the torch is sufficiently hot to melt and join the parent metal.The oxyacetylene flame reaches a temperature of about 3200C and thus can melt all

commercial metals which, during welding, actually flow together to form a complete

bond.A filler metal rod is generally added to the molten metal pool to build up the seam

slightly for greater strength. Oxyacetylene welding does not require the components to be

forced together under pressure until the weld forms and solidifies.

Gas Welding Equipment

The basic equipments used to carry out gas welding are:

1. Oxygen gas cylinder.

2. Acetylene gas cylinder.3. Oxygen pressure regulator.

4. Acetylene pressure regulator.

5. Oxygen gas hose(Blue).

6. Acetylene gas hose(Red).7. Welding torch or blow pipe with a set of nozzles and gas lighter

8. Trolleys for the transportation of oxygen and acetylene cylinders9. A set of keys and spanners.

10. Filler rods and fluxes.

11. Protective clothing for the welder (e.g., asbestos apron, gloves, goggles, etc


3/71

Oxygen Gas Cylinder - Oxygen cylinders are painted black and the valve outlets are

screwed right handed. The usual sizes of oxygen cylinders are 3400, 5200 and 6800 litre.

Oxygen cylinder is a solid drawn cylinder out of mild steel or alloy steel. Mild steelcylinder is charged to a pressure of 13660 KN/m 2 (136.6 bar) and alloy steel cylinders to

17240 KN/m2 (172 bar).

The oxygen volume in a cylinder is directly proportional to its pressure. In other words, ifthe original pressure of a full oxygen cylinder drops by 5% during welding, it means 1/20

of the cylinder contents have been consumed.

Because of the possibility of the oxygen pressure becoming high enough to rupture thesteel cylinder in case the temperature rises, an oxygen cylinder is equipped with a safety

nut that allows the oxygen to drain slowly in the event the temperature increases the gas

pressure beyond the safety load of the cylinder.

An oxygen cylinder has an inside diameter of 21.6 cm, wall thickness 6.50 mm andlength 127.5 cm. In order to protect cylinder valve from getting damaged, a removable

steel cap is screwed on the cylinder at all times when the cylinder is not in use. The

cylinder valve is kept closed when the cylinder is not in use and even when cylinder is

empty.Acetylene Gas Cylinder - An acetylene cylinder is painted maroon and the valves are

screwed left handed; to make this easily recognizable they are chamfered or grooved. Anacetylene cylinder is also a solid drawn steel cylinder which is charged to a pressure of

1552 KN/m2 (15.5 bar).

The usual size of acetylene cylinders are 2800 and 5600 litre. An acetylene cylinder has

an inside diameter of 30 cm, wall thickness 4.38 mm and a length of 101.25 cm. Anacetylene cylinder is filled with a spongy (porous) material such as balsa wood or some

other absorptive material which is saturated with a chemical solvent called acetone.

Since high pressure acetylene is not stable, it is dissolved in acetone, which has theability to absorb a large volume of the gas and release it as the pressure falls. The small

compartments in the porous material (filled in the cylinder) prevent the sudden

decomposition of the acetylene throughout the mass, should it be started by local heatingor other causes.

An acetylene cylinder is always kept upright for safety reasons. The acetone in the

cylinder must not be permitted to enter the blowpipe, otherwise an explosion could result.The acetylene cylinder valve can only be opened with a special wrench and this wrench is

kept in place whenever the cylinder is in use.

An acetylene cylinder has a number of fusible plugs, at its bottom, designed to melt at

104C. These plugs melt and release the pressure in case the cylinder is exposed toexcessive heat.

Acetylene Gas Generator - If large quantities of acetylene gas are being consumed, it is

much cheaper to generate the gas at the place of use with the help of acetylene gasgenerators. Acetylene gas is generated by carbide to water method, i.e., the generator unit

feeds controlled amounts of calcium carbide into the water. When these ingredients are

mixed, acetylene gas is produced.


4/71

In order to make the operation of acetylene generators safe, various devices are

incorporated in it. There are two types of acetylene generators.

(i) Low pressure generator whichdelivers the gas at pressures of less

than 0.1 bar. With this kind of

generator only the injector type ofblow pipes can be used. Low pressure

generator is considered portable and it

produces acetylene above 15litres perminute.

(ii) Medium pressure generator which

delivers the gas at a pressure of up to

0.6 bar.Medium pressure generator is

considered stationary and it can

produce acetylene up to 3000 litres per

minute. This generator is the one that is more commonly used. In control valve opensand closes automatically as the acetylene in the chamber decreases or increases. This

automatically regulates the amount of calcium carbide falling in water.Acetylene generators have certain disadvantages:

1. Greater safety precautions are required.

2. Labour is required to charge carbide and clean out sludge.

3. Gas obtained is not so pure as available in cylinders.4.There is a tendency towards pressure fluctuations with resultant

unsteady flame, if the low pressure type of generator is used.

Pressure Regulators - The pressure of the gases obtained from cylinders/generators is

considerably higher than the gas pressure used to operate the welding torch.The purpose

of using a gas pressure regulator is, therefore(i) to reduce the high pressure of the gas in the

cylinder to a suitable working pressure, and (ii)

to produce a steady flow of gas under varyingcylinder pressures. A pressure regulator is

fitted with two pressure gauges. One indicates

the gas pressure in the cylinder and the other

shows the reduced pressure at which the gas isgoing out. A pressure regulator is connected

between the cylinder/generator and the hose

leading to welding torch.(ii)Gas pressure regulators may be classified as: 1.Single stage Regulator

2. Two stage Regulator.

Welding Hoses and Clamps-

(a) Hoses:The hose for the supply of oxygen (from the pressure regulator) to the welding

torch is coloured blue or black and has right handed thread connections, whereas the

acetylene hose is coloured red or maroon and has left handed thread connections with


5/71

chamfers or grooves on the nuts. For welding purposes, the hoses to be used should be

strong, non porous, flexible and not subject to kinking. Welding hose has a seamless

lining which is manufactured from rubber (or a rubber compound) which is reinforcedwith canvas or wrapped cotton plies.

The hose is resistant to the action of gases normally used in welding. The outer casing is

made of tough abrasion resistant rubber. The hose is very robust and capable ofwithstanding high pressure. Some precautions are to be taken when using reinforced

rubber hoses:

(i) Only one gas should be used in a hose. For example, using an oxygen hose to carryacetylene could cause a serious accident.

(ii) The hose should never be patched or repaired.

(iii) Hot metal (job) should never be placed on the hose.

(b) Hose Clamps (Clips): A metal clamp is used to attach the welding hose to a nipple.The clamp squeezes the hose around the nipple to prevent it from working loose. A nut

on the other end of the nipple is connected to the regulator or torch.

Welding Torch and Blow Pipe - Oxygen and the fuel gas having been reduced inpressure by the gas regulators are fed through suitable hoses to a welding torch whichmixes and controls the flow of gases to the welding nozzle or tip where the gas mixture is

burnt to produce a flame for carrying

out gas welding operation. There aretwo types of welding torches, namely:

(i) High pressure (or equal pressure)

type. (ii) Low pressure (or injector)type. High pressure blowpipes or

torches are used with (dissolved)

acetylene stored in cylinders at a pressure of 8 bars. Low pressure blowpipes are used

with acetylene obtained from an acetylene generator at a pressure of 200 mm head ofwater (approximately 0.02 bars).

(a) Working of a low pressure blowpipe: It is termed as a low pressure blowpipe because

it can be operated at low acetylene pressures; it is frequently used with acetylenegenerators. As acetylene is of low pressure, it is necessary to use oxygen at a high

pressure (2.5 bar). The oxygen enters the mixing chamber through a passage located in

the centre of the torch. The oxygen passage is surrounded by the one carrying the

acetylene. The high pressure oxygen passes through a small opening in the injectornozzle, enters the mixing chamber and pulls (or draws) the acetylene in after it. An

advantage of low pressure torch is that small fluctuations in the oxygen supplied to it will

produce a corresponding change in the amount of acetylene drawn; thereby making theproportions of the two gases constant while the torch is in operation.

(b) Working of a high pressure blowpipe: In this type of blowpipe both the oxygen and

acetylene are fed to the blow pipe at equal pressures and the gases are mixed in a mixing

chamber prior to being fed to the nozzle tip. The equal pressure or high pressure type ofblowpipe is the one most generally used because (i) It is lighter and simpler. (ii) It does

not need an injector. (iii) In operation, it is less troublesome since it does not suffer from


6/71

backfires to the same extent. To change the power of the welding torch, it is only

necessary to change the nozzle tip (size) and increase or decrease the gas pressures

appropriately

Welding Nozzles or Tips: Depending upon the design of the welding torch (or the blow

pipe) the interchangeable nozzles may consist of :(i) Either, a set of tips which screw onto

the head of the blowpipe, or(ii) As a set of gooseneck extensions fitting directly onto themixer portion of the blowpipe. The welding nozzle or tip is that portion of the torch

which is located at the end of the torch and

contains the opening through which theoxygen and acetylene gas mixture passes

prior to ignition and combustion. A welding

nozzle enables the welder to guide the flameand direct it with the maximum ease and

efficiency. The following factors areimportant in the selection of appropriate welding nozzle:

(i) The position of the weld.(ii) The type of joint.(iii) Job thickness and the size ofwelding flame required for the job.(iv) The metal/alloy to be welded. To provide for

different amounts of heat, to weld metals of different thicknesses, welding tips are made

in various sizes. The size of a welding tip is determined by the diameter of the opening ororifice in the tip. As the orifice size

increases, greater amounts of the

welding gases pass through and are burnt

to supply a greater amount of heat. The

choice of the proper tip size is veryimportant to good welding. A chart

giving sizes of tips for welding various thicknesses of metal along with oxygen andacetylene pressures used is generally provided by the manufacturers.


7/71


8/71

Gas Lighter A gas (spark) lighter provides a convenient, safe and inexpensive means of

lighting the torch. Match sticks should never be used for this purpose because the puff ofthe flame produced by the ignition of the acetylene flowing from the tip is likely to burn

the welder's hand. Spark lighters are constructed from flint and steel.

Gas Cylinder Trolleys - Trolleys should be capable of accommodating one oxygen

cylinder and one acetylene cylinder required for gas welding. Normally cylinders can be

mounted on a trolley side by side, but where work has to be done on plant with accessonly by narrow gangways the, has an advantage. Trolleys may have rubber tires or steel

rim wheels. The gas cylinders are held in place with chains and supported on the bottomwith a steel platform.

Types of Flames

1. Neutral Flame (Acetylene oxygen in equal proportions)

2. Oxidising Flame (Excess of oxygen)

3. Reducing Flame (Excess of acetylene)In oxyacetylene welding, flame is the most important tool. All the welding equipment

simply serves to maintain and control the flame. The correct type of flame is essential for

the production of satisfactory welds. The flame must be of the proper size, shape andcondition in order to operate with maximum efficiency.

Neutral Flame - A neutral flame is produced when approximately equal volumes of

oxygen and acetylene are mixed in the welding torch and burnt at the torch tip. (Moreaccurately the oxygen-to-acetylene ratio is 1.1 to 1). The temperature of the neutral flame

is of the order of about 3260C. The flame has a nicely defined inner cone which is light

blue in colour. It is surrounded by an outer flame envelope, produced by the combinationof oxygen in the air and superheated carbon monoxide and hydrogen gases from the inner

cone. This envelope is usually a much darker blue than the inner cone.


9/71

A neutral flame is named so because it

effects no chemical change in the molten

metal and therefore will not oxidize orcarburize the metal. The neutral flame is

commonly used for the welding of: (i) Mild

steel (ii) Stainless steel (iii) Cast Iron (iv)Copper (v) Aluminium

Oxidising Flame - If, after the neutral flame

has been established, the supply of oxygen is further increased, the result will be anoxidising flame. An oxidising flame can be recognized by the small white cone which is

shorter, much bluer in colour and more pointed than that of the neutral flame. The outer

flame envelope is much shorter and tends to fan out at the end on the other hand the

neutral and carburizing envelopes tend to come to a sharp point. An oxidising flameburns with a decided loud roar. An oxidising flame tends to be hotter than the neutral

flame. This is because of excess oxygen and

which causes the temperature to rise as high as

3500C. The high temperature of an oxidizingflame (O2: C2H2 = 1.5: 1) would be an

advantage if it were not for the fact that theexcess oxygen, especially at high

temperatures, tends to combine with many

metals to form hard, brittle, low strength

oxides. Moreover, an excess of oxygen causesthe weld bead and the surrounding area to have a scummy or dirty appearance. For these

reasons, an oxidising flame is of limited use in welding. It is not used in the welding of

steel. A slightly oxidising flame is helpful when welding most (i) Copper base metals (ii)Zinc base metals, and (iii) A few types of ferrous metals, such as manganese steel and

cast iron

The oxidizing atmosphere, in these cases, creates a base metal oxide that protects the basemetal. For example, in welding brass, the zinc has a tendency to separate and fume away.

The formation of a covering copper oxide prevents the zinc from dissipating.

Reducing Flame - If the volume of oxygen supplied to the neutral flame is reduced, theresulting flame will be a carburising or

reducing flame, i.e. rich in acetylene. A

reducing flame can be recognized by

acetylene feather which exists between theinner cone and the outer envelope. The outer

flame envelope is longer than that of the

neutral flame and is usually much brighter incolour. A reducing flame does not

completely, consume the available carbon;

therefore, its burning temperature is lower and the left over carbon is forced into themolten metal.

With iron and steel it produces very hard, brittle substance known as iron carbide. This

chemical change makes the metal unfit for many applications in which the weld may


10/71

need to be bent or stretched. Metals that tend to absorb carbon should not be welded with

reducing flame. A reducing flame has an approximate temperature of 3038C. A reducing

flame may be distinguished from a carburizing flame by the fact that a carburizing flamecontains more acetylene than a reducing flame. A carburizing flame is used in the

welding of lead and for carburizing (surface hardening) purposes.

A reducing flame, on the other hand, does not carburize the metal, rather it ensures the

absence of the oxidizing condition. It is used for welding with low alloy steel rods and for

welding those metals, (e.g. non ferrous) that do not tend to absorb carbon. This flame isvery well used for welding high carbon steel

To conclude, for most welding operations the Neutral Flame is correct, but the other

types of flames are sometimes needed for special welds, e.g., non-ferrous alloys and highcarbon steels may require a reducing flame, whilst zinc bearing alloys may need an

oxidising flame for welding purposes.

Welding Technique: To light the flame, the acetylene valve on the torch is opened

slightly and lighted with the help of a friction spark lighter. The flame draws the oxygenfrom the atmosphere and thus results in a reducing flame. Then the acetylene valve is

opened to get the required flow of acetylene. The oxygen valve is then slowly opened till

The neutral flame, which results fromburning a mixture

containing approximatelyequial volumes of oxygen and

acetylene. Thewell-defined core of the flame (extremelybrightpale blue) is known as the inner cone.

The excess acetylene flame, which has awhitish featheraround and beyond the inner cone.

The oxidizing flame, which results from anexcess of

oxygen in the gas mixture, has a shorter,more sharply-pointed inner cone than the neutralflame.


11/71

the intermediate flame feather of the reducing flame recedes into the inner white cone.

The actual adjustment of the flame depends on the type of material to be joined.

The choice of the torch size depends on the thickness of the metal to be joined. Larger

torch tip sizes cause higher amount of oxygen and fuel to flow out causing the release of

more heat. All joints except outside corner joint require a filler metal to be added to fillthe joint. This is done with the help of a welding rod whose composition depends on the

parent metal of the joint. The torch tip should be positioned above the metal plate so that

the white cone is at a distance of 1.5 to 3.0 mm from the plate. The torch should be held

at an angle of 30 to 450 from the horizontal plane. The torch movement along the joint

should be either oscillating or circular. In forehand welding, the torch is moved in thedirection of the tip. This tends to preheat before the white cone of the tip melts it. In

backhand welding the torch moves backwards. The outer blue flames are directed on thealready welded joint. This allows the joint to be continuously annealed relieving the

welding stresses. This welding allows a better penetration as well as form bigger weld.

Backhand welding is generally used for thicker materials. When the welding rod is usedto provide filler material, it is necessary to hold it at a distance of 10 mm from the flame

and 1.5 to 3.0 mm from the surface of the weld metal pool or puddle. This way the rod

gets preheated and when dipped into the puddle would readily get melted.

Oxy-fuel welding can be used for all the types of joints in all positions. Overhead usage

requires additional skill to safeguard the welder. The various butt joint edge preparationsare shown in the adjacent figure. Thicker plates require more than one pass of the gas

torch along the length to complete the joint. In multi pass welding, the first pass (root

pass) is very critical in any welding operation.


12/71

Gas Cutting: It is possible to rapidly oxidise (burn) iron and steel when it is heated to a

temperature between 800 to 1000 0C. When a high pressure oxygen jet with a pressure ofthe order of 300 KPa is directed against a heated steel plate, the oxygen jet burns the

metal and blows it away causing the cut. This process is used for cutting steel plates of

various thicknesses (can go up to 2 m) mainly because the equipment required is simple

and can be carried anywhere without handling the heavy steel plates. Oxy-acetylene gascutting outfit is similar to that of the oxy-acetylene welding except for the torch tip. Here

the torch tip has a provision for preheating the plate as well as providing the oxygen jet.

Thus the tip has a central hole for oxygen jet with surrounding holes for preheatingflames. The cutting tip should be chosen for the intended application. The size is

normally dependent on the thickness of the plate which determines the amount of

preheating as well as the oxygen jet flow required for cutting. After the steel is heated tothe kindling temperature which is about 870 0C, it gets readily combined with oxygen

giving iron oxide with the following reactions:3 Fe + 2 O2-- Fe3O4 + 6.67 MJ/Kg of iron2Fe + O2-- 2FeO + 3.18 MJ/Kg of iron4 Fe + 3 O2-- 2Fe2O3 + 4.90 MJ/Kg of ironAll the above reactions are exothermic in nature and as such would provide a good

amount of heat to preheat the steel. But this energy may not be sufficient to bring the

steel to its kindling temperature, and hence preheating flames may have to be continued

as somewhat lower rate. The heat generated causes the metal to melt and get blown awayby the oxygen pressure. About 30 to 40 % of metal is simply blown away, while the rest

is oxidised.The cutting can start at the edge or in the middle of the plate. After the plate has reachedthe kindling temperature, the operator should release the oxygen jet to start the cutting,

moving the torch in the forehand direction to achieve the desired cut. Drag is the amount

by which the lower edge of the drag line trails from the top edge.


13/71

A good cut is characterised by very small or negligible drag. When the torch is moved

too rapidly, the metal at the bottom does not get sufficient heat to get oxidized and cut

and hence there is a large drag. When the torch is moved slowly, all the preheated metalis burnt away by the oxygen jet and a large amount of slag is generated.

Though the gas cutting is more useful with thick plates, thin sheets (less than 3 mm) can

also be cut by this process taking special precautions. Tip size chosen should be as small

as possible. If small tips are not available, then the tip is inclined at an angle of 15 to 20degrees. Gas cutting can be done manually or by a machine. The manual cutting is used

for general purpose work and for straight line cutting. In machine cutting the torch is

mounted on a rail and both rail and the torch can move simultaneously along the two

mutually perpendicular axes in the horizontal plane with the help of servo motors. Thereis provision in the machine to hold more than one torch so that large number of identicalpieces can be cut at the same time.


14/71

Oxygen cutting would be useful only for those materials which readily get oxidised and

the oxides have lower melting points than the metals. So it is most widely used forferrous materials. But it cannot be used for materials like aluminium, bronze, stainless

steel which resist oxidation. Cutting of high carbon steels and cast irons require special

attention due to formation of heat affected zone (HAZ) where structural transformationoccurs.

Advantages of gas welding:

It is one of the versatile methods of welding. The same equipment with a range oftorches would be used for welding, cutting, brazing and braze welding.

Rate of heat generation is less so thin sheets can be welded.

As the source of heat and filler metal are separated, the metal deposition can be easily

controlled and heat properly adjusted giving rise to a satisfactory weld. Welding equipment is portable and can be operated at remote places.

The cost of equipment is not so high.

Heat affected zone (HAZ) is very narrow.

Limitations of gas welding:

Heavy sections cannot be joined efficiently.

For heavy sections proper penetration may not be achieved. Slower speed of welding compared electric arc welding.

Flux used in the filler metal provides fumes which are irritating to the eyes, nose, throatand lungs.

More safety is recommended in gas welding.

Acetylene and oxygen are expensive gases. Prolonged heating of the joint may results in large HAZ.

Applications:

For joining of thin materials. For joining materials in whose case excessively high temperature or rapid heating and

cooling of the job would produce unwanted changes in the metal.

For welding both ferrous and non-ferrous metals. In automotive &aircraft industries, project site works, workshops etc.

Arc Welding

Arc welding is the fusion of two pieces of metal by an electric arc between the piecesbeing joined the work pieces and an electrode that is guided along the joint between

the pieces. The electrode is either a rod that simply carries current between the tip and the

work, or a rod or wire that melts and supplies filler metal to the joint.


15/71

The basic arc welding circuit is an alternating current (AC) or direct current (DC) power

source connected by a work cable to the work piece and by a hot cable to an

electrode. When the electrode is positioned close to the work piece, an arc is createdacross the gap between the metal and the hot cable electrode. An ionized column of gas

develops to complete the circuit.

The arc produces a temperature of about 3600C at the tip and melts part of the metalbeing welded and part of the electrode. This produces a pool of molten metal that cools

and solidifies behind the electrode as it is moved along the joint. There are two types of

electrodes. Consumable electrode tips melt, and molten metal droplets detach and mix

into the weld pool. Non-consumable electrodes do not melt. Instead, filler metal is meltedinto the joint from a separate rod or wire. The strength of the weld is reduced when

metals at high temperatures react with oxygen and nitrogen in the air to form oxides and

nitrides. Most arc welding processes minimize contact between the molten metal and theair with a shield of gas, vapour or slag. Granular flux, for example, adds deoxidizers that

create a shield to protect the molten pool, thus improving the weld.

The Five Most Common Arc Welding Processes


16/71

Welding Power Sources:

The main requirement of a power source is to deliver controllable current at a voltage

according to the demands of the welding process being used. Each welding process hasdistinct differences from one another, both in the form of process controls required to

accomplish a given operating condition and the consequent demands on the power

source. Therefore, arc welding power sources are playing very important role in welding.The conventional welding power sources are:

Power Source Supply

Types of Power Source sand characteristicsTwo types of electrical devices can be used to produce low-voltage, high-amperage

current combination that arc welding requires. One type uses electric motors or internalcombustion engines to drive alternators or generators. The other types use step-down

transformers. Because transformer-type welding transformers are quieter, are more

energy efficient, require less maintenance and are less expansive, they are now the

industry standards. However, engine-powered generators are still widely used forportable welding.

Welding transformers, rectifiers and DC generators are being used in shop while enginecoupled AC generators as well as sometimes DC generators are used at site where line

supply is not available. Normally rectifiers and transformers are preferred because of low

noise, higher efficiency and lower maintenance as compared to generators. Selection ofpower source is mainly dependent on welding process and consumable. The open circuit

voltage normally ranges between 70-90 V in case of welding transformers while in case

of rectifiers it is 50-80 V. However, welding voltages are lower as compared to opencircuit voltage of the power source.

Based on the static characteristics power sources can be classified in two categories

Constant current or drooping or falling characteristic power source.

Constant potential or constant voltage or flat characteristic power source.Constant voltage power source does not have true constant voltage output. It has a

slightly downward or negative slope because of sufficient internal electrical resistance

and inductance in the welding circuit to cause a minor droop in the output volt amperecharacteristics.

With constant voltage power supply the arc voltage is established by setting the output

voltage on the source. The power source shall supply necessary current to melt theelectrode at the rate required to maintain the preset voltage or relative arc length. The

speed of electrode drive is used to control the average welding current. The use of such

power source in conjunction with a constant electrode wire feed results in a self

regulating or self adjusting arc length system. Due to some internal or external fluctuation

Power Source Supply

(i) Welding

TransformerAC

(ii) Welding

RectifierDC

(iii) WeldingGenerators

AC or DC (Dependingon generator)


17/71

if the change in welding current occurs, it will automatically increase or decrease the

electrode melting rate to regain the desired arc length.

Fig 4.1: Constant Potential or Constant Voltage or Flat Characteristic.

Fig 4.2: Drooping or Constant current or Falling Characteristic.

The volt ampere output curves for constant current power source are called drooper' because of substantial downward or negative slope of the curves. The power source mayhave open circuit voltage adjustment in addition to output current control. A change in

either control will change the slope of the volt ampere curve. With a change in arcvoltage, the change in current is small and, therefore, with a consumable electrodewelding process, electrode melting rate would remain fairly constant with a change in arc

length. These power sources are required for processes using relatively thicker

consumable electrodes which may sometimes get stubbed to workpiece or with

nonconsumable tungsten electrode where during touching of electrode for starting of arcmay lead to damage of electrode if current is unlimited. Under these conditions the short

circuiting current shall be limited leading to safety of power source and the electrode.


18/71

Some power sources need high frequency unit to start the arc, which may be requirement

of processes like TIG and plasma arc. High frequency unit is introduced in the welding

circuit but in between the control circuit and HF unit, filters are required so that highfrequency may not flow through control circuit and damage it. High frequency unit is a

device which supplies high voltage of the order of few KV along with high frequency of

few KHz with low current. This high voltage ionizes the medium between electrode andworkpiece/nozzle starting pilot arc which ultimately leads to the start of main arc.

Although high voltage may be fatal for the operator but when it is associated with high

frequencies then current does not enter body but it causes only skin effect i.e. currentpasses through the skin of operator causing no damage to the operator.

Duty Cycle:

Duty cycle is the ratio of arcing time to the weld cycle time multiplied by 100. Welding

cycle time is either 5 minutes as per European standards or 10 minutes as per Americanstandard and accordingly power sources are designed. It arcing time is continuously 5

minutes then as per European standard it is 100% duty cycle and 50% as per American

standard. At 100% duty cycle minimum current is to be drawn i.e. with the reduction of

duty cycle current drawn can be of higher level. The welding current which can be drawnat a duty cycle can be evaluated from the following equation;

Duty cycle and associated currents are important as it ensures that power source remainssafe and its windings are not getting damaged due to increase in temperature beyond

specified limit. The maximum current which can be drawn from a power source depends

upon its size of winding wire, type of insulation and cooling system of the power source.

Table 4.1: Welding Processes, Type of Current and Static Characteristic

Welding ProcessType of

Current

Static Characteristic of

The Power Source

Manual Metal Arc Welding Constant Current

Tungsten Inert Gas Welding Constant Current

Plasma Arc Welding Constant Current

Submerged Arc Welding

Constant Current (if

electrode

= 2.4 mm )Constant Potential (if

electrode = 2.4 mm )

Gas Metal Arc Welding / Metal

Inert Gas Welding / Metal ActiveGas Welding

Constant Potential


19/71

Static Type Power Sources - Static type power sources are all of those that use

commercially generated electrical power to energize a transformer that, in turn, steps the

line voltage down to useable welding voltages.The two major categories of static power sources are the transformer type and the

rectifier type. The transformer type produce only alternating current. They are commonly

called "Welding Transformers." All AC types utilize single-phase primary power and areof the constant current type. The rectifier types are commonly called "Welding

Rectifiers" and produce DC or, AC and DC welding current. They may utilize either

single phase or three phase inputpower. They contain a transformer, but rectify the ACor DC by the use of selenium rectifiers, silicon diodes or silicon controlled rectifiers.

Available in either the constantcurrent or the constant voltage type, some manufacturers

offer units that are a combinationof both and can be used for coated electrode welding,

non-consumable electrode weldingand for welding with solid or flux cored wires.

Rotating Type power Sources - Rotating type power sources may be divided into two

classifications:

1.Motor-Generators2.Engine-Driven.

Motor-generator types consist of an electric motor coupled to a generator oralternatorthat produces the desired welding power. These machines produced excellentwelds, but

due to the moving parts, required considerablemaintenance. Few, if any, arebeing built today.

Engine driven types consist of a gasoline or

diesel engine coupled to a generator or

alternator that produces the desired welding power. They are used extensively on jobs

beyond commercial power lines and also as

mobile repair units. Both rotating types candeliver either AC or DC welding power, or a

combination of both. Both types are available

as constant current or constant voltage models.

AC Transformers or AC welding machine: A

welding transformer uses the alternating current

(AC) supplied to the welding shop at a highvoltage to produce the low-voltage power. As electrons flow through a wire they produce

a magnetic field around the wire. If the wire is wound into a coil the weak magnetic field

of each wire is concentrated to produce a much stronger central magnetic force. Because

the current being used is alternating or reverse each 1/60 of a second, the magnetic fieldis constantly being built and allowed to collapse. By placing a second or secondary

winding of wire in the magnetic field produced by the first or primary winding a current

will be induced in the secondary winding. The placing an iron core in the center of thesecoils will increase the concentration of the magnetic field as shown in Fig.3-14.

A transformer with more turns of wire in the primary winding than in the secondary

winding is known as step-down transformer. A step-down transformer takes a high-voltage, low-amperage current changes it into a low-voltage, high-amperage current.

Except for some power lost by heat within a transformer, the power (Watts) into a

transformer equals the power (Watts) out because the volts and amperes are mutually

increase and decreased.


20/71

A transformer welder is step-down transformer. It takes the high line voltage (220V, 440

V etc.) and low amperage current (50A, 60 A etc.)and changes it into 17V to 45V at

190A to 590 A.Welding machines can be classified by the method by which control or adjust the welding

current. The major classifications are multiple-coil, called taps, movable coil or movable

core, Fig. 3-15, and inverter type.The multiple-coil or tap-type machine, allows the selection of different current settings

by tapping into the secondary coil at a different turn value. The greater the number of

turns, the higher is the amperage induced in the turns. These machines may have a largenumber of fixed amperes or they may have two or more amperages that can be adjusted

further with a fine adjusting knob. The fine adjusting knob may be marked in amperes, or

it may be marked in tenths, hundredths, or in any other unit.

DC Welding Machine: Although much welding is accomplished with AC welding

power sources, the majority of industrial welding is done with machines that produce a

direct current arc. The commercially produced AC power that operates the welding

machine must then be changed (rectified) to direct current for the DC arc. This isaccomplished with a device called a rectifier. Two types of rectifiers have been used

extensively in welding machines, the old selenium rectifiers and the more modern silicon

rectifiers, often referred to as diodes. See Figure 16

SILICON RECTIFIER SELENIUM RECTIFIER Fig. 16


21/71

The function of a rectifier in the circuit can best be shown by the use of the AC sine

wave. With one diode in the circuit, half-wave rectification takes place as shown in

Figure 17. The negative half-wave is simply cut off and a pulsating DC is produced.During the positive half-cycle, current is allowed to flow through the rectifier. During

the negative half-cycle, the current is blocked. This produces a DC composed of 60

positive pulses per second. By using four rectifiers connected in a certain manner, abridge rectifier is created, producing full wave rectification. The bridge rectifier results

in 120 positive half-cycles per second, producing a considerably smoother direct current

than half-wave rectification. See Figure 18.

Fig. 17SINGLE PHASE HALF WAVE RECTIFICATION Fig. 18 SINGLE PHASE FULL WAVE RECTIFICATION

Three-phase AC can be rectified to produce an even smoother DC than single-phase AC.

Since three-phase AC power produces three times as many half-cycles per second as

single- phase power, a relatively smooth DC voltage results as shown in Figure 19.

Fig. 19. 3 PHASE FULL WAVE RECTIFICATION

Alternating-Current Transformer Welding Machines. Practically all thealternating current (AC) arc-welding machines

in use are the static-transformer type, as shown

in figure. These types of machines are thesmallest, least expensive, and the lightest

type of welders made. Industrial applications

for manual operation use machines having 200,

300, and 400 ampere ratings. Machines with a150- ampere rating are used in light industrial,

garage, and job/shop welding. The transformers

are usually equipped with arc- stabilizingcapacitors. Current control is provided in

several ways by the welding transformer

manufacturers. One such method is anadjustable reactor that is set by turning a

crank until the appropriate setting is found.

Another method is by plugging the electrodecable into different sockets located on the front of the machine. One major advantage

of ac transformers is the freedom from arc blow, which often occurs when welding

with direct-current (dc) machines. Arc blow causes the arc to wander while you are

welding in corners on heavy metal or using large coated electrodes.

DC Generator Sets - A DC welding generator produces direct current in either straight or

reverse polarity. The polarity selected for welding depends upon the kind of electrode

1 Cycle


22/71

used and the material to be welded. A DC generator is powered either by an electric

motor or a diesel engine. Diesel operated generator sets are suitable for out-door

applications or other areas where power is not available.The current supplied by a DC generator is created by an armature rotating in an electrical

field. The armature is rotated by an electric motor or an engine. The current is drawn off

for welding use by a commutator. A polarity switch on most machines provides reversedor straight polarity. Generators are designed to rotate at speeds of 1500, 1800 or 3600

rpm to give optimum current values. Generator supplies voltage usually in the range from

15 to 45 volts across the arc. The open circuit voltage is between 50 and 100 volts.Current output will vary depending upon the type of unit. A generator is designed such

that it will compensate for any change in the arc column voltage, thus ensuring a

stabilized arc. Three V-I (Voltage-current) characteristics used in arc welding DC

machines to help control fluctuating currents are:(i) Drooping arc voltage or constant current.

(ii) Constant arc voltage.

(iii) Rising arc voltage.

In drooping characteristics as the arc length increases, arcvoltage rises and the current decreases and vice versa. Machine

with drooping characteristics is used for standard shielded arcmanual welding. Constant voltage characteristics are preferred

for semi- automatic (MIG) or automatic welding processes,

because they maintain a preset voltage regardless of the

amount of current being drawn from the machine. In risingvoltage characteristics, as the current increases, voltage also

increases. Fully automatic welding processes use rising voltage characteristic machines.

Advantages of DC Generator Sets

(i) Straight and reverse polarities can be employed to advantage.

(ii) Welding can be carried out in all positions.

(iii) Nearly all ferrous and non-ferrous metals can be welded.(iv) Diesel driven generators form self-contained units.

(v) Generator output (as it does in transformer and rectifier sets) is not affected by normal

variations in power line voltage.(vi) DC is most universal in application; it can be used in practically all welding

operations. An exception is TIG welding of Al and Mg, usually done with AC.

Disadvantages of DC generator sets

(i) Higher initial cost.(ii) Higher maintenance cost.

(iii) Noisy machine operation.

With ac welding machines, polarity is not a problem. When using dc welding machines,you can weld with either straight polarity or reverse polarity. Polarity is the direction of

the current flow in a circuit, as shown in figure 7-9. In straight polarity, the electrode is

negative and the workpiece positive; the electrons flow from the electrode to theworkpiece. In reverse polarity, the electrode is positive and the workpiece negative; the

electrons flow from the workpiece to the electrode. To help you remember the difference,


23/71

think of straight polarity as a SENator and reverse polarity as a REPresentative. Use only

the first three letters of each key word. SEN stands for Straight Electrode Negative; REP

for Reverse Electrode Positive.On some of the older machines, polarity is changed by switching cables. On many of the

newer machines, the polarity can be changed by turning a switch on the machine. Polarity

affects the amount of heat going into the base metal. By changing polarity, you can directthe amount of heat to where it is needed. When

you use straight polarity, the majority of the heat

is directed toward the workpiece. When you usereverse polarity, the heat is concentrated on the

electrode. In some welding situations, it is

desirable to have more heat on the workpiece

because of its size and the need for more heat tomelt the base metal than the electrode; therefore,

when making large heavy deposits, you should

use straight polarity. On the other hand, in

overhead welding it is necessary to rapidly freezethe filler metal so the force of gravity will not

cause it to fall. When you use reverse polarity,less heat is concentrated at the workpiece. This

allows the filler metal to cool faster, giving it

greater holding power. Cast-iron arc welding is another good example of the need to keep

the workpiece cool; reverse polarity permits the deposits from the electrode to be appliedrapidly while preventing overheating in the base metal. In general, straight polarity is

used for all mild steel, bare, or lightly coated electrodes. With these types of electrodes,

the majority of heat is developed at the positive side of the current, the workpiece.However, when heavy-coated electrodes are used, the gases given off in the arc may alter

the heat conditions so the opposite is true and the greatest heat is produced on the

negative side. Electrode coatings affect the heat conditions differently. One type of heavycoating may provide the most desirable heat balance with straight polarity, while another

type of coating on the same electrode may provide a more desirable heat balance with

reverse polarity. Reverse polarity is used in the welding of nonferrous metals, such asaluminum, bronze, Monel, and nickel. Reverse polarity is also used with some types of

electrodes for making vertical and overhead welds. You can recognize the proper polarity

for a given electrode by the sharp, crackling sound of the arc. The wrong polarity causes

the arc to emit a hissing sound, and the welding bead is difficult to control. Onedisadvantage of direct-current welding is arc blow. As stated earlier, arc blow causes

the arc to wander while you are welding in corners on heavy metal or when using large-

coated electrodes. Direct current flowing through the electrode, workpiece, and groundclamp generates a magnetic field around each of these units. This field can cause the arc

to deviate from the intended path. The arc is usually deflected forward or backward along

the line of travel and may cause excessive spatter and incomplete fusion. It also has thetendency to pull atmospheric gases into the arc, resulting in porosity. Arc blow can often

be corrected by one of the following methods: by changing the position of the ground

clamp, by welding away from the ground clamp, or by changing the position of the

workpiece.


24/71

Inverter

Since the advent of high-power semiconductors such as the insulated gate bipolar

transistor (IGBT), it is now possible to build a switching power supply capable of copingwith the high loads of arc welding. These designs are known as inverter welding units.

They generally first rectify the utility AC power to DC; then they switch (invert) the DC

power into a step down transformer to produce the desiredwelding voltage or current. The switching frequency is

typically 10,000 Hz or higher. Although the high switching

frequency requires sophisticated components and circuits, itcan drastically reduce the bulk of the step down transformer.

The circuitry can also provide features such as power control

and overload protection. The high frequency inverter-based

welding machines can be more efficient and have bettercontrol than non-inverter welding machines.

The IGBTs in an inverter based machine are controlled by a

microcontroller, so the electrical characteristics of the

welding power can be changed by software in real timeupdates. Typically the controller software will implement

features such as pulsing the welding current, variable ratios and current densities througha welding cycle, variable frequencies, and automatic spot-welding; all of which would be

prohibitively expensive in a transformer-based machine but require only program space

in software-controlled inverter machine

Manual Metal Arc Welding:

Manual metal arc welding (MMAW) or shielded metal arc welding (SMAW) is the oldest

and most widely used process being used for fabrication. The arc is struck between a fluxcovered stick electrode and the workpieces. The workpieces are made part of an electric

circuit, known as welding circuit. It includes welding power source, welding cables,

electrode holder, earthclamp and the consumable

coated electrode. Figure 5.1

Shows details of weldingcircuit.

Figure 5.2 shows the fine

molten droplets of metaland molten flux coming

from the tip of the coated

electrode. The flux meltsalong with the metallic core

wire and goes to weld pool

where it reacts with moltenmetal forming slag which floats on the top of molten weld pool and solidifies after

solidification of molten metal and can be removed by chipping and brushing.
http://en.wikipedia.org/wiki/IGBThttp://en.wikipedia.org/wiki/IGBThttp://en.wikipedia.org/wiki/Switching_power_supplyhttp://en.wikipedia.org/wiki/Microcontrollerhttp://en.wikipedia.org/wiki/IGBThttp://en.wikipedia.org/wiki/IGBThttp://en.wikipedia.org/wiki/Switching_power_supplyhttp://en.wikipedia.org/wiki/Microcontroller


25/71

Welding power sources used may be transformer or rectifier for AC or DC supply. The

requirement depends on the type of electrode coating and sometimes on the material to be

welded.The constant-current or

drooping type of power

source is preferred formanual metal arc welding

since it is difficult to hold a

constant arc length. Thechanging arc length causes

arc voltage to increase or

decrease, which in turn

produces a change inwelding current. The

steeper the slope of the

volt-ampere curve within

the welding range, thesmaller the current change

for a given change in arcvoltage. This results into

stable arc, uniform penetration and better weld seam in-spite of fluctuations of arc length.

The welding voltages range from 20 to 30 V depending upon welding current i.e. higher

the current, higher the voltage. Welding current depends on the size of the electrode i.e.core diameter. The approximate average welding current for structural steel electrodes is

35.d (where d is electrode diameter in mm) with some variations with the type of coating

of electrode. Table 5.1 shows influence of welding parameters on weld characteristics.

Table 5.1: Welding Variables and Their Influence

Welding Condition Main EffectsCurrent in excess of

optimum

Excess spatter. Flat wide deposit. Deep crater. Deep penetration.

Electrode overheats.

Current less thanoptimum

Slag difficult to control. Metal piles up. Poor dead shape. Poorpenetration.

Voltage in excess of

optimum

Deposit irregular and flat. Arc wander. Porosity. Spatter.

Voltage less thanoptimum

Irregular piling of weld metal. Arc extinctions. Littlepenetration.

Travel speed in excess of

optimum

Narrow thin weld bead. Undercut.

Travel speed less thanoptimum

Wide thick deposit. Difficulty in slag control.

Optimum Welding

conditions

Smooth even weld deposit. Stable arc condition. Easily

controlled slag. Little spatter produced.

The output voltage of the power source on no load or open circuit must be high

enough to enable the arc to be started. A value of 80 V is sufficient for most electrodesbut certain types may require more or less than this value.


26/71

A manual welding power source is never loaded continuously because of operations such

as, electrode changing, slag removal etc. Most MMA welding equipment has a duty cycle

of around 40% at maximum welding current.

Equipment & Operation - One reason for the wide acceptance of the SMAW process is

the simplicity of the necessary equipment. The equipment consists of the following items.

1. Welding power source 2. Electrode holder 3. Ground clamp 4. Welding cables andconnectors 5. Accessory equipment (chipping hammer, wire brush) 6. Protective

equipment (helmet, gloves, etc.)

Welding Power Sources - Shielded metal arc welding may utilize either alternatingcurrent (AC) or direct current (DC), but in either case, the power source selected must be

of the constant current type. This type of power source will deliver a relatively constant

amperage or welding current regardless of arc length variations by the operator. The

amperage determines the amount of heat at the arc and since it will remain relativelyconstant, the weld beads produced will be uniform in size and shape.

Whether to use an AC, DC, or AC/DC power source depends on the type of welding to

be done and the electrodes used. The following factors should be considered:

Electrode Selection - Using a DC power source allows the use of a greater range ofelectrode types. While most of the electrodes are designed to be used on AC or DC,

some will work properly only on DC.

Metal Thickness - DC power sources may be used for welding both heavy sections and

light gauge work. Sheet metal is more easily welded with DC because it is easier to

strike and maintain the DC arc at low currents.

Distance from Work- If the distance from the work to the power source is great, AC isthe best choice since the voltage drop through the cables is lower than with DC. Even

though welding cables are made of copper or aluminum (both good conductors), the

resistance in the cables becomes greater as the cable length increases. In other words, avoltage reading taken between the electrode and the work will be somewhat lower than a

reading taken at the output terminals of the power source. This is known as voltage drop.

Welding Position - Because DC may be operated at lower welding currents, it is moresuitable for overhead and vertical welding than AC. AC can successfully be used for out-

of-position work if proper electrodes are selected.

Arc Blow- When welding with DC, magnetic fields are set up throughout the weldment.In weldments that have varying thickness and protrusions, this magnetic field can affect

the arc by making it stray or fluctuate in direction. This condition is especially

troublesome when welding in corners. AC seldom causes this problem because of the

rapidly reversing magnetic field produced.

Combination power sources that produce both AC and DC are available and provide the

versatility necessary to select the proper welding current for the application.

When using a DC power source, the question of whether to use electrode negative or

positive polarity arises. Some electrodes operate on both DC straight and reversepolarity, and others on DC negative or DC positive polarity only. Direct current flows in

one direction in an electrical circuit and the direction of current flow and the composition

of the electrode coating will have a definite effect on the welding arc and weld bead.

Figure 3 shows the connections and effects of straight and reverse polarity.


27/71

Electrode negative (-) produces welds with shallow penetration; however, the electrode

melt-off rate is high. The weld bead is rather wide and shallow as shown at "A" in

Figure 3. Electrode positive (+) produces welds with deep penetration and a narrowerweld bead as shown at "B" in Figure 3.

While polarity affects the penetration and burn-off rate, the electrode coating also has a

strong influence on arc characteristics.Electrode Holder- The electrode holder connects to the welding cable and con- ductsthe welding current to the electrode. The insulated handle is used to guide the electrode

over the weld joint and feed the electrode over the weld joint and feed the electrode into

the weld puddle as it is consumed. Electrode holders are available in different sizes andare rated on their current carrying capacity.

Ground Clamp - The ground clamp is used to connect the ground cable to the work

piece. It may be connected directly to the work or to the table or fixture upon which thework is positioned. Being a part of the welding circuit, the ground clamp must be

capable of carrying the welding current without overheating due to electrical resistance.

Welding Cables - The electrode cable and the ground cable are important parts of the

welding circuit. They must be very flexible and have a tough heat-resistant insulation.Connections at the electrode holder, the ground clamp, and at the power source lugs must

be soldered or well crimped to assure low electrical resistance. The cross-sectional area

of the cable must be sufficient size to carry the welding current with a minimum ofvoltage drop. Increasing the cable length necessitates increasing the cable diameter to

lessen resistance and voltage drop.

Coated Electrodes - Various types of coated electrodes are used in shielded metal arc

welding. Welding electrodes are used in welding various metals in the fabrication of

equipment for chemical & Allied industries, construction of steel structures such as

bridges, factory sheds, in the manufacture of ships, Vehicles and engineering equipment.

Mild steel is welded by electrodes to a maximum among all the metals & Alloys.Therefore M.S. Welding Electrode is the most widely used core wire. Besides this,

special grade electrodes are being developed for specific applications.Welding electrodes comprise basically of steel core wire and coating ingredients or flux

mild steel core wires are used in majority of unalloyed steel electrodes. Besides mild

steel, nickel, Nickel-copper, Nickel irons are also used in MIG & TIG welding. Stainlesssteel wires are also used for welding in fertilizer, chemical & surgical instrument making

industry. Coating ingredients are basically rutile, potassium silicate, sodium silicate and

Fig.3

DC PowerSource

DC PowerSource

Electrode Electrode

Workpiece Workpiece

Straight polarity Reverse polarity

Higher burn-off rate

Less penetration

Low burn-off rate

Deep penetration

A B


28/71

minerals like quartz, calcite and mica. Ferro-alloys are also used in the formulations of

fluxes.

Coated Electrodes are specified based on core wire diameter. Commonly used electrodediameters are 2, 2.5, 3.18, 4, 5 and 6 mm. Length of electrodes may depend on diameter

of core wire ranging from 250 to 450 mm i.e. larger the core diameter larger the length.However, special electrodes may be of 8-10 mm diameter. Table 5.2 gives the details of

electrode sizes and currents.

Table 5.2: Size and Welding Current for Stick Mild Steel Electrodes

Diameter d

mm

2.0 2.5 3.18(1/8") 4.0 5.0 6.0

Length L

mm

250/300 350 350/450 450 450 450

Welding I

Current A

50-80 70-100 90-130 120-160 160-200 190-240

Electrode coating performs many functions depending upon coating constituents, during

welding to improve weld metal properties. The important functions are as follows:

1. Improve the electric conductivity in the arc region to improve the arc ignition andstabilization of the arc.

2. Formation of slag, which;

(a) Influences size of droplet.(b) Protects the droplet during transfer and molten weld pool from atmospheric gases.

(c) Protects solidified hot metal from atmospheric gases.

(d) Reduces the cooling rate of weld seam.3. Formation of shielding gas to protect molten metal.

4. Provide deoxidizers like Si and Mn in form of FeSi and FeMn.

5. Alloying with certain elements such as Cr, Ni, Mo to improve weld metal properties.6. Improve deposition rate with addition of iron powder in coating.

Various constituents of electrode coating are cellulose, calcium fluoride, calcium

carbonate, titanium dioxide, clay, talc, iron oxide, asbestos, potassium / sodium silicate,

iron powder, ferro-maganese, powdered alloys, silica etc. Each constituent performseither one or more than one functions.

Electrode metallic core wire is the same but the coating constituents give the different

characteristics to the welds. Based on the coating constituents, structural steel electrodescan be classified in the following classes;

1. Cellulosic Electrodes

Coating consists of high cellulosic content more than 30% and TiO2 up to 20%.These are all position electrodes and produce deep penetration because of extra heat

generated during burning of cellulosic materials. However, high spatter losses are

associated with these electrodes.

2. Rutile ElectrodesCoating consists of TiO 2 up to 45% and SiO2 around 20%. These electrodes are

widely used for general work and are called general purpose electrodes.

3. Acidic Electrodes


29/71

Coating consists of iron oxide more than 20%. Sometimes it may be up to 40%, other

constituents may be TiO2 10% and CaCO3 10%. Such electrodes produce self

detaching slag and smooth weld finish and are used normally in flat position.4. Basic Electrodes

Coating consist of CaCO3 around 40% and CaF2 15-20%. These electrodes

normally require baking at temperature of approximately 250 C for 1-2 hrs or asper manufacturer's instructions. Such electrodes produce high quality weld deposits

which has high resistance to cracking. This is because hydrogen is removed from

weld metal by the action of fluorine i.e. forming HF acid as CaF2 generates fluorineon dissociation in the heat of arc.

Table 5.3: Coating Constituents and Their Functions

Coating Constituent Functions

Main Functions Other Functions

Cellulose Gas former Coating Strength and

Reducing agentCalcium Fluoride (CaF2) Slag basicity and metal

fluidity, H2 removalSlag former

Clay (Aluminum Silicate) Slag former Coating strength

Talc (Magnesium Silicate) Slag former Arc stabilizer

Rutile (TiO2 ) Arc stabilizer, Slag former,

Fluidity

Slag removal and bead

appearance

Iron Oxides Fluidity, Slag former Arc Stabilizer, improved

metal transfer,

Calcium Carbonate Gas former, Arc stabilizer Slag basicity, Slag former

Asbestos Coating strength Slag former

Quartz (SiO2 ) Slag fluidity, Slag former Increase in current carryingcapacity.

Sodium Silicate / Potassium

Silicate

Binder, Arc stabilizer Slag former

FeMn / FeSi Deoxidizer -

Iron Powder Deposition Rate -

Powdered Alloys Alloying -

Starting the Arc:

Two basic methods are used for starting the arc:the striking or brushing method (fig. 7-10) and

the tapping method (fig. 7-11).In either method,

the arc is started by short circuiting the welding

current between the electrode and the worksurface. The surge of high current causes the

end of the electrode and a small spot on the base

metal beneath the electrode to melt instantly. In


30/71

the striking or brushing method, the electrode is brought down to the work with a lateral

motion similar to striking a match. As soon as the electrode touches the work surface, it

must be raised to establish the arc (fig. 7-10). The arclength or gap between the end of the electrode and the

work should be equal to the diameter of the electrode.

When the proper arc length is obtained, it produces asharp, crackling sound. In the tapping method, you

hold the electrode in a vertical position to the surface

of the work. The arc is started by tapping or bouncingit on the work surface and then raising it to a distance

equal to the diameter of the electrode (fig. 7-11).

When the proper length of arc is established, a sharp,

crackling sound is heard. When the electrode iswithdrawn too slowly with either of the starting

methods described above, it will stick or freeze to the

plate or base metal. If this occurs, you can usually free the electrode by a quick sideways

wrist motion to snap the end of the electrode from the plate. If this method fails,immediately release the electrode from the holder or shutoff the welding machine. Use

alight blow with a chipping hammer or a chisel to free the electrode from the base metal.

Setting the Current

The amount of current used during a welding operation depends primarily upon thediameter of the electrode. As a rule, higher currents and larger diameter electrodes are

better for welding in the flat position than the vertical or overhead position.

Manufacturers of electrodes usually specify a current range for each type and size ofelectrode; this information is normally found on the face of the electrode container. Since

most recommended current settings are only approximate, final current settings and

adjustments need to be made during the welding operation. For example, when the

recommended current range for an electrode is 90-100 amperes, the usual practice is toset the controls midway between the two limits, or at 95 amperes. After starting the weld,

make your final adjustments by either increasing or decreasing the current. When the

current is too high, the electrode melts faster and the molten puddle will be excessivelylarge and

irregular. High

current alsoleaves a

groove in the

base metalalong both

sides of theweld. This is

calledundercutting,

and an

example isshown in

figure 7-12, view C.


31/71

With current that is too low, there is not enough heat to melt the base metal and the

molten pool will be too small. The result is poor fusion and a irregular shaped deposit

that piles up, as shown in figure 7-12, view B. This piling up of molten metal is calledoverlap. The molten metal from the electrode lays on the work without penetrating the

base metal. Both undercutting and overlapping results in poor welds. When the electrode,

current, and polarity are correct, a good arc produces a sharp, crackling sound. When anyof these conditions are incorrect, the arc produces a steady, hissing sound, such as steam

escaping.

Procedure for Welding

1 Workpiece

Make sure workpiece is clean before welding.

2 Work Clamp

Place as close to the weld as possible.3 Electrode

Before striking an arc, insert an electrode in the electrode holder. A small diameter

electrode requires less current than a large one. Follow recommendations of the electrode

manufacturer when setting weld amperage .4 Insulated Electrode Holder

5 Electrode Holder Position6 Arc Length

Arc length is the distance from the electrode to the workpiece. A short arc with correct

amperage will give a sharp, crackling sound. Correct arc length is related to electrode

diameter. Examine the weld bead to determine if the arc length is correct. Arc length for1/16 and 3/32 in diameter electrodes should be about 1/16 in (1.6 mm); arc length for 1/8

and 5/32 in electrodes should be about 1/8 in (3 mm).

7 SlagUse a chipping hammer and wire brush to remove slag. Remove slag and check weld

bead before making another weld pass.


32/71

WELD JOINTS

The weld joint is where two or more metal parts are joined by welding. The five basictypes of weld joints are the butt, corner, tee, lap, and edge, as shown in figure 3-6.

A butt joint is used to join two members aligned in the same plane (fig. 3-6, view A).This joint is frequently used in plate, sheet metal, and pipe work. A joint of this type may

be either square or grooved.

Corner and teejoints are used to join two members located at right angles to each other(fig. 3-6, views B and C). In cross section, the corner joint forms an L-shape, and the tee

joint has the shape of the letterT. Various joint designs of both types have uses in many

types of metal structures.A lapjoint, as the name implies, is made by lapping one piece of metal over another (fig.

3-6, view D). This is one of the strongest types of joints available; however, for

maximum joint efficiency, you should overlap the metals a minimum of three times thethickness of the thinnest member you are joining. Lap joints are commonly used with

torch brazing and spot welding applications.An edgejoint is used to join the edges of two or more members lying in the same plane.Inmost cases, one of the members is flanged, as shown in figure 3-6, view E. While this

type of joint has some applications in platework, it is more fixquently used in sheet metal

work An edge joint should only be used for joining metals 1/4 inch or less in thickness

that are not subjected to heavy loads. The above paragraphs discussed only the five basictypes of joints; however, there are many possible variations.

PARTS OF JOINTS


33/71

While there are many variations of joints, the parts of the joint are described by standard

terms. The root of a joint is that portion of the joint where the metals are closest to each

other. As shown in figure 3-7, the root may be a point, a line, or an area, when viewed incross section. A groove (fig. 3-8) is an opening or space provided between the edges of

the metal parts to be welded. The groove face is that surface of a metal part included in

the groove, as shown in figure 3-8, view A. A given joint may have a root face or a rootedge. The root face, also shown in view A, is the portion of the prepared edge of a part to

be joined

by agroove

weld that

has not

beengrooved.

As you

can see,

the rootface has

relativelysmall

dimensions. The root edge is basically a root face of zero width, as shown in view B. Asyou can see in

views C and D

of theillustration,

the groove

face and theroot face are

the same

metal surfacesin some joints.

The specified

requirements

for a particular

jointareexpressed

in such termsas bevel

angle, grooveangle, groove

radius, and

root opening. A brief description of each term is shown in figure 3-9. The bevel angle is


34/71

the angle formed between the prepared edge of a member and a plane perpendicular to

the surface of the member. The groove angle is the total angle of the groove between the

parts to be joined. Forexample, if the edge of each

of two plates were beveled to

an angle of 30 degrees, thegroove angle would be 60

degrees. This isoften referred

to as the included anglebetween the parts to be joined

by a groove weld. The

groove radius is the radius

used to form the shape of aJ- or U-groove weld joint. It

is used only for special

groove joint designs. The

root opening refers to theseparation between the parts to be joined at the root of the joint. It is

sometimes called the root gap. To determine the bevel angle, groove angle, and rootopening for a joint, you must consider the thickness of the weld material, the type of joint

to be made, and the welding process to be used. As a general rule, gas welding requires a

larger groove angle than manual metal-arc welding. The root opening is usually governed

by the diameter of the thickness filler material. This, in turn, depends on the of the basemetal and the welding position. Having an adequate root opening is essential for root

penetration.

Welding Positions

The types of welds, joints, and welding positions used in manual-shielded metal arc

welding are very similar to those used in oxygas welding. Naturally, the techniques are

somewhat different because of the equipment involved is different.

Flat-Position Welding

The welding can be done in any position, but it is much simpler when done in the flat

position. In this position, the work is less tiring, welding speed is faster, the moltenpuddle is not as likely to run, and better penetration can be achieved. Whenever possible,

try to position the work so you can weld in the flat position. In the flat position, the face

of the weld is approximately horizontal. Butt joints are the primary type of joints used in

the flat position of welding; however, flat-position welding can be made on just aboutany type of joint providing you can rotate the section you are welding on to the

appropriate position.

Horizontal-Position Welding

You will discover that it is impossible to weld all pieces in the flat position. Often the

work must be done in the horizontal position. The horizontal position has two basic

forms, depending upon whether it is used with a groove weld or a fillet weld. In a grooveweld, the axis of the weld lies in a relative horizontal plane and the face of the weld is in

a vertical plane (fig. 3.29). In a fillet weld, the welding is performed on the up per side of

a relatively horizontal surface and against an approximately vertical plane. An

inexperienced welder usually finds the horizontal position of arc welding difficult, at


35/71

least until he has developed a fair degree of skill in applying the proper technique. The

primary difficulty is that in this position you have no shoulder of previously deposited

weld metal to hold the molten metal.

Horizontal-position welding can be used on most types of joints. The most common typesof joints it is used on are tee joints, lap joints, and butt joints.

Vertical-Position Welding

A vertical weld is defined as a weld that is applied to a vertical surface or one that is

inclined 45 degrees or less. Erecting structures, such as buildings, pontoons, tanks, and

pipelines, require welding in this position. Welding on a vertical surface is much moredifficult than welding in the flat or horizontal position due to the force of gravity. Gravity

pulls the molten metal down. To counteract this force, you should use fast-freeze or fill-


36/71

freeze electrodes. Vertical welding is done in either an upward or downward position.

The terms used for the direction of welding are vertical up or vertical down. Vertical

down weldingis suited for welding light gauge metal because the penetration is shallowand diminishes the possibility of burning through the metal. Furthermore, vertical down

welding is faster which is very important in production work.

Vertical welding is used on most types of joints. The types of joints you will most oftenuse it on are tee joints, lap joints, and butt joints.

Overhead-Position Welding

Overhead welding is the most difficult position in welding. Not only do you have tocontend with the force of gravity but the majority of the time you also have to assume an

awkward stance. Nevertheless, with practice it is possible to make welds equal to those

made in the other positions.

Electrode Movements

Each welder has a preference in this area. Some prefer a simple, straight-line drag at a

slow and steady pace to get thejob done. Others will make a "C" shape with the tip of the

rod as they weld for better coverage and a slick-looking end product. You also can use a

zig-zag technique, pausing for a second or two on each side before moving diagonally tothe next and pausing there. Most beginning welders simply use the straight drag

technique. Whichever technique you choose, the goal is to get strong, complete coverageover the entire weld area. Some of the electrode movements are shown below.

Pressure Welding

Hot pressure welding:

Hot-pressure-welding is a solid state process that produces joints between the fayingsurfaces of two bodies, by application of heat and of pressure. Fusion temperature is not

reached, filler metal is not needed, and substantial plastic deformation is generated.

Heat is generally applied by flames of oxy-fuel torches directed on the surfaces to bejoined. Upon reaching the correct temperature (about 1200 0C), the torches are suddenly

removed, not to stand in the way, and the two bodies, solid bars or hollow sections, are

brought to contact and upset together under pressure, usually by hydraulic equipment.
http://www.ehow.com/careers/http://www.ehow.com/careers/


37/71

This variant is properly called the open joint process. Alternatively, when the parts are

making contact under pressure before heat application from the outside, also by electrical

induction, it is called the closed joint process.In either case flash material is expelled and a bulge is formed at the joint. Hot-pressure-

welding is similar in a way to both friction welding and flash welding, although the

source of heating is different.For obtaining the best results the surfaces

should be machined square and clean. Some

beveling can be used to control the amountof upset.

The process as described is performed as a

manual operation. The materials to be

welded must exhibit hot ductility orforgeability. Therefore cast iron cannot be

Hot-pressure-welded.

The materials commonly joined by Hot-

pressure-welding are carbon, low alloy steels, and certain nonferrous metals. Certaindissimilar materials combinations are weldable by Hot-pressure-welding.

Materials that easily form on the surface adherent oxides upon heating cannot be eaasilywelded in air by this process, typically among them aluminum alloys and stainless steels.

Tests were performed in a vacuum chamber.

Advantages

Simple process Simple joint preparation

Relatively low cost equipment

Quick weld production

High quality joints

No filler metal needed

Minimally skilled operators required

Limitations

Not all metals are weldable

Not easily automated

Length of cycle dependent on time for heating

Removal of flash and bulge required after welding.

Only simple sections readily butt weldable.

The most important parameter is the pressure sequence cycle, possibly being developedby trial and error. Pressure in the range of 40 to 70 MPa must be available.

Typical application reported, refer to butt Hot-pressure-welding of railroad rails sections

and steel reinforcing bars, especially in Japan.For use in the production of weldments for the aerospace industry with delicate materials

Hot-pressure-welding can be carried out in closed chambers with vacuum or a shielding

medium.Mechanical properties tend to be near those of the base materials, but depend upon

materials composition, cooling rate and quality.

Hot-pressure-welding can be an economic and successful process for performing butt

joints of simple shapes if the materials are easily weldable.


38/71

Cold pressure Welding:

Cold pressure welding is a solid state welding process which uses pressure at room

temperature to produce coalescence of metals with substantial deformation at the weld.Welding is accomplished by using extremely high pressures on extremely clean

interfacing materials. Sufficiently high pressure can be obtained with simple hand tools

when extremely thin materials are being joined. When cold welding heavier sections apress is usually required to exert sufficient pressure to make a successful weld.

Indentations are usually made in the parts being cold welded. The process is readily

adaptable to joining ductile metals. Aluminum and copper are readily cold welded.Aluminum and copper can be joined together by cold welding.

Resistance Welding

Resistance welding processes are pressure welding processes in which heavy current ispassed for short time through the area of interface of metals to be joined. These processes

differ from other welding processes in the respect that no fluxes are used, and filler metal

rarely used. All resistance welding operations are automatic and, therefore, all process

variables are preset and maintained constant. Heat is generated in localized area which isenough to heat the metal to sufficient temperature, so that the parts can be joined with the

application of pressure. Pressure is applied through the electrodes.The heat generated during resistance welding is given by following expression:

H = I 2 R T

Where, H is heat generated

I is current in amperes

Ris resistance of area being welded

T is time for the flow of current.

The process employs currents of the order of few KA, voltages range from 2 to 12 voltsand times vary from few ms to few seconds. Force is normally applied before, during and

after the flow of current to avoid arcing between the surfaces and to forge the weld metal

during post heating. The necessary pressure shall vary from 30 to 60 N mm-2 dependingupon material to be welded and other welding conditions. For good quality welds these

parameters may be properly selected which shall depend mainly on material of

components, their thicknesses, type and size of electrodes.Apart from proper setting of welding parameters, component should be properly cleaned

so that surfaces to be welded are free from rust, dust, oil and grease. For this purpose

components may be given pickling treatment i.e. dipping in diluted acid bath and then

washing in hot water bath and then in the cold water bath. After that components may bedried through the jet of compressed air. If surfaces are rust free then pickling is not

required but surface cleaning can be done through some solvent such as acetone to

remove oil and grease.The current may be obtained from a single phase step down transformer supplying

alternating current. However, when high amperage is required then three phase rectifier

may be used to obtain DC supply and to balance the load on three phase power lines.The material of electrode should have higher electrical and thermal conductivities with

sufficient strength to sustain high pressure at elevated temperatures. Commonly used

electrode materials are pure copper and copper base alloys. Copper base alloys may

consist of copper as base and alloying elements such as cadmium or silver or chromium


39/71

or nickel or beryllium or cobalt or zirconium or tungsten. Pure tungsten or tungsten-silver

or tungsten-copper or pure molybdenum may also be used as electrode material. To

reduce wear, tear and deformation of electrodes, cooling through water circulation isrequired. Figure 11.1 shows the water cooling system of electrodes.

Fig 11.1: Water Cooling of Electrodes (a) Spot Welding (b) Seam Welding.

Commonly used resistance welding processes are spot, seam and projection welding

which produce lap joints except in case of production of welded tubes by seam weldingwhere edges are in butting position. In butt and flash welding, components are in butting

position and butt joints are produced.

1. Spot Welding

In resistance spot welding, two or more sheets of metal are held between electrodes

through which welding current is supplied for a definite time and also force is exerted onwork pieces. The principle is illustrated in Figure 11.2.

Fig 11.2: Principle of Resistance spot Welding

The welding cycle starts with the upper electrode moving and contacting the work pieces

resting on lower electrode which is stationary. The work pieces are held under pressure

and only then heavy current is passed between the electrodes for preset time. The area ofmetals in contact shall be rapidly raised to welding temperature, due to the flow of


40/71

current through the contacting surfaces of work pieces. The pressure between electrodes,

squeezes the hot metal together thus completing the weld. The weld nugget formed is

allowed to cool under pressure and then pressure is released. This total cycle is known asresistance spot welding cycle and illustrated in Figure 11.3

Fig 11.3: Resistance Spot Welding Cycle

Spot welding electrodes of different shapes are used. Pointed tip or truncated cones with

an angle of 120 - 140 are used for ferrous metal but with continuous use they may wear

at the tip. Domed electrodes are capable of withstanding heavier loads and severe heatingwithout damage and are normally useful for welding of nonferrous metals. The radius of

dome generally varies from 50-100 mm. A flattip electrode is used where minimum

indentation or invisible welds are desired.

Fig 11.4: Electrode Shapes for Spot Welding

Most of the industri

au pp unit 5 final

Documents