a glance on welding

8/4/2019 A glance on Welding

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INTRODUCTION TOWELDING

ByNAGABHUSHAN C.P.


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WELDING

Welding is a materials joining process which producescoalescence of materials by heating them to suitable

temperatures with or without the application of

pressure or by the application of pressure alone, andwith or without the use of filler material.

Welding is used for making permanent joints.

It is used in the manufacture of automobile bodies, aircraft frames,railway wagons, machine frames, structural works, tanks,furniture, boilers, general repair work and ship building.


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HISTORY


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History of Metalworking

Welding began more than 3000 years ago

Hot or cold metals hammered to obtain forge weld

Bronze developed between 3000 and 2000 B.C.

Iron became known to Europe about 1000 B.C.

Several thousand years after use of copper

Replaced bronze as metal used in manufacture of

utensils, armor and other applications after 800 B.C.

Working of metals followed one another in greatancient civilizations

From copper, to bronze, silver, gold, and iron1 - 4


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History of Welding

Edmund Davy discovered acetylene at beginning of nineteenthcentury

Sir Humphrey Davy discovered the electric arcin 1801

Concerned with use of arc for illumination

Demonstrated possible to maintain high voltage arc forvarying periods of time by 1809

Workable electrical generating devices invented and developedon practical basis by 1850

Electric arc welding method used in US until about 1920

Handicapped because of welds produced by these bareelectrodes not as strong as metal being welded

Welding arc very unstable

1 - 5


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History of Welding

Technology of welding progressed slowly until World War I

Demands of war called for improved methods of

fabricationEnd of war, welding widely accepted

Research on coated electrodes through 1920s resulted inelectrode coatings and improved core wire

1 - 6


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Types of Welding

Plastic Welding or Pressure Welding

The piece of metal to be joined are heated to a

plastic state and forced together by external

pressure

(Ex) Resistance welding

Fusion Welding or Non-Pressure Welding

The material at the joint is heated to a molten stateand allowed to solidify

(Ex) Gas welding, Arc welding


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Classification of Welding Processes:

(i) Arc weldingvCarbon Arc (CAW)

vShielded Metal Arc (SMAW)vGas Metal Arc (GMAW)

vGas Tungsten Arc (GTAW)

vPlasma Arc (PAW)

vSubmerged Arc (SAW)

vElectro-slag (ESW)

vElectro-Gas (EGW)

vFlux Cored Arc (FCAW)

(ii) Gas WeldingvOxy-Acetylene (OAW)

vAir-Acetylene (AAW)

vOxy-Hydrogen (OHW)

(iii) Resistance WeldingvFlash (FW)

vSpot (RSW)

vSeam (RSEW)

vProjection (RPW)vPercussion (RPEW)

(iv)Solid State WeldingvForge (FOW)

vFriction (FRW)

vUltrasonic (USW)vDiffusion (DFW)

vExplosive (EXW)

vCold (CW)

(v)Other Welding ProcessesvElectron-Beam (EBW)

vLaser-Beam (LBW)vThermit (TW)

vInduction (IW)

(vi)Related Process

vOxy-acetylene cuttingvArc cutting

vHard facing

vBrazingvSoldering


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Equipments for Arc Welding

Equipments: A welding generator (D.C.) or Transformer (A.C.)

Two cables- one for work and one for electrode

Electrode holder

Electrode

Protective shield

Gloves

Wire brush Chipping hammer

Goggles


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BASICS OF ELECTRICITY


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Basic Electricity

11

Voltage The electricalpotential or pressure thatcauses current to flow Measured in Volts

Current The movement

of charged particles in aspecific direction Measured in Amps

Polarity DC- (Direct Current

Electrode Negative) DC+ (Direct Current

Electrode Positive) AC (Alternating Current)

DC+

DC -

AC


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Alternating Current

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Alternating current: The type of

current where the flow of

electrons reverses direction

(polarity) at regular intervals.

Recommended current forSMAW general purpose

electrodes and flat position.


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Direct Current

Direct current: The type of

current where the flow ofelectrons (polarity) is in one

direction.

Controlling the polarity

allows the welder to

influence the location of theheat.

When the electrode is

positive (+) DCRP or DCEP it

will be slightly hotter than

the base metal.

When the base metal is positive (+),

DCSP or DCEN, the base metal will be

slightly hotter than the electrode.

DC current is required for GMAW

It is frequently used for SMAW


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Ohms Law

14

Ohm's law states that, in an electricalcircuit, the current passing through amaterial is directly proportional tothe potential difference.

Commonly expressed as:

I = E

R

E = I R

Ohms law also be used to teach a

principle of electrical safety. Amperage is the harmful portion of

electrical current. Rearranging Ohms Law for

amperage shows that amperage(current flow) is determined by the

voltage divided by the resistance. The higher the resistance, the less

current that will flow for a given

voltage.

What does this principlemean for SMAW?


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Conduction of Current in the Arc

Plasma

Electron

Ion

NeutralGas Atom

Ionization Free

RecombinationT>10,000K

Thermal

Cathode

Anode

Electrons Emitted

Electrons Absorbed


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WELDING PROCESSES


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Shielded Metal Arc Welding- SMAW


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Gas Metal Arc Welding -GMAW


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Flux-Cored Metal Arc Welding- FCAW


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Gas Tungsten Arc Welding -GTAW


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Resistance Spot Welding- RSW


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Oxy-Acetylene welding- OAW


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Oxy-Acetylene welding- OAW


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Submerged Arc Welding- SAW


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Electro-Slag Welding- (ESW)


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Joint Design

BUTT JOINT

LAP JOINT

FILLET JOINT

CORNER JOINT EDGE JOINT


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Welding Positions


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Weld Defects

A weld defect is any physical characteristic in the completed weldthat reduces the strength and/or affects the appearance of theweld.

The mark of a good welder is the ability to identify weld defects andadjust the welding parameters to eliminate them.

Defects that are not visible must be detect by using destructive ornondestructive testing.

If the defects in a weld exceed the specifications, the weld must beremoved and redone.

Welds are removed by grinding, gouging and cutting.

Eliminating a weld defect is time consuming and expensive -- youmust be able to complete the weld correctly the first time.

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Typical Arc Welding Defects

Defect Description Causes

The depth of the

weld is less than

specifications.

Excessive heat

Excessive speed.

The weld metal isnot completely

fused to base metal

or passes are not

completely fused.

Incorrect angle Incorrect

manipulation

Insufficient heat

Weld material

flows over, but is

not fused with the

base metal.

Slow speed


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Weld bead does not

extend to the desired

depth.

Low heat

Long arc

Incorrect joint

design

Small indentions in

the surface of the

weld

Excessive gas in the

weld zone.

Moisture

Rust

Dirt Accelerated cooling

Small voidsthroughout the weld

material.


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Usually visible cracks

on the surface or

through the weld

Accelerated cooling

Constrained joint

Small weld volume

Cracks in the transition

zone between the weld

and base metal

Induced hydrogen

Incompatible electrode or wire

Accelerated cooling

Misshapen and/or

uneven ripples

Inconstant speed

Incorrect manipulation

Incorrect welder settings


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Arc welding

Advantages Most efficient way to join

metals

Lowest-cost joining method

Affords lighter weight throughbetter utilization of materials

Joins all commercial metals

Provides design flexibility

Limitations Manually applied, therefore high

labor cost.

Need high energy causing danger

Not convenient for disassembly.

Defects are hard to detect at

joints.


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Comparison of A.C. and D.C. arc welding

Alternating Current (from Transformer)

More efficiency

Power consumption lessCost of equipment is less

Higher voltage hence not safe

Not suitable for welding non ferrous metals

Not preferred for welding thin sections

Any terminal can be connected to the work or electrode


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Comparison of A.C. and D.C. arc welding

Direct Current (from Generator)

Less efficiency

Power consumption more

Cost of equipment is moreLow voltage safer operation

suitable for both ferrous non ferrous metals

preferred for welding thin sections

Positive terminal connected to the work

Negative terminal connected to the electrode


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Brazing

Brazing is a metal-joining processwhereby a filler metal is heatedabove and distributed betweentwo or more close-fitting parts bycapillary action.

The filler metal is brought slightly

above its melting ( liquidus)temperature while protected by asuitable atmosphere, usually aflux. It then flows over the basemetal (known as wetting) and isthen cooled to join the work pieces

together. It is similar to soldering,except the temperatures used tomelt the filler metal is above450 ?C (842 ?F)


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Advantages & Disadvantages of Brazing

Advantages

Dissimilar metals which cannot be welded can be joined by brazing Very thin metals can be joined Metals with different thickness can be joined easily In brazing thermal stresses are not produced in the work piece.

Hence there is no distortion Using this process, carbides tips are brazed on the steel tool holders

Disadvantages

Brazed joints have lesser strength compared to welding Joint preparation cost is more Can be used for thin sheet metal sections


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Soldering

It is a low temperaturejoining process. It isperformed at temperatures

below 450 ?C for joining.

Soldering is used for,

Sealing, as in automotiveradiators or tin cans

Electrical Connections

Joining thermally

sensitive components

Joining dissimilar metals

Hazards


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Hazards

2. Electric Shock

3. Radiation

5. Fire and Burns

1. Fumes and Gases

4. Noise

Personal Protective Equipment


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Personal Protective EquipmentRespiratory protection

Personal Protective Equipment


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Personal Protective EquipmentEye safety

Safety eyewear should always be wornunder the welding helmet to protect

against flying debris when the helmet is

raised to inspect work and when engagedin other welding activities, e.g. grinding,

hammering.


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