Chapter 2 Arc Welding

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Arc WeldingChapter 2 Notes


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IntroductionIn arc welding, two metals are joinedtogether by generating an electricarc between a covered metalelectrode.

The heat produced by the arc meltsthe metal which mixes with themolten deposits of the coatedelectrode.


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Arc is powered by either 1) Direct current or2) Alternating current

The molten metal from the electrode is

deposited on the base metal, and as themolten metal is deposited, a slag forms overthe weld which insulates it against

contaminants of the air while it cools.See figure 24, page 52


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Power SupplyFor effective arc welding, a constantcurrent is required.

Other electrical appliances, the demandfor current is fairly constant, with arcwelding the power fluctuates a great deal.

When the arc is struck with an electrode,a short circuit results which immediatelyinduces a sudden surge of electrical

current.


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Open circuit voltage-the voltage when the

welding machine is running and no welding istaking place (Ranges from 50-100)

Arc Voltage-the voltage after the arc is struck

(Ranges from 18-36) Arc voltage will also varywith the length of the arc.

It is difficult even for a skilled welder to

maintain a perfectly uniform arc length.The amount of current required for any weldingoperation is governed by the thickness of the

work metal.


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Duty Cycle-the amount of time a powersupply can deliver its rated output in tenminute period.

Ex: 30% duty cycle three minutes ofwelding and seven minutes of cooling.

Discussion: Why do you suppose weldersare designed this way?

In what situations would you require ahigher duty cycle?


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DC GeneratorDirect Current- power supply is a generatordriven by an electric motor or gasoline engine.

Polarity-indicates the direction of currentflow in a circuit

Straight Polarity-the electrode is negativeand the workpiece is positive and theelectrons flow from the electrode to theworkpiece.


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Reverse Polarity- the electrode is positiveand workpiece is negative and the electronsflow form the workpiece to the electrode.

Changing polarity is as simple as flipping aswitch on the welding machine.

Alternating Current-The transformerwelding machine takes current directly from a

power supply and transforms it into the voltagerequired for welding.


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An AC welder transformer has a primary and

secondary coil to regulate current.The primary coil receives the alternatingcurrent from the power source and creates a

magnetic field. Creates a burst of current tothe arc then for welding.

When the arc is struck, a sudden burst ofcurrent is sent to start the arc. You will hearthe welding machine bear down when thishappens

After arc is struck, the machine automatically

returns to the current amount set for the job.


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One advantage of the AC welding machine is

the freedom from Arc Blow.Arc Blow-this causes the arc to wander whilewelding.

Arc blow can be corrected by changingposition of ground clamp, changing position ofthe weld piece on the workbench.

Porosity-when impurities get into the weldingarea while the molten metal cools causing tinyholes in the weld. Significantly reduces thestrength of the weld and the asthetic value of

the weld.


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Rectifiers- a rectifier is a transformer which

contain an electrical device that changesalternating current into direct current.

We have these in our shop.

Rectifier materials:

Selenium

Silicon- provides operation at higher temperatures

CD Rom on Arc Welding


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Electrode HolderUsed to hold the electrode and guide it over the seemto be welded

A good holder should be light to reduce excessivefatigue while welding, receive and dispose ofelectrodes easily, and be properly insulated.

Always make sure the holder is connected firmly andproperly to the cable.

Never lay the cable or holder down on the work piecewhile the welder is running.


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A 30 lead cable of a given size may be

satisfactory to carry the required current, but ifanother 30 of cable is added, the combinedresistance of the two leads reduces the currentoutput of the machine.

If then you turn the power output up, it couldpossibly cause the cable to overheat.

This concept is why OSHA is so tough onschools for extension cords etc.Ex: No growlights in classroom anymore,fridge has to be unplugged , shop lights hooked

direct.


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Ground ClampWithout proper grounding, the full potential of the circuitwill fail to provide the required heat for welding.

The place where the ground clamp is connected should be

clean and free of rust.You should not have arcing between the work and theground

Never start the arc on the ground clamp

The closer the ground clamp to the work, the better theconnection for welding.

Rule of thumb: Ground clamp should never produce steamif a small drop of water is placed on it.


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Protective ShieldA suitable helmet or handshield is required forall arc welding.

An electric arc produces a brilliant light andgives off invisible ultravioletand infraredrayswhich can burn the eyes and skin.

Neverlook at the arc from within a distance of50 feet.

Better yet, never look at it at all.


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Lens ShadesShade 5 Light spot welding

Shade 6/7 Welding up to 30 ampsShade 8 Welding from 30-75 amps

Shade 10 Welding from 75-200 amps

Shade 12 Welding form 200-400 ampsShade 14 Welding over 400 amps


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Welders ClothingGloves- leather gloves are acceptable, andwelding gloves are fine as well.

However, use clamps, pliers, etc to pick up hotmetal instead of your gloves.

Use long sleeves that fit tightly so they do not

get in the way of moving equipmentAprons- aprons shield the welding sparks fromburning through onto your clothing/skin.

Helmet of appropriate lens shade


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Welding Safety RulesBe sure the welding area has a cement ormasonry floor

Remove all combustible materials from the workarea

Do not use gloves or other clothing which containoil or grease.

Be sure all power wiring is correctly installed andmaintained. Dont overload the welding cables.

Always be sure your machine is properlygrounded. Never work in a damp area


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Welding Safety Rules cont.Shut off the welding machine before makingrepairs or adjustments to avoid shocks.

Follow the manufacturers rules on operatingthe switches, and making adjustments.

Protect others with a screen and yourself with a

protective shield. Flying sparks are a danger toyour eyes. Arc rays can cause painful burnsand blindness.

Keep firefighting equipment handy at all times.


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Types of Welds/Joints to be

WeldedFlat position

Horizontal Position

Vertical PositionOverhead Position

Fillet welds

Groove welds

Lap welds

T-fillet welds

Butt welds


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Starting The ArcScratching method- almost identical to thesame motion as striking a match.

Tapping method- simply tapping the electrodeon the workpiece.

As soon as the arc is started, immediately raisethe electrode a distance equal to the diameterof the electrode. Usually 1/8-1/4 inch.


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If an electrode does stick, it can be brokenloose quickly by twisting or bending it.

If it still does not unlodge, release the electrode

from the electrode holder.You still of course can always turn off thepower to the machine.

Just dont panic over the situation


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Setting the CurrentCurrent depends upon:

The thickness of the metal to be welded

The actual position of the weldingThe diameter of the electrode

The rule of thumb: higher currents and largerdiameter electrodes can be used for welding inflat positions than in vertical or overhead

welding.


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Setting the CurrentWhenever the current is too high, the electrodewill melt too fast and the molten puddle will betoo large and irregular.

Whenever the current is set too low, there willnot be enough heat to melt the base metal and

the molten pool will be small. The result is notonly poor fusion but the deposit will pile up andbe irregular in shape and aesthetically look

poor as well.


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Setting the Current contToo high current may also produce undercutting,which leaves a groove in the base metal long both

toes of the weld deposit.A current that is set too low will cause cold laps toform where the molten metal from the electrodefalls on the work without sufficiently fusing orpenetrating the base metal.

Both these problems cause weak welds.

For examples turn to page 24 for diagrams.


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Length of ArcIf the arc is too long the metal melts off theelectrode in globules which wonder from sideto side as the arc wavers and does not fuse.

An arc that is too short fails to penetratesufficient heat to melt the base metal properly.

Generally, the length of arc should beapproximately equal to the diameter of theelectrode.


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A shorter arc is usually better for verticalwelding and overhead welding because youhave better control of the molten pool.

When the arc is too long, heat from the arc

stream is dissipated too rapidly, causing muchmetal spatter.

When the electrode, current, and polarity are

correct, a good short arc will produce asharp, crackling sound. A long arc soundslike escaping steam.


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Travel SpeedTravel speed refers to the rate the electrodetravels along the weld seam.

The maximum welding speed is influenced by:

1) the skill of the operator

2) the position of the weld3) the type of electrode

4) required weld penetration


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Travel SpeedNormally, if the speed is too fast, the moltenpool will solidify or freeze quickly trappingimpurities in the weld deposit.

If the speed is too slow, the metal piles upexcessively and the weld bead is high and

wide.

Page 25, figure 40


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Forming the CraterAs the arc comes in contact with the basemetal, a pool or pocket is formed which iscalled a crater.

The size and depth of crater indicates theamount of penetration.

General rule; the depth of penetration shouldbe from 1/3 to the total thickness of theweld bead.


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For a sound weld, the metal deposited from theelectrode must fuse completely with the basemetal.

Fusion will result only when the base metal hasbeen heated to a liquid state and the moltenmetal from the electrode readily flows into it.

Thus if arc is too short, there will be insufficient

spread of heat.If arc is too long, heat is not centralizedenough to form the desired crater.


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Single & Multiple Pass WeldsA single pass weld involves depositing one layerof weld metal. In welding light and moderate

materials, a single pass weld is usuallysufficient.

Multiple pass welds are when a heavier plated

metal is utilized. This may require two or morelayers of weld passing lapping over the other.

Whenever a multiple pass weld is used, theslag on each welded bead must be removed

completely before the next layer is deposited.


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Types of ElectrodesThe type of electrode selected for arcwelding depends upon:

1) quality of weld required

2) weld position

3) Joint design4) welding speed

5) composition of the base metal


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Electrodes classified into 5 main groups:

1) mild steel

2) high carbon steel

3) special alloy steel

4) cast iron

5) nonferrous


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Electrode flux materials:Electrodes are coated with:

Cellulose

Titanium dioxide

Ferro-manganeese

Cilica flour

Calcium carbonate


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Purpose of the flux coating:Assist with easier arc starting

Stabilize the arc

Improve weld appearance andpenetration

Reduce spatterProtect the molten materialcontamination


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Identifying electrodes:Electrode requirements andspecifications are determined by:

1) AWS (American Welding Society)

2) ASTM (American Society for Testing& Materials)


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Electrode ClassificationExample Electrode number: ( E-6013)

E= electric arc welding

First two digits= minumum allowable tensile strength

60,000 lbs.Third digit stands for welding position 1=any position

2=horizontal or flat positions

3=flat position onlyFourth digit represents some special characterisic:(weld quality, type of current, amount of penetration)

See page 31


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Test Preparation

Notes on internet web site

Review guide on internet web site

Assignments

Questions on page 33 of your packetwill be due before the test.

Chapter 2 Arc Welding

Documents

Transcript of Chapter 2 Arc Welding