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Transcript of Arc Weld Processes
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Electric Arc Welding
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Introduction
A group of fusion welding processes that use anelectric arc to produce the heat required for meltingthe metal.
AdvantagesInexpensive power sourceRelatively inexpensive equipmentWelders use standard domestic current.Portable equipment is available
Process is fast and reliableShort learning curveEquipment can be used for multiple functions
Electric arc is about 9,000 oF
Electric arc welding
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Introduction-cont.
All fusion welding process have thee requirements.HeatShieldingFiller metal
The method used to meet these three requirementsis the primary difference between arc weldingprocesses.
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Arc Welding Requirements
Process Heat Shielding Filler Material
SMAW
GMAW
Electric
Arc
Electric Arc
Inert Gas
(Flux)
Inert Gas(Cylinder)
Stick Electrode
Wire Electrode
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Seven Additional Electric Arc Welding Processes
1. FCAW
2. GTAW
3. SAW
4. ESW
5. EGW
6. PAW
7. ASW
Flux Core Arc Welding
Gas Tungsten Arc Welding
Submerged Arc Welding
Electroslag Welding
Electrogas Welding
Plasma Arc Welding
Arc Stud Welding
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Safe Practices
Welders need protection from:
Arcs rays
Welding fumes
Sparks
Contact with hot metal
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Arc Welding Power Supplies--cont.
The type of current and the polarity of the weldingcurrent are one of the differences between arc weldingprocesses.
SMAW Constant current (CC), AC, DC+ or DC-
GMAW Constant voltage (CV) DC+ or DC-
GTAW Constant Current (CC) ), AC, DC+ or DC-
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Amperage Output & Duty cycle
The optimum output amperage is determined by the thicknessof the metal , the type of joint ,welding position and type of electrode .
The amount of continuouswelding time a power supplycan be used is determined bythe duty cycle of the power
supply.Duty cycle may be 100%, butusually is less.
Duty cycle is based on a 10 minuteinterval.
Many power supplies have a sloping duty cycle.
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Five Common Output Currents
1. AC (Alternating Current)
2. DC (Direct Current)
3. ACHF (Alternating Current-High Frequency)
4. PC (Pulsed Current)
5. Square wave
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Electric Arc Welding Electrical Terms
1. Electrical Circuit
2. Direct current (DC)
3. Alternating current
(AC)
4. Ampere
5. Volt
6. Resistance
7. Ohms Law
8. Constant potential
9. Constant current
10. Voltage drop11. Open circuit voltage
12. Arc voltage
13. Polarity
To understand how an electric arc welder works, you must understand the
following thirteen electrical terms.
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Electrical Circuit An electrical circuit is a completepath for electricity.Establishing an arc completes anelectric circuit .
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Alternating Current
Alternating current : The
type of current where
the flow of electrons
reverses direction
(polarity) at regular
intervals.
Recommended current for SMAW general purpose
electrodes and flat position.
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Ampere
Amperes : the unit of measure for
current flow. One ampere is equal to
6.2415094810 18 electrons passing bya point per second.
Electricity passing through a resistancecauses heat.
An air gap is a high resistance The greater the amperage flowing through the resistance (air gap)--the
greater the heat. The electrode also has resistance. Excessive amperage for the diameter of the electrode (current density)
over heats the electrode. Insufficient amperage for the diameter of electrode makes the electrode
hard to start.
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Voltage
Voltage is the amount of
electromotive force.Measured in units of volts
One (1) volt is defined as the
potential difference across aconductor when a current of oneampere dissipates one watt of power.
The voltage at the electrode determines the ease of starting and theharshness of the arc.
Voltage is adjustable in dual control SMAW machines. Changing the voltage adjusts a GMAW machine for different metal
thickness.
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Resistance
Def: that characteristic of a material that impedes theflow of an electrical current.Measured in units of Ohms ( )When an electrical current passes through aresistance heat is produced.The amount of heat produced is a function of theamount of resistance (Ohms) and the amount of current (amps).
Is the resistance adjustable in the SMAW process?
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Ohms Law Ohm's law states that, in anelectrical circuit, the currentpassing through a material isdirectly proportional to the potentialdifference.Commonly expressed as:
Ohms law also be used to teach aprinciple of electrical safety.
Amperage is the harmful portion of electrical current.
Rearranging Ohms Law for amperageshows that amperage (current flow) isdetermined by the voltage divided bythe resistance.The higher the resistance, the lesscurrent that will flow for a givenvoltage.
R
V=I
R I=V
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Constant Current
In the normal operation of a transformer as amperage
is increased, the voltage decreases, and vies versa. Electrical arc welding power supplies are constructed
so that either the voltage or the amperage is relativelyconstant as the other factor changes.
This allows two different types of power supplies:Constant current
Constant potential In a constant current power supply, the current (amperage) stays
relatively constant when the voltage is changed. GMAW
In a constant potential power supply, the voltage stays relatively
constant when the amperage is changed. SMAW
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Constant Current--cont. Characteristics of constant current power supply.
The machine provides a high voltage for striking the arc. Open circuit voltage (OCV) OCV is not adjustable for most machines
When the arc is struck the voltage drops to the welding voltage. Arc voltage Arc voltage varies with the arc length.
As the welding proceeds the current will not vary much as the arc
length changes.
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Constant Current-cont.
Increasing the voltagefrom 20 to 25 volts (25%)only decreases theamperage from 113 to120 Amp (5.8%).
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Voltage DropVoltage drop is the reduction in voltage in an electrical
circuit between the source and the load.
Primary cause is resistance.
When an excessive voltage drop exists, the electrical
circuit will not perform as designed.Localized resistance (connection) can cause excessive heat.
Excessive heat can cause component failure.
When extra long welding leads are used, theamperage must be increased to have the same heat at
the weld.
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o n s e s
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o n s, e sPositions Five types of
welds
1. Surface2. Groove3. Fillet4. Plug5. Slot
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1. Surface Welds
Surface welds are welds were a material has been applied to the surface of another material.
May or may not be blended with the work piece.
Two common applications are for hard
surfacing and padding.
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2. Groove Welds
Groove welds are used tofuse the sides or ends of two pieces of metal.
The primary use of groove welds is
to complete butt joints.
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3. Fillet Welds
Fillet welds have a triangular cross section and areused to fuse two faces of metal that are at a 90 degreeangle to each other.
Lap Joint Outside Corner T Joint
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4. Plug Welds
Plug welds are used toattach two surfacestogether when acomplete joint is notrequired and the designdoes not allow for anyweld bead outside thedimensions of themetal.
The holes can be made with a drill bit or punch.
The weld is completed by establishing the arc on the bottom plate and thencontinuing to weld until the hole is full.
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5. Slot Welds
Slot welds are identical to plug welds except for theshape of the holes. For slot welds, slots are machinedor stamped in the upper plate.
They are complete the same as plug welds.
Joints Welds & Positions
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Joints, Welds & Positions Arc Welding Positions
Horizontal Flat
Vertical Up
Overhead
Vertical Down
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Weld Nomenclature
Penetration Bead
Base metal
Joint Angle Reinforcement
Bead
Root Face Excessive Penetration
Root Opening
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Weld Nomenclature-cont.
Root
Throat
FaceToe
Reinforcement
Leg
Leg
Toe
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Weld Nomenclature-cont.
In multiple pass welds, each pass has a specific function.
Tack Weld
Root Pass Filler Pass
Cover Pass
A tack weld is used to hold the joint at the desired gap.
If it is not used, the heat of theweld will cause the joint toclose.
The filler pass is used to fill in the joint. A pattern bead or multiple stringer beads will be used.
The root pass is used to fuse the root of the weld.
If the root pass does not haveadequate penetration, it must be cut or gouged out before the weld iscompleted.
The cover pass isnt used for strength. It is used for appearance and to fill in surface voids.
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Bead Patterns
Pattern beads areused whenever awider bead is needed.
HardsurfacingFiller passCover passReduce penetration
Common patterns:CircleCrescentFigure 8
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Weld Defects
A weld defect is any physical characteristic in the completed weld thatreduces the strength and/or affects the appearance of the weld.The mark of a good welder is the ability to identify weld defects andknow their cause(s).Some weld defects are visible, but many are not.Defects that are not visible must be detect by using destructive or nondestructive testing.If the defects in a weld exceed the specifications, the weld must beremoved and redone.Welds are removed by grinding, gouging and cutting.Correcting a weld is time consuming and expensive -- you must be ableto complete the weld correctly the first time .
Common Defects and
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Common Defects and Causes
The depth of theweld is less thanspecifications.
Excessive heat Excessive speed.
The weld metal is not
completely fused to basemetal or passes are not completely fused.
Description Cause(s)
Incorrect angle
Incorrect manipulation
Insufficient heat
Weld material flowsover, but is not fused with the base metal.
Slow speed
Common Defects and Causes--
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Common Defects and Causescont.
Weld material doesnot extendcompletely throughthe base metal
Description Cause(s)
Low heat Long arc Incorrect joint design
Small indentions in the
surface of the weld Excessive gas in theweld zone.
Moisture Rust Dirt
Accelerated cooling
Small voidsthroughout the weld material.
Common Defects and Causes--
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Usually visiblecracks on thesurface or throughthe weld
Common Defects and Causescont.
Description Cause(s)
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
Misshapenand/or unevenripples
Irregular speed Incorrect manipulation Incorrect welder settings
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Arc Welding Processes
Lesson Objectives
When you finish this lesson you willunderstand: The similarities and difference betweensome of the various arc welding processes
Flux and gas shielding methods Advantages and disadvantages of the arcwelding processes Need to select between the processes
KeywordsWelding Flux, Inert Shielding Gas, Shielded Metal Arc Welding(SMAW), Gas Metal Arc Welding (GMAW), Metal Transfer Mode,Flux Cored Arc Welding FCAW), Submerged Arc Welding (SAW),
http://et1.net.ohio-state.edu:7070/ramgen/UTS/we300/we3001-1.rmhttp://we019.eng.ohio-state.edu/we300/w300t/we300.htmhttp://we019.eng.ohio-state.edu/wenc/wenc.cfm -
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Arc Welding Processes
Welding processes that employ an electric arc arethe most prevalent in industry
Shielded Metal Arc WeldingGas Metal Arc WeldingFlux Cored Arc WeldingSubmerged Arc WeldingGas Tungsten Arc Welding
These processes are associated with molten metal
Electric Arc
http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=761575532 -
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Linnert, Welding Metallurgy,AWS, 1994
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Protection of the Molten Weld Pool
Molten metal reacts with the atmosphereOxides and nitrides are formedDiscontinuities such as porosity
Poor weld metal properties All arc welding processes employ some means of shielding the molten weld pool from the air
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Welding Flux
Three formsGranular Electrode wire coating
Electrode coreFluxes melt to form a protective slag over the weld poolOther purposes
Contain scavenger elements to purify weld metal
Contain metal powder added to increase deposition rate Add alloy elements to weld metalDecompose to form a shielding gas
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What would happen if there was no flux on the wire todecompose into gas or no inert shielding gas was provided? What would the weld metal look like?
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Shielded Metal Arc Welding (SMAW)
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SMAW Electrode Classification Example
E7018E indicates electrode70 indicates 70,000 psi tensile strength1 indicates use for welding in all positions
8 indicates low hydrogen
E7018-A1-H8R
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ANSI/AWS - 5.1 : Specification for Covered Carbon Steel ANSI/AWS - 5.5 : Specification for Low Alloy Steel ANSI/AWS - 5.4 : Specification for Corrosion Resistant Steel
AWS Website:http://www.aws.org
Coating Materials Partial List
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Coating Materials - Partial List
Arc StabilizersTitania TiO 2
Gas-Forming MaterialsWood PulpLimestone CaCO 3
Slag-Forming MaterialsAlumina Al 2O 3
TiO 2 SiO 2Fe 3O 4
Slipping Agents to Aid ExtrusionClayTalcGlycerin
Binding AgentsSodium SilicateAsbestosStarchSugar
Alloying and Deoxidizing ElementsSi, Al, Ti, Mn, Ni, Cr
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Linnert, Welding MetallurgyAWS, 1994
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Linnert, Welding MetallurgyAWS, 1994
Shielded Metal Arc Welding
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SMAW Advantages
Easily implementedInexpensiveFlexible
Not as sensitive to partfit-up variances
Shielded Metal Arc Welding
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Advantages
Equipment relatively easy to use, inexpensive, portableFiller metal and means for protecting the weld puddle areprovided by the covered electrode
Less sensitive to drafts, dirty parts, poor fit-upCan be used on carbon steels, low alloy steels, stainlesssteels, cast irons, copper, nickel, aluminum
Shielded Metal Arc Welding
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Quality Issues
Discontinuities associatedwith manual weldingprocess that utilize fluxfor pool shielding
Slag inclusionsLack of fusion
Other possible effects onquality are porosity, andhydrogen cracking
Shielded Metal Arc Welding
Shileded Metal Arc Welding
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Limitations
Low Deposition RatesLow ProductivityOperator Dependent
Shileded Metal Arc Welding
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Other Limitations
Heat of welding too high for lead, tin, zinc, and their alloys
Inadequate weld pool shielding for reactive metals suchas titanium, zirconium, tantalum, columbium
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Wood (cellulose) and limestone are added to the coating onSMAW Electrodes for gas shielding. What gases might beformed? How do these gases shield?
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Gas Metal Arc Welding
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Gas Metal Arc Welding
Gas Metal Arc Welding
Gas Metal Arc Welding
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GMAW Modes of Metal Transfer
Spray Globular
Short Circuiting Pulsed Spray
Gas Metal Arc Welding
Gas Metal Arc Welding
http://www-iwse.eng.ohio-state.edu/we300/kurz.avihttp://www-iwse.eng.ohio-state.edu/we300/puls.avihttp://www-iwse.eng.ohio-state.edu/we300/sprueh.avi -
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GMAW Filler Metal Designations
ER - 70S - 6
Electrode
Rod (can be usedwith GMAW) Minimum ultimate tensilestrength of the weld metal
Solid Electrode
Composition6 = high silicon
Gas Metal Arc Welding
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AWS Specifications for GMAW Wire
AWS A5.18 - Carbon Steel ElectrodesAWS A5.28 - Low Alloy Steel Electrodes
Gas Metal Arc Welding
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Shielding Gas
Shielding gas can affectWeld bead shape
Arc heat, stability, andstartingSurface tensionDrop sizePuddle flowSpatter
Ar Ar-He He CO 2
W g
Gas Metal Arc Welding
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GMAW Advantages
Deposition rates higher than SMAWProductivity higher than
SMAW with no slagremoval and continuousweldingEasily automated
g
Gas Metal Arc Welding
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Quality
Spatter Droplets of electrodematerial that land outsidethe weld fusion area and
may or may not fuse to thebase material
PorositySmall volumes of entrapped gas in solidifyingweld metal
g
Gas Metal Arc Welding
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Limitations
Equipment is moreexpensive and complexthan SMAWProcess variants/metaltransfer mechanismsmake the process morecomplex and the processwindow more difficult to
controlRestricted access
GMAW gun is larger thanSMAW holder
g
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When comparing processes that have spray and globularmetal transfer, which type of transfer mode do you thnk results in more spatter? Why?
Flux-Cored Arc Welding
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Flux Cored Arc Welding (FCAW)
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Linnert, Welding Metallurgy,AWS, 1994
Flux-Cored Arc Welding
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FCAW Electrode Classification
E70 T - 1Electrode
Minimum UTS70,000 psi
Position
Flux Cored /Tubular
Electrode
Type Gas, Usabilityand Performance
American Welding Society SpecificationAWS A5.20 and AWS A5.29 .
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Flux-Cored Arc Welding
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Limitations
Slag must be removedMore smoke and fumesthan GMAW and SAW
Spatter FCAW wire is moreexpensiveEquipment is more
expensive and complexthan for SMAW
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What do you suppose would happen if the powder inside thecore did not get compacted good?
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Submerged Arc Welding
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Submerged Arc Welding
Submerged Arc Welding
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Advantages
High deposition ratesNo arc flash or glareMinimal smoke and fumesFlux and wire addedseparately - extra dimension of
controlEasily automatedJoints can be prepared withnarrow groovesCan be used to weld carbonsteels, low alloy steels,stainless steels, chromium-molybdenum steels, nickelbase alloys
Submerged Arc Welding
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Limitations
Flux obstructs view of joint during weldingFlux is subject tocontamination porosityNormally not suitable for thin materialRestricted to the flatposition for grooves - flatand horizontal for filletsSlag removal requiredFlux handling equipment