arc weld processes

7/27/2019 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


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.


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



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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



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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


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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GMAW Modes of Metal Transfer

Spray Globular

Short Circuiting Pulsed Spray


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



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AWS Specifications for GMAW Wire

AWS A5.18 - Carbon Steel ElectrodesAWS A5.28 - Low Alloy Steel Electrodes



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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



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GMAW Advantages

Deposition rates higher than SMAWProductivity higher than

SMAW with no slagremoval and continuousweldingEasily automated

g



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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



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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



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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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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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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



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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

arc weld processes

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