Smaw

1

Shielded Metal Arc Welding

By L D Poyyara

POYYARA CONSULTANTS 2

What Is Welding ?

Process of joining metals / alloys

The process performed by Heat with or without Pressure

Filler metal may or may not be used

The joint will be homogeneous


Classification Of Welding

1 Pressure Welding – With Heat & Pressure

2 Fusion Welding – With Heat & mostly with Filler


Pressure Welding Process

Metal parts heated to forging temperatureHeating by Oven, Oxy fuel flame or Electric ResistancePressure applied on heated parts – by Hammer, Hydraulic Press or Mechanical leverThe Parts remain permanent homogeneousjoint


Types Of Pressure Welding

Forge WeldingResistance Butt / Flash Butt / Stud WeldingResistance Spot WeldingResistance Seam welding


Fusion Welding Process

Metal parts locally heated to melt along the joint.

Heating by oxy fuel flame or electric Arc.

Invariably filler metal added to molten pool.

On cooling, molten puddle solidifies to permanent homogeneous joint.


Types Of Fusion Welding

Shielded Metal Arc Welding- SMAW Gas Tungsten Arc Welding - GTAWGas Metal Arc Welding – GMAW (MIG / MAG / FCAW)Submerged Arc Welding – SAWGas welding – Oxy Fuel GasElectron Beam Welding - EBWThermit Welding


An electric Arc struck between electrode and base metal jointBase metal melts under arcElectrode tip melts in drops and transfers to molten pool of BMElectrode with Arc moves along the joint keeping constant arc lengthOn cooling pool solidifies

Arc

Base Metal

Flux Coating

Core Wire

Pool

SMAW Process+

_


Equipment, Accessories & tools

Power Source

Welding Cables, Holder & EarthingClamp

Head Screen, Hand gloves, Chipping Hammer & Wire Brush


Types Of Power Source

Inverter- DC

Thyristor – DC

Diesel Generator Set -DC

Rectifier – DC

Transformer - AC


Characteristic Of Power Source

VVLinear – Cons. V Drooping – Cons. A

Machine weldingManual welding

A A

Vertical Curve

Horizontal Curve

V1

V2

A1 A2

V1V2

A1 A2


Electrode

Consumable

Metallic Wire Coated with Flux

Conducts Current and generates Arc

Wire melts & deposited as filler in joint


Flux Coating On Electrode

Sodium ChloridePotassium ChlorideTitanium DioxideSodium SilicateFerrosilicon Iron Powder Alloying ElementsBinding Material


Function Of Flux In welding

Stabilizes ArcPrevents contamination of weld metalCleans the weld from unwanted impuritiesIncreases fluidity of molten metalGenerates inert gas shielding while metal transfers


Function Of Flux In welding

Forms slag after melting & covers weldAllows deposited metal to cool slowlyIntroduces alloying elements in the weldIncreases deposition efficiencyMinimizes the spatter generationHelps in even & uniform bead finish


CS & LAS Electrode Sizes & Recommended Currents

60 -70

80 - 90

110 - 120

120 - 140

---

---

Average electrodes

consumed in 8 Hrs shift

240 – 250 A

150 – 210 A

130 – 180 A

100 – 130 A

60 – 85 A

40 – 60 A

CurrentLengthDia

95-10044506.36

90-95645055

85-90845044

80 -85104503.153

60-65123502.52

50-551430021

Time required for burning in seconds

Gage

Core Wire (in mm)

No


Appx. Cost Of CS & SS Electrodes

E309

E308L

E7018

E6013

AWS Classification LengthDia

31.603503.15

22.303503.15SS

6.304503.15

3.004503.15CS

Cost Per Piece( in Rupees)

Core Wire ( in mm )Electrode Quality


ASME Classification Of Electrodes

SFA 5.1E 7018

E = Electrode70 = UTS in 1000 psi ( 60/70/80/90/100/ 110)

1 = Position (1= all, 2= 1G, 1F & 2F, 3= 1G & 1F)

8 = Type of coating (0,1,2,3,5,6,8)


Baking Of Basic Coated Electrodes

Bake the loose electrodes in a baking OvenBaking Temperature 250° C to 300° CBaking Time 2Hrs to 3 HrsReduce the temperature to 100° C Hold the electrodes at this temperature till useUnused / left over electrodes to be re-baked


Why Baking?

To remove the moisture (H2O) from coating to avoid possible cracking of weld


How Does Moist Electrode Generate Crack Within Weld?

Moist electrodes introduce atomic hydrogen at high temperature in weldOn cooling, atomic hydrogen try to form moleculesThe reaction results in stresses and fine cracksCracks occur within hardened metal - HAZ Known as “Hydrogen Embrittlement” , “Under Bead Crack”, HIC, Delayed Crack, Cold Crack.


Important Terminologies used in Critical Welding Operation

PreheatingPost Heating or DehydrogenationIntermediate Stress leavingInter pass TemperaturePost Weld Heat Treatment


What Is Preheating?

Heating the base metal along the weld joint to a predetermined minimum temperature immediately before starting the weld.Heating by Oxy fuel flame or electric resistant coilHeating from opposite side of welding wherever possibleTemperature to be verified by thermo chalks prior to starting the weld


Why Preheating? Preheating eliminates possible cracking of weld and HAZ

Applicable toHardenable low alloy steels of all thicknessCarbon steels of thickness above 25 mm.Restrained welds of CS & LAS of all thickness

Preheating temperature vary from 75°C to 300°C depending on hardenability of material, thickness & joint restraint


How does Preheating Eliminate Crack?

Preheating promotes slow cooling of weld and HAZ Slow cooling softens or prevents hardening of weld and HAZSoft material not prone to crack even in restrained condition


What Is Post Heating/Dehydrogenation?

Raising the pre heating temperature of the weld joint to a predetermined temperature range (250°C to 350° C) for a minimum period of time (3 Hrs) before the weld cools down to room temperature.Post heating performed when welding is completed or terminated any time in between.Heating by Oxy fuel flame or electric resistant coilHeating from opposite side of welding wherever possibleTemperature verified by thermo chalks during the period


Why Post Heating?Post heating eliminates possible delayed cracking of weld and HAZApplicable to

Thicker hardenable low alloy steelsRestrained hardenable welds of all thickness

Post heating temperature and duration depends on hardenability of material, thickness & joint restrain


How does Post Heating Eliminate Crack?

SMAW introduces hydrogen in weld metal

Entrapped hydrogen in weld metal induces delayed cracks unless removed before cooling to room temperature

Retaining the weld at a higher temperature for a longer duration allows the hydrogen to come out of weld


What Is Intermediate Stress Relieving?

Heat treating a subassembly in a furnace to a predetermined cycle immediately on completion of critical restrained weld joint / joints without allowing the welds to go down the pre heat temperature. Rate of heating, Soaking temperature, Soaking time and rate of cooling depends on material quality and thicknessApplicable to

Highly restrained air hardenable material


Why Intermediate Stress Relieving?

Restrained welds in air hardenable steel highly prone to crack on cooling to room temperature.

Cracks due to entrapped hydrogen, hardened HAZ and built in stress

“Intermediate stress relieving” makes the joint free from crack prone by

- Relieving built in stresses- Relieving entrapped hydrogen.- Softening HAZ.


What Is Inter- Pass Temperature?

The temperature of a previously layed weld bead immediately before depositing the next bead over itTemperature to be verified by thermo chalk prior to starting next beadApplicable to

Stainless SteelCarbon Steel & LAS with minimum impact


Why Inter Pass Temperature?

Control on inter pass temperature avoids over heating, there by

Refines the weld metal with fine grainsImproves the notch toughness propertiesMinimize the loss of alloying elements in weldsReduces the distortion


What Is Post Weld Heat Treatment?

Heat treating an assembly on completion of all applicable welding, in an enclosed furnace with controlled heating/cooling rate and soaking at a specific temperature for a specific time. Rate of heating, Soaking temperature,

Soaking time and rate of cooling depends on material quality and thicknessApplicable to

All type of CS & LAS


Why Post Weld Heat Treatment?

Welded joints retain internal stresses within the structure HAZ of welds remains invariably hardened“Post Weld Heat Treatment” relieves internal stresses and softens HAZ. This reduces the cracking tendency of the equipment in service


Welding Terminologies used in Qualifications

Heat In PutHeat Affected Zone – HAZDilutionOverlap In Weld Overlay Tempering Bead


What Is Heat In Put In Welding?

The extent of heat energy generated in Joules per unit length while making each weld bead. “Heat In Put” is the Function of Welding Current, Arc Voltage, And the Welding SpeedIt is measured in Joules -

Heat In Put In Joules / mm= (A x V x 60) ÷ Travel Speed in mm / min


Why Control On Heat in Put?

Heat In put” controls the grain size of weld metal.

Lower the Heat in put finer the grain size. Finer the Grain size Better the impact properties

“Heat In Put” Also controls Dilution, HAZ & Geometry of Bead size


What Is Heat Affected Zone (HAZ)

A small volume of BM adjacent to weld fusion line, which is totally changed in its structure due to intense heat of each weld bead is known HAZ

Weld Zone

HAZDiluted BM

Diluted BM

Weld Zone

Fusion Line


What Is Significant Of HAZ

It is a part and parcel of weld jointIt is inevitableIt has properties different from BM & Weld Metal

Weld Zone

HAZDiluted BM

Diluted BM

Weld Zone

Fusion Line


What Is Dilution In Weld

In all Fusion welding, a small portion of BM very close to the welding heat gets melted and added to weld zone / fusion zone. Dilution is the ratio of molten base metal volume (Area) to the volume ( Area) of total fusion zone

HAZ

Diluted BM

Weld / Fusion Zone

Fusion Line

% Dilution = (Area of Diluted BM ÷Total Fused Area) × 100


What Is Significant Of Dilution Weld metal chemistry changes depending on the extent of dilutionChemical elements influence Physical properties of the joint. Weld chemistry influences corrosion resistance of weld overlays

Weld Zone

HAZDiluted BM

Diluted BM

Weld Zone

Fusion Line


What Is Overlap In Weld Overlay?

The extent of covering or over lapping of previous weld bead by the adjacent bead.

40 to 50 % Over Lap

Less Dilution More Dilution

10 to 15 % Over Lap

More ThicknessLess Thickness


What Is Significant Of Overlap In Weld Overlay?

Overlap of 40 to 50% results in Less Dilution & more weld overlay Thickness per layerLess dilution results weld metal chemistry more towards filler metal chemistry

40 to 50 % Over Lap 10 to 15 % Over Lap

Less Dilution More DilutionMore Thickness

Less Thickness


What Is Temper Bead Technique?

In a multi pass groove & Fillet Welds, each bead & its HAZ are getting tempered (heat treated) by the welding heat of the next bead. Thus all beads & their HAZ, except those in last layer, are tempered. Temper beads are the specially & carefully welded temporary beads on the top of final weld reinforcement with out allowing to generate any HAZ within the BM. Temper beads are to be ground flush with the required reinforcement.


Temper Bead

Temper Bead T1 & T2 Not To Generate HAZ In BMTemper Beads To Be Ground Flush

T2T1

21

3 34 4

55

T1 & T2 To be ground Flush

HAZ

Rqd. Reinforcement


Common Defects In SMAW

1. Cracks 2. Lack Of Fusion3. Slag 4. Porosity5. Pinhole 6. Piping 7. Undercut 8. Overlap9. Lack Of Penetration 10. Excess Penetration11. Spatters 12. Suck Back 13. Under Flush 14. Burn Through 15. Uneven Bead 16.Stray Arcing


Crack

1) Use Right Electrode2) Qualify Procedure3) Preheat Uniformly4) Post heating or ISR

1) Wrong Consumable2) Wrong Procedure3) Improper Preheat4) Excessive Restrain

RemedyCause

crack


Lack Of Fusion

1) Use Right Current2) Train /Qualify welder3) Train/Qualify Welder

1) Inadequate Current2) Wrong Electrode angle3) Improper bead placement

RemedyCause

Lack Of Fusion


Slag

1) Clean each bead2) Use Right Current3) Train / Qualify welder4) Train / Qualify Welder

1) Inadequate Cleaning2) Inadequate Current3) Wrong Electrode angle4) Improper bead placement

RemedyCause

Slag


Porosity

1) Bake the electrodes2) Replace the electrodes3) Clean & warm the BM4) Replace the electrodes

1) Damp Electrode2) Damaged coating3) Wet surface of BM4) Rusted core wire

RemedyCause

Porosity . .


Pinhole

1) Bake the electrodes2) Replace the electrodes3) Clean & warm the BM4) Replace the electrodes

1) Damp Electrode2) Damaged coating3) Wet surface of BM/WM4) Rusted core wire

RemedyCause

Pinhole•


Piping

1) Bake the electrodes2) Replace the electrodes3) Clean & warm the weld4) Replace the electrodes

1) Damp Electrode2) Damaged coating3) Previous beads wet4) Rusted core wire

RemedyCause

Piping•


Undercut

1) Reduce the Current2) Reduce Arc length3) Train & Qualify the

Welder1) Replace the electrode

1) Excess Current2) Excess Voltage3) Improper Electrode

angle1) Eccentric Coating

RemedyCause

Under cut


Overlap

1) Train & Qualify welder

2) Increase the current

1) Wrong Electrode Angle

2) Inadequate current

RemedyCause

Overlap


Lack Of Penetration*

1) Reduce Root Face2) Increase Root Opening3) Reduce electrode size4) Train / Qualify Welder5) Train / Qualify Welder6) Train & Qualify Welder

1) Excess Root Face2) Inadequate Root opening3) Over size electrode 4) Wrong Electrode angle5) Improper bead placement6) Improper weaving

technique

RemedyCause

LOP

* Applicable to SSFPW


Excess Penetration*

1) Reduce root gap2) Reduce Current3) Increase Root face4) Train / Qualify Welder

1) Excess root opening2) Excess Current3) Inadequate root face 4) Wrong Electrode angle

RemedyCause

Excess Penetration

* Applicable to SSFPW


Spatters

1) Reduce to Right Current2) Reduce Arc length3) Correct the polarity4) Use Baked electrodes5) Clean BM surface6) Replace the electrodes7) Replace the electrodes

1) Excess Current2) Excess Voltage3) Wrong Polarity4) Wet Electrodes5) Rusted BM surface6) Rusted Core wire7) Eccentrics coating

RemedyCause

Spatters• • •


Suck Back*

1)Reduce weaving2) Reduce Current3) Increase Root face4) Train / Qualify Welder

1) Excess weaving in root2) Excess Current3) Inadequate root face 4) Wrong Electrode angle

RemedyCause

Suck Back

* Applicable to SSFPW in 4G, 3G & 2G


Under Flush

1) Weld some more beads in final layer

2) Train / Qualify welder

3) Train / Qualify Welder

1) Inadequate weld beads in final layer

2) Inadequate understanding on weld reinforcement requirement

3) Wrong selection of Electrode size for final layer

RemedyCause

Under flush


Burn through*

1) Reduce the Current2) Reduce root opening3) Increase root face4) Train / Qualify Welder

1) Excess Current2) Excess Root opening3) Inadequate Root face4) Improper weaving

RemedyCause

Burn trough *Applicable to root pass


Uneven Bead Finish

1) Train & Qualify the Welder

2) Reduce Arc length3) Train & Qualify the

Welder

1) Improper bead placement

2) Excess Voltage3) Excess / inadequate

current

RemedyCause

Uneven bead finish


Stray Arcing

1) Train the Welder2) Train the Welder

1) Wrong Arc Striking Practice2) Inadequate Skill of Welder

RemedyCause

Arc Strikes

63

Good Engineering Practices in

Shielded Metal Arc Welding


Good Engineering Practices In SMAW

Do Welding with properly baked electrodes Basic coated CS electrodes to be baked to 250°C to 300°C for two hours Baked electrodes to be directly used on job or to be retained in a hold over oven at 100°C to 150°C until useUnused balance electrodes shall be returned to baking oven


Do not weld with damp ElectrodesDo not try to heat electrodes by touching the job (Short circuiting)Do not use electrodes with damaged coating Do not use electrodes with cracked coatingDo not bend the electrodes after holding it in the holder



Do not weld on groove / surface with mill scale or rustingPrior to welding, clean the weld groove with power wire wheelDo not weld with unidentified electrodesDo not leave balance electrodes unattended on shop



All connections with earthing and welding cables shall be tight fittedEarthing clamp shall always be tightly connected to the jobBurn the full length of electrode till 37 mm stub length



Earthing cable shall directly connect to the job with an earthing clamp.Tacks for set up shall be minimum 5 times the electrode diameterWeaving shall be limited to three times the electrode diameter.Only trained & qualified welders shall be employed for welding



Do not direct fan or blower to welding arcRemove paint if any from the area near weldingWhile welding in open, area shall be covered to protect from rain water & breezeWeld edge preparation shall be free from serrations



Use poison plates between the job material & structural supports.Do not damage parent metal while removing temporary supports.Locations where from temporary supports are removed shall be touched up by welding / grinding and PT checked.



Remove visible defects from welds before placing the subsequent beadsDo not weld over a visible crackElectrodes kept out side more than 2 Hrs shall be returned to baking ovenMaximum 15 electrodes at a time shall be taken from oven for welding



When preheat is required, heat from opposite side of welding.Use temperature indicating crayons for checking temperatureDo not Weld more than specified weld size- Fillet / Reinforcement.


73

Safety Precautions in Shielded Metal Arc Welding


Safety Precautions In SMAW

Welders shall use safety devises – Hand gloves, Head screen with right glass & Safety shoesWelders shall use full sleeve boiler suit Use welding glass-DIN 11/12 up to 250 Amps and 13 above 250 AmpsDo not look at the arc with naked eyes


Do not throw Stubs on ground. They shall be placed in stub collector.Do not keep electrode in the holder when work is not in progressDo not touch the electrode held on holder and the the job when the power source is onKeep welding cables duly wound near power source when no welding is done



Do not breath welding fumesWhen working in confined area, ensure adequate ventilation / exhaustGas cutting torch / preheating burner shall not be taken inside confined area unless the flame is lit When not in use, switch off the the power source from electric supply



Acetone / inflammable liquids (Chemical for dye penetrant test) shall not be brought near weldingGas cutting unit / fuel gas cylinders shall be away from welding area Wet safety Shoes or wet hand gloves shall not be worn while welding


Thank You

Smaw

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