20.welding revised

8/8/2019 20.Welding Revised

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WeldingIn engineering, any process in which twoor more pieces of metal are joinedtogether by the application of heat,pressure, or a combination of both. Mostof the processes may be grouped into twomain categories: pressure welding, in

which the weld is achieved by pressure;and heat or fusion welding, in which theweld is achieved by heat.



FUSION WELDINGAny Welding Process in which the weld ismade between metals in a state of fusionwithout hammering or applied pressure.The pieces to be joined are heated to themelting point at the joint, a filler metal is

added, the weld pool is stirred and theweld is allowed to form on cooling.



GAS WELDING In this fusion weldingprocess the base metal

or metals being joinedare brought to the stateof fusion with a hightemperature flame

produced by burning acombustible gas.



Atmospheric Contamination± Oxidation of molten metal by atmospheric

Oxygen

PreventionC 2H 2 + O 2 -- 2CO +H 2

CO +H 2 + O 2 -- CO 2 + H 2O± Fluxes

GAS WELDING



GAS WELDINGFiller Wires and RodsMetal Transfer



GAS WELDING EQUIPMENTS

Gas CylindersPressure RegulatorsHoses and connectors

Welding TorchNozzles



GAS WELDING TECHNIQUES

Rightward Technique



GAS WELDING TECHNIQUES Leftward Technique



MANUAL METAL ARC

WELDINGThe most versatile welding processAn electrical arc produced between an

electrode and the work piece provides theheat source.Electrode provides filler material andconsumable



MANUAL METAL ARC

WELDINGARC± The arc produced consists of large current

discharge sustained through thermallyionised column termed as plasma.



MANUAL METAL ARC

WELDINGMetal Transfer



MANUAL METAL ARCWELDING

Atmospheric Contamination



ElectrodesCore Wire

± Conduct Electricity to thearc

± Supply filler material to theweld

Coating± Stabilize arc± Produce Gas shield

± Flux and clean away theimpurities

± Form protective slag

± Add alloying constituents



ARC WELDING MACHINES

FunctionTo provide electrical energy for convenientstarting, maintaining and controlling the arc.

Open Circuit VoltageVoltage between the secondary terminals of awelding machine when no current is flowingin the circuit.High O.C.V. is required to ionise the air gapat the time of initiating the arc.




Requirements± No load voltage should be high enough to

strike the arc but not so high to endanger thewelder.

± Should be capable of giving the requiredwelding current

± Provision for continuous adjustment of

welding current± Should be light in weight, small in size and

low cost.




Transformers - A.C.OutputGenerators - D.C.OutputRectifiers - D.C.Output




TransformersLess initial costNo moving partsAir or Oil cooling

Step DownNot suitable for

nonferrous metals

Current Regulation

Moving CoreTapped Type




GeneratorsDrooping

Characteristics




Rectifiers



COMPARISON OF WELDINGMACHINES

D.C.MACHINES± More cost± No risk of shock

± Polarity can beselected

± Easy arc maintenance± Less spattering

± More voltage dropacross cables

± All types of electrodescan be used

A.C.MACHINES± Less cost± Risk of shock

± Polarity can not beselected

± Arc maintenancedifficult

± More spattering± Less voltage drop± Special electrodes

with arc stability onlycan be used



ARC WELDING ACCESSORIESElectrode holder± Light in weight± Firm contact between

holder and electrode± Sound mechanical

joint between holderand welding cable

± Adequate rating forwelding current

± Good insulation± Flexibility

Screw head typeJaw head type



ARC WELDING ACCESSORIES

Cables± Welding cable± Welding return cable

Requirements± Flexible± Wear resistant

insulated covering



ARC WELDING ACCESSORIES

Cable Joints & EarthclampsTools± Wire Brush

± Chipping Hammer

Protective Gear± Eye protection± Gloves± Cap & Apron



TUNGSTEN INERT GAS WELDING

Heat Source± Electric arc maintained between the tip of

non consumable electrode and work piece.

Electrode± Pure Tungsten± Alloyed Tungsten

Thoriated TungstenZirconiated Tungsten




Advantages of alloyedTungsten electrodes

± Better arc initiationand re ignition

± Arc stability at lowcurrent values

± Carry high currentwithout tip erosion± Longer life




Use of Inert GasProvide shield around weld pool to preventatmospheric contamination

HeliumHigh CostUsed to weld high thermal conductivity metals

such as copperArgon

Most widely used



TIG Welding Torch



TIG Welding Torch (Water Cooled)



TIG Welding Torch with cable andinert gas supply



TIG Welding Metal Transfer



TIG Welding Electrodes and Nozzles



TIG Welding Machine



E lectrode type Oxygen % Nitrogen % Hydrogen %

Bare wire 0.25 0.15 -Iron Oxide 0.22 0.06 -Acid 0.12 0.035 0.0015Rutile 0.06 0.025 0.0020

Basic 0.03 0.010 0.0004



FUNCTIONS OF ELECTRODECOATING

COATING FUNCTIONCellulose Shielding gasCalcium carbonate Shielding gasFlurospar Slag formation

Dolomite Shielding gasTitania (Rutile) Slag formation

Potasium Titanate Arc stabilizerFeltspar Slag formationIron powder Deposition rateFeromagnise Alloying elementPotasium Siligate Arc stabilizer



I.S.815.1974 CODING OF E LE CTROD E S Prefix E /R extruded/reinforced First digit: Type of covering Second digit : Welding position Third digit: Welding current condition

Fourth and Fifth digit: Tensile stress and yieldstrength

Sixth digit: Percentage elongation and impactvalue

Suffix: Optional



FIRST DIGIT

1. Coating having High Cellulose contentContains atleast 15 % of material having high

cellulose content and upto 30% of titania

Properties± Deep penetrating arc and rapid burn of rate± High spatter loss± voluminous gas shield± coarser weld finish± thin friable slag easy to remove± easy to weld in any position± suitable for D.C positive



FIRST DIGIT2. Coating having High content of titania and

producing fairly viscous slagProperties

± High content of titania and ionisers provide

excellent welding properties± Smooth arc characteristics and less spattering± Dense slag covering the deposit completely and

easily detachable except first run in Deep V Groove.

± Suitable for butt and fillet in all positions± suitable for use with AC, DC(Both polarities)



FIRST DIGIT3. Coating having appreciable amount of titania

and producing fluid slagcoating content appreciable amount of titania butthe addition of basic material yield a much morefluid slag

Properties± Smooth arc characteristics and medium penetration± welding in over head and vertical positions is far

easier

± slag is easy to detach± suitable for both AC & DC



FIRST DIGIT4. Coating producing and inflated slag having

oxides and / or silicates of iron and manganeseProperties

± thick covering

± suitable only for flat position± fluid voluminous slag which freezes with a

characteristics internal honeycomb of holes hallinflated slag which is easy to detach

± smooth weld finish± suitable for Deep Groove welding in thick plates



FIRST DIGIT5. Coating having a content ion oxide and or

silicates producing a heavy solid slagProperties

± thick covering

± suitable for single run fillet where appearance is of primary importance

± Degree of penetration is low± heavy solid slag self detachable

± weld metal has low carbon content± low mechanical property

FIRST DIGIT



FIRST DIGIT

6. Coating having a high content of calcium

carbonate and fluoridesProperties

± Slag is fluid± deposit is convex to flat profile

± suitable for all positions± known as basic coated± suitable for welding medium and high tensile

structural steels and other applications where high

mechanical properties and resistance to cracking orrequired

± to be properly stored and dried before use± correct electrode angle and short arc to be

maintained.

SECOND DIGIT



SECOND DIGIT

Indicates the position in which the electrode may

be used0 F, H, V, D, O1 F, H, V, O2 F, H,3 F4 F, H (Fillet)9 Any other position not classified above

THIRD DIGIT



THIRD DIGIT

Indicates the welding current conditions

recommended0 D +1 D +, A 902 D -, A 703 D -, A 504 D +, A 705 D+, A 90

6 D+, A 707 D+, A 509 Any other current condition

FOURTH & FIFTH DIGIT



FOURTH & FIFTH DIGIT

Indicates two ranges of tensile strength and

appropriate yield stress41 410 to 510 N/mm 2

51 510 to 610 N/mm 2

SUFFIX



SUFFIX

Indicates the deposition weight / penetration/

hydrogen controlJ 110 to 130 %K 130 to 150 %L More than 150%P Deep penetrationH Hydrogen control

LENGTH OF ELECTRODES



LENGTH OF ELECTRODES

Normal length of electrodes - 300 mm to 450 mm

± smaller diametershorter

± larger electrode

reduce in stub lengthfaster welding rate

HEAT TREATMENT OF WELDS




Requirements for Preheating

1. Reduce the strains introduced into the weldmetal as it cools, by reducing thermalgradients.

2. Increase the diffusion rate of Hydrogen, whichfacilitates its removal from the weld metal

3. Compensate for high heat losses caused by themass of metal adjacent to the weld

4. Drive of moisture from the surfaces adjacent tothe weld.5. Retard the formation of undesirable

metallurgical structures in the weld metal and

heat affected zones





Requirements for Post weld heat treatment

1. Allow the reduction of tensile stresses byallowing plastic deformation to occur withinthe work piece.

2. Remove any residual hydrogen from weldmetal.

3. Temper any metallurgically hard zones in theweld metal or heat affected zones.




Methods of Heating± Furnace methods

Box or muffle type

Bogie hearth typeChest of drawerstype




Methods of Heating± Local methods

Gas flame

Radiant gas heatersInduction coils

Exo thermic heatingElctric resistanceheating



PRE EAT I REQU IREMENTS R ELD ING

MATER IAL M IN .TEMP . R T IG ROOT

RUN

HYD . ON . ELD METAL NON HYD . ON . ELD

METALCARBON STEEL ROOT RUNCLASS I I CA

T ION e 127mm J e 12.5mmt ick

" 127mm J " 12.5mmt ick

MATL .TH ICKNESSmm

M IN .PREHEATTEMP .C

MATL .TH ICKNESSmm

M IN .PREHEATTEMP .C

CARBONSTEELe 0.25 C

5 e 20mm : 5" 20mm : 100

e 30" 30

5100

e 20" 20

5100

CARBONSTEEL" 0.25 Ce 0.4 C

50 100 ALL 150 ALL 200

C -M o 5 100 e 12.5" 12.5

20100

e 38 150

1C r 1/2 M o 5 100 e 12.5" 12.5

100150

e 12.5" 12.5e 20.0

150200200

11/4 C r 1/4 M o 5 100 e 12.5" 12.5

100150

e 12.5" 12.5e 20.0

150200200

21/4 C r1 M o 50 100 e 12.5" 12.5

150200

e 12.5 200

POST HEATING



POST HEATINGREQUIREMENTS

FOR Cr,Mo,V STEEL± Rate of Heating 200 0C/Hr± Max. Temp. 710 -730 0C± Soaking Period 2 - 2.5 m/mm± Rate of Cooling 2.5 - 3 0C /m up to 300 0C

WELD DEFECTS CRACKS



WELD DEFECTS - CRACKS

Discontinuity produced either by tearing of metalwhile in plastic condition or by fracture whilein cold condition

CAUSE

± High localised stresses arising from shrinkage of weld metal and the resistance to the movement of parts

REMEDY

± Pre and post heat treatment

WELD DEFECTS CAVITIES OR



WELD DEFECTS - CAVITIES OR POROCITY

Group of small cavitiescaused by gas entrappedin weld metal

CAUSE± Condition of electrode± moisture content of coating± too low welding current

± presence of high sulfur inparent metal or in core wire



WELD DEFECTS - SOLID INCLUSION

Solid foreign substances entrapped in the weldmetal

CAUSES± Electrode coating± dirty surfaces± incorrect joint design± use of too large electrode± inadequate removal of slag between the deposition

of successive runs± too high speed± too long arc

WELD DEFECTS - LACK OF FUSION



WELD DEFECTS LACK OF FUSIONAND PENETRATION

CAUSES± Too low welding current± Wrong polarity connection± Bad incorporation of tack welds



WELD DEFECTS - IMPERFECT SHAPE

Burning away of the side walls of the joint recessor reduction in base metal thickness at the linewhere the last bead is fused to the surface

CAUSES± design of the joint± damp electrode± excessive weld current± improper welding technique

20.welding revised

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