Welding - Made Easy

WELDINGCLASSIFCATION:

Dr. G. R. C. PRADEEP Email: [email protected]

Welding: It is the process of joining similar dissimilarmetals with / without application of heat, with / withoutapplication of pressure and with / without addition of fillermaterial.

Weldability: It is the capacity of being welded intoinseparable joints having specified properties such asdefinite weld strength, proper structure etc. Weldabilitydepends on : (1) Melting point (2) Thermal conductivity (3)Thermal expansion (4) Surface condition (5) Change inMicro structure etc.These characteristics may be controlled / corrected byproper shielding atmosphere, proper fluxing material, properfiller material, proper welding procedure, proper heattreatment before and after deposition.


Metallurgy of Weld


Metallurgically there are 3 distinct zones in a welded partnamely. In the weld, the metal solidities from the liquidstate. Hence fusion welds are considered as castings andthe crystalline structure will usually be columnar(Dendritic). The metallurgical changes are due to theheating and subsequent cooling of the weld and the heataffected zone of the parent materials. A random graingrowth take place in the melt boundary. Within the heataffected zone, the grains become coarse due to heat inputand a partial recrystallization takes place. With increasingdistance from the melt boundary, the grains become lineruntil the heat unaffected zone with original grains isreached.Note: Further discussion on HAZ is given in the next slides


Grain-Coarsened-HAZ:The peak temperatures reached in the grain-coarsened-HAZregion range extends from much above the upper criticaltransformation temperature to just below the solidustemperature (2000 to 2700oF). The microstructure is austenite(for the most part). Any carbides, which constitute the mainobstacle to growth of the austenite grains, dissolve resultingin coarse grains of austenite and the likelihood of martensitecan be considered. It depends on the carbon content of steel.

Grain-Refinement-HAZ: This region comprisestemperature from just above the lower critical transformationtemperature and up to 200oF higher. Austenite is stillproduced and the likelihood of martensite can be considered.It depends on the carbon content of steel.


Intercritical-HAZ: The temperatures in this region includethe intercritical ranges, between the lower and upper criticaltemperatures. Some austenite is produced in this partiallytransformed range, such that very high potential formartensite transformation exists. In medium and highcarbon steels, this austenite can contain large amounts ofcarbon which has a higher tendency to producemartensite on cooling.

Subcritical-HAZ: The subcritical-HAZ includes thetempered area of the Fe-Fe3C phase diagram(since the heatof welding may be sufficient for further tempering). Thereare no phase transformations which take place in this areasince the lower critical transformation temperature is notexceeded.


Types of Welds & Welded joints:The different types of joints are Lap, Butt, Corner, etc.Butt Joints require edge preparation like V, U, J, Bevel.V Joints are easier to make but amount of metal to befilled increases with thickness. Hence other preparations arepreferred for higher thicknesses.Double preparation is done for still higher thicknesses.



EDGE PREPERATIONS:

UV

J

BEVEL

Gas Welding:It is the process of generating the heat required for meltingthe joint by burning a combustible gas with air/oxygen in aconcentrated flame at high temp. It can weld mostcommon materials.Fuel Gases for welding operations:Commercial fuel gases have one common property: they allrequire oxygen to support combustion. To be suitable forwelding operations, a fuel gas, when burned with air/oxygen,must have the following:1. High flame temperature2. High rate of flame propagation3. Adequate heat content4. Minimum chemical reaction of the flame with base and

filler metalsDr. G. R. C. PRADEEP Email: [email protected]

Among the commercially available fuel gases hydrocarbongases such as propane, butane, LPG, natural gas are NOTsuitable for welding ferrous materials due to their oxidizingcharacteristics and are suitable for heating, bending, cutting.MAPP gas is a liquefied petroleum gas mixed withmethylacetylene-propadiene (acetylene + propane) and has aheat value a little less than acetylene and suitable for weldingand cutting. Hydrogen also produces low-temperature flameand is best for aluminium. Hydrogen flame is non-luminous,commonly used for underwater welding and cutting.Acetylene most closely meets all the above requirementsAcetylene is also a hydrocarbon gas and when it reaches itskindling temperature; the bond breaks and releases energy. Inother hydrocarbons, the breaking of the bonds between thecarbon atoms absorbs energy.


(a) Oxy-Acetylene Welding:This is suitable for joining metal sheets and plates havingthickness of 2 to 50 mm. Additional metal called fillermetal is added to the weld in the form of welding rodswhose composition is same as the part being welded. Oxygenis stored at a pressure of 14 MPa. Acetylene decomposes in tocarbon and hydrogen if stored as a gas and increases thepressure which may cause explosion. Hence Acetylenecylinders are packed with porous material (balsa wood,charcoal, corn pith, or portland cement) that is saturated withacetone to allow the safe storage of acetylene. These porousfiller materials help in the prevention of high-pressure gaspockets forming in the cylinder. Acetone is a liquid capableof absorbing 25 times its own volume of acetylene gas atnormal pressure without changing the nature of the gas.



Video 1,2


Chemistry of Oxy Acetylene ProcessThe most common fuel used in welding is acetylene. It hasa two stage reaction;(1) The first stage primary reaction involves theacetylene disassociating in the presence of oxygen toproduce heat, carbon monoxide, and hydrogen gas.2C2H2 + 2O2 = 4CO + 2H2 + Heat ---------- (1)(2) A secondary reaction follows where the carbonmonoxide and hydrogen combine with more oxygen toproduce carbon dioxide and water vapour.4CO + 2H2 + 3O2 = 4CO2 + 2H2O + Heat--------- (2)When you combine equations (1) and (2) you will noticethat about 5 parts of oxygen is necessary to consume 2parts of acetylene2C2H2 + 5O2 = 4CO2 + 2H2O + Heat ----------- (3)

Hence it can be seen that 2.5 volumes of oxygen is requiredfor consuming of 1 volume of acetylene. In the first reaction35.6% of total heat is generated at the inner cone by burningone volume of Oxygen and one volume of acetylene suppliedfrom the cylinders. The remaining 1.5 volumes of oxygen issupplied fromatmosphere.Types of Flames:1. Neutral Flame: When oxygen and acetylene are suppliedin nearly equal volumes, this is produced having a max.temperature of 3200oC. This is desired in most weldingoperations. It has sharp brilliant Inner cone and outer conefaintly luminous with bluish colour. Used for most weldingapplications for many metals like Mild steel, Stainless steel,Cast Iron, Copper, Aluminium etc.


Carburizing Flame:There is excess of acetylene. This has 3 zones, sharp innercone, intermediate whitish cone, bluish outer cone. Thelength of the intermediate cone is an indication of theproportion of excess acetylene. If little excess of acetyleneis used it is called reducing condition and is used for weldingHigh carbon steel, Ni, non-ferrous Alloys, low alloy steel etc.If more excess of acetylene is used it is called carburizingcondition and is used for low carbon steels for carburizingheat treatment purpose.


Oxidizing Flame:There is excess oxygen. It has inner cone with purplish tingeand outer cone. This is used for non-ferrous alloys. Such asCu-base and Zn-base alloys like Brass (Cu-Zn) and bronze(Cu-Sn). The oxidizing atmosphere, in these cases, creates abase metal oxide that protects the base metal. For example, inwelding brass, the zinc has a tendency to separate and fumeaway. The formation of a covering copper oxide prevents thezinc from dissipating.


(b) Air Fuel Gas Welding:This process uses a torch similar to a Bunsen burner andoperates on Bunsen burner principle. The air is drawn intothe torch as required and mixed with fuel gas. The gas isthen ejected and ignited, producing an air-fuel flame. Thecommon fuels used are natural gas, propane & Butane.This type of welding has limited application because of lowtemp. This is suitable for low melting point metals andalloys such as lead etc.

c) Oxy Hydrogen Welding:-This was once used for welding low temperature metals suchas Al, lead, Mg. The process is similar to oxygen acetylene system with the only difference being a specialregulator used in metering the hydrogen gas.


Gas Welding procedures:a) Leftward / Forward welding: The weld is made working

from right to left. This is found most advantageous onplates up to about 3 mm.

b) Right ward / back ward welding: The weld is madeworking from left to right. This method provides bettershielding against oxidation and slows down its cooling.Hence the weld metal is denser, stronger and tougher.Welding speed is 20% to 25% higher and fuelconsumption is 15% to 25% lower in this proceduresuitable for over 12mm thick plates.

c) Vertical welding: This is often advantageous forthickness of 6mm and above. It does not require edgepreparation up to 15mm thickness. Here the operatorstarts at the bottom and proceeds to the top.



Leftward Welding

Rightward Welding

Gas cutting / Oxygen cutting:This used for cutting plates of large thickness and also whencut is to be made along a specified contour. The equipmentis similar to that of Gas Welding, but with a different tip thesite of the hole depends on thickness to be cut. The metal isheated to ignition / kindling temp. the Jet of Oxygen causesrapid oxidization and blows away the oxide and moltenmetal particles thus creating the cut (Kerf) (KindlingTemperature Kindling temperature is the lowesttemperature at which a substance bursts into flame)


Video 3

Oxygen Lance cutting: It is the process by which holes arepierced in heavy blocks of metal by a jet of oxygen passingthro a consumable steel pipe (M.S) called lance creatingvery high temperatures (45000C) due to reaction of oxygenwith hot metal. The pipe is packed with mixed metal wires ofiron, Al, Mg etc. Pure oxygen gas is passed through the pipefrom one end from an oxygen cylinder and regulator. Theother end of the pipe is preheated to its kindling temperaturewith an oxy-acetylene torch. The wires in the pipe burns inthe oxygen coming down the pipe to produce enormous heatand a liquid slag of iron oxides and other materials, whichdribbles and splashes out to longer distances depending onoxygen flow rate. The flow of gas creates a combustion-friendly environment and the high-temperature flameproduced can easily cuts through steel, concrete (18500C)etc.



Application: Also used for opening of tap holes in blastfurnace, making centering holes in heavy shafts, Cutting largemetal castings or frozen metal spills in foundries,cutting concrete slabs and large steel beams in demolition andrenovation of buildings etc.

Video 4

Arc Welding:It is a process of generating the heat required for melting thejoint by means of an electric arc. This is most widely usedthan Gas welding because of the ease of use and highproduction rates.

Principle of Arc:An Arc is generated between two conductors of Electricity,Cathode and Anode, when they are touched to establish theflow of current and then separated by a small distance. Anarc is a sustained electric discharge through the ionized gascolumn called plasma between the two electrodes. Theelectrons liberated from the cathode strike the anode at highvelocity, generating large amount of heat (6000oC). About65% to 75% of total heat is liberated at anode.


It should be noted that Arc temperature depends upon theenergy density of the arc column.

With AC the cathode and anode change continuously and asa result temp. across the arc would be more uniformcompared to a DC arc.

Straight Polarity / DCEN (Direct Current Electrode ve) isused for thick sheets. Here the W.P. is anode, thus moreheat is liberated at the anode which gives deeper penetration.

Reverse Polarity / DCEP (Direct current Electrode +ve) isused for thin sheets. Here penetration is small.

In AC welding, the penetration obtained is medium.

DC welding is more expensive and is used for difficulttasks.


Electrodes:The electrodes used can be consumable (same base material)(or) Non-consumable (Tungsten, Carbon or Graphite). Theconsumable electrode can be either coated (stick electrode)or uncoated (bare electrode). The coatings serve a No. ofpurposes.1. To facilitate establishment and maintenance of arc2. To produce shield gas around arc & weld pool3. To provide formation of slag to reduce rapid cooling.4. To introduce alloying elements not contained in corewire.


Shielded Metal Arc Welding: (SMAW)Here a metal rod is used as electrode. The temp. is about2400oc on -ve and 2600oc on +ve electrodes respectively.This is called Shielded Metal Arc Welding (SMAW) whenstick (coated) electrodes areused. This is a manual processand used for general purposewelding. A.C is the currentsource. D.C also can be used.This can be used forthicknesses above 3mm.The main disadvantages areslow speed, slag inclusion,moisture pick up by coatings,wastage of electrode material etc.


Video - 1

Flux cored Arc Welding: (FCAW)This is a variant of GMAW, where a consumable tubularelectrode wire containing flux at the centre is fed from a reel.DC is used. It is limited to steel and some types of S.S.


Video 2

Carbon Arc Welding:Here, one or two rods ofcarbon are used as veelectrodes and work is +ve.The temp. is about 3200ocon ve and 3900oc on +veelectrodes respectively.Here DC is always used asfixed polarity is notobtained with A.C. This isused where no addition offiller metal is required.Used for welding sheetsteel, Al, Cu alloys likeBrass, Bronze etc.


Video 3,4

Atomic Hydrogen Welding: (AHW)Here an arc is maintained between two non-consumabletungsten electrode, while a stream of hydrogen gas underpressure is passed through the arc and around theelectrodes. As the molecules of H2 pass thro the arc, theychange into atomic state absorbing considerable amount ofenergy. Just outside the arc, the atoms of H2 recombine intomolecules liberations large amount of heat and produces atemp. of the order of 4000oC. This process removes alloxygen and other gases which form oxides and impuritiesand thus produces smooth, uniform, strong and ductile weld.This is used for welding alloy steel, stainless steel and mostnon-ferrous metals. This method is now obsolete afterdevelopment of MIG and TIG.



Video 5

Submerged Arc Welding: (SAW)This is an automatic process developed for high quality buttwelds in steel plates like large container manufacturing,bridges construction, ship building, penstocks, pressurevessels, other structural applications etc. The arc is formedunder the layer of flux (Granular flux of coarse size) and isnot visible. The bare electrode is fed from a reel through agun/nozzle. Speeds up to 80 mm/s on thin plates anddeposition rates up to 45 Kg/hr on thick plates are possible.Plate thicknesses up to 25 mm can be welded in a single passwithout edge preparation using DCEP. Deep penetration withhigh quality weld is possible. gouge


Video 6

Stud Arc Welding (SW)It is a process for faster joining of the studs to the workpieces such as M/C assemblies, motor assemblies, automobileassemblies, structural assemblies etc. The equipment consistsof a Gun similar to GMAW torch which holds the stud toweld. An are is initiated between the stud and the workpiece which melts the end of the stud and contact area ofwork piece. The stud is pushed into the weld pool andcurrent is switched off simultaneously and thus the stud getswelded to the work piece.


Video 7

Gas Metal Arc Welding (GMAW) (or) Metal Inert Gas(MIG) Welding:This is a gas shielded, metal are welding process, where, theconsumable electrode wire is continuously fed from a reeland the welding area is flooded with a inert gas which willnot combine with metal. The wire is often bare (or) verylightly coated. This is advantageousbecause of high welding speeds,No flux requirement, welds manymetals The welding gun is eitherAir cooled / Water cooled.D.C is the current source andmainly used for thick plates.


Video 8

Inert gases used:1. CO2 is used for steel,2. Ar (or) Ar He mixture is used for Al (or) Cu3. Ar O2 [1 to 5 % of Oxygen is added for better fluidityand improved arc stability] (or) He Ar mixture is used forstainless steel4. Pure Ar gas is used for Titanium5. Ar He mixture is used for Cu-Ni and high-Ni alloys.

Helium has higher thermal conductivity. So it gives higherarc voltage for a given current and higher heat input.However, helium being lighter (than argon and air) rises inturbulent manner and tends to disperse into air. So higherflow rate will be required in the case of helium shielding.


Modes of metal transfer in GMAW Welding:In GMAW, the filler metal is transferred from the electrodeto the joint. Depending on the current and voltage used,different ways of transfer occurs.

1. Short circuit / Dip Transfer: Here the electrode tip meltsand forms a Globule of molten metal at tip. As theelectrode advances it touches the W.P. short circuitoccurs. The tip is pinched by electromagnetic forces andtransferred by surface tension into the weld pool. This isused up to thicknesses of 5mm with small diameter wires (upto 0.9 mm). Best for vertical welding and overhead welding.



1)

2)

3) 4)

2. Globular / Drop Transfer: It occurs at higher currents thanthe first. The melted tip forms a big size drop (twice thewire dia) at tip which is pinched by electromagnetic forcesand pulled by gravity in to the weld pool. It causesexcessive spatter hence usually avoided mode of transfer. Itmay sometimes cause short circuit also.3. Spray Transfer: It occurs at higher currents than thesecond. Here the molten metal is detached from tip by theincreased electromagnetic pull irrespective of gravity force.It produces very little spatter & used for thick plates (>6 mm)in flat and horizontal positions only. Wire diameters are more.4. Pulsed Spray Transfer: The current is pulsed between spraytransfer range and nearer to globular range cyclically so thatit is suitable for all positions of welding. It is mainly used forS.S as it reduces distortion and inter granular corrosion.


Tungsten Inert Gas (TIG) welding:This process was invented for welding Al as Al forms anoxide layer immediately on exposing to atmosphere. DCEPwas used in welding Al as it causes peeling of oxide layer(Cathode cleaning process). A.C. was later found to givebetter result. Filler material can be used if required in TIGwelding by feeding as if in Gas welding. Pure tungsten isused for DCEN for welding most of the metals. Thoriatedtungsten or Zirconated tungsten is used for A.C and DCEPfor welding Al and Mg alloys.This process is being widely usedfor thin sheets for precisionwelding in nuclear, air craft, space craft,chemical industries.



Video 9

Plasma Arc Welding: (PAW)It is extension a TIG. Difference is constriction of arccolumn. Plasma is a high-temp. ionized gas and occurs inany electric are between two electrodes. The ionized gasgets hotter by resistance heating from the current passingthrough it. If the arc is constrained by an orifice, theproportion of ionized gas increases and plasma are weldingis created. A non-consumable tungsten electrode with awater-cooled nozzle is enveloped by a gas. The gas isforced past an electric arc thro the constrained opening ofthe nozzle. The gas passing thro the arc is dispersed andtemp. raises to the order of 11000oC to 14000oC.Application is in electronic, instrumentation, aero spaceindustries. It can also weld Carbon steels, S.S, Cu, Brass, Al,Ti, Monel, Inconel, Mo, Tantalum, Haste Alloys etc.



Video 10


A lower flow rate of the orifice inert gas is maintained, asexcessive flow rate may cause turbulence in the weld pool.This flow rate is insufficient to shield the weld pooleffectively. Hence inert gas at higher flow rate is also passedthrough outer gas nozzle to protect the weld pool.

Arc Blow:Due to fixed polarity in D.C. Welding, magnetic lines formin the W.P. When welding at the centre of W.P. these lines areequally distributed on both sides so Arc will be straight. Butwhile welding at the edges, the magnetic lines will try to pullback the arc and itwill be deflectedtowards the W.P., asthese lines will beformed only in thematerial. Thisphenomenon is calledarc blow and causesspatter and improperbead geometry.


Keeping metal plates at entry and exit of the arc. Holding as short an arc as possible to help the arc force

counteract the arc blow. Reducing the welding current - which may require a

reduction in arc speed Changing the ground

positions. Inclining the electrode

with the work oppositeto the direction of arcblow as shown:

Reducing the Arc blow:


Arc cutting:This is based on melting the metal by the heat of anelectric arc and blowing molten metal by a jet of airsupplied along the electrode and into the cut. This is usedfor cutting small sections like pipes, angle channels,separation of gating system from castings, etc.

Power sources in Arc Welding:Selection of power source is mainly dependent on typewelding process. The open circuit voltage normally rangesbetween 70-90 V and short circuit current ranges between600-1000A in any welding transformer. Welding voltages andwelding currents are lower as compared to open circuitvoltage of the power source.


a) Constant current type transformer (Non-Linear):In manual arc welding since are length cannot becontrolled, the arc current is controlled by the transformer.It has the drooping V-I characteristic curve as shown. It canbe observed that a major change in Arc voltage causes insignificant change in Arc current.

b) Constant Voltage Transformer (Linear):It has a flat V-I characteristic with a slight droop. This isused for continuous electrode wire welding like GMAW,SAW and other automatic welding processes. It can beobserved that a major change in Arc current causes insignificant change in Arc voltage.


Note:1. Voltage required to generate arc at no load condition iscalled Open Circuit Voltage (VOC )2. Current required during arc generation is called ShortCircuit Current (ISC).

Duty Cycle:Duty cycle is the ratio of arcing time to the weld cycle timeexpressed as percentage. If arcing time is continuously 5minutes then as per European standard it is 100% duty cycleand 50% as per American standard. At 100% duty cycleminimum current is to be drawn. The welding current whichcan be drawn at a duty cycle can be evaluated from thefollowing equation:


DR x IR2 = I2 x D100

Where I = Current at 100% duty cycleD100 = 100 % Duty cycleIR = Current at required duty cycleDR = Required duty cycle

Duty cycle and associated currents are important as itensures that power source remains safe and its windings arenot getting damaged due to increase in temperature beyondspecified limit. The maximum current which can be drawnfrom a power source depends upon its size of winding wire,type of insulation and cooling system of the power source.


Expressions:1.For a linear power source characteristic, the arc voltageis given by :

V = Voc ((Voc / ISC) x I)Where I = Arc current

2.For a stable arc, in a constant voltage transformer,Varc = Vtransformer

3.For a stable arc, in a constant current transformer,Iarc = Itransformer

3.For a linear power source, the Arc length Voltagecharacteristic is given by

V = a + blwhere l = Arc length, a, b = constants.


4. The equation of the line can also be written as(V-V1) = {(V2-V1) / (I2-I1)} (I-I1)

5. Heat required for melting =Volume melted x rate of meltingVolume melted = Area of Joint x welding speed

6. Net heat supplied = HT x V x IHT = Heat transfer Efficiency

7. Melting = Heat Reqd. to melt the joint / Net heat supplied


Resistance welding: (RW)This is a fusion welding process where both heat and pressureare applied on the joint, but no filler metal (or) flux is added.The heat necessary for the melting of the joint is obtained bythe heating effect of the electrical resistance of the joint.Here a low voltage (typically 1 V) and very high current(typically 15000 A) is passed thro the joint for a very shorttime (typically 0.25 sec.). This heats the Joint due to thecontact resistance at the joint and melts it. The pressure onthe Joint continuously maintained fuses the metal parts.Electrodes:Copper in alloyed form is used for making electrodes.Cu - Cd Alloys for non-ferrous materials like Al & Mg.Cu Cr Alloy for mild steels and low alloy steelsCu with Be & Co for S.S., Tungsten steels.



Note: The transformer in the machine converts low amperage,240V shop line current in to high secondary amperage, lowvoltage welding current, safe from electrical shock. Properearthing is also important. (Range: 125V, 1000100,000 A)

Heat Balance:Proper fusion is obtained only when proper heat balance isthere. This can be provided by increasing or decreasing thecontact areas of the electrodes as follows for differentcombinations.1.Small contact area for thin sheet, big contact area for thicksheet.2. Large contact area is required for high electricalconductivity and small contact area for low electricalconductivity (Dissimilar metals)3. Smaller contact area is required for higher thermalconductivity and large contact area for low thermalconductivity (Dissimilar metals).


Upset Butt welding: (UW)The parts to be welded are clamped edge to edge in CopperJaws of welding M/c and brought together in Solid contact,which forms a locality of high electric resistance. As thecurrent flows here, the joint gets heated us and the pressureapplied upsets the parts together.This is used for non-ferrousmaterials and is used for weldingbars, rods, wires, tubes, pipes etc.


Video 1

Flash Butt welding: (FW)Here the edges are brought together in light contact. A highcurrent starts a flashing action between the two surfaces andcontinues as reached. Theupsetting action will cause meltedmetal to flash out through thejoint and forms like a fin aroundthe joint. This is used for ferrousmaterials and is used for weldingbars, rods, wires, tubes, pipes etc.This is not suitable formaterials like lead, Tin, Zinc,Antimony, Bismuth etc.


Video 2

Percussion Welding:Here one part is heldstationary, and otherpart is held in a clampmounted on slide whichis backed up against pressurefrom a heavy spring. Duringwelding, the movable clamp released rapid carries the partforward. When the distance between the parts is approx.1.5mm, a sudden discharge of electrical energy is released,causing intense Arc between the two surfaces. To completethe weld it takes about 0.1 sec. No upset / flash occurs at theweld. This is a automatic process and is limited to smallareas of 144 mm2 max. and is suitable for welding smallwires to electrical components.


Video 3

Spot Welding: (RSW)This is employed to join overlapping strips, sheets or platesat small areas. This is widely used in electronic, electrical, aircraft, automobile, home appliance industries for bodyconstructions.

Projection Welding: (RPW)This is modification of spot welding. One (or) both of thework pieces are embossed to produce projections. Thecurrent and pressure employed on the embossing flattens outthis projection resulting in good welds at point of contact. Bythis process fastening attachments like nuts, brackets handlesetc. can be welded to sheet metal in electrical, electronic,domestic equipment.



RSW

RPW

Video 4,5,6,7

Seam Welding: (RSEW)This is a method of making a continuous joint between twooverlapping pieces of steel metal. The work is placedbetween wheels which serve as conductors for producingcontinuous welds. Used for pressure tight / leak proof fueltanks in automobiles, seam welded tubes, drums, smallcontainers etc.


Video 8,9

Expressions:1. Heat required for melting = Vol melted x rate of melting

= mL + mCp (Tm Ta)Where m = mass of metal melted = (vol melted x )

L = Latent heat of fusion of the metalCp = Sp. Heat of metalTm = Melting temp. of metalTa = Ambient temp. = Density of metal

2. Net heat supplied = I2 RT = V2T / R (Since V = IR)Where I = Current (Amp)

R = Resistance ()T = Time for welding (sec)

3.Melting =Heat Reqd to melt the joint / Net heat supplied


Thermit Welding: (TW)This is used for the welding of very thick plates, like shiphulls, broken large castings, rail sections etc. Thermit is amixture of finely divided Al (1 part) and Iron oxide (3parts). The Process is based on the chemical reaction whereOxygen leaves Iron oxide and combines with Al, producingAl. oxide and superheated thermitsteel. [8Al + 3Fe3O4 4 Al2O3 + 9 Fe]The temperature is around 3000oC.A wax pattern is first shaped aroundthe parts to be welded. A sand mouldis prepared around it. Pre heating isdone and wax is drained out. Thethermit mixture is poured in to the mould and then pressureis applied after welding temp. is reached.


Video - 1

Electro slag welding : (ESW)This is developed to weld very large plates (200 mm section)without any edge preparation. Here a consumable electrodeis used for filling the gap between the two heavy plates. Theheat required for melting the plates and electrode is obtainedinitially by means of an arc so that the flux will form themolten slag. Then further heating is obtained by theresistance heating of slag itself. For effective welding,vertical welding is done to maintain a continuous slag pool,which is contained in the gap with the help of water cooledcopper dam plates which move along with the weld.Appln: Frames of heavy presses, rolling mills, Locomotivesetc.


Video2


BRIDGE GIRDER

Electron beam Welding: (EBW)Here a focused beam of electrons are accelerated towardsthe anode from the electron gun which forms the cathode.This is done with the help of a electro magnetic lens. Thematerial in the path on the beam gets melted. Largerpenetrations are possible here. No filler material / flux isneeded here. Here the welding zone is narrow and hence welddistortions are eliminated. (0.25mm 1mm dia beam can bepossible).Appln: Specially suitable for welding dissimilar metals and

super alloys, turbine and air craft engine parts wheredistortion is unacceptable, Air plane, automobile, farmequipment etc.


Video3

Laser Beam Welding (LBW)Here a laser beam is directed on to the joint to be welded.Narrow. Heat Zones (0.05 mm to 0.1mm wide) are possiblehere and hence very small wires used in electronic devicescan be welded. This is called Micro welding. They can alsobe used for joining multi layer materials with differingthermal properties. It can weld dissimilar metals and difficultto weld metals like, Cu, Ni, S.S, Ti, Columbium etc. Widelyused in Aerospace and electronic industries.The lasers used for welding are:Solidstate lasers like Ruby - Neodymium (Nd); Nd - Glass;YAG (Yttrium - Aluminium Garnet) etc.The chief gas laser is CO2 laser.


Video4

Forge Welding:This is a oldest method. The ends to be joined are heated toa temperature slightly below the solidus temperature andpressure is applied so that a fusion joint is obtained. Theforce can be applied by machines / continuously rotatingrolls / manually.


ART METAL HORSE SHOE

Video5

Friction Welding (FRW)One of the parts to be joined is axially aligned and pressedtightly against another part and rotated at a high speed (3000rpm). The friction between the parts rises the temperature ofboth ends. The rotation is stopped abruptly and pressure onfixed part is increased so that joining takes place. Evendissimilar metals can be joined. Thisprocess is limited to parts withrotational symmetry.


Video6


Friction welded parts in production applications span overwide products for aerospace, agricultural, automotive,defense, marine and oil industries.

Right from tong holds to critical aircraft engine componentsare friction welded. Automotive parts like gears, enginevalves, axle tubes, driveline components, strut rods, shockabsorbers are friction welded.

Hydraulic piston rods, track rollers, gears , bushings, axlesand similar parts are commonly friction welded foragricultural equipment.

Diffusion Welding: (DFW)Also called Diffusion bonding is the process of joining twoparts purely by diffusion, which can be achieved bykeeping the two pieces in intimate contact under pressure.This does not necessarily need heat. But its temperature israised, the diffusion rate is increased. The joint is formedwithout any filler metal and the microstructure andcomposition at the interface are the same as those of the basemetals. Pressure is applied which will cause local plastic andcreep deformation at the temperature of operation. Bondingwill take place due to diffusion and will depend ontemperature, time and the pressure applied. An interlayer foilor coating may be used to improve the bondingcharacteristics. This process makes it possible to join metalto metal as well as metal to ceramic also.



Appln: Used by gold smiths tobond gold over copper, mostsuitable for joining dissimilarmetals like Ti, Be, Zr, refractorymaterials, composite materials etc.Diffusion bonding with superplastic forming is widely used inaero space (Wing Structures).

Video7

Explosion welding: (EXW)Here, detonation of explosives is used to accelerate a part tomove towards the other plate at a fast rate, so that theimpact creates the joint. As the plate moves at highvelocity and meets the other plate with a massive impact,very high stress waves (of order thousands of MPa) createdbetween the plates makes a clean joint. Application is forcladding of metals for the purpose of corrosion prevention.Used for joining of dissimilar metals like Titanium to steel,Al. to steel, Al to Cu etc. Tantalum can be explosivelywelded to steel though the welding point is higher thanvapourisation temperature of Steel.

Eg: Ship building, chemical Industry.


Video8,9


TITANIUM TO COPPER ALUMINIUM TO STEEL

Brazing:Here a filler material also called spelter is used, whosemelting point is less than the melting point of parts to bejoined. The parts to be welded are cleaned properly Flux(usually Borax) is applied and then filler material is placed inbetween and the parts are heated which melts the fillermaterial and it flows into the space by capillary action. Thefiller materials are copper-base alloys / silver base alloys.Brass is more commonly used filler metal.Eg: Small LPG cylinders, Hydraulic Fittings, HeatExchangers, Tube Manipulations, Machined AssembliesPressed Assemblies etc


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BRAZING OFWATCH ASSEMBLIES, CONNECTORS IN AUTOMOBILES

BRAZE WELDING OF COPPER TUBES, CYCLE FRAMES

Bronze Welding / Braze Welding:This process requires more heat than Brazing and Tin isadded in filler metal for better flowing of melted filler metal.This process is intermediate between true welding and truebrazing. Here the parts are heated to a temp. of meltingpoint of the bronze filling rod which contains 60% Cu and40% Sn. During the operation, the edges of the parent metalare heated by oxy-acetylene flame or some other suitable heatsource. Here the filler metal reaches the Joint without thecapillary action since the Joint gap is more. The filler metalenters the joint by gravity.

Eg: Carbide inserts in tool shanks, carbide drill bits, repairworks etc


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Soldering:This is a method of joining metal parts by means of afusible alloy called solder, applied in the molten state. Fluxesused in soldering are ammonium chloride, zinc chloride etc.The solder is composed of Pb and Sn with a melting point of150 to 350oCSoft soldering: is used for sheet metal works that are notsubjected to excessive loads.Hard Soldering: employed solders whose melting temp. ishigher than soft solders.

Soft solder - lead 37%, tin 63%Medium solder - lead 50%, tin 50%Plumber solder - lead 70%, tin 30%Electricians solder - Lead 58% , tin 42%


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Note: (1) During brazing or soldering flux is used for:Dissolving oxides from the surfaces to be joined, Reducesurface tension of molten filler metal i.e. increasing itswetting action or spreadability, Protect the surface fromoxidation during joining operation. (2) Any metal which has amelting point of < 4500C cannot be used as filler material inbrazing or braze welding and can only be used in soldering.


SOLDERING OF COPPER TUBES, SEAT BELT BRACKETS, STEEL VALVE

TO SiC PLATE

Weld Defects:The defects in the weld can be defined as irregularities inthe weld metal produced due to incorrect weldingparameters or wrong welding procedures or wrongcombination of filler metal and parent metal.Weld defect may be in the form of variations from theintended weld bead shape, size and desired quality.Defects may be on the surface or inside the weld metal.Certain defects such as cracks are never tolerated but otherdefects may be acceptable within permissible limits.Welding defects may result into the failure of componentsunder service condition, leading to serious accidents andcausing the loss of property and sometimes also life.


1) Poor Fusion Lack ofthorough and complete unionbetween the deposited andpresent metal this appears as adiscontinuity in the weld zone.Lack of fusion is because offailure to raise the temperature ofbase metal or previouslydeposited weld layer to meltingpoint during welding. Lack offusion can be avoided byproperly cleaning of surfaces tobe welded, selecting propercurrent, proper welding techniqueand correct size of electrode.


2) Under cutThis appears as a smallnotch in the weldinterface. Main reasonsfor undercutting are theexcessive weldingcurrents, long arclengths and fast travelspeeds.


3) Porosity Porosity results whenthe gases are entrapped in thesolidifying weld metal. These gasesare generated from the flux or coatingconstituents of the electrode orshielding gases used during welding orfrom absorbed moisture in the coating.Rust, dust, oil and grease present on thesurface of work pieces or on electrodesare also source of gases duringwelding. Porosity can also becontrolled if excessively high weldingcurrents, faster welding speeds andlong arc lengths are avoided, flux andcoated electrodes are properly baked.


4)Slag Inclusion These may bein the form of slag or any othernonmetallic material entrapped inthe weld metal as these may notable to float on the surface of thesolidifying weld metal. However,if the molten weld metal has highviscosity or too low temperatureor cools rapidly then the slag maynot be released from the weldpool and may cause inclusion.Slag inclusion can be prevented ifall the slag from the previouslydeposited bead is removed, lowwelding current are avoided.


5) Cracks Cracks occurwhen localized stressesexceed the ultimate tensilestrength of material. Thesestresses are developed due toshrinkage duringsolidification of weld metal.Cracks may be developeddue to poor ductility of basemetal, high sulphur /phosphorous and carboncontents, high arc travelspeeds i.e. fast coolingrates, high hydrogen contentsin the weld metal etc.


6) DistortionBending of components due to improper thermal expansionsand contractions. Hence proper clamping and preheating is tobe done to avoid distortion.


7) Miscellaneous Defects Multiple arc strikes, spatter,grinding & chipping marks,misalignment of weld beads,un removed slag, etc.


Design Considerations:The selection of a welded joint and a welding processinvolves the following considerations:1. The configuration of the component or structure to bewelded, their thickness and size.2. The service requirements, such as type of loading and thestress generated.3. The location, accessibility and ease of welding.4. The effects of distortion and appearance.5. The costs involved in the edge preparation, the welding,post processing of weld including machining and finishingoperations, Heat treatment etc.Design guide lines:1. Product design should minimize the number of welds.2. Components should fit properly before welding.


3. Select designs that can avoid (or) minimize the need foredge preparation.4. Weld bead size should be kept to a minimum to conserveweld metal.5. Weld location should be selected so as not to interfere withfurther processing of the part.Note:1.The correct sequence in ascending order of their weldabilityfor most common metals is : Al < Cu < CI < MS2. Due to improper surface cleaning, hydrogen may enter into weld pool and get dissolved in the weld metal. Duringcooling it diffuses in to HAZ developing cracks due to theresidual stresses assisted by hydrogen coalescence (growingtogether). This is called hydrogen embrittlement.


Welding of Cast Irons:They are difficult to weld because of high carbon contentsand poor ductility.

Massive carbon deposits have a tendency to form in the areasadjacent to the weld.

Thus a high carbon martensite tends to form in the HAZwhich has very brittle micro structure that may lead to cracksduring welding or after welding under load application.

CI is joined by Oxy Acetylene welding and SMAW. Properpre heating and post heat treatment may be required.



Welding of Stainless Steel:Stainless steel is difficult metal to weld because it containsboth Ni and Cr. The best method for welding stainless steel isTIG welding. SMAW is also used but requires use of a heavilycoated electrode. Low current setting with fast travel speed ispreferred for stainless steel as certain stainless steels aresubjected to carbide precipitation.

Ferritic stainless steels are generally less weldable thanaustenitic stainless steels and require both preheating and postweld treatments. Welding ferritic stainless steels can be doneautogenously (or) with an austenitic stainless steel (or) using ahigh nickel filler alloy (or) type 405 filler containinglow % Cr (11%), low % C(0.08%) and small % Al (0.2%). Itcan be welded by TIG, MIG, SMAW, PAW.


WELD DECAYWeld decay is a form of intergranular corrosion, usually foundin corrosion-resistant alloys like stainless steels or certainnickel-base alloys and occurs as the result of sensitization inthe HAZ during the welding operation. The corrosive attack isrestricted to the HAZ. Positive identification of this type ofcorrosion usually requires microstructure examination under amicroscopy although sometimes it is possible to visuallyrecognize welddecay if parallellines are alreadyformed in the HAZalong the weld asshown.


In this case, the precipitation of chromium carbides is inducedby the welding operation when the HAZ experiences aparticular temperature range (550oC~850oC). The precipitationof chromium carbides will consume the alloying element chromium, from a narrow band along the grain boundary andthis makes that zone anodic to the unaffected grains. Thechromium depleted (consumed) zone becomes the preferentialpath for corrosion attack or crack propagation if under tensilestress. Weld decay can be prevented through: Using low carbon (e.g. 304L, 316L) grade of S.S electrodes. Using stabilized electrode grades alloyed with Ti (type 321)

or Nb (type 347). Ti and Nb are strong carbide- formers.They react with the carbon to form the correspondingcarbides thereby preventing chromium depletion.

Use post-weld heat treatment (PWHT).

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Welding - Made Easy