10 mjfirst.ppt

*Dr. N. RAMACHANDRAN, NITC*

Dr. N. RAMACHANDRAN, NITC

*Dr. N. RAMACHANDRAN, NITC*METAL JOININGEven the simplest object is an assembly of componentsComplex ones - greater number of parts- subassemblies joined to perform the functionMETHODS- WELDING, BRAZING, SOLDERING, ADHESIVE BONDING, MECHANICAL JOININGNITC


*Dr. N. RAMACHANDRAN, NITC*WHY JOINING?IMPOSSIBLE TO MAKE AS ONE PIECEEASINESS AND ECONOMY IN MANUFACTUREEASY IN REPAIRS AND MAINTENANCEFUNCTIONAL PROPERTIES DIFFER- e.g.: Carbide tips of tools,corrosion resistant parts, tungsten carbide tip of pens, brake shoes to metal backing etcTRANSPORTING SITE/ CUSTOMERNITC


*Dr. N. RAMACHANDRAN, NITC*CLASSIFICATIONAccording to the STATE of the materials being joinedExtent of external heating- PRESSUREUse of FILLER materials

NITC


*Dr. N. RAMACHANDRAN, NITC*Joining ProcessesRESISTANCEMECH. JOININGARCCUTTINGCHEMICALCONSUMABLENON CONSUMABLEOxy-fuelThermitLIQUIDSOLIDLIQUID-SOLIDSpotSeamProjectionFlashStudpercussionGTAWPAWEBWLBWSMAWSAWGMAWFCAWEGWESWForgeColdUltrasonicFrictionExplosionDiffusionBrazingSolderingAdhesive BondingFasteningCrimpingSeamingStitchingNITC


Welding

*Dr. N. RAMACHANDRAN, NITC*Partial melting and fusion of jointPhysical and mechanical changes taking placeCan be with application of pressure or by addition of filler material

PARTIAL MELTING

BY 1.CHEMICAL REACTION 2. STRIKING AN ARC 3. MAINTAINING RESISTANCE BETWEEN THE PARTS

Prior to joining, PREPARATION TO BE DONE.STANDARDS- AWS; ASTM- TYPES OF GROOVES, JOINTS

LIQUID STATE PROCESSES


of 18

WELDING TERMINOLOGY

*Dr. N. RAMACHANDRAN, NITC*Standard location of elements of weld symbol LPSSpecification process.No tail- SMAWOther side of arrowNear side of ArrowField weldWeld all aroundSizeLength of weldUnwelded lengthG- Grind C- ChipF-File M-MachineR- RollingReference lineFinish symbolArrow connecting reference line to arrow side of joint /to edge prepared /member or bothNITC


*Dr. N. RAMACHANDRAN, NITC*ROOTGROOVE ANGLEJoint angleRoot FaceGroove faceNITC


*Dr. N. RAMACHANDRAN, NITC*WELDING TECHNIQUES FOREHAND BACKHANDTHINSame direction torchHeat concentrated away from beadEven flow, rippled designTHICKOpposite direction torchHeat concentrated on beadBroad bead


*Dr. N. RAMACHANDRAN, NITC*WELD POSITIONS WELD MOVEMENTSFLATHORIZONTALVERTICALOVERHEADHOCJUZIGZAGNITC


*Dr. N. RAMACHANDRAN, NITC*WELD POSITIONSFLAT HORIZONTAL VERTICAL OVERHEADNITC


*Dr. N. RAMACHANDRAN, NITC*ASME Welding Positions Note the welding progression, (vertically upwards or downwards), must always be stated and it is an essential variable for both procedures and performance qualifications. Welding Positions For Groove welds:-

Welding PositionTest PositionISO and ENFlat1GPAHorizontal2GPCVertical Upwards Progression3GPFVertical Downwards Progression3GPGOverhead4GPEPipe Fixed Horizontal5GPFPipe Fixed @ 45 degrees Upwards6GHL045Pipe Fixed @ 45 degrees Downwards6GJL045


*Dr. N. RAMACHANDRAN, NITC*G for Groove Welds

F for Fillet Welds


*Dr. N. RAMACHANDRAN, NITC*Welding Positions For Fillet welds:-

Welding PositionTest PositionISO and ENFlat (Weld flat joint at 45 degrees)1FPAHorizontal2FPBHorizontal Rotated2FRPBVertical Upwards Progression3FPFVertical Downwards Progression3FPGOverhead4FPDPipe Fixed Horizontal5FPF


*Dr. N. RAMACHANDRAN, NITC*G

for Groove Welds

F

for Fillet Welds


*Dr. N. RAMACHANDRAN, NITC*WELD MOVEMENTSOZIGZAGLISTRAIGHTZ


*Dr. N. RAMACHANDRAN, NITC*ASME P Material Numbers Explained ASME has adopted their own designation for welding processes, which are very different from the ISO definitions adopted by EN24063. Straight polarity = Electrode -ve Reverse polarity = Electrode +ve

DesignationDescriptionOFWOxyfuel Gas WeldingSMAWShielded Metal Arc Welding (MMA)SAWSubmerged Arc WeldingGMAWGas Metal Arc Welding (MIG/MAG)FCAWFlux Cored WireGTAWGas Tungsten Arc Welding (TIG)PAWPlasma Arc Welding


*Dr. N. RAMACHANDRAN, NITC*ASME F Numbers Note:- X represents any number 0 to 9

F NumberGeneral Description1Heavy rutile coated iron powder electrodes :- A5.1 : E70242Most Rutile consumables such as :- A5.1 : E60133Cellulosic electrodes such as :- A5.1 : E60114Basic coated electrodes such as : A5.1 : E7016 and E70185High alloy austenitic stainless steel and duplex :- A5.4 : E316L-166Any steel solid or cored wire (with flux or metal)2XAluminium and its alloys3XCopper and its alloys4XNickel alloys5XTitanium6XZirconium7XHard Facing Overlay


*Dr. N. RAMACHANDRAN, NITC*ASME A Numbers These refer to the chemical analysis of the deposited weld and not the parent material. They only apply to welding procedures in steel materials.

A1Plain unalloyed carbon manganese steels.A2 to A4Low alloy steels containing Moly and Chrome MolyA8Austenitic stainless steels such as type 316.


*Dr. N. RAMACHANDRAN, NITC*Types of welds and symbolsFILLET, SQUARE BUTT, SINGLE V, DOUBLE V, SINGLE U, DOUBLE U, SINGLE BEVEL BUTT, DOUBLE BEVEL BUTT, SINGLE J BUTT, DOUBLE J BUTT, STUD, BEAD(EDGE OR SEAL), PLUG, SPOT, SEAM, MASHED SEAM, STITCH, PROJECTION, FLASH, UPSET etc. (REFER sketches supplied)NITC


*Dr. N. RAMACHANDRAN, NITC*Multiple-pass layers. Weld layer sequence


*Dr. N. RAMACHANDRAN, NITC*Welding Positions QW431.1 and QW461.2 Basically there are three inclinations involved. Flat, which includes from 0 to 15 degrees inclination 15 - 80 degrees inclination Vertical, 80 - 90 degrees For each of these inclinations the weld can be rotated from the flat position to Horizontal to overhead.


*Dr. N. RAMACHANDRAN, NITC*UNDERWATER WELDING


*Dr. N. RAMACHANDRAN, NITC*Partial melting and fusion of jointPhysical and mechanical changes taking placeCan be with application of pressure or by addition of filler material

PARTIAL MELTING

BY 1.CHEMICAL REACTION 2. STRIKING AN ARC 3. MAINTAINING RESISTANCE BETWEEN THE PARTS

LIQUID STATE PROCESSES


*Dr. N. RAMACHANDRAN, NITC*LIQUID STATE PROCESSES

Partial melting and fusion of jointPhysical and mechanical changes taking placeCan be with application of pressure or by addition of filler material

PARTIAL MELTING

BY 1.CHEMICAL REACTION


Oxyacetylene Welding (OAW)The oxyacetylene welding process uses a combination of oxygen and acetylene gas to provide a high temperature flame.


Oxyacetylene Welding (OAW)OAW is a manual process in which the welder must personally control the the torch movement and filler rod application The term oxyfuel gas welding outfit refers to all the equipment needed to weld.Cylinders contain oxygen and acetylene gas at extremely high pressure.

*Dr. N. RAMACHANDRAN, NITC*OXY ACETYLENE WELDING (OAW)


Typical Oxyacetylene Welding (OAW) Station

Oxygen CylindersOxygen is stored within cylinders of various sizes and pressures ranging from 2000- 2640 psi. (Pounds Per square inch) Oxygen cylinders are forged from solid armor plate steel. No part of the cylinder may be less than 1/4 thick. Cylinders are then tested to over 3,300 psi using a (NDE) hydrostatic pressure test.

Oxygen CylindersCylinders are regularly re-tested using hydrostatic (NDE) while in service Cylinders are regularly chemically cleaned and annealed to relieve jobsite stresses created by handling .

Cylinder Transportation Never transport cylinders without the safety caps in place Never transport with the regulators in placeNever allow bottles to stand freely. Always chain them to a secure cart or some other object that cannot be toppled easily.

Oxygen CylindersOxygen cylinders incorporate a thin metal pressure safety disk made from stainless steel and are designed to rupture prior to the cylinder becoming damaged by pressure.The cylinder valve should always be handled carefully

Pressure Regulators for CylindersReduce high storage cylinder pressure to lower working pressure. Most regulators have a gauge for cylinder pressure and working pressure.

Pressure Regulators for CylindersRegulators are shut off when the adjusting screw is turn out completely. Regulators maintain a constant torch pressure although cylinder pressure may vary Regulator diaphragms are made of stainless steel

Pressure Regulators Gauges Using a Bourdon movement Gas entering the gauge fills a Bourdon tube As pressure in the semicircular end increases it causes the free end of the tube to move outward. This movement is transmitted through to a curved rack which engages a pinion gear on the pointer shaft ultimately showing pressure.

Regulator Hoses Hoses are are fabricated from rubberOxygen hoses are green in color and have right hand thread.Acetylene hoses are red in color with left hand thread. Left hand threads can be identified by a grove in the body of the nut and it may have ACET stamped on it

Check Valves &Flashback ArrestorsCheck valves allow gas flow in one direction onlyFlashback arrestors are designed to eliminate the possibility of an explosion at the cylinder.Combination Check/ Flashback Valves can be placed at the torch or regulator.

Acetylene Gas Virtually all the acetylene distributed for welding and cutting use is created by allowing calcium carbide (a man made product) to react with water. The nice thing about the calcium carbide method of producing acetylene is that it can be done on almost any scale desired. Placed in tightly-sealed cans, calcium carbide keeps indefinitely. For years, miners lamps produced acetylene by adding water, a drop at a time, to lumps of carbide.

Before acetylene in cylinders became available in almost every community of appreciable size produced their own gas from calcium carbide.

Acetylene CylindersAcetylene is stored in cylinders specially designed for this purpose only. Acetylene is extremely unstable in its pure form at pressure above 15 PSI (Pounds per Square Inch) Acetone is also present within the cylinder to stabilize the acetylene. Acetylene cylinders should always be stored in the upright position to prevent the acetone form escaping thus causing the acetylene to become unstable.

Acetylene CylindersCylinders are filled with a very porous substance monolithic filler to help prevent large pockets of pure acetylene form formingCylinders have safety (Fuse) plugs in the top and bottom designed to melt at 212 F (100 C)

Acetylene ValvesAcetylene cylinder shut off valves should only be opened 1/4 to 1/2 turnThis will allow the cylinder to be closed quickly in case of fire.Cylinder valve wrenches should be left in place on cylinders that do not have a hand wheel.

Oxygen and Acetylene Regulator Pressure SettingsRegulator pressure may vary with different torch styles and tip sizes.PSI (pounds per square inch) is sometimes shown as PSIG (pounds per square inch -gauge) Common gauge settings for cutting1/4 material Oxy 30-35psi Acet 3-9 psi1/2 material Oxy 55-85psi Acet 6-12 psi1 material Oxy 110-160psi Acet 7-15 psiCheck the torch manufactures data for optimum pressure settings

Regulator Pressure Settings

The maximum safe working pressure for acetylene is 15 PSI !

Typical torch styles A small welding torch, with throttle valves located at the front end of the handle. Ideally suited to sheet metal welding. Can be fitted with cutting

attachment in place of the welding head shown. Welding torches of this general design are by far the most widely used. They will handle any oxyacetylene welding job, can be fitted with multiflame (Rosebud) heads for heating applications, and accommodate cutting attachments that will cut steel 6 in. thick.

A full-size oxygen cutting torch which has all valves located in its rear body. Another style of cutting torch, with oxygen valves located at the front end of its handle.

Typical startup procedures Verify that equipment visually appears safe IE: Hose condition, visibility of gauges Clean torch orifices with a tip cleaners (a small wire gauge file set used to clean slag and dirt form the torch tip) Crack (or open) cylinder valves slightly allowing pressure to enter the regulators slowlyOpening the cylinder valve quickly will Slam the regulator and will cause failure.

Typical startup procedures Never stand directly in the path of a regulator when opening the cylinderCheck for leaks using by listening for Hissing or by using a soapy Bubble solution Adjust the regulators to the correct operating pressureSlightly open and close the Oxygen and Acetylene valves at the torch head to purge any atmosphere from the system.

Typical startup procedures Always use a flint and steel spark lighter to light the oxygen acetylene flame.

Never use a butane lighter to light the flame

Flame Settings There are three distinct types of oxy-acetylene flames, usually termed:NeutralCarburizing (or excess acetylene)Oxidizing (or excess oxygen )The type of flame produced depends upon the ratio of oxygen to acetylene in the gas mixture which leaves the torch tip.

*Dr. N. RAMACHANDRAN, NITC*TYPES of FLAMES

Neutral- with inner cone(30400C-33000C), outer envelope, (21000C near inner cone, 12600C at tip)- high heating

Reducing- Bright luminous inner cone, acetylene feather, blue envelopeLow temperature, good for brazing, soldering, flame hardeningHydrogen, methyl acetylene, propadiene also used as fuel.

Oxidising- pointed inner cone, small and narrow outer envelopeHarmful for steels, good for Cu- Cu based alloys

NITC


*Dr. N. RAMACHANDRAN, NITC*OXY ACETYLENE WELDING (OAW)Types of FlamesNeutralReducingOxidisinghigh heatinglow temperaturegood for Cu- Cu alloys


Pure Acetylene and Carburizing Flame profiles

Neutral and Oxidizing Flame Profiles

Flame definition The excess acetylene flame (Fig. 2), as its name implies, is created when the proportion of acetylene in the mixture is higher than that required to produce the neutral flame. Used on steel, it will cause an increase in the carbon content of the weld metal.

The neutral flame (Fig. 3) is produced when the ratio of oxygen to acetylene, in the mixture leaving the torch, is almost exactly one-to-one. Its termed neutral because it will usually have no chemical effect on the metal being welded. It will not oxidize the weld metal; it will not cause an increase in the carbon content of the weld metal.

The oxidizing flame (Fig. 4) results from burning a mixture which contains more oxygen than required for a neutral flame. It will oxidize or burn some of the metal being welded.

*Dr. N. RAMACHANDRAN, NITC*THERMIT WELDING

LIQUID STATE JOINING PROCESSPARTIAL MELTING BY CHEMICAL REACTION

USE OF Fine particles of iron oxide, aluminium oxide, iron & aluminium Termed THERMITE- based on Therm, meaning heatInvolves exothermic reactions between metal oxides and metallic reducing agentsHeat of reaction used for welding.Reactions are:(3/4) Fe3 O4 + 2 Al --- (9/4) Fe + Al2O3 + Heat 3 FeO + 2 Al --- 3 Fe + Al2O3 + HeatFe2O3 + 2Al --- 2Fe + Al2O3+ Heat


THERMIT WELDING

*Dr. N. RAMACHANDRAN, NITC*Slide 13 of 18

THERMIT WELDING


*Dr. N. RAMACHANDRAN, NITC*Mixture is non explosive. Produces temperature of 32000 C within a minute

Practically about 22000- 24000 C. Other materials to impart special properties added. Applying a Mg fuse of special compounds of peroxides, chlorates/ chromates.

Welding copper, brasses, bronzes and copper alloys to steel using oxides of copper, nickel, aluminium, manganese temperatures of 50000 C obtained


*Dr. N. RAMACHANDRAN, NITC*THERMIT WELDING OF RAILS


*Dr. N. RAMACHANDRAN, NITC*Effects of expansion and contraction


*Dr. N. RAMACHANDRAN, NITC*CONTROLLING DISTORTION


*Dr. N. RAMACHANDRAN, NITC*HEAT AFFECTED ZONE


*Dr. N. RAMACHANDRAN, NITC*SOLID STATE PROCESSESJoining without fusion of work piecesNo liquid (molten ) phase present in jointPrinciple: If two clean surfaces are brought into atomic contact with each other - made with sufficient pressure -(in the absence of oxide film and other contaminents) they form bonds and produce strong jointTo improve strength, heat and some movement of mating surfaces by plastic deformation employed. Eg: USW, Friction Welding (FRW)


*Dr. N. RAMACHANDRAN, NITC*FORGE WELDING (FOW)

Both elevated temperature and pressure applied to form strong bond between membersComponents heated and pressed/ hammered with tools, dies or rollers Local plastic deformation at interface breaks up the oxide films improves bond strength. Not for high load bearing applications.


*Dr. N. RAMACHANDRAN, NITC*COLD WELDING (CW)Pressure applied to work pieces either through dies or rollsOne (or both) of the mating parts must be ductileInterface cleaned prior to welding- brushing etc.

RollRolling metalBare metal


*Dr. N. RAMACHANDRAN, NITC*EXPLOSIVE WELDING (EXW)Solid state bonding processJoining by the cohesive force between atoms of two intimate contact surfaces High pressure waves- thousands of MPa created-To weld dissimilar metals, thick to thin, high difference in Melting Point metals.Not a costly processExtremely large surfaces can be joined (2m X 10 m)Welding of heat treated metals without affecting the processNo HAZIncompatible metals joined(thin foils to heavy plates) severe deformation needed for joining.


*Dr. N. RAMACHANDRAN, NITC*Principle: Explosive Impulse used to produce extremely high normal pressure and a slight shear or sliding pressure ( uses a detonator for this) Two properly laid metal surfaces brought together with high relative velocity at high pressure

Large amount of plastic interaction between surfaces results.

Two ways


*Dr. N. RAMACHANDRAN, NITC*Plastic interaction by positioning explosive charge to deliver shock waves at an oblique angle to parts to be welded- Less frequently used.(1)Contact technique


*Dr. N. RAMACHANDRAN, NITC*(2) Impact techniqueTwo pieces explosively projected towards each other. Impact with high velocity (200 400 m/s)


*Dr. N. RAMACHANDRAN, NITC*Severe deformation needed for joining (minimum 40 to 60%), as welding is by pressure.


*Dr. N. RAMACHANDRAN, NITC*Detonation velocity approx. 7000 m/s in the detonation front.Produces pressure at interface 7000 to 70,000 atms. Parts driven at an angle Velocity of impact and angle of collapse selected. Joining as s result of intense plastic flow at the surface called surface jettingFor good joint, surface to be free from contaminantsPressure sufficient to bring surfaces within interatomic distances of each other [ In a range of speed and angle of impact, a high velocity metal jet forms. Removes surface contamination. Speed, angle(10 to 100) of detonation important]


*Dr. N. RAMACHANDRAN, NITC*Bond as strong as the weaker of the two obtained. 100 % efficient joint, (eg. In sheet forming in aerospace industries)At the interface, microhardness slightly increased. (because of plastic deformation and strain hardening- a very thin hardness zone)


*Dr. N. RAMACHANDRAN, NITC*Titanium cladding commonOthers- Ni, SS(50 mm), tantalum, carbon steels, for heat exchangers, tubes, pressure vessels, etc.No change in chemical and physical properties of parent metalBut, not for brittle alloys. Metal must possess some ductility. [Quantity of charge, detonation velocity, and deformation characteristics of flyer plate decide the weld]Also spot welding by small charge. Handy explosive spot welding sets available (for 10mm to 12 mm spots)


*Dr. N. RAMACHANDRAN, NITC*Minus points: : Severe deformation needed for joining (minimum 40 to 60/ 50, as welding is by pressure.


*Dr. N. RAMACHANDRAN, NITC*LIQUID STATE PROCESSESPartial melting and fusion of jointPhysical and mechanical changes taking placeCan be with application of pressure or by addition of filler material

Prior to joining, PREPARATION TO BE DONESTANDARDS- AWS; ASTM- TYPES OF GROOVES, JOINTSNITC


*Dr. N. RAMACHANDRAN, NITC*LIQUID STATE PROCESSPARTIAL MELTING BY STRIKING AN ARC

AFTER THE INVENTION OF ELECTRICITY

HOW ARC STRUCK? ARC COLUMN THEORY


ANODE +

CATHODE -

ELECTRICAL / IONIC THEORY

IONS FROM ANODE TO CATHODE,AS METAL IONS ARE +VE CHARGED

DCARC COLUMN THEORY

TOUCH AND THEN ESTABLISH A GAP TO BALANCE THE ATOMIC STRUCTURE

IONS COLLIDE WITH GAS MOLECULES

PRODUCES A THERMAL IONISATION LAYER

IONISED GAS COLUMN AS HIGH RESISTANCE CONDUCTOR

ON STRIKING CATHODE, HEAT GENERATEDTERMED AS IONIC THEORY

NOT COMPLETE IN EXPLAINING ARC COLUMN THEORYTHUS, ELECTRON THEORY


ANODE +

CATHODE - ELECTRON THEORY

IONS FROM ANODE TO CATHODEAS METAL IONS ARE +VE CHARGED

-VELY CHARGED ELECTRONS DISSOCIATED FROM CATHODE MOVE OPPOSITE WITH HIGH VELOCITY

DC

(MASS- 9.1x 10-28 gm)CAUSES HEAT IN ARC COLUMNRELEASES HEAT ENERGY IN STRIKING THE ANODE

CALLED ELECTRON IMPINGEMENT AND IONIC BOMBARDMENT

ARC COLUMN THEORY



HIGH HEAT

MEDIUM HEATLOW HEAT

ANODE+CATHODE -

ELECTRON IMPINGEMENTIONIC BOMBARDMENT


*Dr. N. RAMACHANDRAN, NITC*MAGNETIC FLUX THEORYTHE COLUMN NOT FLAIRING DUE TO THE FLUX LINES AROUND THE ARC COLUMN. (Right hand Thumb Rule)

THIS COMPLETES THE ARC COLUMN THEORY


*Dr. N. RAMACHANDRAN, NITC*POLARITYACCurrents higher than those of DCRP can be employed (400A to 500 A for 6 mm electrode)Arc cleaning of the base metalNormal penetrationEqual heat distribution at electrode and jobElectrode tip is colder as compared to that in DCRPAverage arc voltage in argon atmosphere is 16V


*Dr. N. RAMACHANDRAN, NITC*DCRPCurrents generally less than 125 amps (up to 6 mm dia electrodes) to avoid overheating2/3rd heat at electrode and 1/3rd at the jobLeast penetrationAverage arc voltage on argon atmosphere is 19VChances of electrode overheating, melting and lossesBetter arc cleaning action


*Dr. N. RAMACHANDRAN, NITC*DCSPWelding currents up to 1000 amps can be employed for 6 mm electrodes33.33% heat is generated at the electrode and 66.66% at the job.Deep penetrationAverage arc voltage in an argon atmosphere is 12 VElectrode runs colder as compared to AC or DCRP No are cleaning of base metal


SHIELDED METAL ARC WELDING (SMAW)

*Dr. N. RAMACHANDRAN, NITC*Shielded metal arc welding (SMAW), Also known as Manual Metal Arc (MMA) welding Informally as stick welding, is a manual arc welding process that uses a consumable electrode coated in flux to lay the weld.

An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined.

As the weld is laid, the flux coating of the electrode disintegrates, giving off vapors that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination.


*Dr. N. RAMACHANDRAN, NITC*Because of the versatility of the process and the simplicity of its equipment and operation, shielded metal arc welding is one of the world's most popular welding processes. It dominates other welding processes in the maintenance and repair industry, used extensively in the construction of steel structures and in industrial fabrication. The process is used primarily to weld iron and steels (including stainless steel) but aluminum, nickel and copper alloys can also be welded with this method. Flux-Cored Arc Welding (FCAW) , a modification to SMAW is growing in popularity


*Dr. N. RAMACHANDRAN, NITC*SAFETY PRECAUTIONS Uses an open electric arc, so risk of burns to be prevented by protective clothing in the form of heavy leather gloves and long sleeve jackets. The brightness of the weld area can lead arc eye, in which ultraviolet light causes the inflammation of the cornea and can burn the retinas of the eyes. Welding helmets with dark face plates to be worn to prevent this exposureNew helmet models have been produced that feature a face plate that self-darkens upon exposure to high amounts of UV light. To protect bystanders, especially in industrial environments, transparent welding curtains often surround the welding area. These are made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets.


*Dr. N. RAMACHANDRAN, NITC*Arc eye, also known as arc flash or welder's flash or corneal flash burns, is a painful condition sometimes experienced by welders who have failed to use adequate eye protection. It can also occur due to light from sunbeds, light reflected from snow (known as snow blindness), water or sand. The intense ultraviolet light emitted by the arc causes a superficial and painful keratitis. Symptoms tend to occur a number of hours after exposure and typically resolve spontaneously within 36 hours. It has been described as having sand poured into the eyes.ARC EYE


*Dr. N. RAMACHANDRAN, NITC*SignsIntense lacrimation Blepharospasm Photophobia Fluorescein dye staining will reveal corneal ulcers under blue light ManagementInstill topical anaesthesia Inspect the cornea for any foreign body Patch the worse of the two eyes and prescribe analgesia Topical antibiotics in the form of eye drops or eye ointment or both should be prescribed for prophylaxis against infection


*Dr. N. RAMACHANDRAN, NITC*EQUIPMENT


*Dr. N. RAMACHANDRAN, NITC*Various welding electrodes and an electrode holder


*Dr. N. RAMACHANDRAN, NITC*PURPOSE OF COATINGGives out inert or protective gas- shieldsStabilizes the arc- by chemicalsLow rate consumption of electrode- directs arc and molten metalRemoves impurities and oxides as slagCoatings act as insulators- so narrow grooves weldedProvide means to introduce alloying elements

Bare electrodes - carbon- more conductive- slow consumption in welding


*Dr. N. RAMACHANDRAN, NITC*ELECTRODE COATING INGREDIENTSSlag forming ingredients- silicates of sodium, potassium, Mg, Al, iron oxide, China clay, mica etc.Gas shielding- cellulose, wood, starch, calcium carbonateDe-oxidising elements- ferro manganese, ferro silicon- to refine molten metalArc stabilizing calcium carbonate, potassium silicate, titanates, Mg silicate etc..Alloying elements- ferro alloys, Mn, Mo., to impart special propertiesIron powder- to improve arc behaviour, bead appearanceOther elements - to improve penetration, limit spatter, improve metal deposition rates,


*Dr. N. RAMACHANDRAN, NITC*Submerged arc welding


*Dr. N. RAMACHANDRAN, NITC*CONTROL PANEL


*Dr. N. RAMACHANDRAN, NITC*Submerged Arc Welding (SAW)Is a common arc welding process. A continuously fed consumable solid or tubular (metal cored) electrode used. The molten weld and the arc zone are protected from atmospheric contamination by being submerged under a blanket of granular fusible flux. When molten, the flux becomes conductive, and provides a current path between the electrode and the work


*Dr. N. RAMACHANDRAN, NITC*Normally operated in the automatic or mechanized mode. Semi-automatic (hand-held) SAW guns with pressurized or gravity flux feed delivery are available. The process is normally limited to the 1F, 1G, or the 2F positions (although 2G position welds have been done with a special arrangement to support the flux). Deposition rates approaching 45 kg/h have been reported this compares to ~5 kg/h (max) for shielded metal arc welding.Currents ranging from 200 to 1500 A are commonly used; currents of up to 5000 A have been used (multiple arcs).


*Dr. N. RAMACHANDRAN, NITC*Single or multiple (2 to 5) electrode wire variations of the process exist SAW strip-cladding utilizes a flat strip electrode (e.g. 60 mm wide x 0.5 mm thick). DC or AC power can be utilized, and combinations of DC and AC are common on multiple electrode systems. Constant Voltage welding power supplies are most commonly used, however Constant Current systems in combination with a voltage sensing wire-feeder are available.


*Dr. N. RAMACHANDRAN, NITC*Material applicationsCarbon steels (structural and vessel construction); Low alloy steels; Stainless Steels; Nickel-based alloys; Surfacing applications (wearfacing, build-up, and corrosion resistant overlay of steels).


*Dr. N. RAMACHANDRAN, NITC*Advantages of SAW

High deposition rates (over45 kg/h) have been reported; High operating factors in mechanized applications; Deep weld penetration; Sound welds are readily made (with good process design and control); High speed welding of thin sheet steels at over 2.5 m/min is possible; Minimal welding fume or arc light is emitted.


*Dr. N. RAMACHANDRAN, NITC*Limitations of SAW

Limited to ferrous (steel or stainless steels) and some nickel based alloys; Normally limited to the 1F, 1G, and 2F positions; Normally limited to long straight seams or rotated pipes or vessels; Requires relatively troublesome flux handling systems; Flux and slag residue can present a health & safety issue; Requires inter-pass and post weld slag removal.


*Dr. N. RAMACHANDRAN, NITC*Key SAW process variables

Wire Feed Speed (main factor in welding current control); Arc Voltage; Travel Speed; Electrical Stick-Out (ESO) or Contact Tip to Work (CTTW); Polarity and Current Type (AC or DC). Other factors

Flux depth/width; Flux and electrode classification and type; Electrode wire diameter; Multiple electrode configurations.


GAS TUNGSTEN ARC WELDING (GTAW)

*Dr. N. RAMACHANDRAN, NITC*Gas tungsten arc welding


*Dr. N. RAMACHANDRAN, NITC*GTAWFusion Welding ProcessArc Between Non-Consumable Tungsten Rod And WorkArc & Weld Pool Shielded By Argon/GasFiller Wire Separately Added To Weld PoolWelding Torch & Tungsten Rod Cooled by Flow OF Argon / Cooling Water


*Dr. N. RAMACHANDRAN, NITC*GAS TUNGSTEN ARC WELDING (GTAW)ELECTRODE NOT CONSUMEDTUNGSTEN ELECTRODES USEDARGON- HEAVIER FOR NARROW AND LIMITED EXPANSION,WIDER, DEEPER PUDDLE HELIUM FOR EVEN EXPANSIONLIMITED STRESS BUILDUPMORE He, MORE HEAT IN ARCAr-He MIX FOR AUTOMATIC GTAWAr- CO2 FOR CARBON STEELS, ECONIMICAL, INCREASES WETTING ACTIONGTAW TORCH- WATER OR AIR COOLED CONSTANT CURRENT SOURCE.(IIIr TO SMAW)


*Dr. N. RAMACHANDRAN, NITC*GTAW Equipment & AccessoriesPower Source Inverter, Thyrister, Rectifier, GeneratorHigh Frequency UnitWater Cooling SystemWelding Torch- (Ceramic Cup, Tungsten Rod, Collet, Gas-lens) Pedal SwitchArgon Gas Cylinder Pressure Gauge, Regulator, Flow Meter Earthing Cable With Clamp


*Dr. N. RAMACHANDRAN, NITC*Equipment & Accessories +Argon Gas InFlow MeterWelding Cable & Cooling Water In TubeHF Unit & Water Cooling SystemArgon CylinderPressure RegulatorCooling Water InCooling Water OutArgon ShieldingTungsten RodPower Source Work Arc+High Frequency Connection Solenoid ValveCeramic CupPedal Switch Gas Lens


*Dr. N. RAMACHANDRAN, NITC*Equipment

GTAW torch, disassembledGTAW torch with various electrodes, cups, collets and gas diffusers


*Dr. N. RAMACHANDRAN, NITC*Gas tungsten arc welding (GTAW), commonly known as Tungsten Inert Gas (TIG) weldingIs an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.


*Dr. N. RAMACHANDRAN, NITC* Most commonly used to weld thin sections of stainless steel and light metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing procedures such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.


*Dr. N. RAMACHANDRAN, NITC*GTAW system setup


*Dr. N. RAMACHANDRAN, NITC*Applications

Aerospace industry is one of the primary users of gas tungsten arc welding, the process is used in a number of other areas. Many industries use GTAW for welding thin workpieces, especially nonferrous metals. It is used extensively in the manufacture of space vehicles, and is also frequently employed to weld small-diameter, thin-wall tubing. Is often used to make root or first pass welds for piping of various sizes. In maintenance and repair work, the process is commonly used to repair tools and dies, especially components made of aluminum and magnesium. Because the welds it produces are highly resistant to corrosion and cracking over long time periods, GTAW is the welding procedure of choice for critical welding operations like sealing spent nuclear fuel canisters before burial.


*Dr. N. RAMACHANDRAN, NITC*ABOUT THE POWER SOURCEDCRP, DCSP, ACHF USED ELECTRODES OF 0.25 mm TO 6.4 mm FOR DIFFERENT APPLICATIONSELECTRODES CODED, WITH COLOR STRIPSBEST FOR ALUMINIUM, SINCE OXIDE FILM BREAKS BY PENETRATION

Frequent cleaning and shaping of electrode tip to be done


*Dr. N. RAMACHANDRAN, NITC*QualityGTAW ranks the highest in terms of the quality of weld produced.Operation must be with free from oil, moisture, dirt and other impurities, as these cause weld porosity and consequently a decrease in weld strength and quality. To remove oil & grease, alcohol or similar commercial solvents used, while a stainless steel wire brush or chemical process remove oxides from the surfaces of metals like aluminum. Rust on steels removed by first grit blasting the surface and then using a wire brush to remove imbedded grit. These steps important when DCEN used, because this provides no cleaning during the welding process, unlike DCEPor AC. To maintain a clean weld pool during welding, the shielding gas flow should be sufficient and consistent so that the gas covers the weld and blocks impurities in the atmosphere. GTA welding in windy or drafty environments increases the amount of shielding gas necessary to protect the weld, increasing the cost and making the process unpopular outdoors.


*Dr. N. RAMACHANDRAN, NITC*Because of GTAW's relative difficulty and the importance of proper technique, skilled operators are employed for important applications. Low heat input, caused by low welding current or high welding speed, can limit penetration and cause the weld bead to lift away from the surface being welded. If there is too much heat input, the weld bead grows in width while the likelihood of excessive penetration and spatter increase. If the welder holds the welding torch too far from the workpiece, shielding gas is wasted and the appearance of the weld worsens.


*Dr. N. RAMACHANDRAN, NITC*If the amount of current used exceeds the capability of the electrode, tungsten inclusions in the weld may result. Known as tungsten spitting, it can be identified with radiography and prevented by changing the type of electrode or increasing the electrode diameter. If the electrode is not well protected by the gas shield or the operator accidentally allows it to contact the molten metal, it can become dirty or contaminated. This often causes the welding arc to become unstable, requiring that electrode be ground with a diamond abrasive to remove the impurity.


*Dr. N. RAMACHANDRAN, NITC*GTAW welding torches designed for either automatic or manual operation and are equipped with cooling systems using air or water. The automatic and manual torches are similar in construction, but the manual torch has a handle while the automatic torch normally comes with a mounting rack. The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, can be varied on some manual torches according to the preference of the operator. Air cooling systems are most often used for low-current operations (up to about 200A), while water cooling is required for high-current welding (up to about 600A). The torches are connected with cables to the power supply and with hoses to the shielding gas source and where used, the water supply.


*Dr. N. RAMACHANDRAN, NITC*The internal metal parts of a torch are made of hard alloys of copper or brass in order to transmit current and heat effectively. The tungsten electrode must be held firmly in the center of the torch with an appropriately sized collet, and ports around the electrode provide a constant flow of shielding gas. The body of the torch is made of heat-resistant, insulating plastics covering the metal components, providing insulation from heat and electricity to protect the welder.


*Dr. N. RAMACHANDRAN, NITC*GTAW TORCHTungsten RodCeramic CupArcArgon Gas InletCooling Water OutletCooling Water Inlet Tube with cableBase MetalTorch HandleCap with collet For Holding TungstenArgon Shielding GasEarthing Cable


*Dr. N. RAMACHANDRAN, NITC*The size of the welding torch nozzle depends on the size of the desired welding arc, and the inside diameter of the nozzle is normally at least three times the diameter of the electrode. The nozzle must be heat resistant and thus is normally made of alumina or a ceramic material, but fused quartz, a glass-like substance, offers greater visibility. Devices can be inserted into the nozzle for special applications, such as gas lenses or valves to control shielding gas flow and switches to control welding current.


*Dr. N. RAMACHANDRAN, NITC*Power supply

GTAW uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult.


*Dr. N. RAMACHANDRAN, NITC*The preferred polarity of the GTAW system depends largely on the type of metal being welded.

DCEN is often employed when welding steels, nickel, titanium, and other metals. It can also be used in automatic GTA welding of aluminum or magnesium when helium is used as a shielding gas. The negatively charged electrode generates heat by emitting electrons which travel across the arc, causing thermal ionization of the shielding gas and increasing the temperature of the base material. The ionized shielding gas flows toward the electrode, not the base material, and this can allow oxides to build on the surface of the weld.

DCEP is less common, and is used primarily for shallow welds since less heat is generated in the base material. Instead of flowing from the electrode to the base material, as in DCEN, electrons go the other direction, causing the electrode to reach very high temperatures. To help it maintain its shape and prevent softening, a larger electrode is often used. As the electrons flow toward the electrode, ionized shielding gas flows back toward the base material, cleaning the weld by removing oxides and other impurities and thereby improving its quality and appearance.


*Dr. N. RAMACHANDRAN, NITC*AC commonly used when welding aluminum and magnesium manually or semi-automatically, combines the two direct currents by making the electrode and base material alternate between positive and negative charge. This causes the electron flow to switch directions constantly, preventing the tungsten electrode from overheating while maintaining the heat in the base material. This makes the ionized shielding gas constantly switch its direction of flow, causing impurities to be removed during a portion of the cycle. Some power supplies enable operators to use an unbalanced alternating current wave by modifying the exact percentage of time that the current spends in each state of polarity, giving them more control over the amount of heat and cleaning action supplied by the power source. In addition, operators must be wary of rectification, in which the arc fails to reignite as it passes from straight polarity (negative electrode) to reverse polarity (positive electrode). To remedy the problem, a square wave power supply can be used, as can high frequency voltage to encourage ignition.


*Dr. N. RAMACHANDRAN, NITC*Tungsten Rod

Non Consumable Electrode.

Maintains Stable Arc

Tip to be Ground to a cone Shape of 60 to 30 angle

Thoriated Tungsten for General Application, Zerconiated Tungsten for Aluminium Welding

Sizes :- 2, 2.4 & 3 mm Tungsten RodGround to 50 angle


*Dr. N. RAMACHANDRAN, NITC*The electrode used in GTAW is made of tungsten or a tungsten alloy, because tungsten has the highest melting temperature among metals, at 3422C. The electrode is not consumed during welding, though some erosion (called burn-off) can occur. Electrodes can have either a clean finish or a ground finishclean finish electrodes have been chemically cleaned, while ground finish electrodes have been ground to a uniform size and have a polished surface, making them optimal for heat conduction. The diameter of the electrode can vary between 0.5mm and 6.4mm, and their length can range from 75 to 610mm .

ISO ClassISO ColorAWS ClassAWS ColorAlloy [18]WPGreenEWPGreenNoneWC20GrayEWCe-2Orange~2% CeO2WL10BlackEWLa-1Black~1% LaO2WL15GoldEWLa-1.5Gold~1.5% LaO2WL20Sky-blueEWLa-2Blue~2% LaO2WT10YellowEWTh-1Yellow~1% ThO2WT20RedEWTh-2Red~2% ThO2WT30Violet~3% ThO2WT40Orange~4% ThO2WY20Blue~2% Y2O3WZ3BrownEWZr-1Brown~0.3% ZrO2WZ8White~0.8% ZrO2


*Dr. N. RAMACHANDRAN, NITC*A number of tungsten alloys have been standardized by the International Organization for Standardization and the American Welding Society in ISO 6848 and AWS A5.12, respectively, for use in GTAW electrodes- refer table

Pure tungsten electrodes (classified as WP or EWP) are general purpose and low cost electrodes. Cerium oxide (or ceria) as an alloying element improves arc stability and ease of starting while decreasing burn-off. Using an alloy of lanthanum oxide (or lanthana) has a similar effect. Thorium oxide (or thoria) alloy electrodes were designed for DC applications and can withstand somewhat higher temperatures while providing many of the benefits of other alloys. However, it is somewhat radioactive, and as a replacement, electrodes with larger concentrations of lanthanum oxide can be used. Electrodes containing zirconium oxide (or zirconia) increase the current capacity while improving arc stability and starting and increasing electrode life.

Electrode manufacturers may create alternative tungsten alloys with specified metal additions, and these are designated with the classification EWG under the AWS system.

Filler metals are also used in nearly all applications of GTAW, the major exception being the welding of thin materials. Filler metals are available with different diameters and are made of a variety of materials. In most cases, the filler metal in the form of a rod is added to the weld pool manually, but some applications call for an automatically fed filler metal, which is fed from rolls.


*Dr. N. RAMACHANDRAN, NITC*shielding gasesNecessary in GTAW to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come in contact with the electrode, the arc, or the welding metal. The gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc.The selection of a shielding gas depends on several factors, including the type of material being welded, joint design, and desired final weld appearance. Argon is the most commonly used shielding gas for GTAW, since it helps prevent defects due to a varying arc length. When used with alternating current, the use of argon results in high weld quality and good appearance. Another common shielding gas, helium, is most often used to increase the weld penetration in a joint, to increase the welding speed, and to weld conductive metals like copper and aluminum. A significant disadvantage is the difficulty of striking an arc with helium gas, and the decreased weld quality associated with a varying arc length.


*Dr. N. RAMACHANDRAN, NITC*Shielding GasInert Gas - Argon , HeliumCommon Shielding Gas ArgonWhen Helium Is Used Called Heli Arc WeldingWhen Argon Is Used Called Argon Arc WeldingInert Gas Prevents Contamination Of Molten MetalIt Prevents Oxidation Of Tungsten RodIt Ionizes Air Gap and Stabilizes ArcIt Cools Welding Torch & Tungsten Rod


*Dr. N. RAMACHANDRAN, NITC*Shielding GasArgon - Purity 99.95%

Impure Argon Results In Porosities

Purity Verified by Fusing BQ CS plate

Leakage of Argon in Torch Results in Porosity.

Check Leakage by Closing the Ceramic Cup With Thump


*Dr. N. RAMACHANDRAN, NITC*Argon Gas CylinderLight Blue In Colour

Full Cylinder Pressure: 1800 psi ( 130 Kgs / cm2 ) Volume Of Argon In Full Cylinder: 7.3 M3

Commercial Argon (99.99%) Cost: Rs 70/- Per M3

High Purity Argon (99.999) Cost: Rs 87/- Per M3


*Dr. N. RAMACHANDRAN, NITC*Back PurgingPurging Gas Commercial Argon or NitrogenApplicable to Single Sided full penetration Prevents oxidation of root pass from opposite side of weldEssential for high alloy steels, nonferrous metals and alloysDesirable For All MaterialWelding TorchRoot PassPurging Gas InPurging Gas OutPurging chamberFiller Wire


*Dr. N. RAMACHANDRAN, NITC*Argon-helium mixtures are also frequently utilized in GTAW, since they can increase control of the heat input while maintaining the benefits of using argon. Normally, the mixtures are made with primarily helium (often about 75% or higher) and a balance of argon. These mixtures increase the speed and quality of the AC welding of aluminum, and also make it easier to strike an arc. Argon-hydrogen, is used in the mechanized welding of light gauge stainless steel, but because hydrogen can cause porosity, its uses are limited. Nitrogen can sometimes be added to argon to help stabilize the austenite in austentitic stainless steels and increase penetration when welding copper. Due to porosity problems in ferritic steels and limited benefits, however, it is not a popular shielding gas additive.


*Dr. N. RAMACHANDRAN, NITC*Materials

Most commonly used to weld stainless steel and nonferrous materials, such as aluminum and magnesium, but it can be applied to nearly all metals, with notable exceptions being lead and zinc. Its applications involving carbon steels are limited not because of process restrictions, but because of the existence of more economical steel welding techniques, such as gas metal arc welding and shielded metal arc welding. GTAW can be performed in a variety of other-than-flat positions, depending on the skill of the welder and the materials being welded.


*Dr. N. RAMACHANDRAN, NITC*A TIG weld showing an accentuated AC etched zoneCloseup view of an aluminium TIG weld AC etch zone


*Dr. N. RAMACHANDRAN, NITC*Aluminum and magnesium are most often welded using alternating current, but the use of direct current is also possible, depending on the properties desired. Before welding, the work area should be cleaned and may be preheated to 175-200C for aluminum or to a maximum of 150C for thick magnesium workpieces to improve penetration and increase travel speed. AC current can provide a self-cleaning effect, removing the thin, refractory aluminium oxide (sapphire) layer that forms on aluminium metal within minutes of exposure to air. This oxide layer must be removed for welding to occur. When alternating current is used, pure tungsten electrodes or zirconiated tungsten electrodes are preferred over thoriated electrodes, as the latter are more likely to "spit" electrode particles across the welding arc into the weld. Blunt electrode tips are preferred, and pure argon shielding gas should be employed for thin workpieces. Introducing helium allows for greater penetration in thicker workpieces, but can make arc starting difficult.


*Dr. N. RAMACHANDRAN, NITC*Direct current of either polarity, positive or negative, can be used to weld aluminum and magnesium as well. DCEN allows for high penetration, and is most commonly used on joints with butting surfaces, such as square groove joints. Short arc length (generally less than 2mm or 0.07in) gives the best results, making the process better suited for automatic operation than manual operation. Shielding gases with high helium contents are most commonly used with DCEN, and thoriated electrodes are suitable. DCEP is used primarily for shallow welds, especially those with a joint thickness of less than 1.6mm. While still important, cleaning is less essential for DCEP than DCEN, since the electron flow from the workpiece to the electrode helps maintain a clean weld. A large, thoriated tungsten electrode is commonly used, along with a pure argon shielding gas.


*Dr. N. RAMACHANDRAN, NITC*SteelsFor GTA welding of carbon and stainless steels, the selection of a filler material is important to prevent excessive porosity. Oxides on the filler material and workpieces must be removed before welding to prevent contamination, and immediately prior to welding, alcohol or acetone should be used to clean the surface. Preheating is generally not necessary for mild steels less than one inch thick, but low alloy steels may require preheating to slow the cooling process and prevent the formation of martensite in the heat-affected zone. Tool steels should also be preheated to prevent cracking in the heat-affected zone. Austenitic stainless steels do not require preheating, but martensitic and ferritic chromium stainless steels do. A DCEN power source is normally used, and thoriated electrodes, tapered to a sharp point, are recommended. Pure argon is used for thin workpieces, but helium can be introduced as thickness increases.


*Dr. N. RAMACHANDRAN, NITC*Dissimilar metals

Welding dissimilar metals often introduces new difficulties to GTA welding, because most materials do not easily fuse to form a strong bond. Welds of dissimilar materials have numerous applications in manufacturing, repair work, and the prevention of corrosion and oxidation. In some joints, a compatible filler metal is chosen to help form the bond, and this filler metal can be the same as one of the base materials (eg:, using a stainless steel filler metal stainless steel and carbon steel as base materials), or a different metal (such as the use of a nickel filler metal for joining steel and cast iron). Very different materials may be coated or "buttered" with a material compatible with a particular filler metal, and then welded. In addition, GTAW can be used in cladding or overlaying dissimilar materials.When welding dissimilar metals, the joint must have an accurate fit, with proper gap dimensions and bevel angles. Care should be taken to avoid melting excessive base material. Pulsed current is particularly useful for these applications, as it helps limit the heat input. The filler metal should be added quickly, and a large weld pool should be avoided to prevent dilution of the base materials.


*Dr. N. RAMACHANDRAN, NITC*Process variationsPulsed-currentIn the pulsed-current mode, the welding current rapidly alternates between two levels. The higher current state is known as the pulse current, while the lower current level is called the background current. During the period of pulse current, the weld area is heated and fusion occurs. Upon dropping to the background current, the weld area is allowed to cool and solidify. Pulsed-current GTAW has a number of advantages, including lower heat input and consequently a reduction in distortion and warpage in thin workpieces. In addition, it allows for greater control of the weld pool, and can increase weld penetration, welding speed, and quality. A similar method, manual programmed GTAW, allows the operator to program a specific rate and magnitude of current variations, making it useful for specialized applications.


*Dr. N. RAMACHANDRAN, NITC*DabberThe Dabber variation is used to precisely place weld metal on thin edges. The automatic process replicates the motions of manual welding by feeding a cold filler wire into the weld area and dabbing (or oscillating) it into the welding arc. It can be used in conjunction with pulsed current, and is used to weld a variety of alloys, including titanium, nickel, and tool steels. Common applications include rebuilding seals in jet engines and building up saw blades, milling cutters, drill bits, and mower blades


*Dr. N. RAMACHANDRAN, NITC*Heat-affected zone

The cross-section of a welded butt joint, with the darkest gray representing the weld or fusion zone, the medium gray the heat affected zone, and the lightest gray the base material.


*Dr. N. RAMACHANDRAN, NITC*The heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding. The heat from the welding process and subsequent re-cooling causes this change in the area surrounding the weld. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input by the welding process.The thermal diffusivity of the base material plays a large role if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Alternatively, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat inputted by the welding process plays an important role as well, as processes like oxyfuel welding use high heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input


*Dr. N. RAMACHANDRAN, NITC*To calculate the heat input for arc welding procedures, the formula used is:where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8.


*Dr. N. RAMACHANDRAN, NITC*Types Of GTAW Power SourceInverter- DC

Thyrister DC

Motor Generator DC

Rectifier DC

Transformer AC (For Aluminium Welding Only)


*Dr. N. RAMACHANDRAN, NITC*Power Source Provides Electric Energy Arc Heat

Drooping Characteristic

OCV Approx. 90V, Current Range 40 A to 300 A ( Capacity Of M/s)

Arc Voltage 18V to 26V


*Dr. N. RAMACHANDRAN, NITC*Characteristic Of GTAW Power Source

AVertical CurveV1V2A1A2V

Drooping Constant Current


*Dr. N. RAMACHANDRAN, NITC*High Frequency Unit Provides High Voltage Electric Energy With Very high Frequency 10000 Cycles / Sec.

Initiates low energy Arc / Spark & Ionize Air Gap.

Electrically charges Air Gap For welding Current to Jump Across the Tungsten Tip & BM to Form An Arc.

HF Gets Cut Off, Once Welding Arc Struck.


*Dr. N. RAMACHANDRAN, NITC*Water Cooling System

Provides Cooling Water To Welding Torch.

Cools Tungsten Rod, Torch handle & Welding Cable.

Cooling Water Returns through Flexible Tube Which Carries welding cable within.


*Dr. N. RAMACHANDRAN, NITC*Pedal SwitchSwitches system on And off in sequenceWhen Pedal PressedSolenoid valve opens, Argon gas flowsHigh Frequency current jumps from tungsten rod generating sparksWelding current flows generating an arc across tungsten rod and work. High frequency gets cut off from the system & welding continues. When Pedal ReleasedCurrent gets cut off, Arc extinguishes Gas flow remains for few more seconds before it stops.


*Dr. N. RAMACHANDRAN, NITC*Argon Gas Cylinder- Pressure Regulator + Flow MeterCylinder Stores Argon At High Pressure

Regulator Regulates Cylinder Pressure to Working Pressure

Flow Meter Controls Flow RateArgon CylinderFlow MeterPressure RegulatorFlow RegulatorPressure gaugesCylinder ValveConnection To Torch


*Dr. N. RAMACHANDRAN, NITC*Tools For GTAW

Head Screen

Hand gloves

Chipping Hammer

Wire Brush

Spanner Set


*Dr. N. RAMACHANDRAN, NITC*Filler Wire

Added Separately to the weld pool.

Compatible to base metal

Used in cut length for manual welding.

Used from layer wound spool for automatic welding.

Sizes :- 0.8, 1, 1.2, 1.6, 2, 2.4 & 3 mm


*Dr. N. RAMACHANDRAN, NITC*ASME Classification Of Filler WireSS Filler Wire:SFA-5.9, ER 308, 308L, 316, 316L, 347, 309 LAS Filler Wire:SFA 5.28, ER 70S A1, ER 80S B2, ER90S D2, ER 80S Ni2 CS Filler Wire: SFA- 5.18 , ER 70S2 C = 0.07%, Mn = 0.9% 1.4%, Si = 0.4 0.7%, P = 0.025%, S = 0.035%


*Dr. N. RAMACHANDRAN, NITC*Dos & Don'ts In GTAWAlways Connect Electrode VeKeep Always Flow Meter VerticalCheck & Confirm Argon PurityClean Groove & Filler wire With Acetone Grind Tungsten Tip to PointDont Strike Arc With Electrode + VeDont strike Arc Without Argon FlowDont Strike Arc By touching Tungsten Rod Dont Touch Weld Pool With Tungsten Rod Dont Lift and break Arc

Dos Donts


*Dr. N. RAMACHANDRAN, NITC*Dos & Don'ts In GTAWBreak The Arc Only By Pedal SwitchLift The Torch only After 5 Sec Of Arc Break.Ensure Pre Purging & Post Purging of 5Sec Ensure Argon Flow & Water Circulation To Torch When Arc is Stopped Dont Lift Torch immediately.Dont Weld With Blend Tungsten RodDont Weld With Argon Leaking TorchDont Weld Without Water CirculationDos Donts


*Dr. N. RAMACHANDRAN, NITC*Dos & Don'ts In GTAWProvide Back Purging For Single Sided Full Penetration WeldsUse N2 or Argon as Back Purging Gas For CS & LASUse Argon As Back Purging Gas For SS & Non Ferrous Alloys Dont Weld Single Sided Full Penetration Welds Without Back PurgingDont Use N2 As Back Purging Gas For Non Ferrous AlloysDont Empty Ag Cylinders Fully.

Dos Donts


*Dr. N. RAMACHANDRAN, NITC*Defects In GTAW 1. Cracks 2. Lack Of Fusion 3. Porosity 4. Undercut 5.Lack Of Penetration 6. Excess Penetration 7.Overlap 8. Suck Back 9. Under Flush 10. Burn Through 11. Tungsten Inclusion 11.Stray Arcing


*Dr. N. RAMACHANDRAN, NITC*Crack

CauseRemedyWrong ConsumableWrong ProcedureImproper PreheatInadequate Thickness In Root PassUse Right Filler WireQualify ProcedurePreheat UniformlyAdd More Filler Wire in root Pass


*Dr. N. RAMACHANDRAN, NITC*Lack Of FusionLack Of Fusion

CauseRemedy Inadequate Current Wrong Torch angle Improper bead placement Use Right Current Train /Qualify welder Train/Qualify Welder


*Dr. N. RAMACHANDRAN, NITC*Porosity

CauseRemedy Impure Argon Gas Argon Leak Within Torch Defective Filler Wire Wet surface of BM Rusted / Pitted Filler wire Improper Flow Of Argon Replace Argon Cylinder Replace Leaking Torch Replace Filler Wire Clean & Warm BM Clean Filler Wire Provide Gas lens


*Dr. N. RAMACHANDRAN, NITC*UndercutUnder cut

CauseRemedy Excess Current Excess Voltage Improper Torch angle Reduce the Current Reduce Arc length Train & Qualify the Welder


*Dr. N. RAMACHANDRAN, NITC*Lack Of Penetration*LOP* Applicable to SSFPW

CauseRemedy Excess Root Face Inadequate Root opening Over size Filler Wire Wrong Direction of Arc Improper bead placement Improper weaving technique Reduce Root Face Increase Root Opening Reduce Filler Wire size Train / Qualify Welder Train / Qualify Welder Train & Qualify Welder


*Dr. N. RAMACHANDRAN, NITC*Excess Penetration*Excess Penetration* Applicable to SSFPW

CauseRemedyExcess root opening Excess Current Inadequate root face Excess Weaving Wrong Direction Of ArcReduce root gapReduce Current Increase Root faceTrain Welder Train Welder


*Dr. N. RAMACHANDRAN, NITC*Overlap Overlap

CauseRemedy1) Wrong Direction Of ArcInadequate CurrentExcess Filler WireTrain & Qualify WelderIncrease CurrentReduce Filler Metal


*Dr. N. RAMACHANDRAN, NITC*Suck Back*Suck Back* Applicable to SSFPW in 4G, 3G & 2G

CauseRemedy Excess weaving in root Excess Current Inadequate root face Wrong Electrode angle Reduce weaving Reduce Current Increase root face Train / Qualify Welder


*Dr. N. RAMACHANDRAN, NITC*Under flushUnder flush

CauseRemedy Inadequate weld beads in final layer2) Inadequate understanding on weld reinforcement 3) Wrong selection of filler wire size Weld some more beads in final layer2) Train / Qualify welder

3) Train / Qualify Welder


*Dr. N. RAMACHANDRAN, NITC*Burn through*Burn trough*Applicable to root pass

CauseRemedy Excess Current Excess Root opening Inadequate Root face Improper weaving Reduce the Current Reduce root opening Increase root face Train / Qualify Welder


*Dr. N. RAMACHANDRAN, NITC*Tungsten InclusionTungsten Inclusion

CauseRemedy Ineffective HF Improper Starting of Arc

3) Tungsten Tip Comes in Contact With Weld Rectify HF Unit Never Touch Weld With Tungsten Rod3) Train / Qualify welder


*Dr. N. RAMACHANDRAN, NITC*Stray ArcingArc Strikes

CauseRemedy HF Not In Operation Inadequate Skill of Welder Rectify HF Unit Train the Welder


What Is GMAW ?A Fusion Welding Process Semi AutomaticArc Between Consumable Electrode &WorkArc Generated by Electric Energy From a Rectifier / Thyrester / InverterFiller Metal As Electrode Continuously fed From Layer Wound Spool. Filler Wire Driven to Arc By Wire Feeder through Welding TorchArc & Molten Pool Shielded by Inert Gas through Torch / Nozzle

Gas Metal Arc WeldingMIG Shielding Gas Ar / Ar + O2 / Ar + Co2 MAG Shielding Gas Co2

FCAW Shielding Gas Co2 With Flux cored WireNote:- Addition of 1 5% of O2 or 5 10% of Co2 in Ar. increases wetting action of molten metal

Power Source For MIG / MAGInverter- DCThyrister DCMotor Generator DCRectifier DC

Characteristic Of GMAW Power SourceAppx. Horizontal CurveV1V2A1A2AV

Constant V / Linear Characteristic

Current & PolarityDC- Electrode +VeStable ArcSmooth Metal TransferRelatively Low SpatterGood Weld Bead CharacteristicsDC- Electrode Ve, Seldom UsedAC- Commercially Not In use

Accessories Of GMAW Power SourceWire Feed UnitShielding Gas Cylinder, Pressure gauges/ Regulator, Flow meter (Heater For Co2 )Welding Torch Water Cooling System (For Water cooled Torch)Earthing Cable With Clamp

Tools For GMAW Head Screen With DIN 13 / 14 Dark GlassHand Wire Brush / Grinder With Wire Wheel Cutting PliersHand Gloves Chipping Hammer / Chisel & hammer Spanner SetCylinder KeyAnti-spatter SprayEarthing Cable With Clamp

GMAW Torch Torch HandleSpring ConduitJobArcGas CupShielding GasFiller Wire - ElectrodeNozzle TipOn / Off Switch

Equipment & Accessories+ Wire Inside Spring LiningFlow MeterWelding TorchWire FeederShielding Gas CylinderPressure RegulatorArgon / Co2 ShieldingPower Source With InductanceWork ArcSolenoid ValveCopper CupWire SpoolElectrode / WireShielding GasHeater (Only For Co2)Contact TipSwitchTorch With Cable Max. 3Mtr

Types Of Wire Feeding In GMAWPush TypeWire fed in to The torch by Pushing through Flexible Conduit From A Remote SpoolPull TypeFeed Rollers Mounted on The Torch Handle Pulls the Wire From A Remote spoolSelf ContainedWire Feeder & The Spool On the Torch

Function Of Shielding Gas In GMAWPrevents Air contamination of weld PoolPrevents Contamination During Metal TransferIncreases fluidity of molten metalMinimizes the spatter generationHelps in even & uniform bead finish

Shielding Gases For GMAWMIG: Argon Or Helium For SS, CS, LAS & Non-ferrous Mt & AlMIG: Ar + 1 to 2 % O2, Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & AlMIG: Ar + 5 to 20 % Co2 Wire With Add. Mn & Si For SS, CS, LAS & Non-ferrous Mt & AlMAG: Co2 With Solid Wire For CS & LAS FCAW: Co2 With Flux Cored Wire For CS, LAS & SSOverlay

ASME Classification For CS GMAW WireSFA 5.18 : - CS Solid WireER 70 S 2, ER 70 S 3ER 70 S 6, ER 70 S 7

SFA 5.20 :- CS Flux Cored WireE 71 T-1, E 71 T-2 ( Co2 Gas )E 71 T-1M, E 71 T-2M ( Ar + Co2 Mix)

GMAW CS WireGenerally Copper CoatedPrevents Oxidation / rusting in StoragePromotes Electric Conductivity in ArcingAvailable In Solid & Flux CoredSize in mm 0.8, 1, 1.2, 1.6, 2, 2.4, 3Manganese & Silicon ( Mn 1 2 %, Si Max 1%)Act As Deoxidizing AgentsEliminate PorosityIncrease Wetting Of Molten Pool

Metal Transfer In MIG Short-Circuiting / Dip Transfer

Globular Transfer

Spray Transfer

*Dr. N. RAMACHANDRAN, NITC* GAS METAL ARC WELDING (GMAW)ALMOST REPLACING SMAW, FASTER, INTRODUCED IN 1940S, DCRP GENERALLY EMPLOYED, CONTINUOUS WIRE FEEDING MODES OF METAL TRANSFER

1SPRAY2SHORT CIRCUIT3GLOBULAR4BURIED ARC5PULSED ARCHIGH VOLTAGEHIGH AMPERAGE(WIRE FEED)VERY LOW VOLTAGEMODERATE WIRE FEEDBETWEEN 1&2UNIQUE IN GMAW, HIGHER WIRE FEEDPULSING BETWEEN MODESDROPLETS-DEEP Penet.FOR THICKCOOLEST MODE, LEAST Penetration.FOR CARBON STEELS, 6 TO 12 MMHIGH SPPED, LOW SPATTER, DEEP Penet., FOR MS AND SSNO GUN OSCILLATIONARGON ST.(FOR NARROW)75 % Ar + 25% CO290%Ar + 7.5% CO2 +2.5% HeFOR THICK TO THIN, DISSIMILAR


Metal Transfer In MIG Dip/Short CircuitingGlobularSprayCS Solid Wire 1.2 mm Above230A24 35 V120 to 250A16 24 VUp to 120A14 22VCo2 or ArCo2 or ArOnly Ar / Ar+O2

Short-Circuiting / Dip TransferWire In Contact With Molten Pool 20 to 200 times per SecondOperates in Low Amps & Volts Less DepositionBest Suitable for Out of Position WeldingSuitable for Welding Thin SheetsRelatively Large opening of Root Can be WeldedLess DistortionBest Suitable for Tacking in Set up Prone to Get Lack of Fusion in Between Beads

Globular TransferMetal transferred in droplets of Size grater than wire diameterOperates in Moderate Amps & Volts Better DepositionCommon in Co2 Flux Cored and Solid WireSuitable for General purpose Welding

Spray TransferMetal transferred in multiples of small droplets100 to 1000 Droplets per SecondMetal Spray Axially DirectedElectrode Tip Remains pointedApplicable Only With Inert Gas Shielding Not With Co2Operates in Higher Amps & Volts Higher Deposition RateNot Suitable for Welding in Out of Position.Suitable for Welding Deep Grooves

Pulsed Spray WeldingPower Source Provides Two different Current LevelsBackground and Peakat regular intervalBackground & Peak are above and below the Average CurrentBest Suitable for Full Penetration Open Root Pass WeldingGood Control on Bead Shape and Finish

Synergic Pulse GMAWParameters of Pulsed Current (Frequency, Amplitude, Duration, Background Current) Related to Wire feed Rate One Droplet detaches with each pulseAn Electronic Control unit synchronizes wire feed Rate with Pulse Parameters Best Suitable for Most Critical Full Penetration Open Root Pass WeldingGood Control on Open Root penetration, Bead Shape and Finish

*Dr. N. RAMACHANDRAN, NITC*GASESPUROPOSE- 1.TO SHIELD MOLTEN PUDDLE FROM CONTAMINATION 2.CREATE A SMOOTH ELECTRICAL CONDUCTION PATH FOR ELECTRONS IN ARCSOME GASES (ARGON)MAKE SMOOTH PATH, BUT SOME RESISTS (CO2) PATH.STRAIGHT ARGON FOR NARROW BEADS98% Ar+ 2 OXYGEN FOR SPRAY, He FOR COPPER, THICK Al (WITH Ar).75 % Ar + 25% CO2 FOR SHORT CIRCUIT., STRAIGHT CO2 ECONOMICAL, BUT SPATTERING.90%Ar + 7.5% CO2 +2.5% He FOR BURIED ARC, SS.90% Ar + 10% He FOR AUTOMATIC V, WIRE FEED SYSTEMS

A CONSTANT VOLTAGE POWER SOURCE USED.


*Dr. N. RAMACHANDRAN, NITC*+ POINTS OF GMAWHIGH WELDING SPEEDNO NEED TO CHANGE ELECTRODES (ONLY WIRE SPOOL IN GMAW)HAZ SMALLVERY LITTLE SMOKE AND VERY LIGHT SiO2 SLAG(CALLED GLASS SLAG)LEAST DISTORTIONEASE OF OPERATION (QUICK LEARNING)GUN MANIPULATION EASIERMOST FLEXIBLE PROCESS- VERSATILEVERY FEW MACHINE ADJUSTMENTS FOR THICK TO THIN CHANGEMS, MCS, TOOL STEEL GRADES, SS, COPPER, Al, Mg WELDEDFCAW, SAW, ESW- OTER FORMS OF GMAW


GMAW Process VariablesCurrentVoltageTravel SpeedStick Out / Electrode Extension Electrode Inclination Electrode SizeShielding Gas & Flow RateWelding Position

Parameter For 1.2 FC WireCurrent 200 to 240 AVoltage 22-24Travel Speed 150 to 250 mm / minStick Out / Electrode Extension 15 to 20 mmElectrode Inclination Back Hand TechniqueShielding Gas Co2, 12 L/Min

Parameter For 1.2 Solid WireCurrent 180 to 220 AVoltage 20-22Travel Speed 150 to 200 mm / minStick Out / Electrode Extension 10 to 20 mmElectrode Inclination Back Hand TechniqueShielding Gas Co2 12 L/Min

Results In Change Of ParametersIncrease In Current More deposition, More Penetration, More BM FusionIncrease In VoltageMore Weld Bead Width, Less Penetration, Less Reinforcement, Excess SpatterIncrease In Travel SpeedDecrease in Penetration, Decrease in Bead Width,Decrease In Gas Flow rateResults In porosityLong Stick Out / Electrode ExtensionExcess Weld Deposit With Less Arc intensity, Poor Bead Finish, Shallow Penetration

Common Defects In GMAW 1. Porosity 2. Spatters 3. Lack Of Fusion 4. Under Cut 5. Over Lap 6. Slag 7. Crack 8. Lack Of Penetration 9. Burn Through 10. Convex Bead 11. Unstable Arc 12. Wire Stubbing

Porosity

CauseRemedy Less Mn & Si In Wire Rusted / Unclean BM / Groove Rusted wire Inadequate Shielding Gas Use High Mn & Si Wire Clean & warm the BM Replace the Wire Check & Correct Flow Rate

SpattersSpatters

CauseRemedy Low Voltage Inadequate Inductance Rusted BM surface Rusted Core wire Quality Of Gas Increase Voltage Increase Inductance Clean BM surface Replace By Rust Free wire Change Over To Ar + Co2

Lack Of FusionLack Of Fusion

CauseRemedy Inadequate Current Inadequate Voltage Wrong Polarity Slow Travel Speed Excessive Oxide On Joint Use Right Current Use Right Voltage Connect Ele. + Ve Increase Travel speed Clean Weld Joint

UndercutUnder cut

CauseRemedy Excess Voltage Excess Current Improper Torch angle Excess Travel Speed Reduce Voltage Reduce Current Train & Qualify the Welder Reduce Travel Speed

Overlap Overlap

CauseRemedy1) Too Long Stick Out

2) Inadequate Voltage Reduce Stick Out

2) Increase the Voltage

Slag

CauseRemedy Inadequate Cleaning Inadequate Current Wrong Torch angle Improper bead placement Clean each bead Use Right Current Train / Qualify welder Train / Qualify Welder

Crack

CauseRemedyIncorrect Wire Chemistry Too Small Weld BeadImproper PreheatExcessive Restrain Use Right WireIncrease wire FeedPreheat Uniformly Post heating or ISR

Lack Of Penetration*LOP* Applicable to SSFPW

CauseRemedy Too Narrow Groove Angle Inadequate Root opening Too Low Welding current Wrong Torch angle Puddle Roll In Front Of Arc Long Stick Out Widen The Groove Increase Root Opening Increase Current Train / Qualify Welder Correct Torch Angle Reduce Stick Out

Burn through*Burn trough*Applicable to root pass

CauseRemedy Excess Current Excess Root opening Inadequate Root face Too Low Travel Speed Quality Of Gas Reduce the Current Reduce root opening Increase root face Increase Speed Use Ar + Co2

Convex Bead FinishUneven bead finish

CauseRemedy Low Current Low Voltage Low Travel Speed Low Inductance Too Narrow Groove Increase Current Increase Voltage Increase Travel Speed Increase Inductance Increase Groove Width

Unstable arc

CauseRemedy Improper Wire Feed Improper Gas Flow Twisted Torch Conduit Check Wire Feeder Check Flow Meter Straighten Torch Cab

Wire Stubbing

CauseRemedy Too Low Voltage Too High Inductance Excess Slope Too Long Stick Out Increase Voltage Reduce Inductance Adjust Slope Reduce Stick Out

Important Terminology used in Critical Welding PreheatingPost Heating or DehydrogenationIntermediate Stress leavingInter pass TemperaturePost Weld Heat Treatment

PreheatingHeating 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 HAZApplicable toHardenable low alloy steels of all thicknessCarbon steels of thickness above 25 mm.Restrained welds of all thickness

Preheating temperature vary from 75C to 200C depending on hardenability of material, thickness & joint restrain

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? 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 toThicker hardenable low alloy steelsRestrained hardenable welds of all thicknessPost heating temperature and duration depends on hardenability of material, thickness & joint restrain

How does Post Heating Eliminate Crack? SMAW introduces hydrogen in weld metalEntrapped hydrogen in weld metal induces delayed cracks unless removed before cooling to room temperatureRetaining 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 toHighly 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 and built in stress

Intermediate stress relieving relieves built in stresses and entrapped hydrogen making the joint free from crack prone

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 toStainless 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 toAll type of CS & LAS

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

*Dr. N. RAMACHANDRAN, NITC*Weldability

The weldability of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others. It greatly influences weld quality and is an important factor in choosing which welding process to use.


*Dr. N. RAMACHANDRAN, NITC*Weldability might be considered by some a subjective term but basically it refers to a material's capacity to be welded under the conditions of design and fabrication and then to perform as expected during its service life. One of the most significant factors determining steels weldability is its ability to resist cracking in the weld area, and its weldability is sometimes evaluated on the basis of cracking sensitivity.


*Dr. N. RAMACHANDRAN, NITC*Carbon steels are the most widely used metals in industry, construction and transportation. They are categorized by carbon content.Low-carbon steel contains 0.15% or less carbon. It is generally considered the most easily welded. Mild steel has a range of 0.15 to 0.30% carbon, and its weldability is generally considered good. Medium carbon steel has a carbon range of 0.30 to 0.50%. Its weldability is considered fair. High-carbon steels have carbon levels of 0.50 to 1.00%, and they are considered very difficult to weld or even unweldable.Although carbon content is not the only factor affecting weldability, it is generally thought that as carbon content increases, weldability decreases. When carbon levels reach the 0.30 to 0.35% range, special precautions, such as preheating, controlling heat input and postweld heat treating, are normally required. The use of low-hydrogen electrodes is often recommended for successful welding.


*Dr. N. RAMACHANDRAN, NITC*Low-alloy steels usually have low carbon levels, but other alloying elements such as nickel, chromium, molybdenum, manganese and silicon are added to impart properties of corrosion resistance, impact toughness at low temperatures and strength. Depending on the combination and percentage of these alloying elements, the welds on these steels can be susceptible to cracking.The introduction of hydrogen into the weld metal of low-alloy steels must be prevented. Hydrogen can come from a variety of sources, but moisture and hydrocarbons are two of the main culprits where welding is concerned. Moisture on the steel or in the internal or external flux of the electrode will dissociate in the arc and release hydrogen into the weld metal. Likewise, hydrocarbons in machining oils, solvents, lubricants and general purpose oils will break down in the arc and transfer hydrogen into the weld metal. Precautions should be taken to avoid the introduction of hydrogen into the weld. Using low-hydrogen consumables, thoroughly cleaning the weld area before welding, and making sure dry conditions prevail are ways of preventing the transfer of hydrogen


*Dr. N. RAMACHANDRAN, NITC*Preheating and maintaining a controlled cooling rate are two methods used to deal with hydrogen that has entered the weld. By controlling heat input and allowing the weld to cool slowly, hydrogen will diffuse from the weld as it solidifies


*Dr. N. RAMACHANDRAN, NITC*Stainless steels are widely used high-alloy steels. Generally, these steels contain 10 to 30% chromium, with a certain group of them also alloyed with a significant level of nickel. Stainless steel is categorized as austenitic, martensitic or ferritic. Of the three types, the austenitic grades are probably considered the most weldable. It must be kept in mind when welding these grades that their coefficient of expansion is greater than plain carbon steel, but heat conducts only about one-third as fast as through plain carbon steel. Induced heat is slow to dissipate; therefore, attention must be paid to controlling distortion or warping.


*Dr. N. RAMACHANDRAN, NITC*The martensitic grades exhibit high hardenability and can be prone to cracking if care is not taken to compensate for this charact

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