Ai- Welding Process

Welding Process

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WELDING PROCESSES  Earliest known form of welding, called forge

welding, dates back to 2000 B.C.  Forge welding is a primitive process of joining metals by heating and hammering until the metals are fused (mixed) together  Now limited to the blacksmith trade.

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Welding Processes  Shielded metal arc welding-SMAW  Gas metal arc welding-GTAW  Flux cored arc welding-FCAW  Gas tungston arc welding-GTAW  Plasma arc welding-PAW  Submerged arc welding-SAW  Oxy acetylene welding-OAW

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Difference between Processes  Shielding used  Weld metal composition  Joint preparation & fit-up  Type of electrode/filler –metal  Welding pattern

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Factors that affect weld Quality          

Shielding quality Base metal quality Filler-metal quality Base metal weld metal compatibility Heat input Electrode size Travel speed Current and Voltage Welding position Environment for welding 

Wind, temperature & fit-up

 Knowledge & Experience of welders 5

Filler metals  filler metal or material  material added to fill-up the space in between two welding pieces during the welding process  Two types of filler metals commonly used  welding rods  welding electrodes.  welding rod refers to a form of filler metal that does

not conduct an electric current during welding process  

The purpose of a welding rod is to supply filler metal to the joint. used for gas welding.

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Electrode  Electrode

component that conducts the current from the electrode holder to the metal being welded.  Electrode types:  consumable and non-consumable.  Consumable electrodes  provide a path for the current and also supply filler metal to the joint.  Eg.electrode used in shielded metal-arc welding.  Non-consumable electrodes  used as a conductor for the electrical current-GTAW  filler metal for GTAW, hand fed consumable welding rod. 

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FLUXES  Base metal has always impurities, called oxides

result from oxygen combining with metal & other contaminants in the base metal.  if these oxides are not removed a faulty weld may result Fluxes Cleaning agents that dissolve oxides and release trapped gases combines with impurities in the base metal, floating them away in the form of a heavy slag which shields the weld from the atmosphere. allow the filler metal and the base metal to be fused formulated for a specific base metal on the expected welding temperature Available in the form of a paste, powder, or liquid 

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ARC WELDING  common to all arc-welding processes

a heat source, filler metal, and shielding source of heat  by arcing of an electrical current between two contacts. concentration of heat  less heat spread reduces buckling and warping  increases depth of penetration and  speeds up welding operation A distinct advantage of arc welding over gas welding more practical and economical than gas welding In gas welding flame spreads over a large area, causing heat distortion 

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Shielded Metal Arc Welding (SMAW)  performed by striking an arc between a coated-metal

electrode and the base metal.  molten metal from the tip of the electrode flows together with the molten metal from the edges of the base metal to form a sound joint, process known as fusion  The coating from the electrode forms a covering over weld deposit, shielding it from contamination  common types of welding   

Oxy-fuel gas welding (OFW) arc welding resistance welding 10

SMAW high-quality welds are made rapidly at a low cost  Weld surfaces have valleys and ripples  Makes interpretation difficult  Discontinuities have random orientation in the weld with other welding processes  Contains entire spectrum of weld discontinuities 

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Oxy-fuel gas welding (OFW)

Shielded metal arc welding (SMAW)

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GAS Metal Arc WELDING  source of heat  oxy-fuel gas, such as acetylene, mixed with oxygen  used in maintenance and repair works  Primary gases used  helium, argon, carbon dioxide or a mixture of these gases  Difference between SMAW & GMAW

type of shielding  GTAW   

both the arc and the molten puddle covered by a shield of inert gas. The shield of inert gas prevents atmospheric contamination-producing a better weld. 13

GMAW  Gas metal arc welding   

No flux used Suitable for thin wall sections < 10 mm Has Low base metal penetration characteristics leading to   

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Incomplete penetration Cold lap Porosity -if loss of shielding occurs Slag ???  Oxides in base metal may be drawn into the weld as slag

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GTAW  Gas Tungsten arc

welding 

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High quality welds with good base metal penetration with operator skill Discontinuities common to GTAW  Incomplete fusion  Cold lap  Porosity -if loss of shielding occurs  Tungsten inclusions 15

FCAW  Flux cored arc welding  Has

good penetrating capability  Discontinuities common to FCAW Slag  Porosity 

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SAW  Submerged arc welding  Has

good penetrating capability  Discontinuities common to SAW Slag  Lack of fusion 

 Follow welding direction and will be in Straight lines 

Porosity 17

Evaluation of weldments

Welding discontinuities

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Welding discontinuities  Discontinuities become defects

when exceed welding standard requirements  Cracks  Incomplete penetration (ICP)  Incomplete fusion (LF & SWLF)  Slag Inclusions (isolated & linear)  Porosity 

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Welding discontinuities  Defective profile  Undercut  overlap  Under-fill  Excess reinforcement  Excess root reinforcement  Root concavity  burn through  spatter

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Welding defects- Causes  Cracks Hydrogen Assisted cold cracking (HACC)  Hydrogen induced cold cracking (HICC)  solidification, liquation causes 

 Incomplete fusion Sidewall, inter run, root pass, weld toes ( cold lap )  Electrode angle implicated or poor joint profile 

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Welding defects- Causes  Porosity  

Gas entrapment / ejection poor shielding

 Inclusions  

Slag, oxide, tungsten Usually operator induced

 Defective weld profile / finish 

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Under-weld, over-weld, lack of root bead, burn through, undercut Usually operator induced 22

weld defects

Incomplete sidewall fusion

Slag inclusion

Incomplete root fusion 23

weld defects

Undercut Cold lap

Incomplete penetration

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Solidification cracks Crater crack

Longitudinal crack

Centreline Crack

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Solidification cracking  Low melting point constituents  Grain boundary segregation  Strains arising during solidification 

Expansion coefficient 

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Differing between base material and weld material Clad materials

Weld pool shape and size

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Methods of control  Preheat

Slow down cooling rate  between 800°C and 500°C 

 Remove hydrogen before weld cools below

150°C Stress relief immediately after welding  Low temp temperature heat treatment  (150°C to 250°C, known as out-gassing) 

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Lamellar tearing  Separation or cracking along planes parallel

to the principal plane of deformation.  Occurs in rolled sections mainly but can also occur in extrusions and forgings.  Does not occur in castings  Not to be confused with plate lamination.

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Lamellar tearing

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