Basic Welding Theory

Welder Handbook – PT McDermott Indonesia

TEORI DASAR PENGELASAN

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POLARITY

A welder should understand the meaning of polarity, and recognize what effect it has on the welding process. For proper penetration, uniform bead appearance and good welding result, the correct polarity must be used when welding with any wire or electrode. Incorrect polarity will will cause: poor penetration, Irregular bead shape, excessive spatter, difficult to control the arc, overheating, and rapid burning of wire or electrode. ARC BLOW      

Occurs when the arc refuses to go where it is supposed to, blows wildly forward or bac, and produces spatters badly. !ost fre"uently encountered at the start and finish of #oints, and in corners and deep grooves, particularly when high amperages are being used in welding thic. thic . !aes welding very difficult, reduce speed and lowers weld "uality $hen the arc blow opposite to the direction of travel it is called bac blow. blow. $hen the arc blows with the direction of travel, it is called forward blow. Is caused by magnetic force acting on the arc, maing the arc blow from its normal path.

TO REDUCE ARC BLOW:     

%educe current $eld toward a heavy tac or toward a weld already made &se bac stepping on long welds 'lace ground connection as for from #oint to be welded as is possible If bac blow is in the problem, place ground connection at start of weld and weld








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$rap ground cable around the wor piece and pass ground current through it in such a direction that magnetic field will be set(up to neutralize the magnetic field causing the blow  )old as short and arc possible to help the arc force counteract the arc blow  If the machine bring used is of type producing both A*+*, switch to A*.








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2. THE EFFECT EF FECT OF THE WELDING HEAT ON METAL

!etals become larger when heated and become smaller upon cooling. uring welding the arc heats the metal being welded, causing it to become larger or expand. As the heat is removed, the surrounding metal and air cause a cooling c ooling effect upon the heated area area,, which which resu results lts in the the meta metall beco becomin mingg smalle smallerr, or cont contra ract cting ing.. $h $hen en this this expansion expansion and contraction contraction is not controlled, controlled, distortion distortion -warping -warping is liely to result. On the other extreme, if expansion and contraction is restrained, or controlled too rigidly, severe stress and strain may result and impair the weld. Three rules can be followed to aid in the prevention and control of distortion:  %educe the forces that cause shrinage.  !ae shrinage forces wor to reduce distortion  /alance shrinage force with other forces. Reduce the force that cause shrinkage:  Avoid over welding  Over welding causes distortion, it is i s a waste of time and money. money. In certain cases it may even weaen the #oint.  &se proper #oint preparation and fit(up  &se intermittent welds  &se 0bac step1 welding method Make shrinkage force work to minimize distortion  're(set parts out of position  2pace parts to allow for shrinage  're(bend -pre(camber Balance shrinkage forces with other forces








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&se of #igs and fixtures such as clamps, #igs, strong bacs to hold the wor in a rigid position during welding.

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3egends: *ontraction in the weld + )A4 along the length. /owing 5 due to the greater volume of metal at the top of the weld 'eaing due to the 6 angle %ipple -in sheet away from the weld *ontraction in the weld metal and )A4 transverse to the weld

“BACK STEP” METHOD










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3. TO STRIKE AND ESTABLISH ESTABLISH AN ARC

7he basis of arc welding is the continuous electric arc. 7he arc is maintained when the welding current is force across gab between the electrode tip and and bar metal. A welder welder must be able to strie and establish the correct arc easily and "uicly.








. Tra!el ra!el "#ee "#ee$ $ too %a" %a"tt  /ead will be thin + stringy  'oor penetration c. Tra!el ra!el "#ee "#ee$ $ too "lo& "lo&  $eld metal will pile up and roll over  8xcessive overlap $. A'#e A'#era rage ge too too h(gh h(gh  /ead will be flat  8xcessive spatter  8xcessive porosity  8lectrode becomes overheated

e. A'#e A'#era rage ge too too lo& lo&

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4. RUNNING BEAD WITH WEAV WEAVING MOTION MOTI ON

$eaving is an oscillating motion, bac and forth, crosswise to the direction of travel. 7hese motions are used to:  Flat out slag,  eposit a wider bead,  2ecure good penetration at the edge of the weld,  Allow gas to escape,  Avoid porosity. 7ypes of weaving motion:








La# &el$ (n the hor()ontal an$ %lat #o"(t(on

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5. WELDING TECHNIQUES IN ALL POSITIONS

*(llet &el$ (n the hor()ontal or %lat #o"(t(on

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R+nn(ng a ea$ (n the !ert(cal #o"(t(on &el$(ng $o&n

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R+nn(ng a ea$ (n the o!erhea$ #o"(t(on 6. REHEAT AND INTERPASS INTERPASS CONTROL

7he main reason for preheating and interpass control is to lower the cooling rate in the $eld !etal -$! and )eat Affected 4one -)A4. 7he lower cooling rate, the lesser the chance $! and /ase !etal -/! cracing. It lowers the chances of shrinage stresses. It will maintain the desired properties of the weld and the base material.

PREHEAT METHODS  'ortable heating torches with rosebud tips  *eramic heating elements PREHEAT AREAS  At least ;1 -<=mm away from the #oint towards the center of the #oint.  'reheat both external and internal -if accessible areas of the #oint.








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7. HOW TO READ AND USE THE WELDING PROCEDURE SPECIFICATION WPS!

$elding 'rocedure 2pecification is a specific guide for welders in execute their #ob. $elder must understand the contents of a welding procedure and must be able to identify the parameters stated in the $'2. $hen there is doubt in executing a weld, always refer to the $'2 for guidance. Always follow the parameter in the welding procedure. O O7 $83 $I7)O&7 A

Basic Welding Theory

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