Basic Welding Theory

Welder Handbook – PT McDermott Indonesia

TEORI DASAR PENGELASAN


1. 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 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 back, and produces spatters badly.

Most frequently encountered at the start and finish of joints, and in corners and deep grooves, particularly when high amperages are being used in welding thick.

Makes welding very difficult, reduce speed and lowers weld quality When the arc blow opposite to the direction of travel it is called back blow. When the arc blows with the direction of travel, it is called forward blow. Is caused by magnetic force acting on the arc, making the arc blow from its normal

path.TO REDUCE ARC BLOW:

Reduce current Weld toward a heavy tack or toward a weld already made Use back stepping on long welds Place ground connection as for from joint to be welded as is possible If back blow is in the problem, place ground connection at start of weld and weld

towards a heavy tack If forward blow causes trouble, place ground connection at the end of weld Wrap ground cable around the work 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

Hold as short and arc possible to help the arc force counteract the arc blow If the machine bring used is of type producing both AC&DC, switch to AC.

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2. THE EFFECT OF THE WELDING HEAT ON METAL

Metals become larger when heated and become smaller upon cooling. During 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 effect upon the heated area, which results in the metal becoming smaller, or contracting. When this expansion and contraction is not controlled, distortion (warping) is likely 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: Reduce the forces that cause shrinkage. Make shrinkage forces work to reduce distortion Balance shrinkage force with other forces.Reduce the force that cause shrinkage: Avoid over welding Over welding causes distortion, it is a waste of time and money. In certain cases it

may even weaken the joint. Use proper joint preparation and fit-up Use intermittent welds Use “back step” welding methodMake shrinkage force work to minimize distortion Pre-set parts out of position Space parts to allow for shrinkage Pre-bend (pre-camber)Balance shrinkage forces with other forces Balance one shrinkage force with another, i.e. by welding alternatively on both

sides. Peering (but no advisable) Use of jigs and fixtures such as clamps, jigs, strong backs to hold the work in a

rigid position during welding.

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Welder Handbook – PT McDermott Indonesia 4

Legends:Contraction in the weld & HAZ along

the length.Bowing – due to the greater volume

of metal at the top of the weldPeaking due to the V angleRipple (in sheet) away from the weldContraction in the weld metal and

HAZ transverse to the weld

“BACK STEP” METHOD

Welder Handbook – PT McDermott Indonesia 5


3. TO STRIKE AND ESTABLISH AN ARC

The basis of arc welding is the continuous electric arc. The arc is maintained when the welding current is force across gab between the electrode tip and bar metal. A welder must be able to strike and establish the correct arc easily and quickly.

a. Too long arc length Increase of spatter Poor penetration Sound of arc will be more

of a hiss than a crackle Metal will melt off the electrode

in large drops Slag removal will be difficultb. Travel speed too fast Bead will be thin & stringy Poor penetrationc. Travel speed too slow Weld metal will pile up and roll over Excessive overlapd. Amperage too high Bead will be flat Excessive spatter Excessive porosity Electrode becomes overheated

e. Amperage too low Difficult in striking the arc

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Welder Handbook – PT McDermott Indonesia Difficult in maintaining

correct arc length Weld metal pile up Excessive overlap Poor penetration

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LEGENDS:Made by a correct procedureAmperage too lowSpatter and undercut caused by excessively high amperageVoltage too lowExcessively high voltage or long arcTravel speed too slowTravel speed too fast

To restart a continuous bead:Strike the arcMove the electrode to the crown of the craterResume forward travel


4. RUNNING BEAD WITH WEAVING MOTION

Weaving is an oscillating motion, back and forth, crosswise to the direction of travel. These motions are used to:

Flat out slag, Deposit a wider bead, Secure good penetration at the edge of the weld, Allow gas to escape, Avoid porosity.

Types of weaving motion:

The weave should not be wider than three times the diameter of electrode. And the purpose accomplished by both these motions is substantially the same and their usage is largely a matter of preference.

Lap weld in the horizontal and flat position

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5. WELDING TECHNIQUES IN ALL POSITIONS

Fillet weld in the horizontal or flat position

Butt Weld in the horizontal position

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Running a bead in the vertical position welding down

Running a bead in the vertical position uphill progression.

Running a bead in the overhead position

6. REHEAT AND INTERPASS CONTROL

The main reason for preheating and interpass control is to lower the cooling rate in the Weld Metal (WM) and Heat Affected Zone (HAZ). The lower cooling rate, the lesser

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Welder Handbook – PT McDermott Indonesiathe chance WM and Base Metal (BM) cracking. It lowers the chances of shrinkage stresses. It will maintain the desired properties of the weld and the base material.PREHEAT METHODS Portable heating torches with rosebud tips Ceramic heating elementsPREHEAT AREAS At least 3” (75mm) away from the joint towards the center of the joint. Preheat both external and internal (if accessible) areas of the joint.MONITORING OF PREHEAT TEMPERATURES Use temperature indicating crayon (ex: tempil stick) with the appropriate preheat

temperature requirements. Apply to the joint at least one minute after preheated, 3” away from the joint.MONITORING OF INTERPASS TEMPERATURE Use temperature indicating crayons (tempil stick) Apply to the joint at least one inch away from the welded joint.

7. HOW TO READ AND USE THE WELDING PROCEDURE SPECIFICATION (WPS)

Welding Procedure Specification is a specific guide for welders in execute their job. Welder must understand the contents of a welding procedure and must be able to identify the parameters stated in the WPS.

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Welder Handbook – PT McDermott IndonesiaWhen there is doubt in executing a weld, always refer to the WPS for guidance. Always follow the parameter in the welding procedure. DO NOT WELD WITHOUT A WELDING PROCEDURE IN YOUR WORK AREAS.

HOW TO READ METHOD STATEMENT AND SPECIAL INSTRUCTION?

Certainly critical applications will require additional instructions to ensure that the work will be done correctly. This instruction will supplement the welding procedure and should also be followed strictly. The method statement will be displayed in the work areas.

7. HOW TO READ AND USE THE WELDING PROCEDURE SPECIFICATION (WPS)

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Basic Welding Theory

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