1. Teknologi Dan Rekayasa
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2. Correcting Faults in Welds
Objective :
After studying this lesson, the student can be able to ;
identify the method of correcting faults in welds.
do the correcting faults in welds.
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3. Correcting Faults in Welds
Welds may contain many faults which can be corrected by the experienced tradesman during the welding operation.
The most common faults and methods for correcting them are listed bellow.
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4. 1. Undercut
This usually occurs in fillet welds where the weld face meets the parent metal. It can be caused :
1. By too much current overheating the parent metal.
Cure : use a lower current
undercut
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5. 1. Undercut
2. By not allowing the electrode to dwell long enough at the edge of the weld pool especially during weaving.
Cure : After welding technique to allow a pause at the undercut edge. This allows more weld metal to be deposited in the area already heated.
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6. 2. Over roll
This fault occurs when heated weld metal rolls over onto a section of the weld metal or parent metal that is not hot enough to allow fusion. An excessive deposit of weld metal can often cause this fault.
Cure : The electrode must move faster and deposit less metal. The angle of the electrode must not allow too much metal to be deposited in one side of the weldment.
Over roll
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7. 3. Slag Inclusions
During welding, the molten flux from the electrode absorb oxides from the surface of the parent metal and floats them to the surface of the weld where they cool to from slag. Two things can happen to stop this taking place.
a. If the weld pool is not hot enough, the molten slag will solidify inside the weld and form a slag inclusion. This may be caused by insufficient current (too little heat input) or by excessive weaving ( too wide a weld for the amount of heat available).
Cure : Raise the current or make the weave narrower.
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8. 3. Slag Inclusions
b. A restricted space between plate fit ups or perhaps an undercut in a previous run of weld can create a narrow area from which the slag cannot float to the surface.
Cure : Grind previous runs of weld or prepare plates so that there is no restriction to the slag floating to the surface.
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9. 3. Slag Inclusions Slag Inclusion Slag Inclusion Slag inclusion caused
by cold weld pool
Slag inclusion caused by a sharp
corner on run 2 of 3 run fillet weld
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10. 4. Porosity
This can be caused by an overheated electrode, a wrong electrode for the metal being welded or a wrong moisture content in the electrode being used.
Overheated Electrode : This is normally caused by too much current in the case of general purpose electrodes.
Cure : Use a lower current
Wrong Electrode : Type many metals need a specific type of electrode for satisfactory welding

11. 4. Porosity
Wrong Moisture Content in Electrode : General purpose electrodes and most others weld best if they are relatively dry. This is best accomplished by keeping electrodes, especially opened packets, in a warm, dry place. There are, however, some exceptions, the most important being cellulose electrodes. Cellulose electrodes must have a certain minimum moisture content which is sealed into the packet during manufacture.

12. 4. Porosity
After opening the packet, the electrodes must not be dried by heating in an electrode oven.
Cellulose electrodes often dry out too much as they are being deposited. This is caused by the heat of the arc. For this reason the last 80 to 100 mm of electrodes may be discarded. (This is the driest part of the electrode, and if used may cause porosity).

13. 5. Arc Blow
This is a phenomenon where the arc, instead of moving straight ahead of the electrode, trends to waver and move from side to side. This causes an irregular deposit and is quite unsatisfactory of wedding. It only occurs when using d.c. current and is caused by the magnetic flux that always surrounds direct current flow. This flux follows a circular path and flows in only one direction unless the direction of current flow is also reversed.

14. 5. Arc Blow
The magnetic flux attracts the arc and causes it to move away from its normal path. Thus, the unstable arc condition known as arc blow is formed.
Cure : the cure is always concerned with upsetting the magnetic flux path.
Strike the job with a hammer. This upsets the configuration of molecules in the metal and often causes the magnetic flux to take a different path.

15. 5. Arc Blow
Reverse the polarity (interchange the welding leads on the wedding machine terminals).
Use multiple earth clamps connected to the same earth cable. This redirects the path of current trough the work and stops the arc from being drawn to one side.
Change from d.c. to a.c. current. Arc blow does not occur with a.c. current. Because a.c. current is constantly changing direction and magnetic flux attraction reverse 50 times every second. Thus, there is no constant attraction of the arc in any particular direction.

16. 5. Arc Blow
ARC BLOW
ELECTRODE

17. 6. Overheated Weld-Pool
If the plates being welded become red-hot or the weld-pool is too turbulent, an unsatisfactory weld will result.
Welding electrodes contain small quantities of elements that replace those burnt out during the welding operation. If the weld-pool becomes turbulent and overheated, too much of these elements (carbon, silicon, manganese, etc.) are burnt out, and the weld-ment becomes weaker.
Cure : (a) do not use an electrode too large for the plate thickness being wekded. (b) Do not use too high a current.

18. 7. Excessive Penetration
When we place two plates together edge to edge, we usually require butt weld penetrating through the full thickness of the edges to be joined.
For plates under 3 mm thick a square edge preparation is common ; on thicker plates a vee type preparation is generally used. When full penetration is needed, the chosen edge preparation has a rot gap between the plates through which the weld must penetrate, but excessive penetration is unacceptable.

19. 7. Excessive Penetration
CORRECT PENETRATION
EXCESSIVE PENETRATION

20. 8. Craters and Cavities
These are the names given to irregularities in the weld surface caused by incorrect speed of travel, electrode angle and poor stop and start techniques.

21. 9. Stop and Start technique
To stop a run of welding, simply draw the electrode away from the work. The arc stops when the arc length reaches about 12 mm. It is good practice not to stop on top of the ”stop” or ”start” of a preceding run of weld.
When a perfect start is required, the end of the electrode should be heated by striking it momentarily on a piece of scrap steel. When this hot electrode is used on the actual piece to be welded, it strikes more easily than would a new, cold electrode.

22. 9. Stop and Start technique
Starting at a point 15 mm before the previous weld crater, the hot electrode is “dragged” along the work till the weld crater is reached. At this point the electrode is moved away from the work and an arc length is formed. The arc length may be momentarily lengthened to about 6 mm and then shortened to the working arc length. In the fraction of a second that the arc is lengthened, the arc rays spread over the work, and the operator can see that he is in fact, in the correct starting position.

23. Other Factor Affecting Weld Quality
Choice of Electrode
Electrode Size
Current
Voltage
Alternating or Direct Current
Single Run and Multi Run Beads
Welding Technique
Electrode Size – Current – Heat Input

24. THE END
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