Marine Engineering Study Materials

OVER LAP

This is overflow of weld metal over the base metal without fusion. Failure of the joint is certain when the overlap is located at the toe of the weld.

This is one of the serious welding defects on ships and should be avoided. It may be caused by:

1. Low welding current
2. Fast travel
3. Improper electrode manipulation

UNDER CUT

This is a cutting away of the base metal surfaces at the edge of the weld. It decreases the thickness of the metal at that point. Any material reduction in the metal thickness also reduces the metal strength, thus causing joint failure since the designed load of the joint is based on the original metal thickness. 

The possibilities of failure at this point are increased when under cutting occurs at the toe of the weld; a point there is high stress concentration. This defect is caused by:

1. Improper arc manipulation
2. Slow travel
3. Excessive welding current

POROSITY

It is the presence of pockets containing gas, in the welds. Excessive porosity in metal arc welds has a serious effect on the mechanical properties of the joint. 

Avoiding best prevents porosity:

1. Overheating and under heating of the weld metal,
2. Too high a current
3. Too long in arc.

SLAG INCLUSION

Slag inclusions are elongated or globular pocket of metal oxides and other solid compounds. It may be caused by concentration of the weld metal by foreign bodies. In multi layer welding process, failures to remove the slag between the layer cause slag inclusion. Preparing the groove and weld properly before each beat is deposited can prevent most slag inclusion, making sure that all slag has been removed and cleaned from the surface of the previous bead.

LACK OF PENERATION

This is the failure of the filler in base metal to fuse together at the root of the joint. Lack of penetration will cause weld failure if the weld is subjected to tension or bending stresses.

This may be due to:

1. Incorrect joint design,
2. Fast travel
3. Electrodes too large
4. Current setting too low.

LACK OF FUSION 

Lack of fusion is the failure of a welding process to fuse together layers of weld metal or weld metal and base metal .

This is generally referred to as overlap. Lack of fusion is caused by failure to raise the temperature of the base metal or the previously deposited weld metal to the melting point.

Reason for this failure include:

1. Too small electrode
2. Too fast travel
3. Too close and arc gap
4. Too low welding current.

DISTORTION 

It is caused by uneven heating and cooling, which involve the expansion and contraction of the base metal. The heat can be controlled by back-step welding sequences, by clamping the parts into their original position in a special fixture, and by single beat welding, which means that instead of making two or three passes with small diameter electrodes, one parts is made with a large electrode.

Distortion can be eliminated by increasing the welding speed and by closing the distance between the parts to be welded.

CENTER LINE CRACKING 

It is caused by the inability of base metal to move when the weld solidify and contracts, by using incorrect electrodes, by using in balance of base metal masses or in too high carbon content in the base metal.

This can be eliminated by

1. Design the joint correctly,
2. Preheating the parts to be welded prior to welding,
3. Maintaining the preheat temperature in the base metal during process,
4. Allowing the base metal to move freely as the welding process takes places
5. Stress relieving the welded parts as soon as the operation completed.

Reference:

Oral Guide by – MIN ZAR TAR