Answer Construction Question 15
Question 15: A rudder of a vessel requires extensive welding repairs and as a Chief Engineer you are requested to supervise ;
A. Suggest a suitable type of welding process;
B. State with reasons FOUR common welding defects that can occur there;
C. State what tests may be carried out before returning the rudder to service.
Answer: A. suitable type of welding process:
The
welding process most suitable for rudder repair is manual electric
metal arc process. The reasons for using the basic process are:
(i)
Since the repair has to be an in-situ repair in dry-dock requiring
multi-positional welding (down hand, overhead, horizontal, vertical),
manual metal are welding is versatile in this respect.
(ii) Since there will be no special steel involved in the rudder structure, basic flux coated electrodes may be used, The flux coating melts at a higher temperature than the electrode metal core thus the coating protrudes beyond the core during welding. This gives better stability, control and concentration of arc. The coating also shields the arc and the molten metal from the atmosphere by means of the inert gases given off as it vaporizes.
Silicates formed from the coating, form a slag upon the surface of the hot metal and protects the hot metal from the atmosphere as it cools. Also due to larger contraction of the slag than the metal as cooling is taking place, the slag is easily removed
(iii) For welding a cast structure, if any pre-heating is to be done to relieve stresses and proper electrode to be used.
(iv) Other semi-automatic processes such as MIG or TIG in which inert gas (argon) is used during welding to surround the atmosphere, may be difficult in an exposed environment as the shielding gas may be blown away by air, unless protected by an enclosure.
(v) During welding by the metal are process, some of the points to be observed are : rate of electrode consumption; penetration; fusion; slag control; length and sound of arc. A good sound weld.
There is a problem fitting the closing plate of the rudder since it can not be welded from the inside. Slots are cut in the closing plate and these align with flat bars welded to the vertical and horizontal webs. The closing plate is then securely attached to webs by "plug welding" on to the flat bars.
(ii) Since there will be no special steel involved in the rudder structure, basic flux coated electrodes may be used, The flux coating melts at a higher temperature than the electrode metal core thus the coating protrudes beyond the core during welding. This gives better stability, control and concentration of arc. The coating also shields the arc and the molten metal from the atmosphere by means of the inert gases given off as it vaporizes.
Silicates formed from the coating, form a slag upon the surface of the hot metal and protects the hot metal from the atmosphere as it cools. Also due to larger contraction of the slag than the metal as cooling is taking place, the slag is easily removed
(iii) For welding a cast structure, if any pre-heating is to be done to relieve stresses and proper electrode to be used.
(iv) Other semi-automatic processes such as MIG or TIG in which inert gas (argon) is used during welding to surround the atmosphere, may be difficult in an exposed environment as the shielding gas may be blown away by air, unless protected by an enclosure.
(v) During welding by the metal are process, some of the points to be observed are : rate of electrode consumption; penetration; fusion; slag control; length and sound of arc. A good sound weld.
There is a problem fitting the closing plate of the rudder since it can not be welded from the inside. Slots are cut in the closing plate and these align with flat bars welded to the vertical and horizontal webs. The closing plate is then securely attached to webs by "plug welding" on to the flat bars.
B. Common welding defects that can occur there:
Under-cutting
: A groove is formed in the parent metal along the sides of the weld
bead. Groove reduces the thickness of the plate and thus the area along
the bead, which in turn weakens the weld. Causes are as follows :
1. Wrong manipulation and inclination of electrode and excessive weaving.
2. Too large electrode diameter
3. Higher currents.
4. Longer arc.
5. Faster arc travel speeds.
6. Magnetic arc blow.
7 Rusty and scaly job surfaces.
Overlapping : It is just reverse to undercutting. An overlap occurs when the molten metal from electrode flows over the parent metal surface, and remains there without getting properly fused and united with the same. Overlapping may occur due to the following :
1. Lower arc current.
2. Slower arc travel speed.
3. Longer arcs.
4. Improper joint geometry (i.e. root gap).
Poor Fusion : Sometimes the molten metal deposited by the electrodes does not fuse properly with the cold base metal and the two do not unite properly and completely. The fusion may be lacking at the root, sides or between two runs in a multi-run weld. Causes are:
1. Lower arc current.
2. Faster arc travel speed.
3. Improper weaving technique.
4. Presence of oxides, rusts, scale and other impurities.
5. Incorrect joint preparation.
6. Incorrect electrode manipulation
Slag inclusion : It may be in the form of slag or any other foreign material, which does not get a chance to float on the surface of the solidifying weld metal and thus get entrapped within the same. Inclusions lower the strength of the joint and make it weaker. Causes are: 1. Too high or too low arc current.
2. Long arcs.
3. Too large electrode diameter
4. Insufficient chipping and cleaning of previous passes in multi-pass welding.
Incomplete penetration : It is the distance of fusion from base plate top surface to the maximum extent of weld nugget. Reasons are :
1. Improper joint preparation ( U-Joint is simpler to have good penetration than J butt joint).
2. Too large root face.
3. Root gap too small.
4. Too small bevel angle.
5. Less arc current.
6. Faster arc travel speed.
7. Too large electrode diameter
8. Longer arc length.
9. Incorrect polarity when welding with direct current.
10. Wrongly held electrode.
1. Wrong manipulation and inclination of electrode and excessive weaving.
2. Too large electrode diameter
3. Higher currents.
4. Longer arc.
5. Faster arc travel speeds.
6. Magnetic arc blow.
7 Rusty and scaly job surfaces.
Overlapping : It is just reverse to undercutting. An overlap occurs when the molten metal from electrode flows over the parent metal surface, and remains there without getting properly fused and united with the same. Overlapping may occur due to the following :
1. Lower arc current.
2. Slower arc travel speed.
3. Longer arcs.
4. Improper joint geometry (i.e. root gap).
Poor Fusion : Sometimes the molten metal deposited by the electrodes does not fuse properly with the cold base metal and the two do not unite properly and completely. The fusion may be lacking at the root, sides or between two runs in a multi-run weld. Causes are:
1. Lower arc current.
2. Faster arc travel speed.
3. Improper weaving technique.
4. Presence of oxides, rusts, scale and other impurities.
5. Incorrect joint preparation.
6. Incorrect electrode manipulation
Slag inclusion : It may be in the form of slag or any other foreign material, which does not get a chance to float on the surface of the solidifying weld metal and thus get entrapped within the same. Inclusions lower the strength of the joint and make it weaker. Causes are: 1. Too high or too low arc current.
2. Long arcs.
3. Too large electrode diameter
4. Insufficient chipping and cleaning of previous passes in multi-pass welding.
Incomplete penetration : It is the distance of fusion from base plate top surface to the maximum extent of weld nugget. Reasons are :
1. Improper joint preparation ( U-Joint is simpler to have good penetration than J butt joint).
2. Too large root face.
3. Root gap too small.
4. Too small bevel angle.
5. Less arc current.
6. Faster arc travel speed.
7. Too large electrode diameter
8. Longer arc length.
9. Incorrect polarity when welding with direct current.
10. Wrongly held electrode.
C. Tests may be carried out before returning the rudder to service:
1. Visual inspection
2. NDT by ultrasonic
3. pressure test by filling the rudder with water to a height of 2.45m above it.
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