Underwater welding is generally carried out where the cost or impracticability of bringing the structure to be welded to the surface prohibits the conventional air welding to be carried out. It finds its application in the repair and construction of structures inside water. In countries like USA, USSR, UK and Japan dry and wet processes have been successfully used in the fabrication of structures.

15.5.1 Underwater Manual Metal Arc Welding

Among the wet welding processes used today, manual metal arc welding process is still finding its maximum use in underwater fabrication. This process, therefore, requires especial consideration. The major parameter, for study in this process is the type of electrode. Waterproof coating has already been discussed earlier.

A critical review of literature indicates that almost all the varieties of electrodes have been used with varying degrees of success. From their results and our own experience on

underwater welding some basic conclusions have been drawn and reported in this text. The discussion would logically start with the underwater welding arc.

15.5.2 Underwater Arc

Underwater welding arc is exposed to two basic mechanisms of compression and constriction. Underwater arc is surrounded by a bubble. Hydrogen content (about 93%) of the arc bubble atmosphere together with water surrounding it compresses the arc and at the same time it has a severe cooling effect on arc column compared to normal air welding. This causes arc - constriction. This compression and constriction of arc column result in a higher current density in underwater arc. Further, in straight polarity welding, the limited geometrical dimensions of the electrode end prevent the free expansion of the cathode spot with increase in welding current. The arc is thus constricted. This apparently explains the fact that the volt-ampere characteristic curves of an underwater arc are concave or rising. Due to these compressive forces the increase in the cross-sectional area of the arc lags behind the given increase in the welding current, thereby raising the current density or field intensity (this distinguishes underwater welding with air welding). Thus to maintain same arc conditions the current should be increased by 10% per atmosphere (10 meters of water) of additional pressure. These higher current densities produce higher arc temperatures. Temperature of arc column at different currents and depths is given in Table 15.1.

15.5.3 Arc Shape

Madatov found that the basic shape of the arc column was cylindrical for metal-arc welding and truncated cone with its base on the work for thin wire CO2 welding. Metal transfer characteristics for the two types of welding processes are given in Table 15 .2.

15.5.4 Arc Atmosphere

A peculiar feature of underwater welding is an arc bubble which is maintained around the arc. The size of the bubble fluctuates between a small bubble barely covering the arc column and a large bubble of 10-15 mm diameter, that eventually breaks away from the weld puddle and floats to the surface, leaving behind a nucleus bubble with a diameter of 6-9 mm. This phenomenon of bubble growth and its break away occurs at an approximate rate of 15 times per second at 150 mm of water depth. Gases generated per second for E-6013, E-6027 and E - 7024 are 40 cc, 50 cc and 60 cc respectively. The gas-bubble consists of 62-82 percent hydrogen, 11-24 percent carbon monoxide, 4-6 percent carbon dioxide, and the remaining 3 percent is nitrogen and metallic and mineral salt vapours.

15.5.5 Arc Stability

During underwater welding the arc-voltage and current values fluctuate. A stability factor for comparing arc performance was defined by Madatov as maximum current divided by mini­mum current. The arc is considered to be stable for values of this factor near one. For values much higher than one the arc is considered unstable. One cause of these fluctuations is the variation in voltage due to changes in arc length during metal transfer. Another cause of fluctuation is collapsing of thick flux covering occurring every 0.3 second or less during the arc welding. Different electrodes produce different levels of stability. Silva has found E-7024 more

*Calculations based on assumption that arc column is a cylinder of arc length 2 mm. Stick elec­trode air-arc temperature is 6000 °K.

Table 15.3 Rates of Metallurgical Reactions in various methods of underwater welding

stable than E-6027, while E-6013 was found comparatively unstable because of its coating being thinner than the other two. Arc has been found to be more stable in salt water than in fresh water. This is due to the ease of ionization of sea water. But there is more current leak­age in sea water (upto about 65-110 amp. at an open circuit voltage of 83-99 volts).

15.5.6 Metal Transfer

Normally, the metal transfers in droplets (globules). Occasionally a large drop forms and short circuits the arc. Drop transfer frequency as reported by Brown is 80 to 100 drops per second for the coated electrodes used by him. Madatov reports the frequency to be 44 drops per second for the type of electrode he used. Thus the drop-transfer frequency depends upon the type of electrode in addition to other factors. Underwater arc is constricted and produces a high arc core temperature of 9000°K to 1100°K at 10 m depth) as compared to 5000°K to 6000°K for air welding (Table 14.1). This increased temperature causes fast melting rate for plate as well as electrode. The weld puddle which would otherwise have been uncontrollable solidifies rapidly due to the quenching effect of water.

With the above background of underwater arc and metal transfer mode in mind let us now analyse the work carried out by the various underwater welding investigators on different types of electrodes.

15.5.7 Electrodes Used

Electrodes used by various investigators along with their findings have been listed in chrono­logical order in Table 14.3. Each type of electrode will now be discussed in detail.

Cellulosic. These electrodes give a harsh digging arc resulting in a high penetration. In underwater welding the currents used are high to maintain the arc. This has been found to aggrevate the situation and produce more undercuts and convex bead. The results are not good even with reverse polarity. E-6010 has been found to spatter violently, gives irregular beads, and produces clouds of black smoke while E-6011 (which contains potassium silicate also in its coating) gives almost no spatter, produces continuous bead. It means that the pres­ence of a substance which ionizes easily improves the electrode performance.

Rutile. Rutile electrodes have been found to be superior to cellulosic and second to acidic but Silva and Hazlett have found plain rutile electrodes inferior to iron powder type. Light coated rutile electrodes E-6013 have been recommended by the U. S. Navy in their manual on underwater welding and cutting in 1953.

Oxidizing. Oxidizing electrodes give satisfactory welds but the welds are inferior in strength and ductility as compared to acid and rutile electrodes. A comparison of various electrodes electrodes is given in Table 15.3.

Iron Powder. In 1946, Van Der Willingen developed an electrode with a substantial amount of iron powder in its coating and a high coating material to core wire ratio. These electrodes were found easy to use in low visibility conditions, had excellent drag-welding char­acteristics and higher deposition rates.

Madatov in 1962 found these electrodes to give stable arc and fine droplet transfer with occasional short circuits. Silva and Hazlett found them to be superior to rutile. Masubuchi in 1974 found heavy coated rutile E-7024 and Iron-oxide E-6027 to give higher heat inputs than basic and rutile. For E-6013 better coating has to be designed to eliminate chiping of the out­side of the coating during welding. Arc elongation effect is more serious in E-7024 and E-6027 and therefore the discrepancy between the machine current setting and the actual measured value is 15-25 amp. for E-6013 and E-7014 electrodes and 50-150 amps for E-7024 and E-6027 electrodes. This arc elongation effect is to be avoided.

Acid. Acid electrodes are those electrodes which have higher ratio of (silica + titenia) to Iron-oxide-Manganese-oxide. Acid electrodes have been found to give good results by Berthet. Nobody else reported on acidic electrodes. More work is required to study these and basic electrodes in detail before arriving at a final conclusion.

Basic. The covering has been found to be very brittle. The weld deposit has often been found to contain surface porosity.

From the above discussion it can be concluded that none of the existing electrodes for air welding can be directly used for underwater welding and special electrodes have to be developed to avoid the difficulties encountered in the use of the existing air welding elec­trodes. In the following paragraphs we shall discuss the characteristic requirements for un­derwater welding electrodes.