Underwater welding and cutting of metals

Underwater welding is a method of joining metal products that are in a liquid environment. It is indispensable when constructing bridge supports, building pipelines, working on ships, etc. What equipment to use depends on the depth at which the work will be carried out. What is included in the process and how it is performed, we will discuss below.

Underwater welding is associated with danger, because it is known that water is a strong conductor of electricity. Plus, the human body experiences the pressure of the liquid. But such welding is the fastest way to build a pipeline network for an entire settlement or connect two settlements with a bridge across a wide river, so its demand is high.

How does welding of parts take place?

The gases evaporating during the melting of the metal and coating of the electrode push the liquid out of the weld pool — and in such a "window" it is possible to maintain the arc and weld the seam.

Features of underwater welding

The seams are deeply fused. This is ensured by the arc burning in the air and the water pressure on it, which ultimately creates pressure on the working metal as well.
The seams are rough, which is caused by external pressure and rapid cooling.
Gases that evaporate during operation create water foam, which makes it difficult to observe the welding bath. Visibility also depends on the purity of the water. If visibility is poor, this can result in the weld center being displaced.
The difficulty of welding is in getting the electrode exactly to the place where the parts are connected. The welder has to take the tip of the electrode and direct it to the beginning of the seam.
Underwater welding, unlike land-based work, uses higher current modes. This is dangerous, but it speeds up the cooling of the metal, which ensures good weld tightness. The only downside is that rapid crystallization makes the weld unstable to fracture and impact.
Another difficulty is ignition. Metal structures that are located in water are often subject to corrosion. And this makes it difficult to excite the arc. As a result, the welder practically picks at the metal with the electrode (since it is simply impossible to tap in a liquid medium)
The law of gravity also operates underwater, so vertical seams are created from top to bottom. The only thing is that the arc must burn continuously so that the gas bubble is constant and the parts can be welded well.

Types of underwater welding

All types are united by the fact that they are created as a result of the burning of an electric arc and the formation of a welding pool. We will talk about the differences further.

1. Manual wet

Manual wet underwater welding is the simplest and most accessible. It requires coated electrodes or a torch with wire feed and gas blowing. In this way, you can weld pipe joints, bridge supports, or temporarily repair the bottom of a ship.

Welding with coated electrodes underwater is similar to how it is done on land. Work can be carried out by going underwater for a maximum of 40 meters. In this case, they must be done in stages. Cables in water must be intact to protect the worker from electric shocks, as well as from voltage losses. The welding holder has a minimum number of open parts that conduct current. Electrodes are inserted and secured in one movement.

2. Welding in the working chamber

This type is used where it is important to connect parts as firmly as possible. For this, a small tank is used, which is placed around the pipe and water is pumped out of it. The chamber is transparent, which allows you to observe how the arc burns and the seam is created. The process can be controlled through special holes.

The seam during underwater welding is made in dry conditions, as it happens on land. Cooling is gradual, and this guarantees good impact toughness of the seam. Plus, evaporating gases do not create foam on the water, which interferes with monitoring the process. However, there are still a couple of disadvantages: the camera can only be used for extremely important joints; the maximum depth, as in the previous paragraph, is no more than 40 meters for the safety of the welder.

3. In a dry box

The product and the welder themselves enter a special box, from which water is pumped out. Thus, a completely dry environment is obtained or the box remains half filled with water. This does not prevent the same seams from being obtained as on land. The surrounding liquid does not press on the worker, so it is possible to work longer and faster in such conditions. This method of underwater welding is used for critical joints and at great depths. The downside is the high cost, since it is necessary to rent a box and a crane.

Conclusion: the first method is used most often. The reason is the low cost and the ability to obtain good seams. But for this, it is necessary to set the correct modes. For example, in underwater work, the following is needed:

direct current for stable arc burning;
higher than on land, current strength — to avoid rapid cooling of the metal. The optimal value is 200-250 Amperes for working with electrodes 4-5 mm in diameter. The deposition coefficient is 6 g / A x H.
voltage no more than 35 volts to ensure the safety of the welder.

What kind of electrodes are used?

The materials used for underwater welding are almost the same as those used on land. The electrode is made of low-carbon steel, which ensures a good, pore-free metal connection.

The coating is also similar to that used on land. The only difference is a thicker coating layer. Resin, celluloid varnish and paraffin are also added to the composition to protect the layer from moisture. This prevents the electrode coating from becoming soggy in water.

Underwater cutting of metals

Coated electrodes also allow cutting metal underwater. However, there are faster methods. For example: electrode-oxygen underwater metal cutting. It requires a device that creates direct current, an oxygen cylinder, a sleeve with a hose and cable, and a holder. The point of this cutting method is that the metal is heated by an electric arc from the electrode. The electrode structure is tubular, a stream of oxygen is fed into it through a hole, which separates the molten metal. Water does not penetrate into the working area — it is removed by gases evaporating from the arc.

The main advantage of underwater metal cutting with steel electrodes is their small diameter, which allows them to pass freely into the cutting cavity. The result is easy cutting. It can be carried out even at great depths (up to 100 meters) and can handle metal 100-120 millimeters thick. At the same time, one good electric welding unit is enough to power the electric arc.

The disadvantages are high consumption of steel electrodes. They burn out quite quickly under the action of an electric arc, serving for about 1 minute. As a result — high consumption of electrodes and loss of time for their replacement.

Graphite electrodes are also used. They are covered with a metal shell and waterproof coating on top. On average, they last 10-12 minutes, but have a larger diameter (15-18 millimeters), which complicates cutting, since the electrode has difficulty passing into the cutting cavity.

The graphite electrode was replaced by a carborundum electrode. Its service life is about 15 minutes, but it, like its previous analogue, has a large diameter of 15-18 mm, which makes it difficult to insert into the cutting cavity.

Conclusion

To sum it up, it is worth saying that welding and cutting metals immersed in liquid is always a lot of work, associated with risk. The type of welding to use depends on the depth and degree of responsibility of the joint. But the advantage of this method is obvious — with its help you can quickly lay pipes, build bridges and repair large sea vessels.