1. Underwater cutting equipment
(1) Underwater Arc-Oxygen Cutting
The main equipment used in underwater arc oxygen cutting includes cutting power supply, cutting torch, cutting cable, circuit breaker and oxygen supply system.
1) Power supply cut
The power source used in underwater oxygen cutting is similar to that used in underwater electrode arc welding, which is a power source for direct current arc welding.
However, it has a higher rated power and the rated output current should not be less than 500A.
Common underwater cutting generators include models such as AX1-500 and AX8-500.
In addition, ZDS-500 type underwater welding power supply and ZXG-500 type arc welding rectifier power supply can also be used for underwater cutting.
Particularly, the ZDS-500 type underwater welding power supply, a ship-specific arc welding power supply, has resistance to water, humidity and vibration, high overload capacity, easy arc initiation, stable arc and can improve cutting efficiency.
2) Cutting torch
The arc oxygen underwater cutting torch must meet the following technical requirements:
① The distance from the cutting bar support to the center of the handle should be 150~200mm, and the weight in the water should not exceed 1000g;
② The torch head must have an automatic arc interruption device to prevent the torch head from drying out;
③ The torch must have devices such as a backfire preventer to prevent hot slag from blocking the gas passage and prevent the oxygen valve from burning;
④ The connecting devices between the torch and the oxygen cable and tube must be convenient and reliable, ensuring the solidity and tightness of the connection. The torch cutting bar clamping device should be simple and have a certain clamping force;
⑤ The cable connector is solid, the loaded part must be insulated, and its insulation resistance is not less than 35MΩ and supports 1000V (AC industrial frequency);
⑥ The oxygen valve must open and close flexibly, the connection is solid, does not leak under air pressure of 0.6 MPa, and the gas flow rate is not less than 1000L/min;
⑦ The outer surface of the torch components should be chrome-plated or silver-plated for corrosion resistance, and the coating should not have defects such as peeling.
The image below shows the SG-III type underwater oxygen cutting torch produced in our country. Experience shows that this type of torch is quite suitable.
If properly maintained, it has a long service life. However, after the cutting bar hole in the torch head has been in use for some time, its contact performance with the cutting bar will deteriorate, which often leads to arc generation at this location, causing damage to the torch. .
Furthermore, after prolonged use, the torch's insulation will decrease, which may cause leaks during the cutting process, putting the diver's safety at risk.
Therefore, it is necessary to regularly inspect the torch and repair or replace damaged parts in time.
3) Cutting cables and switches
Cables used in underwater arc oxygen cutting must be marine cables with multifilament copper cores and a rubber coating that resists seawater corrosion. The cross-sectional area of the cable is usually 70-100 mm 2 and its length depends on the depth of the water.
If the water flow speed is high, the cable needs to be extended. If marine cables are not available, welding cables for land use can be substituted but need to be checked regularly. If any aging or cracks in the rubber coating are discovered, the cable must be replaced immediately to prevent leaks.
The cable that connects the power supply and the cutting torch is colloquially called the “torch line,” while the one that connects the power supply to the cut piece is called the “ground line.”
For safe underwater operation, a cutoff switch is connected to the torch line to immediately supply or cut off power based on the diver's needs. The cutting switch may be a single-blade switch or an automatic circuit breaker, and its conductive elements must have sufficient conductive cross-sectional area.
An automatic circuit breaker can quickly increase the voltage to the level required for the arc during ignition and quickly cut off the power supply during arc interruption or electrode replacement.
This device measures 420 mm × 340 mm × 270 mm, weighs about 30 kg and is suitable for direct positive circuits.
4) Oxygen supply system
The underwater oxygen supply system for arc oxygen cutting consists of an oxygen cylinder, a pressure reducer and an oxygen tube.
① Oxygen cylinder:
The volume of the oxygen cylinder is generally 40L, with a weight of 60kg, an outer diameter of 219mm and a height of 1450mm.
It is painted sky blue and marked with the word “oxygen” in black paint. An oxygen cylinder is a high pressure container, with a nominal pressure of 15.15 MPa.
When using an oxygen cylinder, pay attention to the following:
The. It must be placed steadily during use and must not be mixed with other cylinders, especially with flammable gas cylinders or liquid fuel containers.
B. The oxygen cylinder must be kept at a distance of at least 5 m from fire sources and at least 1 m from general heat sources. It must be protected from exposure to strong sunlight and open flames.
w. The oxygen passage must not be contaminated with grease, especially on the oxygen cylinder valve.
d. Do not completely empty the cylinder of oxygen. At least 1-2 gauge pressure must be maintained to remove dust and prevent other gases from entering during refueling.
It is. A vibration-resistant rubber ring must be installed on the cylinder and must be handled carefully to avoid impacts and slipping.
f. Regular hydrostatic tests must be carried out on the oxygen cylinder. Unqualified cylinders must be repaired or decommissioned immediately.
② Pressure reducer:
A pressure reducer is used to reduce high-pressure oxygen in an oxygen cylinder to the pressure required for operation, ensuring the stability of oxygen pressure during work.
Two pressure gauges are mounted on the reducer to indicate the pressure inside the cylinder and the working gas pressure, respectively.
There are many types of reducers, divided according to the principle of operation into direct action and reaction types; by single-stage and multi-stage pressure reduction stages.
In practice, single-stage reaction-type reducers are commonly used for underwater oxygen cutting. When using a pressure reducer, the following points must be observed:
The. Before installing the pressure reducer, the oxygen cylinder valve must be opened first to remove dust and other impurities from the valve nozzle using oxygen. During operation, the valve nozzle of the oxygen cylinder should not be directed towards the body.
B. Check that all connections are tight and that there are no slip threads and adjust the screw to its loose position.
w. After installing the reducer, open the oxygen cylinder valve again, check whether the pressure gauge is working normally and whether there are any leaks. When everything is normal, connect the oxygen hose.
d. If the reducer is contaminated with grease, it must be cleaned before use.
It is. If the reducer freezes, it is not permitted to defrost it with fire. It can be defrosted with hot water or steam.
f. If a self-flow phenomenon is observed in the reducer, that is, when the adjusting screw is loosened, the low pressure gauge still rises automatically, this may be due to dirt on the spool or spool seat of the reducer, or uneven contact surfaces , causing the high-pressure gas to enter the low-pressure chamber.
At this time, the dirt must be removed and the spool smoothed with fine sandpaper. If a crack is found on the reel seat, it must be replaced in a timely manner.
The occurrence of self-flow may also be due to damage to the secondary spring, leading to insufficient pressure, which must be replaced.
(2) Underwater plasma arc cutting
1) Power supply cut:
To meet the special requirements of underwater plasma arc cutting, the underwater plasma arc cutting power supply uses a thyristor transistor switch and rectifier and is water-cooled.
It has a sharp drop characteristic, ensuring cutting parameters and arc stability when the arc length (arc voltage) changes; and the transition from a “small arc” to a cutting arc can smoothly reach the supplied current value without generating surge current in accordance with the natural characteristic of interruption.
This power supply considers reducing the no-load voltage to 110V in the control circuit and obtaining the external characteristic curve required for manual arc welding, making it also suitable for underwater manual welding.
Table 1 lists the main technical parameters of a typical underwater plasma arc cutting power supply.
Table 1: Main technical parameters of a typical underwater plasma arc cutting power supply
| Cutting current/A | 300~600(At a rated continuous load rate of 60%, during a 10-minute cutting cycle.) |
| No-load voltage/V | 180 |
| Maximum Working Voltage/V | 140(When the cutting current is set to 600A.) |
| “Small Arc” Current/A | 50 |
| “Small arc” power supply No-load voltage/V | 180 |
2) Underwater cutting torch
The distinctions between underwater plasma arc cutting and above ground cutting torches are as follows:
① An external shield is added to the nozzle, through which water or cooling gas flows, forming a “water curtain” (or gas curtain) to prevent water from entering the arc area. This allows the arc to burn stably and also prevents seawater electrolysis from affecting normal cutting;
② Each connecting part has good tightness;
③ They have high voltage insulation resistance.
Figure 4 and Figure 5 respectively illustrate two types of underwater plasma arc cutting torch structures. The KB model torch was designed for cutting in fresh water, with dimensions of 160mm×370mm×40mm and a weight of 2.5kg.
The PM model torch is used to cut seawater, with dimensions of 150mm×350mm×35mm and a weight of 2.5kg.
To ensure the tightness of all connecting parts, a pasty organic silicone adhesive is commonly used. This material vulcanizes at room temperature, transforming into a rubber-like substance, providing moisture resistance, thermal insulation and good insulating properties.
Maintains excellent sealing performance over a wide temperature range (-55 to 300 degrees Celsius).
To prevent air from entering the working gas channel and damaging the electrode during arc initiation, a check valve needs to be installed at the gas inlet. The working gas pressure opens the valve, expelling the temporarily stored air.
For the PM model torch, when the open circuit voltage of the power supply is 180V, a seawater leakage test was carried out. The highest leakage voltage was 10 V, which shows that it is safe and reliable for use in seawater with a salt mass fraction of 1.7% to 2.0%.
The nozzles of these two torches can be cooled by fresh water or compressed air. They can be used for underwater cutting of carbon steel, stainless steel and aluminum alloys at a depth of 52 meters.
3) Underwater Water Jet Cutting with Melted Electrode
Underwater water jet cutting with melted electrode is mainly semi-automatic. In China, dedicated cutting equipment model GSS-800 is available.
The cutting equipment consists of a main machine (including cutting power supply, control device, water circuit system and high pressure water pump), wire feeder, cutting torch, remote control box, wire spool combination cable and grounding cable reel.
The power source for underwater molten electrode waterjet cutting is fundamentally the same as the power source for above-ground molten electrode welding with gas shielding, being a natural flat feature arc welding rectifier, but with greater power.
The rated output current is generally 500-1500A. Table 2 lists the main technical parameters of the underwater cutting equipment model GSS-800 using a water jet with a melted electrode.
Table 2: Main Technical Parameters of Underwater Cutting Equipment Model GSS-800 Using Water Jet with Melted Electrode
| Input power supply | Voltage/V | 380V three-phase |
| Frequency/Hz | 50 | |
| Rated input current/A | 100 | |
| Rated input capacity/kW | 65 | |
| Power supply cut | Power Specifications | Direct current, natural flat characteristics. |
| Maximum Cutting Current/A | 800 | |
| Rated Load Continuity Rate/% | 60 | |
| No-load voltage regulation range/V | 50~70 | |
| Torch and wire feeder | Wire cutting diameter/mm | 2.5 |
| Wire feed speed/m.min-1 | 4~9 | |
| Wire feed hose length/m | 4 | |
| Yarn spool capacity/kg | About 15 | |
| Gas supply pressure/MPa | 0.8 | |
| High Pressure Water Pump | Engine power/kW | 3 |
| Operating Hydraulic Pressure/MPa | 0.6~1.0 | |
| External Dimensions (Length × Width × Height) /mm | Main Machine | 2120×1120×1615 |
| Combined Cable Drum | 1552×1620×1805 | |
| Ground cable drum | 1452×1370×1655 | |
| Wire Feed Box | 600×360×660 | |
| Weight/kg | Main Machine | 1300 |
| Combined Cable Drum | 1000 | |
| Ground cable drum | 8,000 | |
| Wire Feed Box | 50 |
This cutting equipment can perform semi-automatic cutting of metals such as carbon steel, stainless steel, copper and aluminum with thickness of 10-28mm in water depth of 60m.
It is especially suitable for underwater metal cutting in projects such as underwater salvage, seabed mining and subsea pipeline laying. It uses a 2.5mm diameter cutting edge and the cutting width is 4-5mm.
2. Material cutting
(1) Underwater Arc-Oxygen Cutting
Underwater arc oxygen cutting is suitable for conductive metals, but is mainly used for cutting easily oxidized low-carbon steel and high-strength low-alloy steel.
Generally, there are three types of cutting strips used in underwater arc oxygen cutting: steel pipe cutting strips, ceramic pipe cutting strips and carbon rod cutting strips.
The oxygen used in underwater arc oxygen cutting is general industrial oxygen, with purity divided into two grades: the first grade is not less than 99.2% and the second grade is not less than 98.5%. The oxygen supply method is bottled: oxygen is compressed to 120-150 atmospheres and placed in oxygen bottles for use and storage.
1) Steel Pipe Cutting Strips
The structure and manufacturing method of steel pipe cutting strips are similar to those of underwater welding rods. They are made using seamless steel tubes as the core and coated with mineral coatings or wrapped in plastic fiber film.
The coating mainly performs waterproofing, insulation and arc stabilization functions.
The waterproof performance of cutting strip can be achieved in two ways: one is to add a waterproof agent to the coating, which has waterproof performance after drying; the other is to apply a layer of waterproof agent to the cutting strip after drying, to achieve the waterproof purpose. The structure of the cutting strip is shown in Figure 6.
The outer diameter of the cutting strip core is generally 6 to 10 mm, the inner diameter is 1.25 to 4.0 mm, and the length is 350 to 400 mm.
Practice has proven that cutting efficiency is strongly related to the inner diameter of the cutting strip. The thickness of the aerated block is also important.
Under the same cutting conditions, as the inner diameter of the cutting strip increases, the cutting speed and efficiency also increase, as shown in Table 3.
Table 3: Cutting efficiency when cutting steel sheets 10-12 mm thick
| Cutting bar outer diameter /mm | Cutting bar inner diameter /mm | Oxygen pressure/MPa | Working current /A | Cutting length per bar/cm | Cutting time per bar/s | Oxygen consumption per bar/m 3 |
| 6 | 1.25 | 0.65 | 240 | 24 | 55 | 0.18 |
| 7 | two | 0.65 | 260 | 28 | 61 | 0:30 |
| 8 | 3 | 0.7 | 340 | 32 | 61 | 0.35 |
Increasing the internal diameter of the cutting strip improves cutting speed, possibly due to accelerated oxidation resulting from increased oxygen supply. At the same time, the blowing force on the molten metal and slag is increased, which helps to quickly remove them from the cutting area.
There are foreign examples of using cutting strips with an outer diameter of 10 mm and an inner diameter of 4 mm, which have good performance in cutting thick steel sheets. However, oxygen supply is a challenge in offshore operations and it is not suitable to consume too much oxygen, so cutting strips with a larger inner diameter are generally not used.
Adding a suitable amount of metal powder to the coating of the cutting strip can improve its electrical conductivity, stabilize the arc, and greatly increase the oxidation reaction heat of the cutting strip, thereby increasing the cutting speed.
Among them, the best effect comes from iron powder, followed by magnesium and aluminum powder. When these metal powders are added separately to the ilmenite-type coating, the iron powder should not exceed 35% and the magnesium and aluminum powder should not exceed 10%.
If too much metal powder is added, the performance of the coating will decrease and its strength and waterproofing capacity will also be reduced. If several types of metal powders are added simultaneously, their proportions must be reduced accordingly.
Furthermore, for coatings with added metal powder, its proportion by weight must be increased appropriately, but must not exceed 30% so as not to impair the performance of the coating. – Jiangsu Jinfeng Underwater Technology Engineering
Steel cutting strips are strong, affordable and offer good cutting quality (narrow cut and smooth cutting surface).
Although they also melt due to the heat of the arc and need frequent replacement, practice has shown that when cutting parts thicker than 19 mm, the overall cutting efficiency of steel pipe cutting strips is higher than that of strips for cutting ceramic tubes. Steel pipe cutting strips are the most commonly used in underwater arc oxygen cutting.
The underwater arc-oxygen steel tube cutting strips produced in our country are type 304 and consist of a seamless low-carbon steel tube with an outer diameter of 8mm and an inner hole diameter of 3mm, coated with a 1 mm thick drug layer.
Its waterproof insulating layer is made of phenolic varnish and is available in two lengths: 350mm and 400mm.
Responsible: Tao Xiaobin. Type 304 cutting strip is an ilmenite-type thick-coated cutting strip with a weight ratio of 20%. Its performance is not inferior to similar foreign products.
The newly developed underwater oxygen arc cutting strip features a mixed binder that replaces the previous water glass as the coating binder material, making the cutting strip more suitable for long-term preservation and deep water cutting.
This cutting strip can still be used after being soaked in seawater for 240 hours, and its cutting efficiency is even higher than type 304 cutting strip.
2) Ceramic tube cutting strips
Cutting strips made with ceramic tube core are known as ceramic tube cutting strips. They typically have an outer diameter of 12 to 14 mm, an inner diameter of 3 mm, and a length of 200 to 250 mm.
During manufacturing, the ceramic tube is first baked at high temperature to obtain a certain strength, and then a steel coating (about 8mm thick) is sprayed on its outer surface to increase the strength of the ceramic tube.
The end of the ceramic tube, about 32 mm long, must be ground to a diameter that matches the size of the cutting torch for clamping purposes. The remaining part is coated with insulation material or wrapped with waterproof insulation material to form the ceramic tube cutting strip.
The metallic exterior of ceramic tube cutting rods not only increases the strength of the rod, but also improves its electrical conductivity and arc initiation performance. During cutting, the outer metal first comes into contact with the part to be cut.
Due to the skin effect of current, as a portion of the current flows from the outer metal to the workpiece, an arc is initially generated between the outer metal and the workpiece, melting the outer metal first.
Simultaneously, the arc and molten metal preheat the diamond grains at the end of the cutting rod, thus increasing its electrical conductivity.
At this time, the cutting current flows not only in the outer metal of the ceramic tube cutting rod, but also in the ceramic tube itself, directing the arc to the end of the rod for stable combustion.
As ceramics have high anti-oxidation ability, a single ceramic tube cutting rod can be used for 40 to 60 minutes, significantly reducing auxiliary time for underwater cutting operations.
However, the cutting speed per unit of pure cutting time is lower than that of steel pipe cutting rods, and the arc stability is also lower. Therefore, in situations where time is limited and only one or two cutting rods are enough to complete the task, it is advisable to use steel pipe cutting rods.
3) Carbon Rod Cutting
Carbon cutting rods are made from hollow carbon rods or graphite tubes, coated with an outer layer of copper.
They have an outer diameter of 10 to 11 mm, an inner hole diameter of 1.6 to 2 mm and a length of 200 to 300 mm.
Carbon cutting rods have lower compressive strength, and to prevent the end of the rod from being crushed by the cutting torch clamp, a brass cap is installed at one end. To begin cutting, the end cap is inserted into the clamp. To prevent electric shock, an insulating layer (plastic or resin) is applied over the copper coating.
The service life of carbon cutting rods is quite long, second only to ceramic pipe cutting rods.
For a 200mm long carbon rod cutting rod, its working time is approximately 10 to 12 times that of a 400mm long steel tube cutting rod; however, the cutting speed per unit of pure cutting time is lower than that of steel pipe cutting rods.
(2) Underwater plasma arc cutting
Underwater plasma arc cutting mainly uses N2, Ar-H2 mixed gas, O2 and compressed air as plasma gases; CO2, Ar, N2 and compressed air can be used as shielding gases.
Different plasma gases require corresponding electrode materials. In general, tungsten electrodes should be chosen when the plasma gas is N2 or Ar-H2 mixed gas, while hafnium electrodes should be used when the plasma gas is O2 or compressed air.
Because underwater cutting requires a large current, water-cooled electrodes must be used to extend their service life.
When using N2 as plasma gas, although the cutting speed and quality are high, the consumption rate is fast and the operator needs a high level of skill. Especially when cutting at depths greater than 40 to 60 m, the nozzle is subject to damage.
Therefore, Ar should be preferred as the plasma gas for deep water cutting, and Ar-H2 mixed gas should be used as the plasma gas for shallow water cutting.
(3) Submerged arc waterjet cutting
The current submerged arc waterjet cutting process employs solid core cutting wire or flux cored cutting wire.
1) Solid core cutting wire
This method uses welding wire shielded with CO2 gas or aluminum wire, typically 2.4 mm in diameter. The use of CO2 welding wire for cutting in 200mm water depth has the following characteristics:
i) Water depth does not significantly affect cuttable thickness or resulting kerf thickness.
ii) As the arc voltage increases, the cut becomes wider and the lower part may even burn. If the water depth increases by 100 m, increasing the arc voltage by 5-10 V can result in a cut shape similar to that obtained in shallow water.
iii) The pressure of the water jet must increase with the depth of the water. The appropriate water pressure is the hydrostatic pressure equivalent to the water depth plus 0.5 MPa (for cutting low carbon steel) or 0.35 MPa (for cutting aluminum).
iv) Aluminum is easier to cut than low carbon steel. Due to aluminum's lower melting point and infrequent arc short circuits, cutting aluminum is 50% faster than cutting low-carbon steel under the same plate thickness and current conditions. cut.
v) When cutting low carbon steel, more slag adheres to the lower edge of the cut; Less slag occurs when cutting aluminum and can be removed with a wire brush. This is due to the fragile iron-aluminum alloy formed during the cutting process.
When using aluminum wire to cut low-carbon steel, no slag sticks to the bottom edge of the cut, and the cutting surface is smooth.
However, to obtain the same cutting current as cutting with CO2 welding wire, the wire feed speed must be increased, often exceeding the range of standard MIG welding wire feed speeds.
2) Flux Core Cutting Wire
Flux core cutting wire uses low carbon steel MIG welding wire, typically 2.4mm diameter. Submerged arc waterjet cutting with flux core cutting wire can cut carbon steel and stainless steel as well as aluminum.
