The flux-cored arc process for wet welding and cutting - an assessment

TWI Bulletin, May - June 1997

Bill Lucas is Welding Technology Manager for the Arc Laser Department at TWI which he joined in 1970. Much of his work has been concerned with assessing advances in processes and advising Members on their industrial applications.

Underwater welding and cutting can be carried out using either an artificially created dry habitat (hyperbaric) or with the welder and electrode in contact with the water (wet welding). Bill Lucas makes an in-depth appraisal of the FCA process for wet welding and cutting.

As a special habitat is not required, wet welding offers the advantages of low equipment costs and versatility in that the operations are only limited by access to the welder. In a specific application, costs for wet underwater welding were estimated to be about half the cost for hyperbaric welding ^[1] . It is not surprising, therefore, that wet welding is often the preferred technique in repair situations.

In Europe and the USA, wet welding is carried out with the manual metal arc (MMA) process using special electrodes which have a water-proof coating. The principal limitation of MMA wet welding is that, as welding is intermittent due to use of short rod electrodes, production rates are low.

The E O Paton Institute has recently developed an innovative wet welding technique, based on the self-shielded flux-cored arc (FCA) process, which can also be used for cutting. The FCA wires have been developed specifically for operating in direct contact with water, and the novel wire feed system can be completely immersed. When used in either welding or cutting operations, the FCA process offers potential for significant productivity benefits through use of a continually fed wire, compared with MMA where the rod electrodes must be changed at frequent intervals. Furthermore, it is claimed that the combination of flux formulation and wire composition produces the desired slag-gas forming reactions which will not only improve the weld bead profile but also reduce the pick up of hydrogen and oxygen in the weld metal.

As the FCA process appears to offer substantial benefits for cutting and welding operations, a series of welding trials was carried out to evaluate the FCA process (consumables and equipment) This was to substantiate claims for wet underwater welding and cutting with regard to the benefits in weld bead characteristics and productivity. Results of the trials, summarised in this paper, were collated over a period of six months using TWI welders and welder-divers from the UK. Several applications carried out in the former Soviet Union countries have been used to illustrate the benefits of the process for underwater welding.

Welding equipment

Diving Tank

Initial evaluation trials were carried out at TWI in a small tank, which had a maximum depth of water of 1.5m. However, subsequent trials were completed at TWI North's facility where the water depth of 8m provided a more realistic evaluation of the equipment and process.

FCA wire feeding systems

The unique feature of the FCA wire feeding system, Fig 1, is that it has been designed to operate fully submerged with only the control panel and power source outside the tank. The wire feed container is filled with water to balance the hydrostatic pressure when operating underwater (at depths up to 40m). The drive motor and reduction gear are contained in a sealed unit which is filled with a dielectric liquid to provide insulation from the water.

The wire is held on a small reel with a capacity of 3.5kg. If necessary, a new reel of wire can be fitted by the welder/diver whilst underwater.

Power source

For welding and cutting, a conventional flat (voltage-current) characteristic power source is used but it must have relatively high open circuit voltage of 60V for operating underwater. The maximum current for the normal range of wire diameters (1.2. to 2.4mm) is normally limited to 500A (60% duty cycle).

System control

The system is controlled remotely by the welder/diver for safety reasons. A control console, Fig 2, contains the electronics for controlling the wire feed motor, circuit tests, parameter measurements and indicators. In a diving situation, the welder will instruct the equipment controller on the surface to switch on/off the power source and wire feed and to regulate the power source voltage and the wire feed speed.

Welding performance

Process characteristics

Stability of FCA wire for wet welding is characterised by the behaviour of the arc and formation and collapse of the vapour-gas bubble which surrounds the arc and weld pool. As the wire is essentially a rutile type, molten metal transfers from wire to weld pool by the short circuiting mode of metal transfer.

When welding in the flat position, the process is stable with a relatively quiescent arc within a stable gas bubble. To achieve process stability (stable metal transfer and gas shield) in welding, the preferred wire diameter for welding is 1.6mm which produces a deposition rate of typically 1kg/hr in the vertical position, and up to 2kg/hr in the flat position. The slag which is formed on the weld bead is relatively easily removed.

When welding in position, the arc/metal transfer and vapour-gas bubble are less stable. A characteristic feature is that the bubble will periodically detach itself from the wire tip, usually during a short circuit, and the arc is momentarily extinguished. Joints in the vertical position are more difficult to weld and greater skill is required to produce satisfactory weld penetration and bead profile. The vertical-up position is recommended for these wires to minimise the risk of fusion defects, but the welder may find it easier to produce a smoother bead profile in the vertical-down position.

Weld bead appearance and penetration

Typical welding parameters for both fillet and butt joints using a 1.6mm diameter, type PPS-AN2 wire, in the flat and vertical positions are given in Table 1 and 2 , respectively; the characteristic weld bead appearance of the flat and vertical welds is shown in Fig 3. Weld bead penetration is particularly good as shown by the visual appearance of the root pass in the vertical position, Fig 4a, and the section through the final weld which shows good sidewall fusion with no indication of cracks or porosity, Fig 4b.

Hardness

The weld metal and heat affected zone hardnesses (HV10) which were recorded for the vertical fillet weld in 8mm thick, C-Mn steel (BS 4360:43A) were as follows:

The higher hardness, in the HAZ compared with the weld metal, can be attributed to the higher carbon equivalent; the IIW CE for the parent metal was 0.32 and 0.19 for the weld metal.

Typical welding costs

Welding times

A breakdown of the time to weld 1m length of 14mm thick steel was produced by the Paton Institute for FCA and MMA wet welding a butt joint in the flat and vertical positions, Table 3. In the flat position, the FCA weld was completed in five passes whereas the MMA weld required 10 passes. The considerably longer (total) welding times for MMA, more than double that for FCA welding, resulted not only from the greater number of passes but also the additional time required for changing electrodes and the deslagging operations. In the vertical position, the welding, or arcing, times are comparable but additional time for electrode changing and deslagging substantially increases the total time required for MMA welding operations.

Cost analysis

Comparative costs of wet underwater welding using the MMA and FCA processes are shown in Fig 5. Costs are based on time required to weld butt joints in 14mm thick plate using a qualified diver/welder skilled in the FCA process. As deslagging the welds between runs was carried out manually, use of a mechanised cleaning tool should substantially reduce the time recorded for this operation. Consumable and labour costs are estimated figures, but are based on typical costs in the UK.

The greatest cost was for MMA welding in the vertical position due to the larger number of passes. As the number of passes was lower in the flat position, the overall cost was reduced.

FCA welding gave a significant reduction in the welding costs primarily because of the continuously fed wire electrode. In comparison, when using the MMA process, the welder required almost a third of the total welding time to replace electrodes, and it should be noted that these times were under ideal welding conditions ie in a tank. There were also significant gains in applying the FCA process from using a slightly higher welding current, fewer weld passes and a reduction in the amount of cleaning.

FCA cutting

The FCA process can also be readily used for cutting operations. A 2.4 mm diameter wire is normally used which generates a more forceful arc and gas 'jet'. The process appears to work equally well in the vertical-down and horizontal-vertical positions. However, when cutting in the vertical-up position, it was significantly more difficult to maintain the cut opening.

Eight millimetre thickness plate can be cut in the horizontal-vertical position at a speed of 260mm/min using cutting parameters of 450A/42V; cutting speeds in the flat position were typically 250mm/min (450A/40V) and in thevertical-down position 156mm/min (460A/35V). Visual appearance of the cut in the horizontal-vertical position is shown in Fig 6a and the cut width is typically 7mm. The cut edge was reasonably square, Fig 6b, but witha rough surface profile.

Application of FCA system

Wet underwater FCA welding and cutting techniques have been used quite widely in member countries of the former Soviet Union in applications such as repair of ships, pipelines and offshore structures ^[2] . For example, the FCA process has been used to repair more than 70 gas, oil and water pipelines since 1970. Samples of the techniques used in the repair of typical defects in pipelines are shown in Fig 7.

The FCA system has also been widely used in the repair of ships and dock structures. Relevant application areas include:

• sealing of hulls damaged by underwater obstructions or icebergs
• repair of corrosion damage
• repair of screw propellers and rudders
• floating dock repairs and harbour installations

More than 200 ship repair and salvaging applications have been successfully carried out. A noteworthy example was in the salvage of the Mozdok ship which had sunk in the Odessa port as a result of a 7x14m breach in its hull. Welding of the repair patch and the sealing of the cargo hold covers before lifting overlap welds at a depth of 12 to 30m, and vertical and flat welds at a depth of 12m, Fig 8, Total length of welding was 100m.

Conclusions

Based on TWI's evaluation tests at Cambridge and the practical experience in the former Soviet Union, there is no doubt that the FCA system offers a substantial advantage over conventional MMA for wet welding and cutting operations, especially in those situations where a large amount of welding/cutting must be carried out. Potential savings from use of FCA welding operations compared with MMA welding should be approximately 50%. The savings will be realised from reduction in the ancillary operations eg electrode changing, and the slightly higher deposition rates.

Although the process was designed for manual welding, it opens up the possibility of remote operation using an ROV. However, successful application of automatic techniques will depend upon the ability of the ROV to mimic the manipulative skill of a human welder. Irrespective of the type of operation (manual or automatic), reliability of the system will be crucial in order to ensure that the economic benefits derived from continuous operation, can indeed be realised.

Acknowledgements

The author would like to acknowledge the financial support of Michael Cooper (MOD) in providing PWI equipment for the trials.

References