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Initial trials of underwater wet flux-coredwire cutting at shallow water

(1. State Key Laboratory of Advanced Welding and Joining, Harbin

Institute of Technology, Harbin 150001, China; 2. Shandong Provincial

Key Laboratory of Special Welding Technology, Harbin Institute of

Technology at Weihai, Weihai 264209, China)

Abstract: Underwater cutting technology is commonly used in

repairing and removement of damaged offshore steel structures.

In this paper, arc stability and cut quality of flux-cored arc cutting

process were investigated at shallow water. Gas-forming flux-

cored wire and exothermic flux-cored wire were selected to

evaluate its future potential use in underwater cutting process.

Underwater flux-cored arc cutting process was realized using the

two types of wires. Acceptable cut appearances were obtained

and no local bridge emerged in two cuts. Arc stability of FCAC

process for exothermic wires was superior to that for gas-forming

wires because exothermic wires contain more arc stabilizers.

Key words: underwater cutting; flux-cored arc cutting; arc stability;

cutting ability

DOI: 10.7512/j.issn.1001-2303.2017.13.17

Dr. Duo LiuEmail: liuduo0376@163.com

An associate professor of School of Materials Science and Engineering in Harbin Institute of Technology at Weihai since 2013. Her professional interests are in underwater wet welding and cutting, joining between ceramic and metal, dissimilar metal joining, brazing and diffusion bonding of advanced materials, et al.

Education experience:2006.08~2009.10 Harbin Institute of Technology,

Materials processing engineering, Ph.D.2004.08~2006.08 Harbin Institute of Technology,

Materials processing engineering, master. 2000.09~2004.07 Dalian University of Technology,

Specialty of material forming and control engineering, bachelor.

0 IntroductionRepairing and removement of damaged offshore steel structures

require the development of underwater cutting technology [1].

Underwater cutting can be classified into underwater cold and thermal

cutting. Underwater cold cutting free of thermal stress and heat-

affected zone possess a wide range of cutting capacity and high cutting

quality [2]. But cold cutting was used in special condition to obtain high

quality cuts because of its relatively huge costs. Underwater thermal

cutting featured by high cutting speed and high cutting efficiency is a

promising cutting method although it produces thermal effects and

rough cuts. Underwater thermal cutting methods typically include flame

cutting, oxy-arc cutting, plasma cutting and consumable electrode

water jet cutting, and only manual oxy-arc cutting method is now an

established technique for practical application [3-5]. But this method is

slow in cutting speed and require high-skilled operation techniques [6].

Researchers worldwide work on the development of underwater

laser cutting technique for steel and alloys for nuclear applications using

Hongliang Li 1, Duo Liu 1, 2, Hongyun Zhao 1, 2, Yaotian Yan 2, Jinghe Liu 2

local-dry-zone creating nozzle [7-10]. During dry laser cutting process,

a high-power laser beam is focused on the job so that the material

reaches its melting temperature and a high-pressure active or inert gas

is used to remove the molten material. This information indicated that

underwater laser cutting could be used as a high-performance tool for

remote cutting in dismantling and repairs of ship and pipe lines in water.

For such operations, it is highly useful to deliver the laser beam through

optical fiber because of its flexibility. However, this method is urgent

for local dry conditions because laser beam was significantly sensitive to

water. Particularly, it is difficult to form completely dry space under great

depth. What’s more, Germany researchers investigated the contact

arc metal cutting (CAMC) process and pointed out that CAMC could

compete in cutting efficiency and pollution emission rate with other

underwater cutting processes [11]. This method requires extremely high

energy (single power source 1000 A) and is favorable for steel plates

with great thickness. Therefore, it is essential to develop underwater

thermal cutting processes of easy automation and high efficiency.

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The E.O. Paton Welding Institute has developed a method of

underwater electric arc cutting with flux-cored wires [12]. The flux core

contains gas-forming and arc stabilizing components. The cutting

mechanism of underwater flux-cored wire arc cutting (FCAC) without

additional supply of oxygen is that resultant thermochemically highly

active gas-flame jet melts, oxidizes and removes the molten metal from

the cutting surface [13]. In contrast to the existing methods of electric

oxygen cutting, air-plasma cutting and electric water jet cutting, this

method does not require any additional feed of gas or water into the

arcing zone. Comprehensive underwater flux-cored wire cutting results

were obtained using this techniques [14].

Overall, literatures on underwater thermal cutting were very little.

It is very important to dedicate special attention to this process. In this

article, cutting arc stability and cutting quality of FCAC process were

analyzed to characterize the potential of this process using two types of

flux-cored wires.

1 Material and Experiments Welding experiments were conducted in a 1000 mm×1000 mm×

600 mm water tank with Higerman Hi800 CNC operating system in

flat position. Aotai NBC 630 as a power source and YW-50KB3HAE

wire feeder were used for all experiments. The power source has an

open circuit voltage of 70 V with a current limit of typically 630 A. DCEN

current polarity was used in all welds at a depth of 0.3 m. The welding

current and voltage signals were acquired in real time by Hall sensors

and NI 6210 USB data acquisition cards at a frequency of 10 kHz/s.

Two types of flux-cored wires were selected to conduct underwater

cutting trials. A FCAC wire (#1) with 2.0 mm of diameter specially

developed by E.O. Paton Welding Institute, called PPR-AN1, was used

in the wet FCAC process. The flux-core of this wire mainly consists

of gas-forming components. The flux core of the other wire (#2) is

with exothermic addition (55% thermite) which was designed for

underwater cutting application. Electrode wire (#2) was produced with

(34~35) wt.% flux fill to steel sheath ratios. The cutting parameters

applied are arc voltage of 30 V, wire feeding speed of 8.5 m/min,

current of 400 A, and cutting speed of 300 mm/min. Q235 steel with

the thickness of 8 mm was used in underwater wet cutting experiment.

2 Results and discussionFig.1 demonstrates the recorded typical electrical signals of

underwater cutting process using wire #2. Obviously, the fluctuation

trend of electrical signals of cutting process was similar to underwater

welding process before the workpiece was pierced. The measured

values of voltage and current were fluctuated greatly during cutting

process. The time when the workpiece was pierced was a symbol of

arc extinction. For underwater FCA cutting process, arc extinction was

very common. Therefore, the ability for an arc to restart was important

to realize a stable cutting process after the arc extinction. In fact, lots of

gas-forming constituents and arc stabilizer in flux core helps to stabilize

ignition and permit longest arc length discharge.

Arc stability of cutting process was altered due to the difference of

flux core components between the two wires. The effect of two types

of wires on probability density distribution of voltage is displayed in

Fig.2. The probability density distribution of voltage with the addition

of exothermic addition had less pronounced arc extinction region. This

figures also illustrated a more intense and narrower large hump in the

center was obtained for the exothermic addition, which implied a more

steady cutting process.

To dynamically and clearly display the fluctuation of the electrical

signal in the whole cutting process, cyclograms of arc voltage and

cutting current (U-I) was selected for the two wires, as illustrated in

Fig.3. It can be observed that the arc burning area for wire #2 became

more concentrated than that for wire #1. Also, compared to wire #1,

the intensity of the working point arc extinction area for wire #2 was

small indicating that a more stable cutting process could be obtained for

wire #2.

The arc burning through capacity, called pierce time, is considered

to be space of time from arc ignition to full burning through of metal.

The pierce time is important because the plate must be pierced prior

Fig.1 Typical waveforms of the underwater cutting process

Fig.2 Probability density distribution of voltage for two types of wires

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to cutting in practical decommissioning situations. The relationship of

pierce time depending on plate thickness is displayed in Fig.4. It can be

seen that pierce time of wire #2 was shorter at different plate thickness

than that of wire #1, which indicated the benefits of exothermic

addition in wire #2. Here it should be mentioned that wire #1 could

function well at greater depth because of arc compression.

Fig.3 Cyclogram for arc voltage and cutting current for the two wires

(b) wire #2(a) wire #1

Fig.4 Piercing time depending on plate thickness for two wires

Fig.5 FCAC cut appearances of two wires

The cut quality was not of primary importance but it was essential

that any dross produced did not cause a local bridge in the cut. Fig.5

demonstrates that the typical flux-cored arc cutting appearances of the

investigated two wires. It is visible that no local bridge emerged in two

cuts. The back width of cut was larger than the front width of the cut.

In addition, a smaller average cut width (6.1 mm) was obtained for wire

#1 than that (7.8 mm) for wire #2. That is, for wire #2, a considerable

amount of energy was conducted into the work piece resulting in

changes in the material properties and the microstructure of the

material leading to large heat affected zone (HAZ).

The flux-cored arc cutting process has the potential to be applied

to decommissioning work since it can be easily started and operated

automatically. However, it is essential to figure out the optimal cutting

parameters (arc voltage, current and cutting speed) for FCAC process

according to the practical conditions. If the unfitted cutting parameters

were selected, the emergency of arc extinction or bridging between

two edges may decrease the efficiency of FCAC process.

3 ConclusionIn this paper, gas-forming flux-cored wire and exothermic flux-cored

wire were selected to evaluate its future potential use in underwater

cutting process. Main conclusions are as follow,

(1) Underwater flux-cored arc cutting process was realized using the

two types of wires. Acceptable cut appearances were obtained and no

local bridge emerged in two cuts.

(2) Arc stability of FCAW process for exothermic wires was superior

to that for gas-forming wires because exothermic wires contain more

arc stabilizers.

20 mm

Plate thickness/mm

· ·169

(3) The flux-cored arc cutting process could be applied to

decommissioning work since it can be easily started and operated

automatically.

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