Initial trials of underwater wet flux-cored wire cutting ...
Transcript of Initial trials of underwater wet flux-cored wire cutting ...
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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: [email protected]
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
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(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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