Abstract of Dissertation (論文の要旨)
Title (題 目): Development of Vertical Welding Technology for Heavy-Thick and Ni
Steel Plate Using Hot-Wire Laser Welding Method（ホットワイヤ・レーザ溶接法による厚鋼板およびニッケル鋼立向き溶接技術の開発）

Sittisak Charunetratsamee
In recent years, the capacity of container shipbuilding has been greatly
expanded with regard to a demand for the maximum loading capacity. The larger
their structures are, the thicker steel plates become. Its thickness up to 100 mm canbe found in the structures of a large-size ship, especially hatch side coaming. For
vertical welding joints of the heavy-thick sections, electrogas arc welding (EGW)
which is a single-pass vertical welding method is generally used in the field ofshipbuilding. As a result of a large heat input welding process, the HAZ width
becomes so wide; the microstructure of heat affected zone (HAZ) is coarsened andthe properties of joints deteriorated. Steelmakers have developed a new technology
which achieves high strength steel to reduce the designed thickness and hightoughness in the HAZ to resist high heat-input welding processes. Consequently, anovel vertical welding process is strongly demanded to achieve higher efficiency andlower heat input for obtaining weld joints with higher strength and higher toughness.
Nowadays, the number of LNG-fueled ships is drastically increasing and more
infrastructure projects are planned to replace the conventional oil-based fuel due tothe particular emission regulations issued for the international maritime. Using theLNG removes SOx and particle PM emission completely and reduces NOx emissionof up to 85%. For an increase in productivity of LNG tanks, weld joints strongly
require the high-efficiency and high-quality welding process to decrease construction
time and cost. Therefore, we would like to develop new welding processes instead of
conventional arc welding processes in vertical welding using hot-wire laser method.
Firstly, a diode laser and a hot-wire system were applied for the novel singlepass vertical welding. The 6-kW diode laser as a heat source irradiated acontrollable rectangular beam vertically fitting a thickness of steel plate and anelectrically heated filler wire up to its melting point as called hot-wire was used as
deposited weld metal. A width of the beam was able to change to optimize power
distribution. The width narrower than a 10-mm gap width was used with a weaving
system to sweep the beam throughout the gap width. 2-mm and 4-mm beam widths
with a 5-Hz exponential waveform having different power density were used. The 2mm beam width had high power density on the groove surfaces that could not keep
heating on the molten pool and some heat lost by high reflecting laser on above theweld pool. In other words, the 4-mm beam width could keep the molten pool andmelt the base metal that was better than the 2-mm beam width. In addition, the beam
could concentrate more power on the beam edges by using a high-edged energy
beam type made by a particular optic lens that increased dilution and reduced lack offusion on the groove edges. Consequently, the beam having the high-edged type
and weaving system could optimal power distribution on the groove area. Then, 5%,
10% and 20% oxygen in argon shielding gas were studied in the effects of oxygen1content on welds compared with argon shielding gas. The result showed that thebetter dilution could be observed by using 20% oxygen-added argon shielding gas
rather than using argon shielding gas.
The novel vertical welding process could apply to a large size specimen similar
to the fields. Due to the limited laser power of 6 kW, its thickness up to 28.5 mm
could be applied. For the welded joint properties, the tensile strength met therequirement of the specification and the V-notch Charpy impact test result of notch
location at weld metal showed high absorbed energy at -200C according to CTOD
test results that exhibited the high resistance to brittle propagation at -100C.
However, since grain boundary ferrite could be observed in the fusion line, the fusionline showed the unstable absorbed energy.
Then, twin diode lasers were employed to enhance the whole amount of laser
power of 12 kW and the heavy-thick steel plate of 50 mm was used. In order tooptimize power distribution on the groove, the first laser (LD1) irradiated a large
rectangular beam stably which fitted the groove area with high energy density on
both edges in its beam. The beam of second laser (LD2) was irradiated in three
beam sizes in different power distribution as follows: a stable large rectangularbeam, a narrow rectangular beam with weaving system having a beam width of 2mm and 5 mm in order. Based on the adequate power laser and hot-wire system, thestable welding process could be observed during welding of three welding
conditions. However, the cross-section of double stable laser beams showed very
low dilution and narrow bead width and revealed the lack of fusion near the wire
feeding side. Consequently, the stable beam with high-edged energy and bothconditions of 2-mm and 5-mm weaving beams that achieved a sound joint could
improve dilution on edges around plate surfaces and groove surfaces where the lackof fusion easily occurred.
To study the feasibility of using the novel vertical welding process for Ni steel
plate, large specimens consisted of three different thicknesses 10 mm, 15 mm and
20 mm were used. At first, the welding conditions were optimized in each thickness
such as laser power, a rectangular beam dimension, welding speed, etc. The
proposed process achieved a stable weld pool formation and steady hot-wire feeding
during welding in all specimens. The complete penetration could be obtained in thesteady state and the cross-sections showed full penetration without lack of fusion.
The joint properties of tensile test and Charpy impact test met the standard
requirements. In case of the tensile strength, the breaking point was located at theweld metal due to the softer material, and Soft/Hard ration also affected the tensile
strength result. The 10-mm specimen having higher Soft/Hard ration showed theleast tensile strength value at the same gap width.

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