| Summary: | 碩士 === 國立海洋大學 === 航海技術學系 === 83 === The properties of metal weldments are changed owing to the
increase in "heat-input". In addition to that, the
microstructure and corrosion behavior of the weldments will not
be the same at different environments. This study is focused
on the properties of microstructure, microhardness and
corrosion of a general marine-used structural steel weldments
produced by conventional and underwater welding respectively.
The welding process of shielded metal arc welding（SMAW）was
applied for both under seawater welding and atmospheric
welding. CNS E4301 wrapped with plastic for water proof was
used as a welding electrode to join JIS SS41 structural steel.
The cooling rate of under seawater was much higher than that of
air. Thus, a large amount of heat was lost into the water
environment. Underwater weldments show a coarser grain size in
the top of bead. The constituents of the weldments in each
zone are summaried as follows:┌─────────┬────┬
────┬───┐│ PROCESS\ZONE │ WM │ HAZ │
BM │├─────────┼────┼────┼───┤│
Underwater │WF+AF+M │ AF+P │ P+F │├────────
─┼────┼────┼───┤│ Air（Land） │AF+GF+WF
│ P+F │ P+F │└─────────┴────┴────┴─
──┘ The microhardness of underwater weldment was about 10 Hv
higher than that of weldment made on land. However, they had
nearly the same microhardness arround the welded metal in both
underwater and atmospheric welding processes. But, it was
wider in underwater weldments for hardness change than that
welded in air through tranversal section. Comparing the
corrosion properties of the weld metal & heat affected zone, it
shows the weld metal in underwater weldments prefers to
corrode. In contrast, the heat affected zone is the most
active to corrode as compared with the weldment made on land.
The results of immersion test are consistent with the
measurement of dynamic polarization curves of the weldments.
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