Abstract
Thermal cycles of welds in underwater wet gravity arc welding were measured. Then the effect of cooling rate on occurrence of underbead cracks and on the mechanical properties of welds were investigated.
Main results are summarized as follows;
(1) In case the shielding processes are used, the cooling rate and the hardness of underwater welds can be lowered certainly.
(2) Underbead cracks occurring in underwater welds of SM50 steel thinner than 9 mm in thickness may be prevented by lowering the cooling rate at 500°C below 35°C/s and the maximum hardness of the weld less than Hv360 approximately. Such lower cooling rate can be obtained by oil putty shielding.
(3) Whether the shielding process are used or not, cooling rate (R) at 500°C and cooling times (S500, S300) of underwater welds from 800°C to 500°C or 300°C may be estimated from the following experimental formula;
R=6.25×105a⋅Q-0.95⋅tβ (°C/s)
S500=455a⋅R-1.09 (s)
S300=780b⋅R-1.09 (s)
where, Q: weld heat input (J/cm), t: thickness of base metal (mm), and a, β, a and b are constants given as follows :
non shielding: α=1, β=0.17, a=1, b=1
refractory shielding: α=0.28, β=0.45, a=1, b=1
oil putty shielding : α=0.036, β=1, a=0.67, b=0.80
(4) The welded joints obtained by oil putty shielding have nearly comparable mechanical properties with those of open air welds.
