To eliminate generation of defects such as blowholes in a welded metal by drilling a through-hole in a steel tube or an annular body at the time of welding the steel tube to the annular body to be inserted into it and thereby discharging an expansion gas due to welding heat from the through-hole to the atmospheric air.
An upper and lower annular bodies 2, 2' are fitted around the outer circumference of a steel tube 1, and welded to the steel tube 1 at an upper weld (a) and a lower weld (b). A space 3 is formed between the steel tube 1 and the inside of each the upper/lower annular bodies 2, 2', and made an enclosed space by the welds (a), (b). Then, a through-hole 4 is drilled in the thickness direction of the steel tube 1. After this through-hole 4 is drilled, the annular body 2 is welded. A through-hole 4a is drilled obliquely in the annular body 2 or a through-hole 4b is drilled in the diameter direction so that the space 3 is directly communicated with the atmospheric air. Even if the space 3 gains a high temperature with the effect of a welding heat, the expansion gas is discharged from the through-hole 4 to the inside of the steel tube 1 or from the annular body 2, 2' directly to the atmospheric air, causing no increase in pressure inside the space 3 and generating no defects such as blowholes inside the welded metal.
MIYAO TOSHIAKI
NAKAMURA NOBUYUKI
ISHIMURA HIROSHI
