裏当て金の材質が完全溶込み溶接部の機械的性能におよぼす影響

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  • INFLUENCE OF BACKING PLATE ON THE MECHANICAL PROPERTY OF WELD METAL IN FULL PENETRATION JOINTS
  • ウラアテ キン ノ ザイシツ ガ カンゼン トケコミ ヨウセツブ ノ キカイテキ セイノウ ニ オヨボス エイキョウ

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 The base materials used were TMCP385B and SA440B with a plate thickness of 40 mm. For the backing plate, a SN490B blast-furnace steel sheet with a thickness of 9 mm and two types of electric furnace steel plates with different nitrogen contents were used; a blast-furnace steel plate stock was also used for comparison. JIS Z 3312 G59JA1UC3M1T was used as the welding wire. MAG welding was used as the welding method. Then, the effect of the material of the backing plate on the chemical composition and on the mechanical properties of the weld metal part in the full penetration weld joint was evaluated. We investigated the case wherein the mechanism of the origin of these properties and the important factors that guarantee these properties fluctuate. The test results led to the following findings.<br> ? In the scope of this research, the material of the backing plate was found to not affect the deterioration in quality of the weld characteristics.<br> · The composition ratio of the backing metal to the weld metal in the one-layer one-pass portion was 33%－39%, two-layers one-pass portion was 5%－10%, and three-layers one-pass portion was 1%－2%; as the lamination progressed, the composition ratio of the backing metal decreased.<br> · The Ceq of the weld metal was lowest for the one-layer one-pass portion closest to the backing plate. However, the tensile strength higher than that of the base metal was guaranteed by cooling the surrounding of welded part.<br> · When an electric furnace steel sheet backing metal was used, the total nitrogen tended to increase as the distance between the weld metal part and the backing metal part was short. In particular, the total nitrogen value of the 60° V-type groove, wherein the composition ratio of the backing metal is high, was high.<br> · When a SN490 electric furnace low-nitrogen steel plate was used as the backing plate, the tensile strength was 24 N/mm2 lower than the samples using the other backing plates. This result is attributed to the Ceq of the SN490 electric furnace low-nitrogen steel plate being 0.05%－0.08% lower than the average Ceq values of the other steel plates. However, the strength of the weld metal was not less than the lower limit of the yield point or the tensile strength of the base metal.<br> · The Charpy absorbed energy at 0°C of the weld metal of this sample was 132－158 J, with a 100% ductile fracture surface.<br> · Spherical nonmetallic inclusions of 5 μm or less scattered in the intergranular ferrite were Ti－Si－Mn－O composite oxides; these inclusions became the starting point of intragranular ferrite and contributed to the improvement of toughness.<br> · When an electric furnace high-nitrogen steel sheet was used as the backing plate, the free nitrogen content of the weld metal was 29 ppm higher; however, the Charpy absorbed energy at 0°C of the weld metal did not decrease.<br> · Weld metal toughness was examined using thermodynamic equilibrium calculations. When the amount of weld metal Ti was one-tenth of the current state, the free nitrogen increased only approximately 2 ppm; however, the reduction in the Charpy absorbed energy due to the reduction in the Ti oxide amount, which is the origin of intragranular ferrite, is a concern.<br> · If the welding metal Ti remains as the current state is and if nitrogen intervention occurs from the outside, because most of the amount of this increase will be the increase in free nitrogen, the decrease in Charpy absorbed energy at 0°C will be a concern.

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