가스 텅스텐 아크 용접을 이용한 고망간강 판재 및 강관 용접부의 미세조직 및 기계적 물성

Abstract

This study investigated microstructural and mechanical characteristics of high manganese steel sheet and pipe welded by gas tungsten arc welding(GTAW). The weld zone showed coarse dendrite due to the heat input, and the heat affected zone(HAZ) showed equiaxed coarser grains than the base metal due to the thermal effect of the welding process. Mn segregation occurred in the inter-dendritic regions of the weld zone and deformation band of the base metal. Mechanical properties of the weld metal lowed than the base metal, which was caused by the coarser grain size. The negative strain rate sensitivity(SRS) observed in the weld zone, and the base metal is considered to have originated from the negative strain rate dependency of twinning nucleation stress. Before the drawing process, the recrystallization of the base metal partially occurred during pre-annealing. However, the recrystallization of the weld metal occurred during the post-annealing. Work hardening by the drawing process increased yield strength of welded steel pipe, and the post-annealing enhanced fracture elongation up to 110%. The pipe annealed at 900℃ and 1080℃ showed a severe degradation of mechanical properties by propagating the intergranular cracks. The mechanism of crack formation was the correlation between the oxide in the grain boundary and the residual stress accumulated during the manufacturing processes.