Three-point bend testing method was employed in the crack tip opening displacement (CTOD) test to investigate the fracture toughness of base metal, heat affected zone (HAZ), and weld of an X100 longitudinal submerged arc welded (LSAW) pipe. The results show that at the same temperature, the δ0.05>δ, and δm in the X100 LSAW pipe distribute ass base metal>weld bead>HAZ; and the crack propagation resistance and tear modulus distribute as; base metal> HAZ>weld bead. Further analysis shows that the toughness of HAZ of X100 LSAW pipe closely relates with its microstructure. Large amount of polygonal ferrite in HAZ microstructure and less content of reinforcement phases, such as granular bainite, can lead to the decrease in fracture toughness, especially at low temperature, and the emergence of local instability.%为了检验X100直缝埋弧焊管的断裂韧性,采用三点弯曲多试样法,在室温、-10℃和-30℃下对该焊管的母材、热影响区和焊缝部位进行了裂纹尖端张开位移(CTOD)试验.测试结果表明:在同一温度下,CTOD试验的表观启裂值δ0.05、条件启裂值δ和最大载荷值δm的分布规律为母材最好,焊缝次之,热影响区最差；采用裂纹扩展阻力和撕裂模量表征的分布规律为母材最好,热影响区次之,焊缝最差；焊接热影响区组织中含有大量呈块状分布的铁素体,而粒状贝氏体等增强相的含量较少,这会造成其韧性下降,使焊管出现局部失稳现象.
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