Heat affected zone (HAZ) along with the weld seam of LSAW has discrete microstructure and usually shows lower toughness than base material. Grain coarsening and formation of M-A constituents governs the HAZ toughness, and a lot of efforts have been made to improve HAZ toughness. However, it is impossible to completely reduce the effect of microstructural change by seam welding. Especially, higher strength or heavy wall linepipes which have richer chemistry tend to have lower HAZ toughness. Therefore, it is important to understand the relationship between microstructural characteristics and toughness of HAZ and fracture behavior of HAZ region in order to ensure the structural reliability of seam welded region. In this paper, fracture behavior of X70 grade heavy wall UOE linepipe was investigated. Wide plate tensile test was conducted with the surface notch introduced on to the coarse grain HAZ, as well as the small scale testing such as Charpy and CTOD test. Fracture mechanics analysis was also carried out to understand the critical condition for brittle fracture of HAZ, and compared with the experimental results.

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