Low temperature toughness of heavy-wall X80 steel welded HAZ along wall thickness direction

To clarify low temperature toughness of heavy-wall X80 steel welded heat-affected zone (HAZ) along wall thickness direction and influence of local brittleness of intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) on welded joints, welded joints of cryogenic steel tubes (ϕ1 422 mm × 22 mm) in the eastern China-Russia station was selected to evaluate impact absorbed energy at 2 mm, 4 mm, 6 mm, and 8 mm from the outer surface by −80～20 ℃ series of temperature impact tests and toughness when the front end of V-notch contained ICCGHAZ. The results showed that brittle-ductile transition rule of heavy-wall X80 steel welded HAZ along wall thickness direction was consistent, with well low-temperature toughness above −60 ℃. However, attention should be paid to occasional embrittlement caused by coarse grain embrittlement of CGHAZ. ICCGHAZ was local brittle zone (LBZ) of welded joints throughout the temperature range, the degree of its influence on toughness of the whole welded joints was related to temperature, and the critical temperature was −45 ℃. At −45 ℃ and below, the overall toughness would be significantly reduced with the decrease of temperature, and probability of brittleness was increased. Under technical conditions for heavy-wall cryogenic pipelines with service temperature below −45 ℃, it was recommended to increase one set of Charpy impact test, in which V-notch went through ICCGHAZ.