Abstract
CB2 steel for thick castings has been designed and investigated in the frame of COST-522 and C0ST-536 R&D actions of EU to work at supercritical conditions of the steam main lines. The steel exhibits loss of impact strength and hardness after long annealing and also of strength and life when creep tested after the long annealing. The most serious decrease of properties appears in weld’s heat-affected zones during the creep tests. Differences in microstructures of base metal (BM) and weld’s heat-affected zone (HAZ) have been studied in initial state and after long-term annealing/aging for 10 kh and 30 kh at 625 °C, followed by creep testing. The initial as-cast microstructure of CB2 is inhomogeneous, and the welding thermal cycle causes additional separation of phases in the intercritical region. TEM observations revealed appearance in HAZ of large ferrite grains between compact colonies of spheroidal carbides, associated with the decrease of creep strength and life in the HAZ.
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