| Simulation study employing metal thermal cycle simulator (MTCS) has been performed to investigate a correlation between the microstructure and fracture properties of the heat affected zone (HAZ) in welded HY-100 steel. Charpy impact energy dropped from 210 J to about 125 J after the first thermal cycle in the range of peak temperature (T_P¹) of 800℃∼1100℃, and decreased rapidly above 1100℃, down to 60 J for T_P¹=1350℃. Decreased Charpy impact energy of T_P¹=1350℃ appears to be due to the change of microstructure from tempered martensite to mixture of fresh martensite and fresh lower bainite. Impact transition temperature increased by the amount of 70℃ after thermal cycle simulation of T_P¹=1350℃, namely -50℃ in HAZ and -120℃ in HY-100 steel. Evaluation of Charpy impact result, after the second thermal cycle in the range of peak temperature (T_P²) of 700℃∼1300℃, revealed that the fracture properties of HAZ are not affected significantly by second thermal cycle in HY-100 steel. |
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