| Abstract: | Charpy impact test and COD test were performed on the specimens subjected to simulated welded thermal cycle and the specimens taken from welded joint. The optical microscope, TEM, SEM, EDAX and XRD analysis have been used to investigate the behaviors of second phase particles and the effects of microstructure on toughness separately. The results are as follows. The dispersed second phase particles can effectively retard the growth of austenite grain in the coarse grained HAZ (CGHAZ), and improve the toughness. When t 8/5 is different, the behaviors of the particles are also different in dissolving, coarsening and re precipitating. The ability of retarding the growth of austenite grain will be affected. When t 8/5 increases from 10 s to 70 s , the microstructure of CGHAZ will transform from upper bainite and granular bainite to granular bainite, and the size of austenite grain will grow a little, thus the toughness of the materials decreases slightly. |
