| It is well known that the crack tip opening displacement (CTOD) test, widely used for heat-affected zone (HAZ) toughness evaluation, is very sensitive to complex distribution of microstructures in HAZ. So, as the second step of the researches on metallurgical analyses of fracture characteristics in HAZ of improved 9% Ni steel, this study aimed to investigate the microstructure distribution including local brittle zone (LBZ) and its influences on CTOD toughness. To study the correlation between microstructures and actual HAZ toughness in more systematic way, K-grooved HAZ specimens were prepared for the CTOD tests. To figure out the microstructure influence, the microstructure-distribution maps were constructed by semi-empirical method including the process of sectioning and observation of tested specimens. In the regions near fusion line, it was found that the crack initiations were ruled by weakest-link-typed failure, which resulted in relatively low toughness of the regions. In the regions of fusion line (F.L.) +2 ㎜ and +3 ㎜, i.e. relatively high toughness regions, the failure was ruled by rule-of-mixture type. It was also revealed that the minimum fraction of LBZs showing the weakest-link-typed failure was 17% in crack tip front of actual HAZ specimens. From the results obtained, the mechanism of toughness change based on metallurgical analyses was proposed and discussed. |
|
