PSI - Issue 33

Saki Hayashi et al. / Procedia Structural Integrity 33 (2021) 1162–1172 Hayashi et al / Structural Integrity Procedia 00 (2019) 000 – 000

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and the holding time is short. The toughness of the steel that is subjected long time temper condition improves as the holding time increases. This is estimated to be because the effect of tempering on toughness is greater than the effect of temper embrittlement when the holding time is long. However, this consideration is quite qualitative and the critical amount of Sn, which is important in engineering aspect, cannot be discussed only with this data. Thus, we tried to establish new segregation model for welding thermal cycle to discuss quantitatively.

Fig. 6. Results of fracture test (specimen without B).

Fig. 7. Results of fracture test (specimen with B).

Fig. 8. Transition temperature shift (specimen without B).

Fig. 9. Transition temperature shift (specimen with B).

3. Discussion 3.1. Detailed observation of Fracture and its mechanism

Fracture appearances of the CTOD tests were observed in detail. Fig. 10 shows the SEM observation for brittle fractured specimens of single cycled and double cycled of steel I. Both showed apparent temper embrittlement. It is important observation fact that all brittle-fractured specimens broke in cleavage manner. This is against the many previous studies which show that Sn segregates at grain boundaries and exhibit intergranular fractures as Inushima et al. (2019) claimed them. We had to identify and clarify this difference from previous knowledge focusing the HAZ microstructure with large grain size. First, let us consider the susceptibility to fracture before tempering, i.e., before Sn segregation. Basically, in the base plate, it is shown dislocations are piled up at the grain boundaries by Kosuge et al. (2020), and this leads a trigger point for fracture which normally exhibits transgranular mode. On the other hand, for the weld heat-affected zone, Nakanishi (2016) showed dislocations likely to pile up not only at grain boundaries but also around precipitates inside of the grain due to the large grain size. Therefore, the probability of the fracture originating inside of the grain