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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increases. The probability of transgranular fracture also increases because dislocations are generated from within the grain. Next, the role of Sn segregation when temper embrittlement occurring for the brittle fracture behavior has to be explained. In the base plate, McMahon et al. (1991) showed Sn is likely to segregate at the grain boundaries under tempering temperature, and since it is shown Sn weakens the bond strength of Fe by Yamaguchi (2015), cracks are more likely to propagate along the grain boundaries. Therefore, the probability of intergranular fracture is much higher than before segregation. On the other hand, in the HAZ, Sn segregates not only at the grain boundary but also at the precipitate interface. Therefore, not only grain boundaries but also precipitate interface are weakened, and furthermore initial microcracks are more likely to occur from inside of the grains because weaker constrained condition is given at inside of the grain. This candidate mechanisms are schematically illustrated in Fig. 11.

(a) Single cycle

(b) Double cycle (5sec.)

Fig. 10. SEM observation around a brittle fracture initiation point (Steel I).

Fig. 11. Schematic illustration for the mechanism of the difference of fracture mode after temper embrittlement between base plate and HAZ.