PSI - Issue 13

Michihiro Kunigita et al. / Procedia Structural Integrity 13 (2018) 198–203 Kunigita / Structural Integrity Procedia 00 (2018) 000 – 000

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by about 20 ℃ and the prediction became poor. It is evident that the GB ferrite deteriorates toughness even at small GB ferrite volume fraction (2.1% in the present case) and the present model can predict its effect quantitatively.

Fig.5 Microstructures, CR: 1℃/s, (a) SEM microstructure, (b) MA particle thickness, (c) inter -MA particle distance and (d) GB ferrite width.

Fig.6 Comparison of experimental and predicted 50J Charpy absorbed energy transition temperature.

5. Conclusions

The present study proposed a model to predict Charpy impact absorbed energy for steel weld HAZ of predominantly upper bainite containing martensite-austenite constituent (MA) as a second phase. Probability distribution of local fracture stress for cleavage crack nucleation is calculated from microstructural parameters. On the other hand, local stress and strain are calculated from dynamic elastic-plastic finite-element analysis. By comparing the local fracture stress and stress exerted on each volume element, probability of cleavage fracture initiation of the volume elements is calculated and the probability of specimen fracture is calculated applying the weakest-link mechanism. Charpy impact tests were conducted for steel simulated HAZ with various cooling rates. As a result, the predicted Charpy absorbed energy transition temperature agreed well with the experiment. Improvement of the model, including the determination of the adjustable parameters, and further validation for wider variety of steel chemical compositions, microstructures with multiple HAZ thermal cycles are necessary as a future work.