PSI - Issue 17

Flavio Pereira de Moraes et al. / Procedia Structural Integrity 17 (2019) 131–137 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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observed at grain boundaries, which have been practically covered by sigma phase. Therefore, the depletion of Cr in the vicinity of the grains boundaries has been due to the precipitation of the sigma phase, confirming the work of Terada et al. (2008), which showed that precipitation of intermetallic phases rich in Cr and Mo at grain boundaries of austenitic stainless steels may lead also to intergranular corrosion and sensitization.

4. Conclusions

Based upon the results and discussion presented, it was possible to conclude that: • During exposure at 640ºC occurred an intense precipitation at grain boundaries and on the interior of the grains. The phase that precipitated most was the sigma phase (  ). Intense precipitation of sigma phase occurred at the austenitic gra in boundaries. Practically all delta ferrite (δ) present initially in the interior of the grains, vanished giving origin to the sigma phase. There are some indications of the presence of M 23 C 6 , chi phase (χ) and Laves phase Fe 2 Mo (η) mainly in the interior of the grains. • The microstructural modifications that occurred during the exposition at 640ºC caused, in comparison to the solution annealed condition, increase in hardness, yield strength and ultimate strength and decrease in ductility (total elongation) and toughness (impact test). • The presence of continuous regions of sigma phase at grain boundaries can justify the fracture mode, the loss in ductility and toughness and the accentuated sensitization, evaluated through the practice “A” of the ASTM A262 standard. • Creep life calculations and microscopic observations allowed to disregard plastic deformation through flow in the pipe during its usage.

Acknowledgements

The authors would like to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil) and the Universidade de São Paulo (USP, Brazil) for the financial support.

References

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