PSI - Issue 42

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Guocai Chai et al. / Procedia Structural Integrity 42 (2022) 155–162 Author name / Structural Int grity Procedia 00 (2019) 000 – 000

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Fig. 5. ECCI images show other types of voids formed in the material, (a) voids formed in the front of a crack in the as-delivered material with hydrogen charging; (b) voids formed at the martensitic lath boundaries in the same material, (c) crack propagation at the martensitic lath boundaries in the same material. Fig. 6a shows separated small cracks formed near the transgranular crack boundary in the tempered material. Fig. 6b shows the details. These small cracks are correlated to the presence of some precipitates such as NbC and austenitic phase. They appear at martensitic lath boundary. It can be the crack initiation stage by hydrogen trapping, formation of void and then small crack by storage of hydrogen in cavity and plastic deformation during SSRT testing. A local crack propagation can then occur at the cavity once the internal pressure of hydrogen in the void is high enough. This leads to slip plane decohesion and formation of “fisheye” at the martensitic lath boundary, Möser et al. (2018).

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Fig. 6. Formation of “fisheye” in the tempered material, (a) overview; (b) enlarged view.

4. Conclusions Martensitic stainless steel 17-4PH is susceptible to hydrogen embrittlement. The heat treatment with a tempering at 510  C for 4 hours reduces the susceptibility to hydrogen embrittlement. Precipitation of different FCC phases in the material during the tempering contributes to hydrogen trapping and consequently improves resistance to hydrogen embrittlement. HEDE is a main hydrogen embrittlement mechanism in the as-delivered material. The tempering provides a transition of hydrogen embrittlement from HEDE to HELP. Acknowledgements This paper is published by permission of Alleima. The supports of Dr Tom Eriksson, Mr Martin Östlund, and the assistances of ECCI study by Mr Jerry Lindqvist and EBSD study by Dr Raveendra Siriki are greatly acknowledged.