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Mécanismes de fragilisation de l’acier inoxydable super-martensitique X4CrNi16-4 Virgo™38 : Effets couplés des traitements thermiques et des milieux corrosifs contenant Na2S ou H2S

Abstract : This PhD thesis is focused on the study of embrittlement mechanisms in a 16Cr-4Ni low carbon supermartensitic stainless steel (Virgo™38) used for the construction of rotating parts of centrifugal compressors produced by GE Oil&Gas.It is shown that the morphologically complex multiphase microstructure of this alloy is very sensitive to the heat treatment conditions. In particular, the fraction and repartition of retained austenite, the presence of chromium carbides and the phosphorous grain boundary segregation (PGBS) are evidenced consecutive to several heat treatments. PGBS is one of the mechanisms leading to brittle failure of Virgo™38 after specific heat treatments.The presence of retained austenite is not only beneficial for toughness but also leads to hydrogen trapping in this phase. Due to the mechanical instability of retained austenite, its transformation under applied stress leads to the release of trapped hydrogen into newly formed martensite and thus to a local overconcentration of hydrogen. Hydrogen embrittlement is the second mechanism studied in this report.Mechanisms involved during the formation of retained austenite lead to the heterogeneous repartition of alloying elements (partitioning). One of the consequences being the selective dissolution of one or the other phase of the alloy as a function of the pH of the environment and of the load applied. Preferential dissolution is the third mechanism studied.
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https://tel.archives-ouvertes.fr/tel-01844249
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Submitted on : Thursday, July 19, 2018 - 11:02:34 AM
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Clément Gayton. Mécanismes de fragilisation de l’acier inoxydable super-martensitique X4CrNi16-4 Virgo™38 : Effets couplés des traitements thermiques et des milieux corrosifs contenant Na2S ou H2S. Autre. Université de Lyon, 2018. Français. ⟨NNT : 2018LYSEM004⟩. ⟨tel-01844249⟩

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