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Application des structures bainitiques par transformation isotherme et d'un traitement de surface adapté aux vis à haute résistance

Abstract : In the objective to prevent brittle fracture due to hydrogen (internal or external origins), the usages of mechanicalfastening parts is restricted above 1000 MPa. As already experienced on low-thickness flat products (clips), thebainitic microstructure generated by salt bath quenching is not subjected to hydrogen embrittlement. The target ofthe present study consists in setting up the required knowledge to extend this process to massive parts.In addition to its resistance to hydrogen, only the lower-bainite microstructure is able to meet the mechanicalpropertyspecifications for fasteners. The optimization tool developed in the present framework, has been designedto integrate the particularities of the austenite to lower bainite transformation in salt bath, as well as the ability tosustain cold forming after annealing treatment.A set of mechanical characterizations has been performed on hydrogen saturated bolts. Under a load of 1370 MPa,the bainitic structure has not shown any sign brittle fracture, while it has systematically been the case for thetempered martensitic structure. Furthermore, among the ternary alloys Al-Zr-Zn that can be deposed in vapor phase,a sacrificial grade presenting a protection effect has been identified. However, this effect must be furtherinvestigated, in order to determine the interest for fastening applications.
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Didier Forgeoux. Application des structures bainitiques par transformation isotherme et d'un traitement de surface adapté aux vis à haute résistance. Matériaux. Université de Technologie de Belfort-Montbeliard, 2016. Français. ⟨NNT : 2016BELF0302⟩. ⟨tel-01872749⟩

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