Influence de la composition chimique et des conditions de refroidissement sur la fissuration à chaud d'alliages de Nickel

Abstract : Ni-based alloys 690 present solidification hot cracks during welding of vapour generators. Hot cracks are qualitatively known to be due to the formation of interdendritic liquid films and of secondary phases down to low temperatures. This study aims at establishing the link between thermodynamics, solidification and hot cracking. Experimental solidification paths of high purity alloys with varying Nb and Si contents) are obtained from quenching during direction al solidification and TIG-welding experiments. They are compared to Thermo-Calc computations, assuming no diffusion in the solid. From directional solidification samples, good agreement between computed and experimental solidification paths is shown in the quenched liquid. Secondary arms of dendrites are affected by solid state diffusion of Nb. Combined effect of diffusion and solute build-up in the liquid phase modifies microsegregation in the sol id region. Solidification paths from welding specimens are similar to those of the solid region of quenched samples. Nb solid state diffusion is negligible but undercooling compensates the effect of solid state diffusion in direction al solidification. Evolution of liquid fraction at the end of the solidification is in accordance with the hot cracking classification of the alloys. Nb favours formation of interdendritic liquid films and eutectic-like phases down to low temperature.
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Theses
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http://docnum.univ-lorraine.fr/prive/INPL_T_2000_DE_VITO_S.pdf
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https://hal.univ-lorraine.fr/tel-01750480
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Submitted on : Thursday, March 29, 2018 - 12:47:10 PM
Last modification on : Monday, April 16, 2018 - 10:40:58 AM

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Sophie de Vito. Influence de la composition chimique et des conditions de refroidissement sur la fissuration à chaud d'alliages de Nickel. Autre. Institut National Polytechnique de Lorraine, 2000. Français. ⟨NNT : 2000INPL094N⟩. ⟨tel-01750480⟩

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