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Précipitation des inclusions de nitrure de titane (TiN) dans un acier maraging au cours de sa refusion à l'arc électrique sous vide (VAR)

Abstract : The titanium contained in maraging steels combines itself with residual Nitrogen to form Titanium nitride precipitates (TiN), which are detrimental to fatigue properties. Understanding their formation may give some ways to reduce their sizes. A Sieverts apparatus was used to study denitriding reactions and precipitation reactions from a thermodynamic and kinetic point of view. According to these experiments, to thermodynamical data from literature, and to the Nitrogen content in the steel, the TiN inclusions are supposed to form during the solidification of the steel thanks to interdendritic segregation. A certain number of the TiN inclusions are found under SEM observations to be located next to another oxide or sulfur particle. A TEM observation reveals the existence of an orientation relationship between a nitride, a sulfur CaS and a spinel MgAl2O4. It suggests an epitaxial growth of the TiN on these two germs. This study was completed with ab initio calculations of surface energies and adsorption energies. This work supports the hypothesis of a heterogeneous nucleation of the Titanium nitrides on preexisting, stable particles in the liquid steel. A numerical model of the precipitation coupled to the interdendritic segregation of solutes is developed and introduced in the SOLAR software modelling the VAR ingot solidification. It evaluates the determinant influence of the initial Nitrogen mass fraction, germ number density and local solidification time on the TiN sizes
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  • HAL Id : tel-01751292, version 1

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Vincent Descotes. Précipitation des inclusions de nitrure de titane (TiN) dans un acier maraging au cours de sa refusion à l'arc électrique sous vide (VAR). Sciences de l'ingénieur [physics]. Université de Lorraine, 2014. Français. ⟨NNT : 2014LORR0262⟩. ⟨tel-01751292⟩

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