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Theses

Influence d’éléments d’addition sur les transformations de la martensite revenue dans les aciers faiblement alliés

Abstract : The purpose of this work is to optimise the strength of a tempered martensitic medium C steel, containing low-level of Cr, V and Si/Al. A series of high purity "model" ternary alloys (Fe-C–X, X=Cr, V, Si, Al) was prepared (EMSE) by the cold crucible method. The initial structures of quenched alloys were optimised through appropriate austenitizing conditions. Analysis of alloys behaviour on tempering was performed through carbide precipitation characterisation (X-ray diffraction, SEM and TEM), completed by hardness and dilatometric measurements. The behaviour of "model" alloys was compared to that of the industrial material.Vanadium addition is responsible for two effects: (i) vanadium carbides lock the grain growth during the austenitizing; (ii) dissolved vanadium promotes the precipitation of (V,Cr)3C, that is at the origin of a secondary hardening peak near 550°C. Silicon addition has an interesting effect, as it stabilises the martensitic matrix and increases the temperature of cementite precipitation. Therefore, a high value of hardness is conserved up to high tempering temperatures.Unfortunately, because of some noxious properties, the industrial process in concern excludes the presence of Si. That’s why two new model alloys have been prepared and tested, in which the silicon addition (i) was absent, or (ii) replaced by Al.Aluminium has been shown to be "neuter" vs tempering behaviour. Moreover, the interesting effect of secondary hardening (550°C) has been observed in both Si-free steels, showing that alloying with only Cr and V can be enough to get the expected behaviour of the studied steel.
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Submitted on : Monday, November 29, 2010 - 4:53:20 PM
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Muriel Hantcherli. Influence d’éléments d’addition sur les transformations de la martensite revenue dans les aciers faiblement alliés. Autre. Ecole Nationale Supérieure des Mines de Saint-Etienne, 2010. Français. ⟨NNT : 2010EMSE0570⟩. ⟨tel-00541049⟩

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