Microstructural evolution of a 2.25Cr - 1 Mo steel during austenitization and temper : austenite grain growth, carbide precipitation sequence and effects on mechanical properties

Abstract : This work aims at optimizing tensile and toughness properties of a 2.25Cr – 1Mo steel by controlling its microstructure through heat treatments. To this aim, phase transformations during austenitization, quenching and tempering have to be understood. Quantitative microstructural analyses were performed by means of SEM, TEM and XRD to characterize and model metallurgical evolution of the steel at each step of the heat treatment. The evolution of austenite during the austenitization stage, and its influence on the resulting as-quenched microstructure were thoroughly investigated. Austenite grain growth was modelled in order to understand its mechanisms, including the limited growth phenomenon observed at lower temperatures. The effect of austenitization conditions on further decomposition of austenite and on mechanical properties after quenching + tempering was experimentally determined. An optimal austenitization condition was selected and applied to study the tempering stage. Carbide precipitation was studied for various tempering temperatures and amounts of time. M3C carbides precipitate first, followed by M2C and M7C3; M23C6 are the equilibrium carbides. The influence of carbide precipitation on mechanical properties was studied. Tensile properties are closely linked to the tempering conditions in the range investigated, while impact toughness remains stable.
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Sylvain Dépinoy. Microstructural evolution of a 2.25Cr - 1 Mo steel during austenitization and temper : austenite grain growth, carbide precipitation sequence and effects on mechanical properties. Materials. Ecole Nationale Supérieure des Mines de Paris, 2015. English. ⟨NNT : 2015ENMP0049⟩. ⟨tel-01304150⟩

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