Abstract
The evolution of secondary phases in Cr-V and Cr-Mo-V low alloy steels was studied. The steels with two levels of chromium (0.9 and 2.5 wt%) were aged in the range of 100 to 5000 hours at 773, 853, 953 and 993 K. In the investigation, a limited transmission electron microscopy experimental program was combined with credible thermodynamic predictions (Thermo-Calc). It revealed the positive influence of higher chromium bulk content on the stability of molybdenum rich M6C carbide. In the steels with a lower bulk chromium content (0.9 wt%), the M7C3 carbide with a slight iron dominancy was identified. Molybdenum was considered to accelerate the M6C precipitation. It was predicted thermodynamically that an increase in the bulk vanadium content favours the phase equilibrium: ferrite + MX (X = carbon + nitrogen).On a étudié l’évolution des phases secondaires des aciers faiblement alliés au Cr-V et au Cr-Mo-V. On a endurci par vieillissement des aciers à deux teneurs différentes en chrome (0.9 et 2.5% en poids) pour une durée de 100 à 5000 heures à 773, 853 et 993 K. Dans cette étude, on a combiné un programme expérimental de microscopie électronique à transmission et des prédictions thermodynamiques crédibles (Thermo-Calc). L’étude a montré l’influence positive d’une plus grande teneur massique en chrome sur la stabilité du carbure M6C riche en molybdène. Dans les aciers à plus faible teneur massique en chrome (0.9%), on a identifié le carbure M7C3 avec légère domination de fer. On considère que le molybdène accélère la précipitation de M6C. On a prédit par la thermodynamique qu’une augmentation de la teneur massique en vanadium favorisait l’équilibre de phase: ferrite + MX (X = carbone + azote).
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