Effect of Al Addition on the Microstructure and Phase Stability of P91 Ferritic-Martensitic Steel

This paper presents the results of an experimental and computational study carried out to elucidate the effect of Al on the microstructure and phase stability of P91 F/M steel in as-cast, homogenized and normalized conditions. Al-added steels followed ‘Ferritic-Austenitic’ mode of solidification and the as-cast microstructures consisted of δ-ferrite + α′-martensite, the volume fraction of ferrite and hardness of martensite increased with Al concentration. Heat treatments and DSC experiments confirmed increased stability for δ-ferrite with Al addition. Systematic change in the phase transformations temperatures and volume fraction of equilibrium phases due to Al addition was estimated with the help of Thermo-Calc^®. Al addition promoted the formation of AlN which was confirmed through electron microscopy-based investigations. AlN dissolution temperature was always above γ-loop which made it impossible to dissolve during austenization. With the help of Scheil and equilibrium simulations using Thermo-Calc^®, elemental partitioning between δ-ferrite and α′ phases was found to be the reason for higher hardness of martensite. Based on experimental evidences, it is concluded that except in the case of 0.48 wt pct Al-added steel it is impossible to obtain single phase γ-field (without ferrite) at high temperature thereby a fully martensite structure on cooling.