Austenitic transformation and influence of prior ferrite formation on martensite transformation characteristics of Al-added P91 ferritic-martensitic steel

Abstract In the present study austenitic transformation and effect of prior ferrite formation on martensite transformation characteristics of 0.48 wt% Al-added P91 steel was investigated by thermal analysis using differential scanning calorimetry and related microstructure characterization. Starting microstructure of as-cast alloy was dual phase consisting of α′-martensite+δ-ferrite. Austenitization heat treatments were carried out in the temperature range of 1273-1673 K at constant heating/cooling rates and hold time. γ-austenite formation from the initial dual phase structure occurred in two stages with increasing temperature: α-ferrite (martensite) → γ-austenite transformation at 1170 K followed by δ-ferrite and Al enriched fibrous α-ferrite → γ formation that begins at 1210 K. Isothermal characteristics of γ-austenite transformations were studied by Dictra® simulations. Elemental segregations were found to be the reason for two stage γ-austenite formation with temperature. These transformations involving α-ferrite, δ-ferrite and γ-austenite and extent of these transformations at various temperatures affected the martensite formation characteristics. Formation of α′-martensite was favored after heat treatment at 1373 K and 1473 K. Dissolution of ferrite and corresponding changes in the stability of γ-austenite at 1273 K and 1373 K, interfacial effects at 1473 K, grain size and new δ-ferrite formation effects at 1573 K and 1673 K dominated martensite transformation of Al-added P91 steel.