KINETICS OF THE AUSTENITE-FERRITE ISOTHERMAL PHASE TRANSFORMATION IN 9 % Cr LOW ACTIVATION MARTENSITIC STEELS

We have studied microstructural and kinetic aspects of the austenite-ferrite isothermal transformation for two 9CrWVTa steels. The kinetics of the austenite-ferrite isothermal transformation was followed by dilatometry and modeled according to the Kolmogorov-johnson-Mehl-Avrami (KJMA) equation. We have considered the influence of the parent austenite grain size distribution (GSD) on isothermal transformation (IT) kinetics, in particular, in the case of a strongly heterogeneous distribution. Two-step thermal cycles -i.e, austenitizing and further isothermal transformation-were designed to that aim. For the Ta-containing Eurofer97 alloy, the parent austenite grain size develops an important heterogeneity at 1050 C-(degrees) under bar when austenitizing is performed at rates higher than 5 C-(degrees) under bar/s. The austenite annealing gives rise to a set of large-sized grains in a << matrix >> of small-sized grains -that is, a bimodal austenite GSD. For the Ta-nearly free LA12LC alloy, the parent austenite grain size remains homogeneous at 1050 C-(degrees) under bar whatever was the heating rate. The experimental isothermal transformation curves obtained in the transformation step reflect the features of the austenite GSD. We have considered a double expression of the KJMA type to model the isothermal transformation austenite-ferrite in the Eurofer97 case whereas the IT curve is well fitted with a single KJMA expression in the LA12LC case.