Heterogeneous austenite grain growth in 9Cr martensitic steels: influence of the heating rate and the austenitization temperature

The characteristics of the austenitization process in two 9Cr martensitic steels, Eurofer97 (9Cr1WVTa) and T91 (9Cr1MoVNb), have been investigated by dilatometry and light microscopy. The martensite–austenite transformation was studied for heating rates ranging from 0.083 up to 100 K s−1. Phase equilibria calculations are presented as a contribution to the understanding of the microstructural evolution upon heating. The combined effect of the austenitization temperature and heating rate on the resulting austenite microstructure was studied. The heating rate to reach the austenite phase field has a strong influence on the austenite grain size distribution. A window of critical values of austenitization temperatures and heating rates for the development of heterogeneous austenite grain growth was established. The critical heating rate diminishes with the increase of the austenitization temperature. In contrast, a 9Cr, Ta-free alloy (LA12LC) with chemical composition close to Eurofer97 did not show heterogeneous grain growth for the same experimental conditions.