Isothermal transformation of austenite to ferrite and precipitation behavior in 9Cr-1.5Mo-1.25Co-0.1C-VNb heat-resistant steel

Abstract This study explored the transformation of austenite to ferrite and precipitation behavior during the isothermal annealing of heat-resistant steel based on Fe-9Cr-1.5Mo-1.25Co-0.1C-0.2 V-0.05Nb (wt%). For this purpose, the samples used here were initially reheated to the full austenitization temperature and thereafter directly annealed at 710 °C and 730 °C, a temperature range in which the transformation to ferrite takes place, for up to 196 h. In addition, we modified the prior austenite grain size (PAGS) by changing the reheating temperature and then analyzed the effect of PAGS on the kinetics of the isothermal transformation to ferrite. In the early stage of annealing, Cr-enriched M23(C,B)6 carbides initially precipitated along the prior austenite grain boundaries (PAGB). As annealing progressed, the ferrite began to nucleate at the PAGB and then grew into the austenite. The initial stage of the transformation to ferrite was then accompanied with the cellular precipitation of M23(C,B)6 particles. Interestingly, we also found the interphase precipitation of V- and Nb-enriched MX particles resulting from the partitioning of the C concentration at the moving austenite/ferrite interface during the austenite to ferrite transformation. The transformation was finished after annealing for more than 48 h. The time taken to complete the process was prolonged with an increase in the PAGS, indicating that the kinetics of the transformation decreased with an increase in the PAGS.