Laves Phase in a 12% Cr Martensitic/Ferritic Steel: Evolution and Characterization of Nanoparticles at 650 °C

A 12% Cr martensitic/ferritic with tailor-made microstructure was characterized by HRTEM after tempering (780 °C/2 h) and isothermal aging at 650 °C during 2 months and 1 year. The investigation was focused on the precipitation behavior of Laves phase nanoparticles and its contribution to creep response at 650 °C. Although, dense precipitation of Laves phase occurs during the heat treatment (196 nm), addition of 3.8% tungsten (W) promotes a high growth rate of this particle at 650 °C. The excessive addition of W induces a high driving force for the growth of this particle reaching a mean size of 300 nm at 2 months and 400 nm at 1 year. However, some small Laves phase nanoparticles inside the grain were detected at 1 year of isothermal aging, suggesting a low coarsening rate maybe due to low diffusion of W in the matrix. Results of creep test at 650 °C suggest most of the precipitation hardening is granted by fine dispersion of Ta-rich carbonitrides (Ta-MX), i.e., Laves phase is too coarse to contribute to the pinning force of the subgrain structure in the designed alloy.