Microstructural stability during creep of Mo- or W-bearing 12Cr steels

TLDR: In this paper, the evolution and stability of particulate phases during creep of molybdenum- or tungsten-bearing 12Cr steels have been investigated in considerable depth, and data on Laves-phase precipitation kinetics as a function of time and temperature were quantified using the Wert-Zener equation in conjunction with the proprietary Thermo-Calc software.

Abstract: The evolution and stability of particulate phases during creep of molybdenum- or tungsten-bearing 12Cr steels have been investigated in considerable depth. The important finding is that the performance of Laves-phase precipitation in the molybdenum-bearing alloy is significantly different from that in the tungsten-bearing alloy. It is generally believed that such differences in kinetics will influence creep behavior. Data on Laves-phase precipitation kinetics as a function of time and temperature were quantified using the Wert-Zener equation in conjunction with the proprietary Thermo-Calc software, to determine equilibrium solute concentrations in these complex steels. The progressive depletion of Mo and W from the matrix as the particles of Laves phase evolve has been quantitatively modeled using experimental data obtained on both steels over a range of times and temperatures. The Isothermal coarsening rates of M23C6 and MX carbide particles were measured and found to occur at a constant volume fraction, in accordance with Ostwald ripening kinetics, with no significant differences in rates found between the two steels. The coarsening rates of M23C6 particles, found on subgrain boundaries, were consistent with a third-power dependence on particle radius, with an activation energy similar to that of volume diffusion. The smaller MX particles, which lay on subgrain-interior dislocation lines, were better explained by dislocation pipe diffusion, with a fifth-power dependence on particle radius and an activation energy approximately half that of volume diffusion.