Accelerated coarsening of MX carbonitrides in 12%Cr steels during creep deformation

MX carbonitrides (M=Nb,V,Ta/X=C,N), which are finely dispersed in the matrix of 12%Cr Ferritic heat resistance steels, increase the creep strength of the steels by the precipitation strengthening mechanism. However, MX particles usually coarsen during high temperature creep deformation, and this results in a decrease of creep strength. In order to clarify the coarsening process of MX particles in 12%Cr steels (10.5Cr-0.1C-(Nb,V,Ta)), TEM samples were prepared from the grip portion (creep stress free zone) and the gauge portion (stress zone) of ruptured creep test pieces. The changes of MX particle radii in the gauge portion were considerably larger than that in the grip portion, indicating that creep stress accelerated Ostwald ripening of the particles in the gauge portion. The Ostwald ripening equation was modified by introducing the effective diffusion coefficient as a sum of the lattice diffusion coefficient and dislocation diffusion coefficient, based on an assumption that Nb and V might diffuse not only by the lattice diffusion mechanism but also by the dislocation diffusion mechanism. The effect of dislocation diffusion in the gauge portion was larger than that in the grip portion because dislocations in the gauge portion were moved by creep stress, and a rate of solute atoms diffusing along the dislocations might be increased. The coarsening process of MX particles in the gauge portion could be simulated successfully by means of a modified ripening equation.