The effect of long-term creep on particle coarsening in tempered martensite ferritic steels

The high creep strength of tempered martensite ferritic steels results from the presence of a dislocation network of subgrain boundaries (SGBs) which is stabilized by carbides. In these microstructures creep accelerated particle coarsening is observed at creep rates of the order of 10⁻¹⁰ s⁻¹ (long-term creep). The microstructural explanations presented in this study, are based on the close contact between particles and SGBs:

• 1.

  (i) the difference in the thermodynamic potential on SGBs perpendicular and parallel to the stress axis results in short range pipe diffusion fluxes over distances of the order of the mean subgrain size (0.5 μm) which are not enforced during stress free ageing;

• 2.

  (ii) at high temperatures, recovery processes can result in a decrease of the dislocation density within SGBs, which results in a decrease of pipe diffusion along SGBs. In the presence of a stress higher dislocation densities in SGBs (higher pipe diffusion fluxes) can be maintained than without stress (ageing);

• 3.

  (iii) during primary creep carbides interrupt “knitting” reactions between “free” dislocations and SGBs. As a result carbides “come in contact” with additional pipe diffusion paths (pinned dislocations). At low stresses the “pinned” dislocations stay in contact with the “pinning” carbides throughout the creep life. These “additional” matrix/carbide pipe diffusion paths do not form during ageing (absence of primary creep). All three effects can contribute to “creep accelerated particle coarsening” in tempered martensite ferritic steels.