Free dislocations and boundary dislocations in tempered martensite ferritic steels

The present paper shows how total dislocation densities evolve during heat treatment, long term aging and creep of a 12 wt.% Cr tempered martensite ferritic steel. Special emphasis is placed on the evolution of the density of free dislocations (dislocation density: ρf) and dislocations in micro-grain boundaries. Only micro-grain boundaries with misorientation angles between adjacent grains between 1 and 5° are considered. These are grouped into (i) sub grain boundaries and (ii) low angle grain boundaries with misorientation angles of 1 and 2–5°, respectively. It is shown that the overall density of free dislocations which form in order to facilitate the formation of martensite decreases during tempering, long term aging and creep. The density of free dislocations and of dislocations in low angle boundaries decreases. The density of dislocations in sub grain boundaries stays constant due to a balance between an increase of the relative frequency of sub grain boundaries and an increase of micro-grain size (which corresponds to a decrease of micro-grain boundary area per volume).