The evolution of the dislocation densities in 2 tempered martensite ferritic steels (12%Cr and 9%Cr) during heat treatment and creep was analyzed by using transmission electron microscopy and X-ray diffraction. Both methods yielded results which were in good agreement with X-ray diffraction data which were based upon the densities of free dislocations. It was shown that, due to the intermediate formation of martensite, standard heat treatments produced very high dislocation densities in tempered martensite ferritic steels. Long-term tempering and creep were characterized by a decrease in dislocation density, but the dislocation densities were still high when compared with alloys in which the formation of the microstructure did not involve a martensitic transformation. An heterogeneous microstructure, after long-term tempering and creep, was a characteristic feature of tempered martensite ferritic steels. Micro grains having high dislocation densities co-existed with micro grains without dislocations. The X-ray diffraction method yielded average data and could not account for this microstructural heterogeneity. However, it supported the transmission electron microscopic results; which offered insufficient statistical data.

The Evolution of Dislocation Density during Heat Treatment and Creep of Tempered Martensite Ferritic Steels. J.Pešička, R.Kužel, A.Dronhofer, G.Eggeler: Acta Materialia, 2003, 51[16], 4847-62