The evolution of dislocation density during heat treatment and creep of tempered martensite ferritic steels

The evolution of dislocation density in two tempered martensite ferritic steels (a 12% and a 9% chromium steel, X20 and P91) during heat treatment and creep is analyzed using transmission electron microscopy (TEM) and V ray diffraction (XRD); both methods yield results which are in good agreement when rationalizing the XRD-data based on densities of free dislocations. It is shown that due to the intermediate formation of martensite, standard heat treatments produce very high dislocation densities in tempered martensite ferritic steels (TMFSs). Long term tempering and creep are characterized by a decrease of dislocation density; but dislocation densities are still high as compared to alloys where the formation of microstructure does not involve a martensitic transformation. A heterogeneous microstructure after long term tempering and creep is a characteristic feature of TMFSs. Micro grains with high dislocation densities co-exist next to micro grains without dislocations. The XRD method yields average data and cannot account for this microstructural heterogeneity; but it supports the TEM results which in isolation suffer from providing insufficient statistics. The results of the present study are discussed in the light of earlier work published in the literature and contribute to a better understanding of the role of free dislocations in TMFSs during tempering, creep and high temperature low cycle fatigue. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.