ON THE CYCLIC SOFTENING MECHANISMS OF REDUCED ACTIVITY FERRITIC/MARTENSITIC STEELS

M.F. GIORDANA; I. ALVAREZ-ARMAS; A.F. ARMAS

Montpellier

Congreso; European Congress and Exhibition on Advanced Materials and Processes, EUROMAT 2011; 2011

Reduced activation ferritic/martensitic (RAFM) steels are leading candidates for blanket/first-wall structures of future fusion reactors. Continuous cycling produces changes in the tempered martensite lath structure and a marked cyclic softening that could deteriorate their mechanical properties. Particularly in RAFM steels, a more pronounced cyclic softening rate and independence of this softening rate with the strain amplitude were also reported. In the literature some authors have rationalized such behavior as a consequence of the higher martensite start temperature of this type of steels. In this work the cyclic behaviour of three ferrite/martensite steels, the European RAFM steel EUROFER 97 and the commercials AISI 410 (similar amount of carbon as EUROFER 97) and AISI 420 (with 0.28% C) have been studied and compared. This last steel was chosen as representative of steel with lower martensite start temperature. The softening behavior as a function of the plastic strain amplitude and temperature has been analyzed between 20ºC and 5500C. Although the three steels show similar behaviors under these test conditions, the cyclic softening presented by AISI 420 at room temperature is clearly weaker than the softening exhibited by EUROFER 97 and AISI 410. At high temperature the three steels show pronounced softening but lower for AISI 420. The evolution of the flow stress during cycling was studied by analyzing the stress components, back and friction stresses, obtained from the hysteresis loops. At room temperature the cyclic softening is produced, principally, by the softening observed in the friction stress which could be caused by the progressive annihilation of dislocations located in the interior of the subgrains. This conclusion was corroborated with transmission electron microscopy observations. In AISI 420 no influence of the back stress was observed. On the contrary, at high temperature the main contribution to the cyclic softening of the materials is provided by the back stress. In AISI 420 almost no contribution of the friction stress was observed. The synergic effect of cycling and temperature contribute to the disappearance of the subgrain dislocation walls. Precipitate fences which decorated the disappearing dislocation walls become visible at high temperatures. Cyclic softening rates increase linearly with temperature up to temperatures at which self-diffusion mechanisms can operate enhancing a cyclic strain assisted dynamic recovery process.