Fatigue behaviour of reduced activity ferritic/martensitic

M. AVALOS; I. ALVAREZ ARMAS; A.F.ARMAS

New Developments on Metallurgy and Applications of High Strength Steels

TMS

Lugar: Warrendale, USA; Año: 2008; p. 343 - 356

The first steel considered for fusion was a modified 9Cr-1Mo steel called MANET. This alloy has particularly good strength properties with adequate ductility. Much of this performance is based on high dislocation densities with a fine, well dispersed precipitate distribution. However, due to the long radioactive decay times for some of the activated species in these steels (especially Ni, Mo and Nb) programs were conducted worldwide to produce variants such as F82H, OPTIFER IV and EUROFER 97 in which the critical alloying elements have been substituted either completely or partly. These alloys are frequently named as Reduced Activity Ferritic/Martensitic (RAF/M) steel. Their ability to resist the effects of high doses of irradiation focused the interest on these alloys for first-wall and blanket structure components of fusion reactors. The low activation 9-12% Cr ferritic/martensitic steels exhibit attractive tensile strength properties up to approximately 6000C. It is also known that 9Cr-1Mo steels subjected to the standard treatment are microstructurally stable during subsequent ageing. However, under cyclic loading conditions are prone to softening during cyclic loading. Microstructural observations show dislocation annihilation and rearrangement could occur in softening thereby leading to break up of the martensite laths and the gradual development of an equiaxed substructure. The initial high strength of hardened steels can be seriously compromised even after a low number of loading cycles, reducing their original strength and load carrying capabilities. The present work is an examination of the fatigue response of alloys in the temperature range between RT and 5500C with particular attention to the tendency of cyclic softening and fracture of materials. The purpose of this work was also to compare with the cyclic behaviour of  RAF/M alloys with commercial AISI 410 and AISI 420  in order to make evident the difference in the observed cyclic softening and fatigue behaviour.