Influence of 160 000 service hours on the high temperature LCF behaviour of 2.25Cr-1Mo steel

The influence of ageing on the high temperature low cycle fatigue (LCF) strength is studied on a ferrito-bainitic 2.25Cr-1 Mo steel. Microstructure of aged steel (after 160000 hours service exposure) and of the same steel after regeneration (by a normalizing-tempering heat treatment) have been studied before and after LCF tests. The long service exposure has induced microstructural evolutions (regarding dislocation arrangement and precipation) especially in bainite. As a consequence this ageing leads to a decrease of the cyclic stress, to a softening instead of a hardening at the beginning of fatigue life and thus to an increase in fatigue resistance. The cyclic plasticity is controlled by the ability to soften the bainite which depends on its density of mobile dislocations before LCF tests. The partition of deformation between bainite and ferrite can be evaluated by the dislocation arrangements in the ferrite after LCF tests.