Mechanisms of cyclic plasticity of a ferrite-bainite 2 1/4 Cr1Mo steel after long-term service at high temperature

LCF tests were carried out at 565 degrees C on a ferrite-bainite 21/4 Cr1 M o steel that has been taken from a power plant component after a 160 000 h service time. Before fatigue, a detailed preliminary analysis of the micros tructure was first performed not only in terms of precipitates but also in terms of dislocation cell subboundaries present in the bainitic grains. Str ain cycling globally led to a softening of the alloy which can be preceded by a hardening period depending on the strain amplitude. A fatigue cellular structure developed in the ferritic grains whatever the strain level. In t he bainite, it was possible by CBED to show a rotation of the preexisting c ells as indicated by the orientation measurements. The mechanisms of cyclic plasticity of the 21/4 Cr1 Mo steel is thus explained by considering the i ndividual behavior of each phase.