The role of microstructure in fatigue crack initiation and propagation in 9-12%cr ferritic-martensitic steels

M.N. BATISTA; M.C. MARINELLI; S. HEREÑÚ; I. ALVAREZ

Verbania, Lake Maggiore

Encuentro; XVII International Colloquium "mechanical fatigue of metals" (ICMFM-XVII),; 2014

Ferritic-martensitic steels of the type 9-12%Cr are of high interest for application as structural materials in advanced fusion systems. After the heat treatments of normalization and tempering, the general microstructures of these steels consist of prior-austenite grain boundaries, lath/subgrain boundaries, dislocations, and precipitates. Laths are contained within the prior-austenite grain boundaries, and they contain a relatively high dislocation density (1013?1014 m-2), depending on the tempering conditions. The dominant precipitates are large M23C6 particles that are mainly located on lath boundaries and prior-austenite grain boundaries. Under low-cycle fatigue tests these steels show, after the first few cycles, a pronounced cyclic softening accompanied by microstructural changes such as the decrease of the dislocation density inside the subgrain and the vanishing of low-angle boundaries. In the LCF regime larger strain amplitudes lead to early microcrack initiation, such that the component?s lifetime is determined by the growth of these microcracks. Transmission and scanning electron microscopy in combination with automated electron back-scattered diffraction techniques are used to study the surface damage as well as the initiation and propagation of short fatigue cracks during low-cycle fatigue.