Cyclic instability of martensite laths in reduced activation ferritic/martensitic steels

Low cycle fatigue tests were performed in the temperature range between room temperature and 550 °C in low activation quenched and tempered steels. After the first few cycles a pronounced cyclic softening that continues up to failure is observed for all these steels. Although this softening is coincident with similar behaviour observed in commercial martensitic alloys, reduced activation steels present some remarkable different features. Almost all cyclic curves never reach a saturation stress and after few cycles, dependent on the total strain range, they converge in a common softening stage independent of the strain range. The mechanism governing this softening stage is almost independent of temperatures below 450 °C. The cyclic softening observed in the reduced activation steels is more pronounced than for MANET II and commercial martensitic steels. This fact is related to the difference in the Martensite Start temperature.