SB-06-2(031) High Temperature Low Cycle Fatigue of 21/4Cr1Mo Steels : Role of Microstructure and Effect of Environment(Changes in Microstructure 1)

概要

論文の詳細を見る
In order to increase the reliability of high temperature components of power plants, the increase in the knowledge of the fatigue mechanisms is necessary. The aim of this paper is to present some results on the high temperature low cycle fatigue of 2.25Cr1Mo steels that have been in service exposed at a temperature of 565℃ for about 110 OOOh and 160 OOOh. The analysis is carried out with special attention to the effect of microstructure and environment. The role of microstructure is studied taking into account the proportion of ferrite and bainite and the effect of the in-service long term ageing on the stress response to strain cycling. Laboratory tests performed at 565℃ in air under different strain levels show a strong influence of the microstructure on the accommodation of the cyclic plasticity but not on the fatigue resistance. 2.25CrlMo steels can exhibit cyclic hardening or cyclic softening. It is shown that increasing ferrite content promotes cyclic hardening. With a volumic fraction of 35% bainite, the 2.25CrlMo exhibits rather softening which intensity and rate is controlled by the state of bainite. Long term aged bainite promotes a softening of the steel. To study the effect of the environment on the nature of crack initiation, low cycle fatigue tests were carried out in vacuum at a reduced pressure of 10^<-4> Pa. Fatigue lives are much longer in vacuum than in air. The density of cracks was particularly decreased in vacuum.
2001-06-03