Strain controlled fatigue tests for hot-extruded tube material of Alloy 690 (Sandvik Sanicro 69) have been performed at room temperature (RT) and at elevated temperature (204°C). The influences of temperature on the cyclic deformation behaviour and the fatigue life have been investigated. The influence of temperature on the fatigue life is relatively small in the temperature range investigated. However, the fatigue behaviour at elevated temperature is quite different from that at RT. A second cyclic strain hardening was observed at 204°C. The possible mechanisms have been investigated using transmission electron microscopy. Besides dislocation mechanism, the interactions between moving dislocations and stacking faults and between interstitial atoms and moving dislocations could also contribute to this secondary cyclic strain hardening. The formation of micro-twins during cyclic loading at 204°C and its influence on the cyclic stress-strain response were also discussed. Temperature affects both fatigue crack initiation and propagation behaviour. Increase in temperature promotes duplex slipping process, which causes the formation of striation.

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