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Tensile tests were carried out on service exposed Alloy 625 ammonia cracker tube used at heavy water production plant to study the effect of microstructure on the serrated yielding and mechanical properties of the material. Owing to temperature gradient during service exposure, the microstructure was different in top, middle, and bottom sections of the tube. Variation of flow stress, ductility, and average work hardening were monitored with temperature. In the present work, emphasis was given on the study of serrated yielding in the service exposed Alloy 625. Detail investigations were made to study the effect of microstructure on the underlying mechanism of dynamic strain aging of the material. The study revealed that both the normal and the inverse Portevin-Le Chatelier effect (PLC) occured in the material at lower and higher temperature regime, respectively. While the normal PLC dynamics was associated with locking of dislocations by interstitial carbon atoms, the inverse one was accomplished by the dislocation pinning by substitutional Mo atoms. Further analyses identified that the basic deformation mechanism was different in middle and bottom samples as that in the top samples which was reflected in the difference in their respective activation energy and stress drop magnitude. [ABSTRACT FROM AUTHOR]