Influence of thermal ageing and creep on ultrasonic velocity in 9Cr-1Mo ferritic steel

Ultrasonic velocity measurements have been performed using 15 MHz longitudi nal beam probe to characterize the microstructural degradation occurring du e to thermal ageing and creep deformation of 9Cr-1Mo ferritic steel at 873 K for durations in the range 10-15175 h. The variation in ultrasonic veloci ty with exposure time in both the conditions exhibited an initial increase up to 10 h (regime-I) followed by a gradual decrease at shorter durations ( regime-Il). At intermediate durations, the ultrasonic velocity displayed a reversal in trend i.e., the velocity increased rapidly with time (regime-II I) followed by a sharp reduction at longer durations (regime-IV). These var iations in the velocity has been correlated with the precipitation behaviou r of the alloy. The decrease in ultrasonic velocity in regime-II is ascribe d to the coarsening and consequent decrease in number density of needle sha ped Cr2N precipitates, whereas the rapid increase in the velocity in regime -III is attributed to the formation of Fe2Mo precipitates. The sharp reduct ion in the velocity in regime-IV in both the conditions can result from the coarsening and decrease in number density of both the Cr2N and Fe,Mo preci pitates. Further, the lower times for the onset of rapid increase in the ve locity in regime-III and rapid decrease in the velocity in regime-IV for cr eep tested specimens can be attributed to faster kinetics. The deformation enhanced precipitation under creep condition is also reflected in the highe r ultrasonic velocity in the regime-III and regime-IV.