Abstract
Hydrogen is usually trapped by vacancy, dislocation, grain boundary, precipitate and void in steels and it is easily released from these trap sites by heating the steels. However, the desorption temperature ranges vary according to the kinds of trap sites and they can be separated in principle, because the binding energy between hydrogen and each trap site is different. It seems that this interesting nature of hydrogen is available for determining microstructural changes and defect formation processes during creep. In this study, the change in hydrogen desorption characteristic due to creep was investigated to discuss the applicability of this technique as a nondestructive procedure for evaluating creep damage accumulation of high Cr ferritic steels. The gauge and grip portions of creep ruptured specimens with different rupture times were charged with hydrogen by means of cathodic electrolysis. Subsequently, the thermal desorption spectroscopic (TDS) analysis was applied to those hydrogen-charged samples for measuring hydrogen desorption curve. Experimental results revealed that a clear peak appeared on the hydrogen desorption curve at around 65°C for the as-tempered. The peak temperature and the peak height of the gauge portion increased significantly resulting in the increase in amount of desorbed hydrogen as the creep rupture time increased, although those of the grip portion showed almost no difference. These changes in hydrogen desorption characteristic seemed to be attributable to the formation of geometric damages such as a creep cavity and microcrack rather than the microstructural changes, because the re-normalizing had almost no influence on the hydrogen desorption curve of the gauge portion.
