A study on creep damage parameters in creep cavity growth dominant region under creep-fatigue loading

Metallographic parameters which can represent the creep-fatigue damage progress under creep damage dominant condition were studied. A series of creep-fatigue tests which have been interrupted at several fractions of the life were carried out on type 304 stainless steel at a low strain range with long strain hold time. All interrupted test specimens were cut off in an axial plane and observed their microstructures by SEM. The observed cavitation of grain boundaries were quantified using several metallographic parameters. These parameters were compared with the estimated creep damage by a mechanical evaluation model based on linear damage rule. The area fraction of cavity per unit area has a good correlation with each stage of estimated creep damage. And the process of cavity nucleation and its growth up to micro-crack initiation along grain boundary was discussed phenomenologically based on the observed data. (author)