The spatial and temporal characterization of type IV creep fracture in notched specimen of modified 9Cr-1Mo steel

In order to observe type IV fracture occurring in the HAZ fine-grained zone of a modified 9Cr-1Mo steel at high resolution, the creep damage process of a single notched specimen was continuously observed in three dimensions. The initiation, growth and coalescence behaviour of individual creep voids were clearly observed. This was combined with an analysis of the local stress state using the finite element method. Furthermore, all creep voids that could be observed in the final loading stage were traced backward in time to determine the loading stages at which the creep voids were initiated together with the growth behaviour of the individual creep voids. Significant creep void formation was observed a little further in from the bottom of the notch, where the maximum principal and hydrostatic stresses were greatest, while the stress triaxiality was greatest a little closer to the centre of the specimen than the maximum principal and hydrostatic stresses, where significant creep void growth was observed. It was found that the creep voids nucleated at an early loading stage dominated the creep failure of the specimen. The growth behaviour could be well approximated by the model that combined the diffusion law and power creep law with the constraint effect from neighbouring grains. Engineering parameters that describe the progress of type IV damage were also investigated.