2019 Volume 68 Issue 8 Pages 599-606
High-chromium ferritic resisting steels have been commonly used for high temperature components of USC boilers. However, it is well known that in the welded joints TYPE IV damage occurs in the fine-grained HAZ during long-term use under elevated temperature, due to the multiaxial stress at the fine-grained HAZ region. TYPE IV damage develops by the creep void nucleation at grain boundaries, following the void growth and coalescence. In order to measure the amount of creep damage, the distribution of voids on two-dimensional cross section is normally observed. However, the damage evaluation has a limitation from the point of accuracy because the void grows three-dimensionally. Recently, X-ray CT has been applied to observe damage distribution in three dimensions, and it is known that X-ray µ-CT in SPring-8, which is synchrotron radiation facility, can detect a void of about 1 µm diameter. In this paper, the TYPE IV damage is observed with X-ray µ-CT. The amount of void nucleation and the size of voids in the fine-grained HAZ region with time are quantitatively evaluated. Comparison between the creep damage evaluations based on two dimensional cross section (average diameter, area fraction and number density) and three dimensional void observations is conducted. It appears that the evaluation based on the area fraction on cross section is closest to that in three dimensions and therefore the most suitable evaluation.
