3D analysis of creep voids in hydrogen reformer tubes

An ex-service hydrogen reformer tube composed of Fe-0.45% C-25% Cr-35% Ni-1.0% Si-1.5% Nb-1.0% Mn (approximate wt.%) was examined via serial sectioning and 3D reconstruction to characterize creep voids in terms of their size, location, and surrounding precipitates. In addition, electron backscatter diffraction (EBSD) technique was used to investigate any crystallographic influence on the location of these voids. Observations in 3D show that void distribution is not uniform in terms of size, shape, and location. Most of the voids are oblong and occur at grain edges and corners. In addition, all observed voids were in contact with the Cr-rich M23C6 precipitates. EBSD results showed that 80% of the M23C6 precipitates surrounding these voids have an irrational crystallographic orientation relationship (OR) with the austenite matrix, contrary to the expectation that grain boundary precipitates nearly always obtain rational OR with respect to one adjacent grain.