Three-dimensional analysis of creep void formation in steam-methane reformer tubes
Degree GrantorUniversity of Canterbury
In methanol processing plants, steam-methane reformers consist of hundreds of vertical tubes operating at temperatures up to 1000°C. These reformer tubes fail by creep through the formation of creep voids during service. Preliminary research showed that the occurrence of these voids was not random and may be related to certain microstructural features of the material. In the present research, the technique of serial sectioning was used to generate threedimensional reconstructions of voids in several steam-methane reformer tube samples with creep damage. The serial sectioning method and subsequent 3D reconstruction revealed creep void information such as size, density, location, and shape in three-dimensions, information that cannot be obtained from two-dimensional micrographs alone. Samples were obtained at various locations along the length of an ex-service reformer tube to investigate the effects of service conditions on the characteristics of creep voids. In addition, samples were taken from various positions along the wall thickness where there were differences in temperature, stress, and microstructure. Additionally, the identity and crystallographic orientations of the phases adjacent to creep voids were studied by electron backscatter diffraction (EBSD) to determine the crystallographic trends in creep void locations. Three dimensional observations revealed that creep voids were indeed not uniformly distributed through the volume in terms of their size, shape, and location. All voids appeared next to carbides and these voids came into contact with M₂₃C₆ precipitates somewhere along their perimeter. Most of the voids were found on austenite (ɣ) grain facets (the interface between two ɣ grains) but the larger voids were generally found at grain edges and corners. The grain boundaries where voids were located were generally oriented at 45 degrees with respect to the hoop stress direction. Here, the effective stress due to a combination of loading and temperature were highest. xviii Abstract EBSD results showed that 80% of the M₂₃C₆ precipitates surrounding these voids have an irrational crystallographic orientation relationship (OR) with the austenite matrix. In contrast, grain boundary precipitates in an aged sample always show a rational OR with respect to one adjacent grain. This implied that the preferred sites for creep voids are low registry boundaries between M₂₃C₆ precipitates and austenite. The data obtained from 3D observations were applied to a classic void growth model. Various permutations of the parameters obtained from this work were applied to the model to simulate conditions that may be beneficial to extending the service lives of reformer tubes. It was shown that the void growth model required accurate and representative materials constants for good estimation of life. Furthermore, the model revealed that more work was required in terms of observations of void nucleation in 3D, in order to fully utilize the model. Finally, it was shown that void density measurements are the most critical item for accurate prediction of growth of voids.