ELECTRON BACKSCATTERED DIFFRACTION (EBSD) OF CREEP-VOID PRECIPITATES IN STEAM-METHANE REFORMER TUBES

Steam-methane reformer tubes are subjected to stresses at high operating temperatures. These centrifugal-cast austenitic stainless steel tubes typically fail by creep through the formation, growth, and coalescence of voids during service. Previous research has shown that the occurrences of these voids were not random. It was observed that these voids were always found next to precipitates, and it was hypothesized that perhaps their formation may be related to certain microstructural features of this material. In this work, samples were obtained from several ex-service steam-methane reformer tubes and prepared using typical metallography methods. The microstructures were initially observed using the optical microscope to locate the creep voids and their vicinity. Once appropriate regions of interest were identified, the samples were studied using a Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) detectors. The precipitates surrounding the voids were then characterized using a combination of EDS and EBSD techniques. It was discovered that the precipitates were either Chromium-rich M23C6, Niobium-rich MC, or Titanium-rich MC carbides. Next, EBSD patterns (or EBSPs) were recorded in spot mode for specific locations of the matrix and precipitates surrounding the creep voids. The
orientation relationship (OR) between each precipitate and the austenite matrix adjacent to each creep void were then determined. Results indicate that the preferred sites for creep voids are low-coherency boundaries (random OR) between M23C6 carbides and austenite.