Tham khảo tài liệu 'failure analysis case studies ii episode 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 338 Fig. 7. Oxide spike under the fractured surface of primary crack. which was located far from the oxide layer and on the tube metal in the vicinity of a primary crack whose composition may be affected by oxide scale formation. These two analysis results were compared to predict the cause of oxidation. Also a piece of oxide scale was detached from the tube s interior surface and analyzed. Compositional analysis of the oxide layer at the crack tip was also conducted. EDAX analysis in the electron microscope was used for composition analysis and the results are summarized in Table 2. It is shown that the Cr content of the metal right beneath the primary crack decreased compared to the sound part of the tube. This is due to the formation of Cr2O3 at the crack surface resulting in a decrease of the Cr content in the neighboring base metal. Analysis results of three oxide scales two pieces detached from different locations and one attached at the crack tip showed that the Ni content did not vary while the Cr content increased and the Fe content decreased considerably. Thus it can be predicted that Cr2O3 is the main Table 2 EDAX analysis results of the radiant tube at several locations in wt Locations Composition Ni Cr Fe Si Mo s V Sound tube metal far from the cracked region Tube metal near to the cracked region Crack tip oxide scale Detached oxide scale 1 44 84 Detached oxide scale 2 18 05 41 24 35 48 339 Fig. 8. Ni distribution map of crack tip region same location as shown in Fig. 6 b oxidation product in the oxide scale. Other minor elements appeared in the scales such as of s and of V apparently produced by contact with combustion gas. A high content of s may generate a corrosion problem vanadium usually forms V2O5 which together with high velocity combustion gas may cause erosion problems. In this tube failure .
