6 Scanning electron microscopic photographs with different magnifications of fracture surface including suspected initiation zones are shown in Fig. 6. Crack initiation sites can be seen on the outer surface, where a single macroscopic direction of crack propagation is impossible to be defined. This is because crack size is still in the microcrack zone, where multiple cracks form at the surface, initiating at different locations and with different orientations. The important notice is the unclear fatigue striations at the fracture initiation zones. 3. Discussion Visual and dye penetrant examinations of the failed spool showed that the fracture zone was confined to the acute or sharp angle of 16" pipe/20" header assembly, just beside circumferential weld line. Such sharp angle is considered as stress concentration zone. The fracture was initiated at the outer surface and propagated through thickness in two opposite directions where it was extended less than half way around the circumference. No indications for corrosion attack were observed on spool outer or inner surface. Only, mechanically damaged zone was observed at the inner surface of 16" pipe and this could be occurred long time ago during construction or repair works. However, such mechanical damage played no role in the current failure. In general, non-destructive investigations showed less weld quality. Meanwhile, fracture was not directly related to welding defects since fracture had occurred in the base metal just beside weld line. Based on the results of chemical analysis, hardness measurements, tensile and impact tests and microscopic examination of the failed spool, materials of both base and weld metals were found to be within the specification of type 316L austenitic stainless steel. Macro- and microscopic examinations of cross sections around the fracture zone showed multi-cracks initiated at the weld toe that is considered as another stress concentration zone. Stereoscopic examination of the fracture surface showed that multi-cracks were initiated at the outer surface where smooth surface with beach marks was observed. Scanning electron microscopic investigation of the fracture surface showed unclear fatigue striations at the fracture suspected initiation zones. These findings support fatigue damage as a failure mechanism [1-5]. Figure 6. Scanning electron microscopic photographs of fracture suspected initiation zones at outer surface showing unclear fatigue striations.
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