2 fracture has occurred at its conjunction zone. A spool including fracture zone was subjected to different nondestructive and destructive investigations for failure analysis. 2. Investigations The failed spool was subjected to different non-destructive and destructive tests including visual investigation, liquid penetrant test, radiographic test, chemical analysis, thickness measurement, macro-, optical and scanning electron microscopic examinations, hardness measurements, tensile and impact tests. General and enlarged views of the fracture zone of the spool are shown in Fig. 1. The important notice is that fracture had occurred at the acute or sharp angle zone of 16" pipe/20" header assembly, just beside its circumferential weld. Based on visual investigation, it can be deduced that fracture was started at its mid-length then, propagated in tow opposite directions along circumference. The fracture extended less than half way around the circumference of 16" pipe/20" header. Visual and dye penetrant inspections indicated surface cracks on both sides of the fracture termination zones (Fig. 1-b). Generally, the outer surface of the failed spool is clean and free from deposits or indications for corrosion. Visual investigation of the weld face and root sides of the circumferential weld of 16" pipe/20" header as well as that of 16" pipe showed less weld quality. However, dye penetrant test of both root and face sides of circumferential weld of 16" pipe showed no surface cracking. Visual investigation of the internal surface of the failed spool showed damage at the inner surface of 16" pipe, which could be related to mechanical action. Except that, the internal surface of the spool showed smooth surface with no indications for internal corrosion. In other words, no thinning or variation in the wall thickness was observed, where almost uniform wall thickness (~6.0mm for 20" pipe and ~4mm for 16" branch) was obtained at both fractured and non-fractured zones. Macro- and microscopic investigations of a cross section taken from mechanically damaged zone showed internal localized reduction of about 0.6mm depth in the wall thickness of mechanically damaged zone. Optical microscopic investigation showed no indication for cracking at mechanically damaged zone. Surface cracks zones observed around the fracture termination zones (zones 1, 2, 3 & 4, in Fig. 1-b) were subjected to detailed investigation. Close-up views of zones 4 and 1 at front of the fracture termination zones are shown in Fig. 2-(a) and (b). It can be noticed that surface cracks are existed just beside Figure 1. General (a) and enlarged (b) views of fracture zone of the spool. Note that fracture had occurred at sharp angle zone of 16" pipe/20" header assembly, just beside its circumferential weld.
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