3 circumferential weld of 16" pipe/20" header. Visual and dye penetrant examinations of zones 3 and 4 showed similar results. After detailed visual and dye penetrant investigations, specimens from both fractured and non-fractured zones of the spool were cut out and prepared for chemical analysis, macro- and microscopic examinations, scanning electron microscopic examination, hardness measurements, tensile and impact tests. Results of chemical analysis of the failed spool together with the specified chemical composition range for type 316L stainless steel are shown in Table 1. It is obvious that the chemical composition of the used spool is a typical for the austenitic stainless steel type 316L. Cracked zones 1, 2, 3 and 4 in Fig. 1-(b) were cross sectioned where its cracking features were clarified. Macrographs of a cross section taken from cracked zone 2 are shown in Fig. 3. It is clear that the observed cracks are weld toe cracking type. Similar results were obtained for samples taken from zones 1, 3 and 4 in Fig. 1-b. Weld toe cracks of zones 2 and 3 (~18cm away from fracture termination zones) were propagated through thickness of 16" pipe. Weld toe cracks of zones 1 and 4 in Fig. 1-b (~35cm away from fracture termination zones) were propagated through thickness of 20" header. It is obvious that cracks were initiated at weld toe on outer surface then, propagated through thickness. Optical micrographs with high magnification of cross sections taken from zones 1 and 4 in Fig. 1-b are shown in Fig. 4. It can be noticed that normal austenitic microstructures for both base metal, HAZ and weld metal were obtained and cracks were propagated through grain boundaries. Survey of the hardness measurements indicated Figure 2. Close-up views of zones 4 and 1 at front of fracture termination zones of Fig. 1-b. Note surface cracks on both sides of circumferential weld of 16" pipe/20" header. Table 1. Results of chemical analysis (wt%) of the failed spool together with the specified chemical composition range for ASME-SA213, 316L stainless steel.
RkJQdWJsaXNoZXIy MjM0NDE=