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Background
Austenitic stainless steels have been the most popular corrosion-resistant materials for a wide variety of applications , with the 300 series being the largest category of stainless steels ( SS ) produced in the US . 1 However , in the presence of aqueous chlorides , tensile stress and temperatures above 140 ° F , chloride stress corrosion cracking ( Cl - -SCC ) of 300 series SSs can occur . 2 Dissolved oxygen in the process stream can exacerbate this cracking .
In this case study , failure analysis was performed on two 316 stainless steel expansion joints that were retrieved from a unit reactor circulating a process fluid containing lubricant additive precursors . The system operated as a
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batch process , with cyclical temperatures ranging from ambient to maximum operating temperatures of 400 ° F . The operating pressure was 7 psig .
The site also reported that the possibility of chlorides in the system could not be ruled out . The two joints were in operation for 10 months , followed by a 1-month shutdown during which they were subjected to steam cleaning .
The leaks were found upon resumption of operations . One joint was located upstream of the reactor tank , while the other joint was located downstream of the reactor , on the suction side . The reactor tank was glass-lined and worked in tandem with a similar reactor vessel circulating the same process stream . The purpose of the expansion joints
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was to reduce the stresses experienced by the glass-lined nozzles in the unit . The tanks were part of a chemical plant manufacturing emulsifiers . A photograph of the leaking upstream expansion joint taken onsite is shown in Figure 1 . Both the upstream and downstream expansion joints were examined in the laboratory to determine the damage mechanism ( s ) responsible and the differences , if any , between the two failures . |
Figure 2 : Photographs of the downstream sample following longitudinal sectioning . Cracks were present on the inside surface of the bellows ( yellow arrows ). Numbered scale divisions are in inches . |
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Figure 1 : Photograph taken onsite showing the leak ( yellow arrow ) at the upstream expansion joint . |
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Laboratory Examination |
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Visual Examination |
Inside surface |
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Dimensional measurements of the two samples indicated that the upstream expansion joint was of 4-inch diameter , 8¾-inch height , and the downstream expansion joint was of 4-inch diameter , 5¾-inch height . The expansion joints contained a braided-hose style wire mesh that encased a bellows region , with flanges present on either end as observed in the longitudinal section of the downstream sample shown in Figure 2 . The close-up photograph of the inside surface of the bellows indicated the presence of cracks ( marked by yellow arrows ). Similar cracks were observed for the upstream sample as well . The upstream flanges were of the floating type , and the downstream flanges were of the welded type .
The cracks were opened using lab overload so that the fracture surfaces could be examined using a stereo microscope . Figure 3 shows a photograph and a stereomicrograph of the fracture surface for the downstream sample .
The fracture morphology appeared to be transgranular . Similar crack morphology was observed for the upstream bellows sample .
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Figure 3 : Photograph showing fracture surface and cracking ( yellow arrow ) on the inside surface of the downstream sample following lab overload ( top , original magnification : 30X ). Stereomicrograph of fracture surface following lab overload ( bottom , original magnification : 100X ).
Scanning Electron Microscopy ( SEM ) and Energy Dispersive X-Ray Spectroscopy ( EDS )
SEM fractography of the two bellows samples was also performed following ultrasonic cleaning . The fracture morphology was transgranular , with the cracking occurring by a cleavage mechanism . Figure 4 shows SEM images for the downstream bellows sample . Similar images were obtained for the upstream bellows sample .
Prior to ultrasonic cleaning , EDS analysis was performed to document the elements present within the deposits / corrosion products on the two fracture surfaces . Standardless EDS analysis does not accurately measure carbon contents ; hence , these values were excluded from analysis . The upstream
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