Case Study |
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Figure 2 : The 2-inch pipe and flange shown as-received at SES for analysis . Approximate location of the leak is indicated by an arrow in the lower photograph . |
Figure 3 : After several attempts , penetrant examination of the outside surface of the pipe revealed a faint indication in the pipe adjacent to the weld . The lower photograph shows a crack-like , linear feature at the location of the penetrant indication . The faint presentation of the indication indicates the “ crack ” was tight and absorbed little penetrant . |
Figure 4 : Fluorescent penetrant examination of the inside surface of the pipe revealed a small , approximately 0.4 inch long crack adjacent to the weld . Slight undercut was observed along the weld toe but was not associated with the crack . |
Figure 5 : An unetched metallographic cross section of the crack responsible for the leak . Crack is branched and transgranular , characteristic of stress corrosion cracking . |
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dark etching , approximately linear features in the microstructure . The residual ferrite bands are typical of formed stainless steels .
Figure 6 : An etched metallographic cross section of the crack responsible for the leak . Crack is branched and transgranular , characteristic of stress corrosion cracking .
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Scanning Electron Microscope ( SEM ) Examination
The crack that resulted in the leak was broken open to expose the two-mating surface of the crack surfaces , shown in Figure 9 . The crack surfaces were analyzed using an SEM in conjunction with energy dispersive x-ray spectroscopy ( EDS ) to determine the cause of the transgranular stress corrosion cracking observed in the pipe ( the leak ). Austenitic stainless steels typically suffer stress corrosion cracking as a result of either chlorides ( chlorine ) or caustic ( sodium hydroxide ) exposure . EDS analysis of the crack surface indicated the presence of oxygen , vanadium , silicon , iron , chromium with traces of nickel , sulfur and chlorine . The presence of chlorine and not sodium indicates the branched , transgranular crack was a result of chloride stress corrosion cracking and not caustic cracking .
Chemical Analysis
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Conclusion
Based on the analyses of the leaking 2-inch , type 321 stainless steel pipe , the pipe leaked as a result of chloride stress corrosion cracking . The leak consisted of a tight , through-wall crack that was highly branched with a transgranular cracking mode , consistent with stress corrosion cracking in austenitic stainless steels . EDS analysis of the crack revealed the presence of significant amounts of chlorine with no sodium detected indicating the cracking resulted from chloride stress corrosion cracking . No defects or anomalies were observed in the pipe , weld or
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flange that contributed to the cracking . The chemistry of the pipe met the compositional requirements of type 321 stainless steel . The weld met the compositional requirements of type 347 stainless steel except for being possibly low on niobium as the tantalum content was not analyzed . The chemistry of the flange was low on chromium concentration , but slightly high on nickel concentration .
References :
1 . JC . Lippold and D . J . Kotecki , “ Welding Metallurgy and Weldability of Stainless Steels ”, John Wiley & Sons , Inc ., 2005 .
2 . A . J . Sedriks , “ Corrosion of Stainless Steels ”, John Wiley & Sons , Inc ., 1996 .
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To confirm the materials of construction of the pipe , weld and flange , optical emission spectroscopy was performed on each to determine their chemical compositions . Table 1 provides the results of the chemical analyses along with the chemical requirements for types 321 and 347 stainless steels for comparison . |
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Figure 7 : Etched micrograph showing the branched , transgranular nature of the crack at higher magnification . |
The pipe satisfied the compositional requirements of type 321 stainless steel . The weld was potentially slightly low on niobium concentration since the tantalum concentration is unknown ; however , there was some minor amount of titanium , another carbide-forming element . The flange was slightly low in chromium concentration , but slightly high in nickel concentration . |
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Figure 8 : General microstructure of the pipe consisted of equiaxed austenite grains with bands of ferrite ( dark , somewhat linear features ). |
Figure 9 : The through-leak / crack in the pipe broken open to expose the mating surfaces of the crack . Multiple secondary cracking visible on the crack surfaces is consistent with the branched cracking seen in Figure 7 . |