Stainless Steel World Americas October 2024 | Page 11

Figure 4 : SEM images showing cleavage facets on the downstream bellows sample fracture surface .

bellows fracture surface primarily consisted of iron , oxygen and chromium with several trace elements including chlorine as shown in Figure 5 . The downstream bellows fracture surface primarily consisted of iron , oxygen , chromium , nickel with several trace elements including sodium and chlorine .

Figure 7 : Optical micrographs showing transgranular cracking across upstream bellows cross-section . [ Top left ] As-polished . Original magnification : 50X . [ Top right ] Etchant : Glyceregia . Original magnification : 50X . [ Bottom ] Etchant : Glyceregia . Original magnification : 200X .

Chemical Analysis

To confirm the material of construction of the two bellows samples , optical emission spectroscopy ( OES ) was performed to determine their chemical

Carbon 0.052 0.043 0.08 max . Manganese 1.02 1.06 2.0 max . Phosphorus 0.028 0.028 0.045 max . Sulfur 0.007 0.008 0.030 max . Silicon 0.51 0.55 1.0 max . Nickel 10.08 10.02 10.0-14.0 Molybdenum 2.05 2.04 2.0-3.0 Chromium 16.25 16.25 16.0-18.0 Table 1 . Chemical Analysis of the Two Bellows Samples .

found on the downstream bellows fracture surface , the possibility of caustic SCC was ruled out because the fracture surface did not contain a black magnetite oxide layer , typically found in caustic SCC failures .

The two expansion bellows met the compositional requirements for UNS S31600 austenitic stainless steel . The general microstructure of the bellows material consisted of austenite grains along with bands of residual ferrite , which are typical of formed austenitic stainless steels .

One way to mitigate against Cl - -SCC is to avoid conditions where chlorides can concentrate or deposit onto the 316 SS surfaces . From the perspective of manufacturing , a stress relief heat treatment of expansion bellows following fabrication can reduce residual stresses . However , care should be taken to avoid sensitization of the 316 SS , which can then lead to intergranular corrosion / cracking . Upgrading to higher nickel base alloys , such as Inconel , can provide increased Cl - -SCC resistance .

References :

1 . A . J . Sedriks . 1996 . “ Corrosion of Stainless Steels .” John Wiley & Sons , Inc .

2 . ANSI / API Recommended Practice 571 . 2020 . “ Damage Mechanisms Affecting Fixed Equipment in the Refining Industry ”, 3rd Ed ., March 2020 .

Figure 5 : EDS analysis of upstream bellows sample fracture surface prior to ultrasonic cleaning .

Metallographic Examination

Two metallographic mounts were prepared using sections of the downstream and upstream bellows samples . Figure 6 shows a photograph of a representative mount . Optical micrographs for the upstream bellows cross-section are shown in Figure 7 . Similar micrographs were taken for the downstream bellows sample . For both samples , the crack morphology was distinctly transgranular with the craze-cracked , lightning bolt appearance . The cracks displayed multiple branching and originated from the inside surface of the bellows . The general microstructure of the bellows material consisted of austenite grains along with bands of residual ferrite , which are typical of formed stainless steels .

Figure 6 : Representative macrograph of metallographic mount across the bellows cross-section .

compositions . The results provided in Table 1 indicate that the two samples met the compositional requirements for UNS S31600 austenitic SS .

Conclusions and Recommendations

Based on the results of the laboratory examination of the two expansion joints , it is evident that the inside surface of the bellows cracked from chloride-induced stress corrosion cracking ( Cl - -SCC ). The fracture morphology was distinctly transgranular with branched cracking occurring in cleavage mode . EDS analysis of the fracture surfaces confirmed the presence of chlorides , which are known to promote Cl - -SCC . Any chloride contamination within the process stream , the steam cleanout and the exposure to chloride-containing environments during shutdown may have subjected the inside surfaces of the bellows to aqueous chloride solutions . Residual stresses present in cold worked components , such as these expansion bellows , are also known to contribute to the damage mechanism .

Another consideration was the similarity in crack morphologies for Cl - -SCC and caustic SCC . Although sodium was also

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