Failure analysis of service exposed austenitic stainless steel pipelines

Failure analysis of service exposed austenitic stainless steel pipelines

Engineering Failure Analysis 108 (2020) 104337 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier...

5MB Sizes 0 Downloads 57 Views

Engineering Failure Analysis 108 (2020) 104337

Contents lists available at ScienceDirect

Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal

Failure analysis of service exposed austenitic stainless steel pipelines

T



N. Sreevidyaa, S. Abhijithb, Shaju K. Alberta, , V. Vinodc, Indranil Banerjeec a b c

Metallurgy and Material Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu 603102, India National Institute of Technology Surathkal, Karnataka 575025, India Fast Reactor &Technology Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu 603102, India

A R T IC LE I N F O

ABS TRA CT

Keywords: Austenitic stainless steel Microstructure Sensitization IGSCC EBSD

Several leaks appeared in Austenitic Stainless Steel (ASS) pipelines installed for transporting water in a test loop after a few years of operation at Indira Gandhi Centre for Atomic Research, Kalpakkam. The locations of leaks were mostly on pipe fittings like bends, but a few were noticed on the pipe away from the fittings too. This paper presents the results of failure analysis carried out on leaking of pipes and fittings. Investigation carried out include optical as well as scanning electron microscopy, energy dispersive spectroscopy, electron back scattered diffraction analysis and microhardness measurements. In addition, double loop electrochemical potentio-kinetic reactivation experiments were conducted on specimens extracted from the pipe side and fitting side of a weld that was found leaking. Further ASS pipe welds prepared with different surface finish conditions were exposed to the environment of the installed pipeline and surface degradation in these pipe welds were compared to reveal the effect of surface treatment on degradation of the welds. It is found that the sensitization along with residual stress generated during welding facilitated intergranular stress corrosion cracking in pipe fittings made of AISI 304 stainless steel resulting in the leaks observed in the pipe fittings. Cracks initiated from the corrosion pits present near the weld, which most likely would have formed due to improper cleaning given to the weld zone after completion of the weld. Leak observed in the pipe is attributed to the crevice corrosion that progressed from a defect present in the pipe making it grown across the thickness. The defect itself was result of an improper repair by arc welding, of a discontinuity that was found in the pipe. The pipe is produced from sheets by resistance welding and the origin of the discontinuity is the poor joint formation during resistance welding. The paper also gives recommendation on good fabrication practices to be followed so that similar kind of failures could be avoided in future.

1. Introduction Failures are common in industry and can occur at any stage of fabrication, testing, transportation or during service and happen without sufficient advance warning. A systematic analysis of such failures helps in pointing out the possible causes of failure and suggesting the ways to improve the design and process parameters, fabrication practices, inspection and operation to prevent such failures in future. Leaks were observed at locations close to their welds and away from the welds on the water pipe lines made of austenitic stainless



Corresponding author. E-mail address: [email protected] (S.K. Albert).

https://doi.org/10.1016/j.engfailanal.2019.104337 Received 10 May 2019; Received in revised form 30 September 2019; Accepted 18 November 2019 Available online 22 November 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

steel (ASS) installed in Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam for testing of components in flowing water. Visual inspection clearly revealed the presence of cracks on pipe fittings like bends at the locations of leaks. A few leaks were noticed on the pipe lines away from these bends too. In spite of the good resistance to uniform corrosion [1,2] and weldability that ASS possess, the components made from it seemed to be prone to failure within short period of their service; as the observed leaks mentioned above were detected within four years of their service. This paper presents the results of various investigations conducted on failed pipes and pipe fittings using different techniques. The first phase (Section 3 of this paper) gives the failure analysis for the leaks noticed at the weld joints of the pipe fittings based on studies like visual inspection, analysis of corrosion products under scanning electron microscope coupled with energy dispersive spectroscopy (SEM/EDS), microstructural analysis, generation of phase as well as microscopic level qualitative strain contouring maps using Electron Back Scattered Diffraction (EBSD) data and micro hardness measurements. The cause of failure found was further confirmed by doing two additional experiments. One was double loop electrochemical Potentio kinetic reactivation (DL-EPR) to compare the degree of sensitisation in pipe and pipe fittings close to the weld. The other was simulating the service condition to study the effect of pickling and passivation after welding as well as the use of fresh and contaminated grinding wheel for grinding of the weld joints. The second phase (Section 4 of this paper) gives failure analysis for the leaks observed in the pipes. Finally, based on the findings from this study, suggestions to prevent such failures in future are proposed. 2. Preliminary investigations 2.1. Checking of records Verification of records indicated that, though entire pipelines to be fabricated using AISI 304L SS as per the design, only the pipes were made of AISI 304L and the pipe fittings like bends used were made using AISI 304 since pipe fitting made from AISI 304L were not readily available during the erection of these pipelines. As the cracks near the welds between pipe fittings and pipes were confined to pipe fitting side of the welds, it is clear that compromise on the choice of material for pipe fitting had a role in the degradation. Further, records also showed that the pipes were resistance welded and detailed non-destructive inspection of the pipes and pipe fittings were not conducted during the procurement time. There were no records of pickling or passivation of the weld zone after completion of welding. 2.2. Chemical composition Specimens extracted from pipe line and pipe fitting (bend) were subjected to Radio Frequency Glow Discharge Optical Emission Spectrometry in order to get the exact chemical composition which is given in Table1. On comparison of this data with that given in material history, carbon percent is found to be a little bit higher on fitting made of AISI 304 (i.e. 0.09%) than 0.08, the maximum specified for this class of steel. 2.3. Visual inspection and dye penetration test The condition of the pipe lines where failure occurred was visually inspected and the relevant photographs were captured. Failed pipes and pipe fittings were also inspected using dye penetrating test (DPT) to see whether any surface discontinuities are present. Visual examination of the regions near to the welds of pipe (made of AISI 304L) to pipe fittings (made of AISI 304) revealed that weld locations have undergone more corrosion than the rest of the regions as shown in Fig. 1a. The cracking near weld on pipe fitting (revealed after dye penetration test) is shown in Fig. 1b. These cracks are seen in narrow regions of the pipe fittings, approximately 7–8 mm from the fusion boundary of the weld. All the observed cracks were parallel to the weld indicating welding has a role in the formation of these cracks. Examination of ID surface revealed not much corrosion has taken place near weld in the ID side even though water has been flowing continuously during service. Water leakage occurred through a pin hole on pipe line too (Fig. 1c). Visual inspection further revealed that, after welding, the weld joints had been subjected to excessive grinding to clean the weld zone (Fig. 2) and the corrosion was more in these ground locations. From the excessive oxidation observed in the ground zone, it appears that grinding wheel used was not meant for exclusive use with austenitic stainless steel material and hence, iron contamination from them might have accelerated the corrosion of the weld zone. A pinhole on the pipe line (made of AISI 304L) was observed at the location of leak from the pipe line far away from the pipe fittings as shown in Fig. 3a and water leakage occurred through this pin hole. Pipe was cut near this pin hole and inner surface of the pipe was examined. Fig. 3b shows the ID of the pipe with pinhole and corrosion products accumulated around this defect. It is clear that location of this pin hole is in the weld line of the pipe produced by resistance welding and there is no full fusion across the Table 1 Chemical composition (in wt. %) by Radio Frequency Glow Discharge Optical Emission Spectrometry. Specimen

Cr

Ni

Mn

C

P

S

Si

Fe

Pipe fitting (3 0 4) Pipe line (304L)

17.8 18.1

8.9 11.6

0.75 1.75

0.09 0.023

0.02 0.03

0.037 0.021

0.65 0.58

Balance Balance

2

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 1. Photographs of failed pipes (a) corroded areas near weld; (b) cracks observed after DPT on pipe fitting side; (c) pinhole on pipe.

Fig. 2. Photographs of the pipe fittings showing their degradation in service.

Fig. 3. Photographs showing lack of fusion in the weld regions of pipes due to defect caused by pitting corrosion (a) covering the entire pipe; (b) the specimen from the location of pit with crack moving from ID to OD side of the pipe.

thickness of pipe in the location of this defect. It appears that welding defect present in the pipe facilitated localized corrosion in this zone resulting in the formation of through thickness pin hole leading to water leakage. Microstructural details of this location is described later in the Section 4 of this paper.

3

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 4. SEM micrographs in (a) SEI mode showing weld and crack regions on pipe fitting; (b) BEI mode showing irregularities of surface near corroded and cracked portion on pipe fitting.

3. Failure analysis of leak in weld joint 3.1. Examination of oxides under scanning electron microscope (SEM) A sample was extracted from the location of joint between pipe and pipe fitting in such a way that this sample contains both the weld and the crack for detailed investigation under SEM. Fig. 4a shows crack propagating direction is almost parallel to the weld fusion line. Corrosion products along the grinding marks are observed. The regions close to the crack appeared to be severely corroded. Fig. 4b is the SEM back scattered image where the cracks associated with the oxide films and grinding marks are clearly seen. Surface damage due to flaking of the oxide films also can be noted. Among the different types of oxides seen, flaky oxides were prominent in close vicinity of crack. These oxides (shown in Fig. 5) appear like clustered spherical particles [3]. The Energy Dispersive Spectrum (EDS) taken from a point on one of these oxide particles is also shown in Fig. 5.which confirms that this is iron rich oxide. Chlorine was detected in the regions close to these oxide films as shown in Fig. 6. Presence of silicon content is also noticed on the oxides present close to the cracked portions (Fig. 7). Iron contamination on the surface was detected at some locations (Fig. 8). Excessive corrosion is seen at the regions of weld metal too. Fig. 9 shows oxide films present on the weld metal with minor cracks and pits. Isolated Si rich oxide inclusions are also seen. Following facts are clear from the examination of the oxide films seen near the crack location as well as seen on the welds. These oxides are rich in iron and they are not adherent to the surface. Surface cracks are seen on these oxide films and spalling of these oxide films also appears to be happening. Contamination with iron and Cl− ions are also detected along with these oxides. These are in contrast with Cr rich adherent oxide film expected to be present on the surface of stainless steels that protect the surface from further oxidation. Based on these observations, the reasons for excessive oxidation seen near the weld locations of the piping are the following: The oxide films formed at high temperature during welding are rich in Fe and they do not protect the underlying metal from further oxidation. In the absence of pickling and passivation, these oxides are not removed and this makes the weld joint susceptible to further corrosion. Grinding operation carried out on the weld joints, apparently using same grinding wheels used for grinding of

Fig. 5. SEM micrograph with EDS spectrum showing crack associated with iron rich oxides on pipe fitting side. 4

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 6. SEM micrograph with EDS spectrum showing Cl enriched areas near crack on pipe fitting.

Fig. 7. SEM micrograph with EDS spectrum showing presence of Si near crack on pipe fitting.

Fig. 8. SEM micrograph with EDS spectrum for embedded Fe rich particles near crack on pipe fitting.

carbon steel, causes iron contamination on the stainless surface which further facilitate galvanic corrosion. Chloride ions present in the sea side atmosphere contribute to this corrosion. 3.2. Metallography 3.2.1. Specimen preparation Specimens were cut from failed weld joints for metallographic examination in such a way that it covers cracks in the pipe fittings. 5

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 9. SEM micrographs for the welds showing the presence of (a) corrosion; (b) micro cracks.

They were prepared by grinding and polishing. Etching was according to ASTM A262 Practice A in order to find out whether the weld joint is sensitized or not. For examination under SEM, specimens were prepared separately with light electrochemical etching using 10% oxalic acid [4], to get an idea on the distribution of precipitates on the surface, if any. For EBSD analysis, specimens were prepared with fine polishing using colloidal silica and with no etching. 3.2.2. Hardness profile across the weld Microhardness profile across the weld from pipe side to pipe fitting side was taken using Vickers micro hardness tester at 300 g load and the result is shown in Fig. 10. It is clear that hardness of the pipe fittings (> 300VHN) is more than that of the pipe (< 200VHN) (a fall of hardness on 304 side seen in the hardness profile given in Fig. 10 corresponds the value near to the location close to crack) indicating that the fittings are in cold worked condition. It appears the fittings have not been subjected to any solution annealing treatment after the forming operation. It should be noted that corrosion/oxidation behaviour of the cold worked austenitic stainless steel would be different from that of the solution annealed steel. Cold work increases the density of dislocations along which the diffusion of chromium occurs easily, which in turn facilitate nucleation and growth of carbides at the grain boundaries. 3.2.3. Microstructure Metallographic specimens prepared from the cracked location of the pipe fitting were examined under optical microscope in order to evaluate the microstructure in the failed location. Optical micrographs of the pipe fittings at the location of crack, ~7 mm from the weld, is shown in Fig. 11. Fine branched cracks that follow the grain boundaries are seen. This indicated the type of cracking as either inter granular corrosion cracking (IGC) or as inter granular stress corrosion cracking (IGSCC). Further, it may be noted that cracks initiated from a corrosion pit present on the surface. Optical micrograph of the regions near to the crack (but without any crack) and near to the weld on pipe fitting side, after etching as per ASTM A262 Practice A is shown in Fig. 12. Microstructure reveals dual structure (mixture of ditched and step) near the location of crack (Fig. 12a) and step structure close to the weld (Fig. 12b). This suggests that material at the location of the crack could be partially sensitized and hence likely to be prone to IGC or IGSCC [5,6,7,8]. Dual structure is observed from a distance of 2 mm from

Fig. 10. Typical micro hardness profile across weld having both pipe and pipe fitting sides. 6

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 11. Optical micrograph for branching cracks propagating through grain boundaries on pipe fitting.

Fig. 12. Optical micrographs from the pipe fitting showing (a) more carbide precipitation near crack; (b) with less carbide precipitation near weld.

the weld up to 11 mm from the weld with more grooves like features close to location of cracking (~7 mm from the weld). Fig. 13a–e, f are the SEM micrographs taken at similar magnifications at distances of 3 mm, 4 mm, 6 mm, 8 mm, 10 mm, and 11 mm respectively from the weld fusion line on pipe fitting side. No precipitate is seen at the grain boundary up to a distance of 2 mm from the weld.It is confirmed from this figure that the carbide precipitation along the grain boundaries increased with increasing distance from the weld. Very fine precipitates formed at the location close to the cracking on fitting side is shown in the SEM micrograph (Fig. 14). It is interesting to note that there was no sign of similar precipitation at a similar distance from the weld on the pipe side. The phase of these precipitates are identified further in the Section 3.2.4 by using EBSD technique. 3.2.4. EBSD analysis Carbide precipitation in the pipe fitting side, where cracks are observed, was also confirmed from EBSD studies. EBSD techniques are widely used for phase analysis in steels [9,10,11,12,13]. The specimens taken from the regions close to the crack and close to the weld on pipe fitting side were subjected to electron back scattered diffraction. The EBSD software could correlate the electron back scattered diffraction pattern (EBSP) consisting of Kikuchi bands obtained with the available crystallographic data of the phases and provide possible crystallographic solutions. This is achieved by comparing measured parameters like interplanar angles, zone axes, band widths and band positions of EBSP, with the software data base. This process is followed by voting of all possible indexing solutions to identify the best fit. The high resolution mapping was carried out with similar SEM-EBSD parameter settings in both the cases with a step size of 0.35 µm on a grid of 645 × 485 pixel. The Mean Angular Deviation (MAD) value for the indexing was 0.28. MAD represents the error in fit in degrees between the pattern expected to be present and the observed pattern for a selected phase. Based on information from the EBSP we could see that the carbides are mainly Cr23C6 and matrix is iron austenite as shown in Fig. 15 as per EBSD data base mentioned in Table 2. Fig. 15a and b represents the phase maps obtained as such (without any noise reduction) close to the crack and that close to the weld. The phase map statistics showed the fraction of Cr23C6 phase as 0.14 near crack and 0.05 near weld which indicates more carbides are present near to the cracked location than those near to weld. Fig. 15c and d represents the simulated EBSP for iron FCC and Cr23C6 near crack and Fig. 15e and f those near welds respectively. Bands shown in the EBSP represent intersections of diffraction cones that correspond to a family of crystallographic planes. Band widths are proportional to the inverse interplanar spacing, and intersection of multiple bands (planes) corresponds to a pole of those planes. Bandwidth for Cr23C6 is lesser than that of iron FCC which indicate that the interplanar distance in the cubic crystal of Cr23C6 is 7

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 13. SEM micrographs depicting the degree of carbide precipitation at a distance of (a) 3 mm; (b) 4 mm; (c) 6 mm; (d) 8 mm; (e) 10 mm; (f) 11 mm from the fusion line on pipe fitting.

Fig. 14. SEM micrograph showing regions of partial precipitation in proximity of intergranular crack on pipe fitting side.

greater than that of iron FCC. The database with crystallographic geometry parameters for EBSD phase identification is given in Table 2. Cr23C6 carbide is having a lattice parameter about three times that of austenite; therefore a real lattice strain in the proximity of the precipitates also could not be excluded [14]. It is possible to obtain strain mapping at a microscopic level using EBSD [15,16]. Accordingly, the strain contours obtained at locations near the crack and near the weld observed in the pipe fittings are given in Fig. 16. The legends for strain contouring map with the corresponding histograms in which X-axis represents maximum misorientation per grain in degree and Y-axis represents the relative frequency are also shown in Fig. 16. The maximum frequency is around 0.1 degree misorientation at location close to fusion line while it is at around 0.4 degree misorientation close to the cracked portion. The range of maximum misorientation per grain values in both the cases are different as shown in the legends for strain contouring map (Fig. 16). Hence the grains near the location of crack are more strained than that near the fusion line. This indicates, residual stress is present in the weld and its value is high at location corresponding to that of cracks. Carbide precipitation along the grain boundaries can lead to sensitization which causes IGC and residual stresses present can promote stress corrosion cracking. IGC accelerates the crack growth resulting in intergranular stress corrosion cracking (IGCSC) and austenitic stainless steels are prone to IGSCC [17]. The location of cracking is same as the location where we find evidence of sensitization and this portion is ~6–8 mm from the 8

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 15. EBSD phase maps on pipe fitting side confirming the presence of (a) more Cr23C6 close to the crack; (b) lesser Cr23C6 close to the weld; Simulated EBSP (c) for iron FCC; (d) for Cr23C6 at a point close to crack; Simulated EBSP (e) for iron FCC; (f) for Cr23C6 at a point close to weld. Table 2 Data base with crystallographic geometry parameters for EBSD phase identification. Phase name

a A°

b A°

c A°

α degrees

β degrees

γ degrees

Space group

EBSD Data base

Iron FCC Cr23C6

3.6599 10.6595

3.6599 10.6595

3.6599 10.6595

90 90

90 90

90 90

225 225

HKL ICSD

Fig. 16. EBSD strain contouring map (a) near crack; (b) near weld; (c) and (d) the corresponding legends.

9

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

fusion line. In the case of solution annealed austenitic stainless steels, this distance is too long to cause the sensitization at this location due to weld thermal cycle experienced for pipe/pipe fittings of this wall thickness. However, it should be noted that the pipe fittings are in cold worked condition and in a systematic study on effect of cold work on sensitization of austenitic stainless steels, it is shown that nose of the CCT curve for sensitization shifts to lower temperatures with degree of cold work [18,19,20]. This means the temperature at which sensitization occurs with minimum time of exposure will be lower for a cold worked austenitic stainless steel than for solution annealed steel. In the case of weld joint, in which base metal HAZ is heated to peak temperatures decreasing with distance from the weld, this translate into a shift in the zone susceptible for sensitization for cold worked material away from the weld from that for the solution annealed material. This could be the reason why sensitization and subsequent cracking is observed in the cold worked pipe fittings of AISI 304 SS at a relatively large distance from the weld. Thus, results from optical microscopy, SEM and EBSD indicate that the sensitization has occurred during welding at a location of ~6–8 mm from the weld and cracking of the fitting occurred in this location during service. In the same zone, from the strain contours obtained, peak residual stresses also appears to be high. Hence, it is reasonable to assume that intergranular stress corrosion cracking (IGSCC) in the part of the pipe fitting which has undergone sensitization during welding is the cause of the leak observed in the pipe fittings. It is already shown that the oxide film present in this location is Fe rich and not adherent. Further, presence of Cl- ions and iron contamination are also observed in this location. Hence, the mechanism of cracking that led to final leaking could be as follows: because of the non protective oxide film, iron contamination, presence of Cl ions, surface irregularities caused by grinding and subsequently by oxide spalling etc., pitting corrosion would have happened on the surface and IGSCC would have then initiated from the pits and progressed to produce the leaks as observed in service. The pit and two independent cracks (shown in Fig. 11) progressed from this pit are proof for this hypothesis. Thus it can be concluded that a combination of various factors, which include use of fittings made from 304 SS instead of 304L SS, cold working in the fittings, use of welding parameters that can sensitize 304 SS, absence of pickling and passivation, improper grinding of the welds, probably by using wheels contaminated with iron and presence of Cl- ions in the atmosphere have all contributed to cracking of the pipe fittings and subsequent leaking. It appears cracks initiated from pits formed on the outer surface of the pipe fittings due to pitting corrosion and then crack grew slowly through thickness resulting in the final leak. 3.3. Confirmation of the cause of failure by additional experiments In order to confirm that the causes identified for the failure of the weld joints between pipe and pipe fittings are correct, two additional experiments are carried out. One was to compare the degree of sensitization in the pipe and pipe fitting material separately using Double loop-Electrochemical Potentio kinetic Reactivation (DL-EPR) technique. Another was to study the effect of surface cleaning (use of fresh and contaminated grinding wheel for grinding) and surface treatment (pickling and passivation) after welding on degradation of the weld joints. These experiments and results obtained are briefly described below. 3.3.1. Double loop –Electrochemical Potentio kinetic reactivation (DL-EPR) experiment DL-EPR test was carried out on specimens at identical distances on either side of the weld for relative comparison of percentage degree of sensitisation (%DOS) between the pipe and fitting material close to the weld. Sensitized stainless steel coupons could be evaluated for degree of sensitization by employing DL-EPR method [21,22,23,24]. Specimens of 10 mm × 10 mm area were moulded and the exposed surface was polished up to 1 µm diamond finish. The edges of the specimens were masked with lacquer to avoid any crevices. The solution used for the experiment was 0.5 M H2SO4 + 0.01 M KSCN at temperature 30 degreesC. Specimens were polarized from −0.5 V to + 0.3 V with respect to saturated calomel electrode and then reversed the scan at a rate of 1.67 mV/s after setting a stable open circuit potential (OCP). The ratio of the peak currents during reactivation to that during activation gives the

Fig. 17. DL-EPR curves at identical distances on either side of the weld. 10

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 18. Photographs of the pipes using iron contaminated grinding wheels (a) before atmospheric exposure; (b) after atmospheric exposure and pipes using fresh alumina wheel (c) before atmospheric exposure; (d) after atmospheric exposure.

percentage degree of sensitization the value of which is greater than one on fitting side of the weld and close to zero on pipe side as per DL EPR curve shown in Fig. 17. This clearly indicates that the fitting material is more prone to sensitization compared to the pipe material [25,26]. Hence the fitting material is likely to have sensitized during the weld thermal cycles experienced and hence become susceptible to IGSCC in the presence of a favourable environment. Here the electrochemical result substantiates the results from metallographic studies.

3.3.2. Simulating the service conditions In order to simulate the corrosion/oxidation damage as observed near the service exposed weld joints, which were not subjected to pickling and passivation and cleaned just by grinding, two sets of sample welds between pipe and pipe fittings were prepared with different post welding cleaning processes and exposed them to the environment for durations more than six months to examine the effect of cleaning process on surface degradation. In this case both pipe and pipe fittings were made of AISI 304L SS. One set consists of an ‘L’ bend fitting which was welded to two pipe pieces on either ends as shown in Fig. 18. In one case, both the welds were ground with a grinding wheel which is already in use in the fabrication shop (for grinding mainly carbon steel). However, one of the welds was pickled and passivated after welding and grinding (marked AA in Fig. 18) while the other (marked AB) left as it is. The second set of welds was ground with a fresh alumina wheel (not having any iron contamination) and in this case also one weld was subjected to pickling and passivation (RB in Fig. 18) while the other (RA in Fig. 18) is left without any further treatment. A comparison of the discolouration observed in the weld joints shows that the weld joints ground using old grinding wheel already used in a general workshop and not subjected to pickling and passivation has under gone maximum corrosion while that ground with fresh grinding wheel and subjected to pickling and passivation has minimum discolouration and hence, minimum damage. It is also observed that the discolouration is not uniform across the circumference for a given weld; but at isolated location distributed randomly along the weld line. Oxidation/corrosion observed in the service exposed welds is also similar. Thus, the results of the laboratory experiments conducted on assessing material susceptibility to sensitization and role of post weld cleaning and surface treatment on surface degradation during service exposure also support the conclusions derived based on the failure analysis carried out on the leaking pipe fittings: i.e. failure is caused by intergranular stress corrosion cracking resulting from a combination of factors involving wrong selection of material and improper cleaning of the weld joint after completion of the weld.

11

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

Fig. 19. Optical micrographs for (a) defect in the weld with crack propagating from it; (b) propagating crack, magnified view.

4. Failure analysis of leak through pin hole in the pipe line From the verification of the records, it is known that pipes used for the construction of these pipeline was produced by resistance welding and visual inspection of the pin hole defect revealed that pin hole was formed from a pre-existing lack of penetration defect present in the pipe weld during production. A comparison of cross section of the weld at the location of the welding defect with that of the weld without defect as shown in Fig. 19(a) revealed that the width of the weld at the location of defect is considerably larger than that of the other. It appears that during production of the pipe, the location where leak occurred during service was not welded properly by resistance welding. Hence, this defect was repaired locally using one of the arc welding processes without any edge preparation. Large width of the fusion zone in this location compared to that of the resistance weld is a proof for this argument. Welding consumables seems to have been employed; but full penetration weld could not be produced; with weld metal fusing only the outer portion of the wall thickness. As a result, though the fresh pipe would have passed a leak test; it had a lack of full penetration which can initiate crevice corrosion in service [27]. Actually Fig. 19 shows the etched microstructure of the weld, close to the location of leak. Microstructure of the fusion zone is revealed after electrochemically etching with 10% oxalic acid. Progress of the corrosion in the fusion zone starting from root of the partial penetration weld can be seen in Fig. 19a with the magnified view of the crack in Fig. 19b; corrosion is progressing from the inner diameter (ID) of the pipe to the outer diameter (OD). Fusion zone seems to contain delta ferrite and the interface between austenite and delta ferrite appears to be the preferred site for corrosion attack at microscopic level. The reason for pinhole leak observed in the pipeline can be attributed to improper repair of the weld defect that formed during fabrication of the pipe using resistance welding. Pipe manufacturer attempted cosmetic repair of original defect formed during pipe production by locally fusing the two edges, without ensuring a full penetration weld across the thickness. An ultrasonic inspection of the pipe would have revealed this defect; but this was not carried out at procurement stage. 5. Conclusion From the detailed analysis of the possible causes for the leaks observed in the austenitic stainless steel pipeline used in various water test loops of Indira Gandhi Centre for Atomic Research, it is clear that cause of leak in the pipe fittings is different from that observed in the pipe. In the former, it is primarily due to use of fittings made of AISI 304 SS while the original choice was AISI 304L SS. As a result, partial sensitization of the HAZ on the fitting side took place during welding of pipe fitting to pipe and this along with residual stress generated during welding facilitated IGSCC in the weld joint. Absence of pickling and passivation after welding and excessive grinding of the weld zone with iron contaminated grinding also seem to have assisted the degradation of the weld zone. The isolated leaks observed in the pipe, is attributed to defects formed during production of resistance welded pipe and improper repair at the defect location to remove the same. Lack of full penetration of these repair welds left crevices at the repair location. Hence, it may be noted that in the case of leaks near the pipe fittings, material degradation initially started from the OD side and progressed to the ID of the pipe. In contrast, in the case of leaks observed in the pipe, away from the welds, degradation started from the lack of fusion defect present in the ID side of the pipe and progressed towards the OD side. 6. Recommendations Based on the above discussed results of analysis of the failed austenitic stainless steel pipe lines, the following recommendations during procurement and fabrication of the components are proposed to prevent such failures in future. (1) Do not change the material selection made at the time of design from low carbon austenitic stainless steel to normal one. If such a 12

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

(2) (3) (4) (5) (6)

change is inevitable due to difficulty in procurement of small quantities, then welding procedure recommended to avoid sensitization in normal stainless steel should be used instead of those finalized for low carbon varieties. It is recommended to ensure proper post cleaning, like pickling and passivation and use of grinding wheel exclusively used for stainless steel fabrication. High grades of austenitic stainless steel such as 316L can be used in the structural components placed in sea shore environments. Solution annealing for stress relieving is recommended as a good practise to dissolve any carbide in austenitic stainless steel formed during welding in case there is no any site constraint. In the case of procurement of resistance welded pipe line, inspection of the pipe using an appropriate NDE technique should be made mandatory. When used in sea shore environment, protecting the pipe line by painting may be considered.

Declaration of Competing Interest No potential conflict of interest Acknowledgement The authors are grateful to all members of Material Joining Section (MMG, IGCAR) who assisted in carrying out this failure analysis study. Our sincere thanks to Mr. Sinu Chandran (WSCD, BARCF, Kalpakkam) for his support to perform DL-EPR experiments and Mr.Sriram who helped in taking material composition by glow discharge sputtering. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.engfailanal.2019.104337. References [1] V. Cihal, Intergranular Corrosion of Steels and Alloys. Material Science Monograph, Elsevier Science Publishing Company, New York, NY, 1984, pp. 146–149. [2] R.K. Dayal, N. Parvathavarthini, B. Raj, A review on the influence of metallurgical variables on the sensitization kinetics of austenitic stainless steels, Int. Mater. Rev (2005) 50129–50155. [3] N. Sreevidya, Sinu Chandran, C.R. Das, Shaju K. Albert, S. Rangarajan, A comparative study on atmospheric oxidation of reduced activation ferritic martensitic steel and grade 91 steel, J. Fusion Eng. Des. 135 (2018) 204–215 PA. [4] ASTM, A 262-93a: Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels. (2004) ASTM International, West Conshohocken, 42. [5] N. Parvathavarthini, R.K. Dayal, J.B. Gnanamoorthy, Influence of prior deformation on the sensitization of AISI Type 316LN stainless steel, J. Nucl. Mater. 208 (3) (1994) 251–258. [6] S.M. Bruemmer, G.S. Was, Microstructural and micro-chemical mechanisms controlling Intergranular stress corrosion cracking in light-water-reactor systems, J. Nucl. Mater. 216 (1994) 348–363. [7] P. Ganesh, et al., Enhancement of Intergranular corrosion resistance of type 304 stainless steel through a novel surface thermo-mechanical treatment, Surf. Coat. Technol. 232 (2013) 920–927. [8] D.A. Jones, Principles and prevention of corrosion, Macmillan, 1992. [9] B S El-Dasher, A Deal: Application of Electron Backscattered Diffraction to Phase Identification. (2008) LLNL book-405555, Electron Backscatter Diffraction in Material science, 2nd edition. [10] Zhongwei Chen, Yanqing Yang and Huisheng Jiao: Some Applications of Electron Back Scattering Diffraction (EBSD) in Material Research. Open access pre reviwed chapter. 2012. DOI: https://doi.org/10.5772/35267. [11] C. Ornek, D.L. Engelberg, Correlative EBSD and SKPFM characterisation of microstructure development to assist determination of corrosion propensity in grade 2205 duplex stainless steel, J. Mater. Sci. 51 (2016) 1931–1948. [12] M. Godec, D.A. Skobir, Balantic: Coarsening behaviour of M23C6 carbides in creep resistant steel exposed to high temperatures, Sci. Rep. (2016), https://doi. org/10.1038/srep29734 nature.com/scientific reports/6:29734/. [13] Edina Kossisova, Maria Domankova, Ivan Slatkovsky, Martin Sahul, Study of the sensitization on the grain boundary in austenitic stainless steel AISI, Research Papers Faculty of Materials Science and Technology Slovak University of Technology, 2014, p. 22. [14] G. Maistro, C. Oikonomou, L. Rogstrom, L. Nyborg, Y. Cao, Understanding the microstructure-properties relationship of low temperature carburized austenitic stainless steels through EBSD analysis, Surf. Coating Technol. 322 (2017) 141–151. [15] Tomoyuki Fujii, Ryohei Yamakawa, Keiichiro Tohgo, Yoshinobu Shimamura, Strain based approach to investigate intergranular stress corrosion crack initiation on a smooth surface of austenitic stainless steel, Mater. Sci. Eng A 756 (2019) 518–527. [16] A.A. Tiamiyu, Vahid Tari, J A Szpunnar, A G Odeshi, A K Khan: Effects of grain refinement on the quasi static compressive behaviour of AISI 321 austenitic stainless steel, Int. J. Plasticity 107 (2018) 79–99. [17] M.A. Strietcher, S. Begum, Corrosion, Intergranular, Reference module in material science and materials engineering, (2016) DOI: https://doi.org/10.1016/ B978-0-12-803581-8.02712-0. [18] N. Parvathavarthini, R.K. Dayal, S.K. Seshadri, J.B. Gnanamoorthy, Continuous cooling and low temperature sensitization of AISI types 316 SS and 304 SS with different degrees of cold work, J. Nucl. Mater. 168 (1–2) (1989) 83–96. [19] C. Garcia, F. Martin, P. De Tiedra, J.A. Heredero, M.L. Aparicio, Effect of Prior cold work and sensitization heat treatment on chloride stress corrosion cracking in type 304 stainless steels, Corrosion Sci. 43 (2001) 1519–1539. [20] Maria Domankova, Marek Peter, Moravcik Roman, The effect of cold work on the sensitization of austenitic stainless steels, Materiali in tehnologije (2007) 131 ISSN 1580-2949 MTAEC9 41(3). [21] Raghuvir Singh, Sandip Ghosh Chowdhury, B. Ravi Kumar, Swapan K. Das, P.K. De, Indranil Chattoraj, The importance of grain size relative to grain boundary character on the sensitization of metastable austenitic stainless steel, Scripta Materilia 57 (2007) 185–188. [22] B Gideon, L P Ward: Intergranular corrosion and residual stress determination of a duplex stainless steel pipe line Girth weld. (In) Proc (2008) Corrosion & Prevention, paper 110. [23] S Rahimi, D L Engelberg, T J Marrow: Characterisation of the sensitisation behaviour of thermo mechanically processed type 304 stainless steel using DL-EPR testing and image analysis methods. (2010) ISBN 978-80-90393-6-3.

13

Engineering Failure Analysis 108 (2020) 104337

N. Sreevidya, et al.

[24] M. Prohaska, T. Wernig, J. Perek, G. Mori, G. Tischler, R. Grill, Application of the DL-EPR method for detecting sensitization to intergranular corrosion in thermo mechanically rolled corrosion resistant alloys 316L, 825L and 926L, International Conference Corrosion and Material Protection, 2nd, 2010, pp. 1–8. [25] M. Bassiouni, L.P. Ward, R.K. Singh Raman, A.P.O. Mullane, B. Gideon, S. Bhargava, Studies on the degree of sensitization of welded 2507 super duplex stainless steel using a modified DL-EPR test procedure, 50th Annual Conference of the Australasian Corrosion Association, Corrosion and Prevention, (2010). [26] Anita Toppo, M.G. Pujar, N. Sreevidya, John Philip, Pitting and stress corrosion cracking studies on AISI type 316N stainless steel weldments, Defence Technol. 14 (3) (2018) 226–237. [27] R.K. Dayal, N. Parvathavarthini, J.B. Gnanmoorthy, P. Rodrig, Intergranular attack during crevice corrosion of stainless steel, Materials letters 2 (3) (1983).

14