Engineering Failure Analysis 18 (2011) 2211–2221

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Engineering Failure Analysis

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Sensitization induced stress corrosion failure of AISI 347 stainlesssteel fractionator furnace tubes

J. Swaminathan a,⇑, Raghuvir Singh b, Manoj Kumar Gunjan a, Bhupeshwar Mahato a

a Materials Science and Technology Division, National Metallurgical Laboratory, Jamshedpur 831 007, Indiab Applied Chemistry and Corrosion Division, National Metallurgical Laboratory, Jamshedpur 831 007, India

a r t i c l e i n f o

Article history:Received 14 April 2011Received in revised form 16 July 2011Accepted 19 July 2011Available online 1 September 2011

Keywords:Polythionic acid induced stress corrosioncrackingAISI 347 SSSensitizationFurnace tube failure

1350-6307/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.engfailanal.2011.07.015

⇑ Corresponding author. Address: National MetTel.: +91 657 2345195; fax: +91 657 2345213.

E-mail address: jsn@nmlindia.org (J. Swaminatha

a b s t r a c t

Austenitic stainless steel tubes are used as furnace tubes in petrochemical industries mainlybecause of their corrosion resistance and mechanical strength. AISI 347 grade stainless steelis used as furnace heater tubes in the fractionator of hydrocracker unit. Even though thisstainless steel is stabilized with the addition of niobium thus preventing sensitizationrelated corrosion failures, operational and maintenance errors may result in prematurefailures if conditions prevail. The present work reports the premature failure of AISI 347grade fractionator furnace tubes after nearly 8 years of service. The failure occurred aftershutdown. Carbonaceous deposits were found on the inner walls of the tube and circumfer-ential cracks were found beneath the deposit. The service exposed 347 SS alloy tube was inthe sensitized condition as confirmed by microstructure and double loop electrochemicalpotentiodynamic reactivation test. The tube material got sensitized possibly by localizedoverheating at the carbon layer deposited site. During shutdown of hydrocracker unitpolythionic acid formation occurred possibly due to errors in shutdown procedures. Sensi-tized alloy 347 tube undergone polythionic acid induced intergranular stress corrosioncracking (PASCC).

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Austenitic stainless steels (ASSs) though corrosion resistant, undergo corrosion in presence of certain specific corrosiveagents and conditions encountered during service leading premature component failures. When exposed in the critical tem-perature range of 500–800 �C for a reasonable time, formation of chromium carbides at grain boundaries and depletion ofchromium adjacent to it occurs, a phenomena known as sensitization [1–4]. Even shorter exposure in the critical tempera-ture range is enough to nucleate carbides without a damaging degree of Cr depletion. The nucleated carbides may grow dur-ing service exposure at temperatures below 500 �C leading to severe Cr depletion, a phenomenon identified as lowtemperature sensitization (LTS) [5]. Sensitized grain boundaries in stainless steels are more prone to intergranular corrosionattack (IGC). Stressed and sensitized materials are prone to intergranular stress corrosion cracking (IGSCC) [1–4,6]. SCC oc-curs if the material is in susceptible condition, subjected to tensile stress and aggressive environment [1–3]. Intergranularstress corrosion cracking (IGSCC) of austenitic stainless steel in petrochemical components are well recorded [6–8]. Stabi-lized stainless steel (SS) grades are also prone to IGSCC once they become sensitized and environmental conditions favorit [9,10]. Reports on sensitization induced SCC failures of stabilized grades of ASS material are very few [7,13]. The presentpaper discusses one such failure happened in a petrochemical processing unit.

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allurgical Laboratory, Council of Scientific and Industrial Research, Jamshedpur 831 007, India.

n).

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2. Technical details of furnace tube

The furnace tube belonged to the convection zone of fractionator in hydrocracker unit. The tube was used to heat the frac-tionator feed. The furnace was single cabin box type. The important details about the furnace tube are as given below:

Figclo

Tube type

. 1. Photographs of failed tube section showing (a) cracksse-up view of the removed porous and cracked deposit lay

Studded

Tube material A 312 TP 347 (AISI 347) Stud material Carbon steel Tube outer diameter 168.3 mm Tube wall thickness 7.11 (average) Corrosion allowance 1.8 mm Design pressure 13 kg/cm2

Design temperature

512 �C Location of failed tube Intermediate Skin temperature reported 480 �C (top row) and 450 �C (bottom row) Service life 68,298 h (total stream hours) i.e. 7.8 years (approx.)

About 1 m length of the failed portion of the tube with failure cracks was received for investigation. The component tubespecifications did not specify stabilization treatment prior to service. It was reported that the failure was detected duringshutdown taken to rectify a plant problem elsewhere.

3. Visual examination

Photographs of the failed tube sent to investigation are shown in Fig. 1a–b. The studs around the cracks were found to begrounded off for dye penetrant test. The studded tube had two numbers of clearly visible cracks on the tube base metal por-tion lying between the studs. The cracks were appeared to have stopped near the welded stud on both side of length of thecrack. The cracks were in the transverse direction i.e. perpendicular to the tube axis. The two cracks were measured to be20 mm and 18 mm in length. No other visible cracks were seen on the surface. The tube did not show bending or any otherdistortions. The studs were oxidized but no thicker scales were observed on it and shape of the studs was intact. The tubebase metal outer surface between the studs did not show heavier scale buildup or deposits.

The inspection of the inner side of the tube revealed presence of an adherent layer of porous, cracked, black colored de-posit mostly on one side of the tube covering about 5 in. width and along the entire length of supplied tube piece. Failure

on the outer surface marked by arrows, (b) deposit on inner wall marked by arrow and (c)er.

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cracks were actually below this layer as could be made out visually. At the opposite half to the cracked location of the tube noadherent deposit was found. The hard and brittle deposit was not sticking firmly on tube metal surface. Cracks were actuallybehind these deposit covered portion. Visual examination of ring piece cut from the cracked location revealed that the crackswere visible on the inner side too i.e. through thickness penetration. The deposit layer was not having high adherence as itcould be removed by sharp metallic tool. No deep pits or other cracks were visible to the naked eye. The wall thickness exam-ination revealed no significant wall thinning and the observed values are very well with in the allowance for corrosion andnormally expected for the expended service life of 7.8 years.

4. X-ray diffraction analysis (XRD)

The internal deposit layer was removed from the tube and analyzed along with base metal specimen using – X-raydiffraction method. Observed XRD plot for the deposit is shown in Fig. 2. From the strong 2h peaks and corresponding ‘‘d’’values the deposit was identified as graphite and sulfur oxide compounds. The XRD pattern for the tube material is givenFig. 3.

For the tube material, major peaks obtained were that of austenite. No significant peak of other phases like sigma phasewas observed.

5. Chemical analysis

A ring piece was cut from the tube end and tube material specimen was taken for chemical analysis by conventionalchemical analysis method. Carbon and sulfur were analyzed by Carbon–Sulfur analyzer. Since failure was observed onlyin the austenitic tube material, the chemical composition of the carbon steel stud was not analyzed. Observed results (Table1) confirms that the base metal is a niobium (Nb) stabilized austenitic steel. However the carbon (C) content is slightly higherand niobium content is slightly lower than the limits specified for the AISI 347 steel. [14].

The inner wall deposit was also chemically analyzed and identified as carbonaceous deposit with carbon content 96.5%(wt.%). This is in conjunction with the XRD result for the deposit.

6. Hardness

Transverse sections of tube were cut from the failed location, 180� opposite to the failed crack and well away from thefailed location. The stud material specimen was also checked. Vickers micro hardness testing was done using Leitz microhardness tester. A load of 50 gm was used. Hardness was measured starting from the inner wall to outer wall at intervalsof 1 mm for the base metal and for the stud material from the welded end to free end. Observed results are presented inTable 2. The hardness value did not indicate any significant variation from inner wall to outer wall region for the base metaland the values are typical of material grade specified in cold rolled condition. The carbon steel stud material also showedhardness value typical of its specification and no significant variation observed along the specimen length.

Fig. 2. XRD pattern of the internal deposit layer.

Fig. 3. XRD pattern of base metal of the failed tube.

Table 1Chemical composition of the failed tube base metal (wt.%).

Element Specified (ASTM A 240 and ASME SA-240) Observed

C 60.08 0.1S 0.030 0.008P 0.045 0.013Mn 62.00 1.87Si 60.75 0.54Ni 9–12 9.45Cr 17–19 17.45Nb 1.0 P Nb P10�% C 0.50Fe Bal. Bal.

Table 2Vickers microhardness test results.

Specimen detail Vickers microhardness

1 2 3 4 5

Near the failed crack 221 218 230 226 224180� opposite to the failure crack 234 230 235 229 236Far away from the failed crack 235 240 237 230 241Stud material 140 154 144 149 146

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7. Fractography

Strip section was cut longitudinally from the failed crack portion of the tube. Care was taken so as to locate the observedcrack in the centre of the 120 mm length � 50 mm wide strip. The carbon steel studs were removed cleanly using a slowspeed saw cutter. This strip was gripped in a tensile testing machine and pulled apart to open the crack fully enabling toexamine the fracture surface of the crack for fractography using scanning electron microscope (SEM) fitted with energy dis-persive X-ray analyzer (EDX). SEM fractographs of the crack are presented in Fig. 4. The EDX plot obtained from the markedlocations on the fracture face was given in Fig. 5.

Observed fractographs clearly reveal intergranular cracking and corroded nature of grain boundaries. Corrosion was ob-served to be comparatively more near the inner wall side than on the outer wall side. Intergranular cracking was observedthrough out the fracture face. Microcracks were clearly visible on the fractured surface (Fig. 4a and c). On the fracture surfacenear the inner wall surface (Fig. 4a), EDX spectra from the marked spot show (Fig. 5a) presence of sulfur, oxygen and carbon,conforming corroded nature of the cracked region. At a place 1 mm away from the inner wall surface, but still showing cor-roded features (corresponds to Fig. 4c) the EDX spectra show similar presence of the elements. But, content of sulfur was lesscomparing to the spot from the fracture surface very near the inner wall.

Fig. 4. Fractographs of opened crack (a) very near inner wall, (b) at mid thickness and (c) 1 mm away from inner wall. EDX analysis of the spot marked aregiven in Table 3.

Fig. 5. Inner wall surface of the failed tube in the un-etched condition showing multiple cracks in the transverse direction of the tube.

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8. Metallography

Metallographic specimens were cut from the tube material after removal of the carbon steel studs on the surface. Trans-verse and longitudinal sections of the tube near the failure crack were cut and polished following standard metallographicprocedures. Optical microscope and scanning electron microscope (SEM) were used to analyze the microstructures. EDXspectra from selective spots were also taken as part of analysis.

Specimens were at first analyzed in un-etched condition. While only two cracks have penetrated to the outer wall side ofthe tube, inner wall surface showed multiple cracks (Fig. 5) with crack branching. Fine cracks were appeared to branch out ofthe main crack though not extensive. Longitudinal section of the tube inner surface at location near the failure crack, in un-etched condition showed (Fig. 6) presence of pits. More number of cracks was observed to emanate from the bottom of pitand penetrating the tube in the thickness direction. The inner wall surface near the failure crack was observed to be corrodedbut the pits were sporadic and isolated. The penetrating cracks were found filled with corrosion products.

All the polished specimens were then electrolytically etched with 10% oxalic acid solution using stainless steel cathodeand current 1 Amp/sq cm as per ASTM A 262 practice A [15] a standard procedure to confirm the sensitization.

Optical micrographs of the failed tube specimens were shown in Figs. 7 and 8. It was clearly revealed that the tube mate-rial was sensitized as marked by the ditch like grain boundary structure after etching according to ASTM A 262 Practice A[15]. The inner wall had corrosion induced pits and these pit bottom had corrosion induced intergranular cracks penetratinginto the alloy interior along the sensitized grain boundaries. Multiple cracks were seen on the inner wall surface of the tube.The cracks have grown only in the circumferential direction of the tube. Sensitized nature of grain boundaries was noticedthrough out the tube section irrespective of its proximity to the failure crack. On the outerwall no such pits with cracks wereobserved. The weld interface between stud and austentinitic tube material did not show any cracking. No cracks were foundon the tube material near the stud weld. The carbon steel stud showed (Fig. 9) ferrite–pearlite structure. The pearlitic regionswere degraded to a significant extent as revealed by the spherodization of the pearlitic regions. This indicates that the studand tube material would have experienced temperatures above the specified service temperature of 480 �C. The observedmicrostructure of the carbon steel also indicate that the carbon steel stud might not have experienced a level of higher tem-perature above A1 which could have dissolved the pearlitic regions completely.

SEM micrographs of the etched specimen (longitudinal section) were presented in Fig. 10. Both secondary electron (SE)and back scattered electron (BSE) mode were used.

The corrosion pitted nature of the innerwall intergranular cracking was well reveled (Fig. 10a) and sensitized grainboundaries are clearly distinguished in the BSE micrograph (Fig. 10b). The EDX spectra taken at the bottom of the pit (spot1 in Fig. 10a) and at cracked grain boundary (spot 2 in Fig. 10a) showed significant presence of carbon, oxygen and sulfur(Table 4). These results are in agreement with the EDX analysis of the fracture surface shown in Table 3.

9. Double loop electrochemical potentiodynamic reactivation (DLEPR) test

Severity of sensitization can be effectively evaluated through the electrochemical method of double loop electrochemicalpotentiodynamic reactivation test [11]. Specimens cut from the failed furnace tubes in the service exposed condition andservice exposed + solution annealed (1100 �C for 1 h) condition were subjected to DLEPR test for quantification of the sen-sitization in terms of ‘degree of sensitization’ (DOS). DL-EPR test was performed at ambient temperature using a test solutioncontaining 0.5M H2SO4 and 0.01M KSCN in distilled water. Specimens were anodically polarized up to +300 mV (with respectto saturated calomel electrode, SCE) and subsequently reversed at a scan rate of 6 V/h. Graphite electrode was used as acounter electrode. The % DOS was evaluated by measuring the ratio of (Ir/Ia) � 100, where Ir is the peak reactivation currentdensity and Ia is the peak activation current density. The DL-EPR experiments on each specimen set were repeated in freshsolution and average values are reported.

Fig. 6. Longitudinal section of tube inner wall side of specimen (un-etched) showing corrosion pits with penetration of intergranular cracks in the tubetransverse direction.

Fig. 7. Optical micrographs of failed tube specimen under etched condition. Inner wall surface view showing (a) intergranular cracking, (b) corrosion filledcrack propagation in the transverse direction and (c) sensitized nature of the grain boundaries marked by the ditch structure.

Fig. 8. Optical micrographs of the failed tube specimen (a) longitudinal section showing corrosion induced pits on inner wall and intergranular crackpenetration at the bottom of pit and (b) sensitized nature of the grain boundaries near the pit marked by ditch structure.

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The service exposed AISI 347 SS sample showed DOS value �44.46 while solution annealed showed it to be �0.75. TheDLEPR curves for the two different conditions are compared in Fig. 11. It is clearly shown that reactivation current density forservice exposed specimen is much larger compared to solution annealed specimen of the same AISI 347 SS tube, indicatingthe severity of sensitization. The qualitative difference in corrosion along the grain boundary for the test samples can be seenfrom the post EPR microstructure shown in Fig. 12. Extensive corrosion occurred along the chromium depleted zone of asservice exposed condition specimen compared to that in the post service solution annealed condition. Such a high DOS valuecould cause the failure of the stainless steels tube by intergranular stress corrosion cracking.

Fig. 9. Optical micrograph of the longitudinal section of stud material welded on to the outer surface of the failed tube.

Fig. 10. Electron micrographs of longitudinal section of failed tube showing innerwall pit and intergranular cracks originating from pits (a) secondaryelectron mode and (b) back scattered electron mode. (EDX analysis results given in Table 4.)

Table 3EDX analysis of spots shown in Fig. 4.

Element wt.%

Spot 1 Spot 2

C 24.28 25.52O 7.73 16.65S 13.64 1.22Si – 0.42Nb – 0.51Cr 12.87 3.40Fe 32.99 52.27Ni 8.48 –

Total 100.00 100.00

Table 4EDX analysis of spots shown in Fig. 10.

Element wt.%

Spot 1 Spot 2

C 45.98 2.40O 16.50 18.42S 3.68 3.25Si – 1.42Cr 20.61 47.65Fe 13.23 26.86

Total 100.00 100.00

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-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.000001 0.00001 0.0001 0.001 0.01 0.1 1

Pote

ntia

l V (S

CE)

Current density, A/cm2

As service exposed

Service exposed + solution annealed - 11000C/1h

Fig. 11. DLEPR plots of AISI 347 specimens tested under as service exposed condition and service exposed + solution annealed conditions.

Fig. 12. Optical photomicrographs of AISI 347 specimens after DLEPR testing (a) as service exposed and (b) service exposed + solution annealed condition.

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10. Discussion

The chromium depleted region near the grain boundaries are prone to preferential corrosion attack under certain condi-tions of environment. A combination of corrodent species and alloy type must exist for SCC to occur [1–4]. Stresses may beeither residual, introduced through fabrication or arise during operation. If a pit or other sharp notch is present, the residualstress is multiplied several times resulting in a stress far in excess of the tensile yield strength. Thus, SCC usually starts withpitting or crevice corrosion as a precursor to forming a stress concentrator.

Possibility of sensitization in ASS material occurs while in service through unwarranted temperature excursions into thesensitizing temperature range 500–815 �C. Sensitization may also occurs if the component material experienced carbide pre-cipitation prior to or during its fabrication stages subjecting the material into the sensitization range due to slow cooling.Any short term exposure in the critical range followed by service temperature range lower to the level at which stabilizationis not effective and at the same time sensitization cannot be avoided, may lead to sensitization [5]. Stabilized grades of AISI321 SS (Ti added) and AISI 347 SS (Nb added) were observed to get sensitized up on long term exposure in the temperaturerange 500–800 �C and for AIS 347 SS grade a narrow temperature range of 500–600 �C and time more than 10,000 h wasfound to sensitize the alloy [12]. Amount of carbon and carbide forming elements in the base metal are among the vital fac-tors affecting sensitization induced SCC [4]. It is to be noted that in the present case of AISI 347 tube material, the carboncontent was slightly higher and the Nb content slightly less.

Chlorides, polythionic acids and caustics are known to cause SCC of austenitic stainless steels [4]. The problem of polyth-ionic acid intergranular SCC (PASCC) is widespread in the oil refining industry, with special reference to desulfurizer, hydro-cracker and reformer processes [13]. PASCC is usually an internal problem, occurring on the process-exposed side of a heatertube, vessel or piping. PASCC cannot be monitored practically because the cracking may not occur until well into a turn-around. Conditions causing the cracking are not usually present while operating the component. During shutdown, on cool-ing to room temperature in a sulfide containing environment that exist in the refining operation, the sulfide (often hydrogensulfide) reacts with moisture and oxygen to form polythionic acids which attack the sensitized grain boundaries. Underconditions of stress, intergranular cracks form. Proper stabilization heat treatment is essential for AISI 347 SS to avoid stress

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corrosion cracking. If the stabilization heat treatment or solution annealing temperature is too high, PASCC susceptibility willbe higher for ASS alloys [16,17].

The salient points of observations to be noted from the current failure investigation are as follows:

� Chemical composition of the failed tube material did not correspond to the specifications exactly. The carbon content isslightly higher (>0.08%) and niobium content is less than the amount 10�% C specified by the standard for AISI 347 ASS.� Only the ASS tube material showed cracking. Carbon steel stud – ASS tube material weld did not show any cracks.� Carbon steel stud microstructure indicated that the component had experienced service temperature exposure above the

specified level of 480 �C.� The ASS tube material was in a sensitized condition.� Failure occurred during shutdown.� Failure cracks appeared on the internal side of the tube portion covered with the porous and cracked layer of carbon

deposit.� The cracks were observed to be grown in the circumferential direction and penetrated through thickness.� The cracks are brittle and intergranular.� Pitting was observed in the region covered by the deposit on the inner wall of the tube. The intergranular cracks were

actually originated from the inner wall side and from the pit head.� General corrosion attack on the inner surface or loss in thickness is negligible.� Sulfur rich corrosion products identified inside the pit and on the fracture surface.� DLEPR tests on the as received tube materials and service exposed + solution annealed (1100 �C) confirmed the sensitized

nature of the service exposed tubes.

Sensitization in the present case of stabilized grade of AISI 347 SS might have occured due to one or a combination ofthe following facts: (i) presence of carbon deposits on the tube internal wall might have disturbed the heat flow condi-tions leading to localized over heating of the component materials thus exposing the base material in the sensitizationrange, (ii) the tube base material might have been exposed to the sensitization conditions before or during fabricationoperations and (iii) though not recorded, possibility of service temperature excursions above the operating level of480 �C during operation or at the time of shutdown even for a shorter time, a common occurrence in refinery operations,resulting in exposure to sensitization range for shorter time and subsequent growth of Cr – carbides during normal ser-vice. Had above such thing happened, service expended time of 7.8 years is fairly enough to time cause sensitization of asignificant degree. The higher amount of carbon, lower amount of niobium/titanium in the base material, absence of astabilizing treatment prior to service are all conducive to favor sensitization during service operation of SS components.

In the present case, sensitized base material had encountered polythionic acid attack during shutdown was supported bythe facts that cracking occurred during shutdown and sulfur compounds were observed at the corrosion pits on the tubeinner wall. The porous and cracked inner wall deposit layer may easily facilitate entrapment of such corrodent. Pitting attackof the inner wall sensitized grain boundaries may lead to cracking under stress which always present in the component.

From the above facts, it is conclusively evident that present failure of the heater tube had occurred through sensitizationinduced polythionic acid intergranular stress corrosion cracking (PASCC).

Detailed instructions on protection against PASCC of austenitic SS material, based on refinery practice may be found in theNACE Standard RP-0170: ‘‘Protection of-Austenitic Stainless Steels in Refineries against Stress-Corrosion Cracking’’.

11. Conclusions

The furnace tube material of AISI 347 SS failed through IGSCC caused by polythionic acid. The material was in a sensitizedcondition possibly through localized overheating caused by the coke deposition. The sensitized alloy enhanced corrosionpolythionic acid formed during shutdown. The accompanied pits acted as stress multiplier and intergranular cracking inthe circumferential direction initiated and propagated through stresses present in the component.

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