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Cracking of radiant tube of heater coil carring heavy oil in a hydrocracking unit
Engineering Failure Analysis 31 (2013) 281–289
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Cracking of radiant tube of heater coil carring heavy oil in a hydrocracking unit H.M. Shalaby a,⇑, N. Al-Sebaii b, W.T. Riad a, P.K. Mukhopadhyay b a b
Petroleum Research & Studies Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, Kuwait Shuaiba Refinery, Kuwait National Petroleum Company, Kuwait
a r t i c l e
i n f o
Article history: Received 15 August 2012 Received in revised form 22 January 2013 Accepted 22 January 2013 Available online 9 February 2013 Keywords: Sulphide corrosion Carbide precipitation Heater tube Brittle failure
a b s t r a c t Failure investigation was made on a cracked radiant tube made of 9Cr–1Mo (wt.%) alloy steel (ASTM A 213-T9). The failed tube was removed from a heater coil that carried crude vacuum residue and on external fuel gas firing in a hydrocracking unit operating at high pressure. The failed tube had been in service for about 15 years. The tube visually exhibited cracks on the external surface in the longitudinal axis direction. Cross-sectional examinations showed a straight and un-branched crack that reaches up to 70% of the tube’s wall thickness. Examination of the internal surface along the cracked area showed a surface of wavy topography. The cracking of the tube was attributed to long-term service at high temperature in sulphur bearing crude. High temperature sulphidic corrosion at the internal tube surface is believed to have aided the dissociation of carbides, resulting in the diffusion of carbon towards the hot external surface where fresh carbides precipitated and accumulated during shut-downs. Areas of carbide accumulation exhibited high hardness and were brittle, facilitating cracking at the external surface during start up operations. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Failure investigation was made on a failed radiant tube section made of alloy steel 9Cr–1Mo (wt.%) (ASTM A 213-T9) [1], and, removed from a heater coil that carries heavy crude bottom oil and on external fuel gas firing in a hydrocracking unit operating at high pressure. The tube section is 68 cm in length, 11.43 cm in outside diameter and 1.00 cm in thickness. The radiant tube was in service for about 15 years and operated at a pressure of 2400–2800 psig. The skin temperature was 780– 850 °F (415–454 °C). The heater was constructed with vertical radiant tubes with a weld joint in the middle of each tube. During shut down, the heater’s tubes were subjected to decoking at temperature of approximately 1100 °F (593 °C) for 8–12 h. The investigation included optical and scanning electron microscopic (SEM) examinations and chemical analyses using energy dispersive X-ray spectroscopy (EDS) as well as microhardness measurements.
2. Visual observations Visual examination of the failed radiant tube before sectioning revealed the presence of an external crack (termed main crack in the present paper) in the longitudinal direction of the tube axis below the weld joint by about 1.2 m (Fig. 1a). In addition, the outside diameter in the cracked area had increased by 1.1 mm. ⇑ Corresponding author. Tel.: +965 23980499; fax: +965 23980445. E-mail address: [email protected] (H.M. Shalaby). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.01.019
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External surface
Internal surface
(a)
(b)
(c)
Fig. 1. Photographs showing: (a) crack at the external tube surface; (b) penetration of the main crack into the tube wall thickness and another narrow smaller crack (marked by the semi-circle); and (c) appearance of the tube internal surface.
Table 1 Chemical composition of failed radiant tube sample (in wt.%). Sample
% Cr
% Mo
% Mn
%C
%S
% Si
%P
Near fracture ASTM A213 T9
8.79 8.00–10.00
1.18 0.90–1.10
0.53 0.30–0.60
0.11 0.15
0.03 0.03
ND 0.05–1.00
ND 0.03
ND = not determined.
External surface External surface
(a)
(b)
Crack
Crack
(c)
(d)
Fig. 2. Optical micrographs of a polished tube cross-section, showing: (a) and (b) views of small cracks filled with scale or deposits and emanating from the external surface; and (c) and (d) crack segments close to the main crack.
Cross-sectional examinations of the failed tube section showed that the main crack was straight, un-branched and reached up to 70% of the tube’s wall thickness. The examination also showed another secondary short and narrow crack, marked by a semi-circle in Fig. 1b. Examination of the internal surface along the cracked area showed a surface of wavy topography and a well defined longitudinal groove near the main crack tip (Fig. 1c). 3. Results 3.1. Chemical analysis Chemical analysis using EDS was conducted on a piece cut from the tube section for determination of the weight percentages of Cr, Mo and Mn elements. Chips were machined from the tube section for determination of C and S by combustion
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Internal surface
(a)
(b)
(c)
Fig. 3. Optical micrographs of a polished tube cross-section, showing: (a) and (b) views of grooves and penetrations emanating from the internal surface; and (c) close up view of one of the grooves.
External surface
Near internal surface
Fig. 4. Optical micrographs of an etched tube cross-section showing transition in the microstructure of the material across the tube wall thickness near the cracked area (almost totally ferrite near internal surface and accumulation of carbides near external surface).
method. Table 1 shows the results of the chemical analysis along with the nominal chemical composition of heat-resistant chromium-molybdenum steel of 9Cr–1Mo type (ASTM specification A213-T9) [1]. The analysis shows that the chemical composition of the failed tube falls within the specification of ASTM A213-T9 [1].
284
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3.2. Optical microscopic examinations Microscopic examinations using optical microscopy were made on several cross-sectional specimens cut from the tube section. The specimens were cut from the areas containing the cracks as well as away from the cracked area. The examinations were made on the specimens after metallographic preparations. The preparations consisted of mounting followed by mechanical grinding and polishing using diamond pastes of up to 3 lm in size. The examinations were made on the specimens before and after etching. Etching was used to reveal the general microstructure. This was achieved using 2% nital etch followed by rinsing in distilled water and drying with hot air, then immersion in coloured etchant of sodium metabisulfite which was composed of 100 ml H2O and 12 g of Na2S2O5. In the polished condition, the optical microscopic examinations of the cross-sectional surfaces showed that the short cracks were filled with scale or deposits (Fig. 2a and b). In addition to these cracks, small segmental cracks were seen near the main crack surface, and within the bulk of the material (see Fig. 2c and d). The examinations did not show signs of corrosion attack initiating at the external surface and the surface was covered with a thin and adherent scale layer. Some small voids or cavities were seen at the surfaces of the polished tube crosssections.
External surface
Fig. 5. Optical micrographs of an etched tube cross-section showing that the crack started from a carbide-rich area at the external surface and accumulated carbides are present close to the crack extension.
Internal surface Fig. 6. Optical micrograph of an etched tube cross-section showing that the grooves emanating from the internal surface are present in ferrite areas depleted in carbides.
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285
Fig. 7. Microhardness values at different areas along the tube cross-section.
(a)
(b)
50
Counts x10 2
40 30 20 10 0
Energy (keV)
(c) Fig. 8. SEM fractographs (a and b) and EDS analysis (c) of the scale found deposited inside the main crack emanating from the external surface.
The optical microscopic examinations of the polished tube cross-section also revealed the presence of scale filled, deep penetrated grooves that were initiated from the internal tube surface (see Fig. 3a–c). It is worth mentioning that the number of grooves emanating from the internal surface was noticed to be much more in the area containing the cracks than away from it. Examinations of an etched cross section containing a crack revealed a change or transition in the microstructure across the tube wall thickness. Near the internal surface, the microstructure was mostly composed of ferrite grains, while accumulated black precipitates of carbides in a ferrite matrix were seen near the external surface (see Fig. 4).
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(a) 30
Counts x102
25 20 15 10 0
Energy (keV)
(b) Fig. 9. SEM micrograph (a) and EDS analysis (b) of the deposits found on the internal tube surface.
The examinations also revealed that the cracks initiated from the carbide-rich areas near the external surface. Moreover, accumulated carbides were also observed at the areas of propagating cracks (Fig. 5). On the other hand, the microstructure at areas surrounding the penetrated grooves was mainly composed of ferrite and little carbides at grain boundary (Fig. 6). 3.3. Microhardness measurements Microhardness measurements were conducted on the cross-sectional surface which contained the main crack. The measurements were made in Vickers in accordance with ASTM E 384-89 [2], using a test load of 4 N. Indentations were made at areas near the internal surface, external surface and around the crack (see Fig. 7). The hardness values are seen to be noticeably higher in the areas containing the accumulated carbides (dark areas) and around the crack when compared with the areas containing ferrite grains depleted in carbides (white areas). 3.4. SEM and EDS analysis The main crack emanating from the tube’s external surface was opened up for SEM examinations and EDS analysis of the scale seen deposited inside the crack. Fig. 8 shows two micrographs of the scale along with its EDS analysis. The analysis showed elements of Ca, Al, Mg and S, in addition to the alloying elements of the steel, namely: Fe, Cr, Si, Mn and C. It is worth mentioning that the specimen was immersed for a long time in Clark’s solution to dissolve the scale and to observe the fracture surface, but the scales were slightly removed. When the internal tube surface was examined using SEM, it was found covered with broken scale and deposits (see Fig. 9a). EDS analysis of the deposits showed elements of Si, Al, and Cl, in addition to the alloying elements of the steel, namely: Fe, Cr, Mn and C (Fig. 9b). It is worth mentioning that it is normal to have chloride in vacuum bottom, but it may not have contributed to the observed corrosion, as it does not hydrolyze to form HCl at such operating temperature. SEM examination of the metallographically prepared cross-sections showed carbide precipitates close to the external surface near a crack. Some of the precipitates appeared to exist at the grain boundaries (Fig. 10a). Also, scale was seen filling the grooves emanating from the internal surface (Fig. 10b). EDS analysis of the later scale revealed a strong sulphur peak in addition to the alloying elements of the tube material (see Fig. 10d and compare with Fig. 10c for the metal). This result suggests that the grooves and penetrations initiating from the internal surface were due to high temperature sulphidic corrosion. 4. Discussion The present investigation revealed the presence of smaller cracks emanating from the external tube surface in addition to the large size crack (see Figs. 1b and 2a and b). All these cracks penetrated the tube material in an almost straight manner up
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Near internal surface
External surface
b a
(a)
(b)
35
Counts x102
30 25 20 15 10 5 0
Energy (keV)
(c) 20
Counts x10 2
15
10
5
0
Energy (keV)
(d) Fig. 10. SEM micrographs showing carbide rich zone close to external surface (a), grooves seen penetrating the tube from the internal surface (b) and EDS analyses (c and d) of the metal (point a) and scale inside the grooves (point b), respectively.
to a depth of about 70% of the tube’s thickness. In addition to these cracks, some small segmental cracks (joined or individual) were seen within the tube material near the main crack (see Fig. 2c and d). These latter cracks appeared similar to stress rupture cracks. The cracks emanating from the external surface were observed to be filled with scale or deposits (see Fig. 5). These deposits were found to be rich in Ca, Al, Mg, and S in addition to the alloying elements of the steel, namely: Fe, Cr, Si, Mn, and C (see Fig. 8c). The analysis suggests that the scale is due to the fuel gas ashes [3]. The investigation also revealed the presence of deposits at the internal tube surface and grooves emanating from this surface (see Figs. 3 and 6). The observed grooves were found filled with scale. EDS analysis of the scale indicated that it was rich in sulphur in addition to the alloying elements of the tube (see Fig. 10d). Thus, the analysis suggested the occurrence of high temperature sulphidic corrosion that was penetrating the metal from the internal tube surface. This form of corrosion is possible in view of the high temperature operating conditions of the heater coil (415.5–454.4 °C) and the flow of heavy oil inside the tube, which is known to be sulphur-rich.
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An important observation in the present work is the change in the microstructure of the tube in the cracked area (see Fig. 4), where the microstructure was almost totally ferritic near the internal tube surface and contained extensive accumulation of black precipitates close to the external surface. This microstructure is unexpected as radiant tubes are usually made in the tempered conditions and the microstructure is usually tempered martensite. The black precipitates are believed to be carbides. The presence of carbides was indirectly confirmed by microhardness measurements conducted at the areas containing the black precipitates and the ferrite grains (see Fig. 7). The hardness values in the carbide-rich areas were in the range of 246–272 Hv, while they were in the range of 224–230 Hv in the white areas dominant in ferrite. The highest hardness values were noticed to exist in the areas surrounding the crack propagating from the external surface. These high hardness values indicate that the tube material is in a brittle condition. Cracks were noticed to initiate from the external surface where accumulation of carbides occurred (see Fig. 5). This in effect indicates that the crack initiated from the high heat flux side of the tube, suggesting that outward carbon diffusion associated with increased carbon diffusivity occurred as a result of the dissociation of the alloy microstructure. This further suggests that the crack initiated from a brittle zone. Sarojas et al. [4] indicated that 9Cr–1Mo steel in its commercially heat treated condition consists of a tempered martensitic microstructure with M2X and M23C6 precipitates. Exposure of the steel in this condition to high temperatures (550– 750 °C) for long time results in the dissociation of martensite, formation of subgrains of ferrite and evolution and growth of secondary carbides. In addition, Hucin´ska [5] indicated that ageing 9Cr–1Mo at 550 °C for 10,000 h causes the steel to suffer from temper embrittlement as a result of the formation of brittle Laves phase (Fe,Cr)2Mo at grain boundaries. On the other hand, Hucin´ska [6,7] also stated that the progress of high temperature sulphide corrosion can be followed by internal carburization of this steel due to a release of carbon from the sulphur-attacked carbides and diffusion of carbon into the steel interior ahead of the advancing sulphide corrosion front, accompanied by formation of new carbides, or growth of the existing M23C6 carbides. Grabke [8] and Schneider et al. [9] indicated that there is an antagonism between the two elements, sulphur and carbon. The presence of sulphur on the steel surface causes the activity of carbon in the steel to increase, the thermal stability of the carbides to decrease, and the carbon that is released from the carbides to diffuse inside the steel. In 9Cr–1Mo steel the carbon loss in M23C6 carbides causes their transformation into ferrite [10]. Diffusion of sulphur inside the steel results in a progressive failure of carbides situated at the front of the diffusion. The carbon profile in the steels differentiates, namely, a layer of low carbon content forms beneath the surface, followed by a zone of higher carbon content. In the decarburized layer chromium/iron sulphides form. The present investigation indicates that the root cause of the cracking of the 9Cr–1Mo heater tube observed at the external surface was due to long-term exposure at high temperature in sulphur-containing crude oil. The sulphur bearing crude caused high temperature sulphidic corrosion, which is known to take place during operation at temperatures 260–500 °C [6]. The flow of heavy crude or recent switching to heavy crude which is known to contain high levels of sulphur compounds may have accelerated the corrosion process. As indicated above, the occurrence of high temperature sulphidic corrosion was found to cause dissociation of carbides ahead of the advancing sulphidation front. In such a case, it is natural for the freed carbon to diffuse towards the hotter side of the tube which is the external surface and causes precipitation of fresh carbides during periods of shut-down. This results in the creation of carbide depleted areas of ferrite and areas rich in precipitated carbides, as was seen in the present areas. The areas containing accumulated carbides near the external surface are brittle and are prone to cracking, which can take place during start-up operation. 5. Conclusions High temperature sulphidic corrosion at the internal tube surface caused dissociation of carbides and diffusion of carbon towards the hot external surface where carbides precipitated and accumulated during shut down periods. Areas of carbide accumulation exhibited high hardness and were brittle. Thus, cracking was facilitated at the external surface most likely during start up operation.
Acknowledgements The authors would like to thank Dr. Nusrat Tanoli for her SEM and EDS work and Mr. Harish Gopal for his optical microscopy work. References [1] ASTM A 213. Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat exchanger tubes. Philadelphia Pennsylvania (USA): American Society for Testing and Materials; 2006. [2] ASTM E384. Standard test method for microhardness of materials. Philadelphia, Pennsylvania (USA): American Society for Testing and Materials; 1989. [3] Pipatmanomai S, Fungtammasan B, Bhattacharya S. Characteristics and composition of lignites and boiler ashes and their relation to slagging: the case of Mae Moh PCC boilers. Fuel 2008;88:116–23.
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[4] Sarojas S, Vijayalakshmim M, Raghunathanv VS. Effect of prolonged exposures of 9Cr–1Mo–0.07C steel to elevated temperatures. Materials transactions – JIM ISSN 0916-1821 1993; 34(10): 901–906. _ [5] Hucin´ska J. Effect of ageing on microstructure and impact properties of 9Cr–1Mo steel. Inzynieria Materiałowa 2007; (3–4): 157–158 [Cena: 6, 10 Zl]. [6] Hucin´ska J. The effect of elevated temperature refinery service on the chemistry of M23C6 carbides in 9Cr–1Mo steel. Mater Corros 2000;51(3):173–6. [7] Hucin´ska J. Carburization and metal dusting like attack of 9Cr–1Mo steel resulting from sulphide corrosion. Mater Corros 1999;50(5):267–72. [8] Grabke HJ. Metal dusting of low- and high-alloy steels. Corrosion 1995;51:711–20. [9] Schneider A, Viefhaus H, Inden G, Grabke HJ, Muler-Lorenz EM. Influence of H2S on metal dusting. Mater Corros 1998;49:336–9. [10] Chmidt HJ. Interstitial alloys. London: Butterworths; 1967.
Contents lists available at SciVerse ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Cracking of radiant tube of heater coil carring heavy oil in a hydrocracking unit H.M. Shalaby a,⇑, N. Al-Sebaii b, W.T. Riad a, P.K. Mukhopadhyay b a b
Petroleum Research & Studies Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, Kuwait Shuaiba Refinery, Kuwait National Petroleum Company, Kuwait
a r t i c l e
i n f o
Article history: Received 15 August 2012 Received in revised form 22 January 2013 Accepted 22 January 2013 Available online 9 February 2013 Keywords: Sulphide corrosion Carbide precipitation Heater tube Brittle failure
a b s t r a c t Failure investigation was made on a cracked radiant tube made of 9Cr–1Mo (wt.%) alloy steel (ASTM A 213-T9). The failed tube was removed from a heater coil that carried crude vacuum residue and on external fuel gas firing in a hydrocracking unit operating at high pressure. The failed tube had been in service for about 15 years. The tube visually exhibited cracks on the external surface in the longitudinal axis direction. Cross-sectional examinations showed a straight and un-branched crack that reaches up to 70% of the tube’s wall thickness. Examination of the internal surface along the cracked area showed a surface of wavy topography. The cracking of the tube was attributed to long-term service at high temperature in sulphur bearing crude. High temperature sulphidic corrosion at the internal tube surface is believed to have aided the dissociation of carbides, resulting in the diffusion of carbon towards the hot external surface where fresh carbides precipitated and accumulated during shut-downs. Areas of carbide accumulation exhibited high hardness and were brittle, facilitating cracking at the external surface during start up operations. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Failure investigation was made on a failed radiant tube section made of alloy steel 9Cr–1Mo (wt.%) (ASTM A 213-T9) [1], and, removed from a heater coil that carries heavy crude bottom oil and on external fuel gas firing in a hydrocracking unit operating at high pressure. The tube section is 68 cm in length, 11.43 cm in outside diameter and 1.00 cm in thickness. The radiant tube was in service for about 15 years and operated at a pressure of 2400–2800 psig. The skin temperature was 780– 850 °F (415–454 °C). The heater was constructed with vertical radiant tubes with a weld joint in the middle of each tube. During shut down, the heater’s tubes were subjected to decoking at temperature of approximately 1100 °F (593 °C) for 8–12 h. The investigation included optical and scanning electron microscopic (SEM) examinations and chemical analyses using energy dispersive X-ray spectroscopy (EDS) as well as microhardness measurements.
2. Visual observations Visual examination of the failed radiant tube before sectioning revealed the presence of an external crack (termed main crack in the present paper) in the longitudinal direction of the tube axis below the weld joint by about 1.2 m (Fig. 1a). In addition, the outside diameter in the cracked area had increased by 1.1 mm. ⇑ Corresponding author. Tel.: +965 23980499; fax: +965 23980445. E-mail address: [email protected] (H.M. Shalaby). 1350-6307/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engfailanal.2013.01.019
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External surface
Internal surface
(a)
(b)
(c)
Fig. 1. Photographs showing: (a) crack at the external tube surface; (b) penetration of the main crack into the tube wall thickness and another narrow smaller crack (marked by the semi-circle); and (c) appearance of the tube internal surface.
Table 1 Chemical composition of failed radiant tube sample (in wt.%). Sample
% Cr
% Mo
% Mn
%C
%S
% Si
%P
Near fracture ASTM A213 T9
8.79 8.00–10.00
1.18 0.90–1.10
0.53 0.30–0.60
0.11 0.15
0.03 0.03
ND 0.05–1.00
ND 0.03
ND = not determined.
External surface External surface
(a)
(b)
Crack
Crack
(c)
(d)
Fig. 2. Optical micrographs of a polished tube cross-section, showing: (a) and (b) views of small cracks filled with scale or deposits and emanating from the external surface; and (c) and (d) crack segments close to the main crack.
Cross-sectional examinations of the failed tube section showed that the main crack was straight, un-branched and reached up to 70% of the tube’s wall thickness. The examination also showed another secondary short and narrow crack, marked by a semi-circle in Fig. 1b. Examination of the internal surface along the cracked area showed a surface of wavy topography and a well defined longitudinal groove near the main crack tip (Fig. 1c). 3. Results 3.1. Chemical analysis Chemical analysis using EDS was conducted on a piece cut from the tube section for determination of the weight percentages of Cr, Mo and Mn elements. Chips were machined from the tube section for determination of C and S by combustion
283
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Internal surface
(a)
(b)
(c)
Fig. 3. Optical micrographs of a polished tube cross-section, showing: (a) and (b) views of grooves and penetrations emanating from the internal surface; and (c) close up view of one of the grooves.
External surface
Near internal surface
Fig. 4. Optical micrographs of an etched tube cross-section showing transition in the microstructure of the material across the tube wall thickness near the cracked area (almost totally ferrite near internal surface and accumulation of carbides near external surface).
method. Table 1 shows the results of the chemical analysis along with the nominal chemical composition of heat-resistant chromium-molybdenum steel of 9Cr–1Mo type (ASTM specification A213-T9) [1]. The analysis shows that the chemical composition of the failed tube falls within the specification of ASTM A213-T9 [1].
284
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3.2. Optical microscopic examinations Microscopic examinations using optical microscopy were made on several cross-sectional specimens cut from the tube section. The specimens were cut from the areas containing the cracks as well as away from the cracked area. The examinations were made on the specimens after metallographic preparations. The preparations consisted of mounting followed by mechanical grinding and polishing using diamond pastes of up to 3 lm in size. The examinations were made on the specimens before and after etching. Etching was used to reveal the general microstructure. This was achieved using 2% nital etch followed by rinsing in distilled water and drying with hot air, then immersion in coloured etchant of sodium metabisulfite which was composed of 100 ml H2O and 12 g of Na2S2O5. In the polished condition, the optical microscopic examinations of the cross-sectional surfaces showed that the short cracks were filled with scale or deposits (Fig. 2a and b). In addition to these cracks, small segmental cracks were seen near the main crack surface, and within the bulk of the material (see Fig. 2c and d). The examinations did not show signs of corrosion attack initiating at the external surface and the surface was covered with a thin and adherent scale layer. Some small voids or cavities were seen at the surfaces of the polished tube crosssections.
External surface
Fig. 5. Optical micrographs of an etched tube cross-section showing that the crack started from a carbide-rich area at the external surface and accumulated carbides are present close to the crack extension.
Internal surface Fig. 6. Optical micrograph of an etched tube cross-section showing that the grooves emanating from the internal surface are present in ferrite areas depleted in carbides.
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285
Fig. 7. Microhardness values at different areas along the tube cross-section.
(a)
(b)
50
Counts x10 2
40 30 20 10 0
Energy (keV)
(c) Fig. 8. SEM fractographs (a and b) and EDS analysis (c) of the scale found deposited inside the main crack emanating from the external surface.
The optical microscopic examinations of the polished tube cross-section also revealed the presence of scale filled, deep penetrated grooves that were initiated from the internal tube surface (see Fig. 3a–c). It is worth mentioning that the number of grooves emanating from the internal surface was noticed to be much more in the area containing the cracks than away from it. Examinations of an etched cross section containing a crack revealed a change or transition in the microstructure across the tube wall thickness. Near the internal surface, the microstructure was mostly composed of ferrite grains, while accumulated black precipitates of carbides in a ferrite matrix were seen near the external surface (see Fig. 4).
286
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(a) 30
Counts x102
25 20 15 10 0
Energy (keV)
(b) Fig. 9. SEM micrograph (a) and EDS analysis (b) of the deposits found on the internal tube surface.
The examinations also revealed that the cracks initiated from the carbide-rich areas near the external surface. Moreover, accumulated carbides were also observed at the areas of propagating cracks (Fig. 5). On the other hand, the microstructure at areas surrounding the penetrated grooves was mainly composed of ferrite and little carbides at grain boundary (Fig. 6). 3.3. Microhardness measurements Microhardness measurements were conducted on the cross-sectional surface which contained the main crack. The measurements were made in Vickers in accordance with ASTM E 384-89 [2], using a test load of 4 N. Indentations were made at areas near the internal surface, external surface and around the crack (see Fig. 7). The hardness values are seen to be noticeably higher in the areas containing the accumulated carbides (dark areas) and around the crack when compared with the areas containing ferrite grains depleted in carbides (white areas). 3.4. SEM and EDS analysis The main crack emanating from the tube’s external surface was opened up for SEM examinations and EDS analysis of the scale seen deposited inside the crack. Fig. 8 shows two micrographs of the scale along with its EDS analysis. The analysis showed elements of Ca, Al, Mg and S, in addition to the alloying elements of the steel, namely: Fe, Cr, Si, Mn and C. It is worth mentioning that the specimen was immersed for a long time in Clark’s solution to dissolve the scale and to observe the fracture surface, but the scales were slightly removed. When the internal tube surface was examined using SEM, it was found covered with broken scale and deposits (see Fig. 9a). EDS analysis of the deposits showed elements of Si, Al, and Cl, in addition to the alloying elements of the steel, namely: Fe, Cr, Mn and C (Fig. 9b). It is worth mentioning that it is normal to have chloride in vacuum bottom, but it may not have contributed to the observed corrosion, as it does not hydrolyze to form HCl at such operating temperature. SEM examination of the metallographically prepared cross-sections showed carbide precipitates close to the external surface near a crack. Some of the precipitates appeared to exist at the grain boundaries (Fig. 10a). Also, scale was seen filling the grooves emanating from the internal surface (Fig. 10b). EDS analysis of the later scale revealed a strong sulphur peak in addition to the alloying elements of the tube material (see Fig. 10d and compare with Fig. 10c for the metal). This result suggests that the grooves and penetrations initiating from the internal surface were due to high temperature sulphidic corrosion. 4. Discussion The present investigation revealed the presence of smaller cracks emanating from the external tube surface in addition to the large size crack (see Figs. 1b and 2a and b). All these cracks penetrated the tube material in an almost straight manner up
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287
Near internal surface
External surface
b a
(a)
(b)
35
Counts x102
30 25 20 15 10 5 0
Energy (keV)
(c) 20
Counts x10 2
15
10
5
0
Energy (keV)
(d) Fig. 10. SEM micrographs showing carbide rich zone close to external surface (a), grooves seen penetrating the tube from the internal surface (b) and EDS analyses (c and d) of the metal (point a) and scale inside the grooves (point b), respectively.
to a depth of about 70% of the tube’s thickness. In addition to these cracks, some small segmental cracks (joined or individual) were seen within the tube material near the main crack (see Fig. 2c and d). These latter cracks appeared similar to stress rupture cracks. The cracks emanating from the external surface were observed to be filled with scale or deposits (see Fig. 5). These deposits were found to be rich in Ca, Al, Mg, and S in addition to the alloying elements of the steel, namely: Fe, Cr, Si, Mn, and C (see Fig. 8c). The analysis suggests that the scale is due to the fuel gas ashes [3]. The investigation also revealed the presence of deposits at the internal tube surface and grooves emanating from this surface (see Figs. 3 and 6). The observed grooves were found filled with scale. EDS analysis of the scale indicated that it was rich in sulphur in addition to the alloying elements of the tube (see Fig. 10d). Thus, the analysis suggested the occurrence of high temperature sulphidic corrosion that was penetrating the metal from the internal tube surface. This form of corrosion is possible in view of the high temperature operating conditions of the heater coil (415.5–454.4 °C) and the flow of heavy oil inside the tube, which is known to be sulphur-rich.
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An important observation in the present work is the change in the microstructure of the tube in the cracked area (see Fig. 4), where the microstructure was almost totally ferritic near the internal tube surface and contained extensive accumulation of black precipitates close to the external surface. This microstructure is unexpected as radiant tubes are usually made in the tempered conditions and the microstructure is usually tempered martensite. The black precipitates are believed to be carbides. The presence of carbides was indirectly confirmed by microhardness measurements conducted at the areas containing the black precipitates and the ferrite grains (see Fig. 7). The hardness values in the carbide-rich areas were in the range of 246–272 Hv, while they were in the range of 224–230 Hv in the white areas dominant in ferrite. The highest hardness values were noticed to exist in the areas surrounding the crack propagating from the external surface. These high hardness values indicate that the tube material is in a brittle condition. Cracks were noticed to initiate from the external surface where accumulation of carbides occurred (see Fig. 5). This in effect indicates that the crack initiated from the high heat flux side of the tube, suggesting that outward carbon diffusion associated with increased carbon diffusivity occurred as a result of the dissociation of the alloy microstructure. This further suggests that the crack initiated from a brittle zone. Sarojas et al. [4] indicated that 9Cr–1Mo steel in its commercially heat treated condition consists of a tempered martensitic microstructure with M2X and M23C6 precipitates. Exposure of the steel in this condition to high temperatures (550– 750 °C) for long time results in the dissociation of martensite, formation of subgrains of ferrite and evolution and growth of secondary carbides. In addition, Hucin´ska [5] indicated that ageing 9Cr–1Mo at 550 °C for 10,000 h causes the steel to suffer from temper embrittlement as a result of the formation of brittle Laves phase (Fe,Cr)2Mo at grain boundaries. On the other hand, Hucin´ska [6,7] also stated that the progress of high temperature sulphide corrosion can be followed by internal carburization of this steel due to a release of carbon from the sulphur-attacked carbides and diffusion of carbon into the steel interior ahead of the advancing sulphide corrosion front, accompanied by formation of new carbides, or growth of the existing M23C6 carbides. Grabke [8] and Schneider et al. [9] indicated that there is an antagonism between the two elements, sulphur and carbon. The presence of sulphur on the steel surface causes the activity of carbon in the steel to increase, the thermal stability of the carbides to decrease, and the carbon that is released from the carbides to diffuse inside the steel. In 9Cr–1Mo steel the carbon loss in M23C6 carbides causes their transformation into ferrite [10]. Diffusion of sulphur inside the steel results in a progressive failure of carbides situated at the front of the diffusion. The carbon profile in the steels differentiates, namely, a layer of low carbon content forms beneath the surface, followed by a zone of higher carbon content. In the decarburized layer chromium/iron sulphides form. The present investigation indicates that the root cause of the cracking of the 9Cr–1Mo heater tube observed at the external surface was due to long-term exposure at high temperature in sulphur-containing crude oil. The sulphur bearing crude caused high temperature sulphidic corrosion, which is known to take place during operation at temperatures 260–500 °C [6]. The flow of heavy crude or recent switching to heavy crude which is known to contain high levels of sulphur compounds may have accelerated the corrosion process. As indicated above, the occurrence of high temperature sulphidic corrosion was found to cause dissociation of carbides ahead of the advancing sulphidation front. In such a case, it is natural for the freed carbon to diffuse towards the hotter side of the tube which is the external surface and causes precipitation of fresh carbides during periods of shut-down. This results in the creation of carbide depleted areas of ferrite and areas rich in precipitated carbides, as was seen in the present areas. The areas containing accumulated carbides near the external surface are brittle and are prone to cracking, which can take place during start-up operation. 5. Conclusions High temperature sulphidic corrosion at the internal tube surface caused dissociation of carbides and diffusion of carbon towards the hot external surface where carbides precipitated and accumulated during shut down periods. Areas of carbide accumulation exhibited high hardness and were brittle. Thus, cracking was facilitated at the external surface most likely during start up operation.
Acknowledgements The authors would like to thank Dr. Nusrat Tanoli for her SEM and EDS work and Mr. Harish Gopal for his optical microscopy work. References [1] ASTM A 213. Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat exchanger tubes. Philadelphia Pennsylvania (USA): American Society for Testing and Materials; 2006. [2] ASTM E384. Standard test method for microhardness of materials. Philadelphia, Pennsylvania (USA): American Society for Testing and Materials; 1989. [3] Pipatmanomai S, Fungtammasan B, Bhattacharya S. Characteristics and composition of lignites and boiler ashes and their relation to slagging: the case of Mae Moh PCC boilers. Fuel 2008;88:116–23.
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