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Failure investigation on a radiant tube in an ethylene cracking unit
Engineering Failure Analysis 104 (2019) 216–226
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure investigation on a radiant tube in an ethylene cracking unit ⁎
H. Pourmohammad, A. Bahrami , A. Eslami, M. Taghipour
T
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
A R T IC LE I N F O
ABS TRA CT
Keywords: Ethylene cracking Radiant tube Petrochemical Failure Thermal shocks
In this study, failure of G4852-Micro radiant tube in an ethylene cracking unit in a petrochemical plant was investigated. The tube was replaced approximately after 38,000 h of service, which was much shorter than the anticipated service lifetime (100,000 h). Failure analysis was performed by conducting visual and microscopic examinations. Both optical and scanning electron microscopes (SEM) were used to analyze the microstructure of damaged tubes. For comparison, microstructure and mechanical properties of a tube after 10,000 h of service were also investigated. Microhardness profilometry across the thickness of the tubes, showed significant decrease in hardness on both internal and external surfaces. This was in accordance with the observed chromium depletion, at both surfaces. There was no indication of creep voids, inferring that the coalescence of voids was not the main cause of the failure. Also, thermal stress simulations showed that by controlling unexpected shutdowns, tube residual stresses can be reduced. Overall, it was concluded that thermal shocks associated with unexpected shut downs, were the main cause of failure.
1. Introduction Ethylene is known to be one of the most strategic and the most widely used materials in petrochemical industry. Ethylene is the product of a cracking reaction in the presence of water steam at high temperatures. Cracking reaction takes place in radiant tubes. A schematic Figure showing radiant tubes is shown in Fig. 1. Integrity and safe operation of the tube is considered to be extremely important. Also, any failure and its consequent shutdown can cause tremendous economic loss for a plant [1–5]. Materials used in the radiant tubes are expected to have excellent resistance against high temperature oxidation, carburization, and creep deformation [6–10]. The working conditions for these tubes are extremely aggressive, with the working temperature in some cases being as high as 1150 °C. Also, the radiant tubes frequently undergo decoking and shutdown-restart cycles [4]. A routine service life for these tubes at 900 °C is 100,000 h, but in reality, depending on the service condition and the quality of tubes, this could vary from 30,000 to 180,000 h [4]. Normally, only after 10,000 h of operation, oxidation and carburization reactions start to degrade the microstructure of the radiant tubes [11–15]. Carburization results in secondary carbide formation and primary carbide coarsening, which is associated with deterioration of high temperature creep ductility of tubes. Carbon deposition on the inner surface of tubes can also result in the development of a thick layer of coke within the tubes, which can then negatively affect the radial heat transfer, decreasing the pressure of fluid inside the tubes, and therefore drop in the efficiency of the production. Therefore, the system is preferably shutdowned only in order to conduct decoking operation, which is essentially burning of the deposited coke layer. On the other hand, high-temperature surface oxidation leads to the formation of surface oxide scales which can accelerate the formation of Cr-depleted regions near the surface [5]. Failure analysis in radiant tubes can be very complicated, due to involvement of several
⁎
Corresponding author. E-mail address: [email protected] (A. Bahrami).
https://doi.org/10.1016/j.engfailanal.2019.05.042 Received 18 February 2019; Received in revised form 6 May 2019; Accepted 31 May 2019 Available online 03 June 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 104 (2019) 216–226
H. Pourmohammad, et al.
Fig. 1. Schematic of ethylene cracking tubes.
factors. A concrete judgment on failure cause of radiant tubes necessitates looking into the history of operation, microstructure evolvement during service, and also processing parameters. 2. Background The Failure occurred in a G4852-Micro radiant tube coil in an ethylene cracking unit. The tube was taken out of the service after 38,000 h (name as the Failed Tube) and replaced with the new tube. Surface cracks were observed on the external surface of tubes. A typical example of such surface cracks is shown in Fig. 2a. For comparison, the microstructure and mechanical properties of a tube (hereafter named as the reference tube) only after 10,000 h of service (Fig. 2b) was also investigated. 10,000 h service is too short to induce any major alteration in the microstructure of tubes. Based on available records in the plant, radiant tubes at this installation were anticipated to be in service at approximately 900 °C
Fig. 2. Digital camera image of: a) Failed Tube (after 38,000 h of service), and b) reference tube (after 10,000 h of service). 217
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Table 1 Chemical composition of the failed tube, compared with standard values. Element (wt%)
C
Si
Mn
P
S
Cr
Mo
Ni
Nb
Add
Fe
Failed tube composition Standard composition(ASTM A608)
0.42 0.38–0.45
2 0.5–1.5
1.19 0.5–1.5
0.012 0.03 max
0.002 0.03 max
25.5 24–27
0.27 0.5 max
34.3 34–37
0.8 0.5–1.5
Ti –
Balance Balance
for 100,000 h. The recorded data from the service history of the failed tube showed that the tube in this unit had undergone 33 shutdowns. Amongst number of shutdowns, 16 were pre-planned shutdowns; planned for decoking operation. The remaining were unexpected out-of-plan shutdowns for different reasons. This means that in average the plant had experienced a shutdown, per each 48 days. 3. Analysis procedure Samples with dimensions of 10 × 15 × 7 mm3 were cut from the as-cast, the reference tube (with 10,000 h service exposure), and the failed tube for microstructural examinations. Tubes are all centrifugally cast. Samples were mounted and grounded with grinding papers of 80 to 2400, followed by polishing with Alumina powder of 0.05 μm. Etching was performed with solution HNO3/HF/H2O according to standard NACE TM0498. Electron Microscope and Energy Dispersive X-ray spectroscopy model Leo Philips XL 30 were used for microstructural characterization. Tensile tests were done according to ASTM E8/E8M standard. Vickers micro-hardness profilometry was done with 100 g force, on the cross section of the tubes starting from the inner surface towards the outer surface. Thermal stress simulation was also carried out using a rather simple analytical approach, by dividing the whole thickness into different region, with each having its own characteristics. 4. Results and discussion 4.1. Chemical composition Table 1 shows the chemical composition of the G4852 Micro alloy. As can be seen from this Table, all alloying elements are within the standard range of the alloy, except for silicone, which is slightly above the standard limit. Alloy G4852-Micro has small amounts of strongly carbide-forming elements (in particular Ti) in addition to Cr and Nb, which are common carbide forming elements. It is well known, that the size, volume fraction, density, and type of carbides play a key role in high temperature performance of a tube (in particular in creep resistance). 4.2. Microstructural analyses Fig. 3 shows a macro-image from the cross section of the failed tube. This Figure shows that grains are elongated from the external surface towards the center. This directional solidified structure is the typical structure of centrifugally cast tubes. Also, this figure shows a thin layer of coke, deposited on the inner surface. Coke formation during the cracking reaction is inevitable [4], and this, in
Fig. 3. Digital camera image from cross section of the Failed Tube. 218
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Fig. 4. SEM image from microstructure of as-cast specimen prior to service.
fact, can be the main reason for most of the shutdowns, planned for decoking. Fig. 4 shows microstructure of the as-cast specimen (prior to service). Fig. 4a shows typical cast structure of connected islands of primary carbides. From Fig. 4b two main types of primary carbides can be seen, lighter ones which are niobium carbide and darker ones which are primary carbides (M23C6, M = Fe,Cr). During the service, primary M23C6 carbides gradually transform to carbides with M7C3 stoichiometry due to the diffusion of carbon into the inner surface of tubes [13,16]. Figs. 5–8 show microscopic images from the inner and outer surfaces as well as from the mid-wall thickness of the reference tube and the failed tube respectively. A clear distinction between the microstructure of the reference tube and the failed tube can be seen. Due to the short service time, the failure in the reference tube has not taken place, i.e. there is no bulging, deformation, microcracking, excessive carbide coarsening, and surface oxidation. The failed tube shows remarkably different microstructures at the exposed areas (the inner and outer surfaces) and areas in the middle. The former areas typically comprise coarser phases, whereas the latter one is more similar to the cast microstructure. In addition, a black phase has appeared at exposed areas, which later is shown to be an oxide-carbide phase. Chromium oxide at the surface acts as a barrier against diffusion of oxygen and carbon [17]. However, during aging, this layer will break down, resulting in the diffusion of carbon and oxygen towards the balk of material. Carbon diffusion is obviously associated with carburization, carbide coarsening and the formation of secondary carbides [4]. Moreover, with the loss of the chromium oxide layer, chromium near the surface areas diffuses towards the inner or outer surface to form new oxide layer, which leads to the formation of Cr-depleted areas. Also, oxygen diffusion expectedly leads to the formation of oxide particles. Detailed analyses of exposed areas were performed using SEM/EDS analysis. Fig. 9 shows typical microstructure at the inner surface of the failed tube at higher magnifications. Pointed marks, shown in this Figure, were analyzed by EDS. Fig. 10 shows EDS analysis from the points. As can be seen from Fig. 10, point 1 is rich in oxygen and carbon, inferring that the black phase is a complex of oxide and carbide phases. High concentration of silicone and chromium is an indication that the oxide and carbide phases are mainly composed of silicon and chromium. Oxygen diffusion has resulted in the formation of internal large oxide particles. Oxide particles can obviously act as crack nucleation sites, due to their inherent brittleness. These oxide phases have also formed on the outer areas. Oxygen diffusion from the furnace atmosphere results in the formation of oxide phases. The formation of chromium oxides as well as diffusion of chromium towards surface depletes the matrix from chromium. This can be clearly seen in the EDS analysis of the matrix (point 2), where chromium content is significantly lower than the chromium concentration in the as-received alloy. Chromium depletion leads to the formation of Cr-depleted layer at both inner and outer surfaces, with the former being
Fig. 5. Optical microscope images from cross section of the reference tube (10,000 h' service): at a) close to the inner surface, b) close to the middle thickness, and c) close to the outer surface. 219
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Fig. 6. Optical microscope images from cross section of the Failed Tube (38,000 h' service) at: a) close to the inner surface, b) close middle thickness, and c) close outer surface.
Fig. 7. SEM images from cross section of the reference Tube (10,000 h' service) at: a) close to the inner surface, b) close to middle thickness, and c) close to the outer surface.
Fig. 8. SEM images from cross section of the Failed Tube (38,000 h' service) at: a) close to the inner surface, b) close to middle thickness, and c) close to the outer surface.
Fig. 9. Typical microstructure from cross section of the failed tube close to the inner surface of tube with EDS analysis presented at different regions. 220
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Fig. 10. EDS analyses from different regions shown in Fig. 9.
221
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Fig. 11. Typical micro-cracks at close to the outer surface.
approximately 170 and the latter being 120 μm (see Fig. 8). As can be seen from Fig. 8 the depletion at the outer surface is not as much as that in the inner surface. This essentially is due to the comparatively lower activity of oxygen at the pipe external surface. The inner surface has comparatively higher amounts of globular carbides. This is not surprising, because the inner surface is in contact with the deposited carbon layer, which obviously causes precipitation of secondary carbides. Point 3 contains a high percentage of chromium (68.5%) with some carbon, showing that this phase is the chromium carbide. Interestingly, chromium carbides/ matrix interface is seemingly the preferred place for the formation of chromium oxide phase. This possibly is due to the fact that there is local depletion of chromium in the vicinity of chromium carbide phase, which makes the alloy less resistant against oxidation [18]. Also, analysis at point 4 in Fig. 9 for the failed tube shows that the lighter phase is niobium carbide. In addition to obvious differences between the amount of oxide phase at the inner and outer surfaces, there is another distinct difference between the microstructure of used pipe at these two regions. While there is almost no crack at the inner surface, the outer surface comprises a considerable number of micro-cracks. A typical example is shown in Fig. 11, where a network of micro-cracks can be clearly seen. Micro-cracks are formed either by carbide defragmentation or interface de-cohesion mechanisms. At some point of service time, the micro-cracks had connected to one another and a network of macro-cracks are formed.
4.3. Tensile properties Results of tensile test for both tubes are shown in Fig. 12. As can be seen from this Figure, there is almost no plastic deformation for the reference tube and failed tube. It is clear that failed tube is much more brittle than the reference tube. Brittleness of failed tubes is due to carburization during service conditions and reduced the plastic deformation capacity of the tubes.
Fig. 12. Engineering stress-strain curve for the material in Failed Tube and the reference Tube. 222
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Fig. 13. Fracture surfaces of a) the reference tube, and b) the failed tube at low magnification.
4.4. Fractography Fig. 13 shows fracture surfaces of failed and reference tubes at low magnification. As can be seen from Fig. 13, the failed tube has a more fibrous oriented fracture alongside directionally solidified grains, as compared to the reference tube. The fracture in the failed tube has propagated along dendrites. So, the fracture in the failed tube can be characterized as an inter-dendritic fracture. The reference tube has two distinct regions. The first region, which is more at the center of the thickness and outer region (Fig. 14a), is more a brittle transgranular fracture with river marks [19], whereas the second region, which is close to the inner surface (Fig. 14b), is a typical image containing dimples, showing ductile fracture. The thickness of this ductile layer is, however, comparatively much lower than the remaining tube thickness. The fracture surface of the failed tube consisted of three regions as shown in Fig. 15a. Region 1 near the inner surface has a flat fracture which can be due to slipping at the edges. For region 2 in the failed tube, the fracture surface is entirely different. This region coincides with the depleted region that was already mentioned. There are dents and bumps, showing relatively ductile fracture has occurred by emptying this area of brittle chromium carbides. Fig. 15b shows this region at a higher magnification. Fine broken particles are visible on the surface. This is probably because of the breakdown of the brittle oxide phases in this layer. Region 3, which is close to the outer surface, has transgranular and interdendritic fractures which is an indication of fracture under abrupt loads. The dendritic structure of these tubes is due to their centrifugal casting process. Under applied stresses, micro-cracks grow from the brittle carbide phases located between dendrites and finally, interdendritic fracture occurs. Fig. 15c shows an example of these micro-cracks. 4.5. Hardness profilometry Results of micro-hardness profilometry are shown in Fig. 16. In general, the Failed Tube has higher hardness compared to the reference Tube. This is due to higher degree of carburization. The internal surface is in contact with the coke layer, which constantly supplies carbon. Carbon diffusion and carburization obviously increase the hardness of the matrix. Excess carbon promotes the formation of secondary chromium carbides, which is associated with hardness increase from the center towards both surfaces. As can be seen from Fig. 16 near both internal and external surfaces, there is a significant drop in the hardness at the chromium-depleted areas, which has to do with chromium depletion. Contrary to the failed tube, the hardness profilometry in the reference sample shows a relatively flat profile with no major change through the thickness, which is an indication that the tube has not been yet severely influenced by the service condition. Variation in the chemical composition and, therefore, the thermal expansion coefficient of the alloy through the thickness can have detrimental influence on the performance of the tube in case of temperature fluctuations and during shutdowns.
Fig. 14. Fracture surface of reference tube at: a) close to the outer surface b) close to the inner surface. 223
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Fig. 15. Fracture surface of the Failed Tube showing: a) three regions b) region 2 and c) region 3.
Fig. 16. Hardness profilometry of the tubes from inner surface towards the outer surface.
5. Thermal stress simulations In normal cooling conditions, when the coke layer is not thick enough, the internal and external temperatures of the tube are approximately equal. The gradual creation of coke on the inner wall creates a temperature gradient between the outside and inside of the tube, so the operator increases the external temperature to increase the internal temperature of the tube. Depending on the controlled or sudden shutdown conditions, there are roughly three cooling regimes: moderate, medium and severe temperature reduction, resulting in moderate, medium and severe thermal shocks within the tube. As shown in Fig. 17, it is assumed that in the moderate cooling regime, temperature on the outer surface is roughly decreased to 700 °C, whereas in the medium and severe conditions, the temperature on the external surface is approximately assumed to be reduced to 550 °C and 400 °C, respectively. Fig. 17, shows schematics of the tube thickness with different layers, starting from the coke layer at the inner surface to the chromium 224
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Fig. 17. Schematic showing cooling regimes within different layers at different service conditions.
oxide layer Cr-depleted layer, the unaffected bulk alloy, Cr-depleted layer, and chromium oxide layer at the outer surface. The thickness of each layer is an approximate value, measured from the images of failed tubes. Using the simple equation σ = Eα(T2 − T1), the amount of thermal stress induced at the tube wall at different cooling regimes can be calculated. Table 2 gives an overview of constants used for each layer in the equation [20–23]. The results are shown in Fig. 18. As shown in this figure, thermal stresses can dramatically increase in case of sudden shutdowns. Thermal shock-induced stresses at the outer surface of the tubes could become so high, ending up in the formation of surface microcracks. The SEM images presented in Fig. 15 also confirm that micro-cracks have formed at the outer surface of the tube.
6. Conclusions In this study, the failure of G4852-Micro radiant tubes in an ethylene cracking unit in a petrochemical plant was investigated. The observed failure was in the form of surface cracks, which led to the replacement of tubes after 38,000 h of service, which is much shorter than the anticipated service life (100,000 h). The recorded data from the service history showed that the tube in this unit had undergone 33 shutdowns. Amongst total number of shut times, 16 were normal shutdowns, planned for decoking operation. The remaining (17) were unexpected shutdowns. Unplanned shutdowns are in most cases associated with uncontrolled cooling, and this causes thermal shocks. The Failed tube had several microstructural features. The internal and external surfaces of the failed tube were heavily oxidized and there was a Cr-depleted area. Also, carbide particles were coarsened due to carburization. Carburization, itself, can deteriorate high-temperature ductility of the alloy. Also, a large variation of hardness was seen profilometry of the failed tube. During unexpected shutdowns, the tube should have experienced fast cooling rates and this obviously could induce thermal stresses/ strains induced by thermal shocks, cannot be accommodated at the carbide/matrix interface at the outer surface, resulting in the interface de-cohesion and formation of micro-cracks. Results showed that there were micro-cracks at the outer surface in the vicinity of carbide particles in the failed tube. Micro-cracks were formed as a result of the carbide/matrix interface de-cohesion. Carbide fragmentation was also seen, which could also result in crack initiation. Also, during shut-downs, the outer surface was cooled faster than the inner surface, which implies that the extent of thermal shock stress at the outer surface is comparatively higher than that at the inner surface. This made the outer surface more prone to crack formation. The situation in this case was even worse, since there was a large variation of hardness throughout the thickness. There was no indication of creep as the dominant failure mechanism. There were not many creep voids observed at both the reference and the failed tubes. For tubes where creep is a major degrading mechanism, creep void coalescence alongside grain boundaries should be observed. Given that thermal shock is the major cause of failure in this case, any mitigation strategy should be based on controlling the cooling regimes during shutdowns.
Acknowledgments The authors would like to thank Isfahan University of Technology, and also Aryasasol Polymer Complex for their support. Table 2 Elastic modulus and thermal expansion coefficient of Cr2O3, G4852 Micro, Coke and Cr-depleted layer (constants for Cr-depleted layer is approximated with those of a low alloy steel). Material Cr2O3 G4852 Micro Coke Low alloy steels
α (1/k)
E (Mpa) −6
10.9 × 10 16.75 × 10−6 3.6 × 10−6 11.84 × 10−6
225
273 × 103 108.5 × 103 28 × 103 209 × 103
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Fig. 18. Relationship between applied stresses at different cooling regime through the tube thickness.
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Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failure investigation on a radiant tube in an ethylene cracking unit ⁎
H. Pourmohammad, A. Bahrami , A. Eslami, M. Taghipour
T
Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
A R T IC LE I N F O
ABS TRA CT
Keywords: Ethylene cracking Radiant tube Petrochemical Failure Thermal shocks
In this study, failure of G4852-Micro radiant tube in an ethylene cracking unit in a petrochemical plant was investigated. The tube was replaced approximately after 38,000 h of service, which was much shorter than the anticipated service lifetime (100,000 h). Failure analysis was performed by conducting visual and microscopic examinations. Both optical and scanning electron microscopes (SEM) were used to analyze the microstructure of damaged tubes. For comparison, microstructure and mechanical properties of a tube after 10,000 h of service were also investigated. Microhardness profilometry across the thickness of the tubes, showed significant decrease in hardness on both internal and external surfaces. This was in accordance with the observed chromium depletion, at both surfaces. There was no indication of creep voids, inferring that the coalescence of voids was not the main cause of the failure. Also, thermal stress simulations showed that by controlling unexpected shutdowns, tube residual stresses can be reduced. Overall, it was concluded that thermal shocks associated with unexpected shut downs, were the main cause of failure.
1. Introduction Ethylene is known to be one of the most strategic and the most widely used materials in petrochemical industry. Ethylene is the product of a cracking reaction in the presence of water steam at high temperatures. Cracking reaction takes place in radiant tubes. A schematic Figure showing radiant tubes is shown in Fig. 1. Integrity and safe operation of the tube is considered to be extremely important. Also, any failure and its consequent shutdown can cause tremendous economic loss for a plant [1–5]. Materials used in the radiant tubes are expected to have excellent resistance against high temperature oxidation, carburization, and creep deformation [6–10]. The working conditions for these tubes are extremely aggressive, with the working temperature in some cases being as high as 1150 °C. Also, the radiant tubes frequently undergo decoking and shutdown-restart cycles [4]. A routine service life for these tubes at 900 °C is 100,000 h, but in reality, depending on the service condition and the quality of tubes, this could vary from 30,000 to 180,000 h [4]. Normally, only after 10,000 h of operation, oxidation and carburization reactions start to degrade the microstructure of the radiant tubes [11–15]. Carburization results in secondary carbide formation and primary carbide coarsening, which is associated with deterioration of high temperature creep ductility of tubes. Carbon deposition on the inner surface of tubes can also result in the development of a thick layer of coke within the tubes, which can then negatively affect the radial heat transfer, decreasing the pressure of fluid inside the tubes, and therefore drop in the efficiency of the production. Therefore, the system is preferably shutdowned only in order to conduct decoking operation, which is essentially burning of the deposited coke layer. On the other hand, high-temperature surface oxidation leads to the formation of surface oxide scales which can accelerate the formation of Cr-depleted regions near the surface [5]. Failure analysis in radiant tubes can be very complicated, due to involvement of several
⁎
Corresponding author. E-mail address: [email protected] (A. Bahrami).
https://doi.org/10.1016/j.engfailanal.2019.05.042 Received 18 February 2019; Received in revised form 6 May 2019; Accepted 31 May 2019 Available online 03 June 2019 1350-6307/ © 2019 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 104 (2019) 216–226
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Fig. 1. Schematic of ethylene cracking tubes.
factors. A concrete judgment on failure cause of radiant tubes necessitates looking into the history of operation, microstructure evolvement during service, and also processing parameters. 2. Background The Failure occurred in a G4852-Micro radiant tube coil in an ethylene cracking unit. The tube was taken out of the service after 38,000 h (name as the Failed Tube) and replaced with the new tube. Surface cracks were observed on the external surface of tubes. A typical example of such surface cracks is shown in Fig. 2a. For comparison, the microstructure and mechanical properties of a tube (hereafter named as the reference tube) only after 10,000 h of service (Fig. 2b) was also investigated. 10,000 h service is too short to induce any major alteration in the microstructure of tubes. Based on available records in the plant, radiant tubes at this installation were anticipated to be in service at approximately 900 °C
Fig. 2. Digital camera image of: a) Failed Tube (after 38,000 h of service), and b) reference tube (after 10,000 h of service). 217
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Table 1 Chemical composition of the failed tube, compared with standard values. Element (wt%)
C
Si
Mn
P
S
Cr
Mo
Ni
Nb
Add
Fe
Failed tube composition Standard composition(ASTM A608)
0.42 0.38–0.45
2 0.5–1.5
1.19 0.5–1.5
0.012 0.03 max
0.002 0.03 max
25.5 24–27
0.27 0.5 max
34.3 34–37
0.8 0.5–1.5
Ti –
Balance Balance
for 100,000 h. The recorded data from the service history of the failed tube showed that the tube in this unit had undergone 33 shutdowns. Amongst number of shutdowns, 16 were pre-planned shutdowns; planned for decoking operation. The remaining were unexpected out-of-plan shutdowns for different reasons. This means that in average the plant had experienced a shutdown, per each 48 days. 3. Analysis procedure Samples with dimensions of 10 × 15 × 7 mm3 were cut from the as-cast, the reference tube (with 10,000 h service exposure), and the failed tube for microstructural examinations. Tubes are all centrifugally cast. Samples were mounted and grounded with grinding papers of 80 to 2400, followed by polishing with Alumina powder of 0.05 μm. Etching was performed with solution HNO3/HF/H2O according to standard NACE TM0498. Electron Microscope and Energy Dispersive X-ray spectroscopy model Leo Philips XL 30 were used for microstructural characterization. Tensile tests were done according to ASTM E8/E8M standard. Vickers micro-hardness profilometry was done with 100 g force, on the cross section of the tubes starting from the inner surface towards the outer surface. Thermal stress simulation was also carried out using a rather simple analytical approach, by dividing the whole thickness into different region, with each having its own characteristics. 4. Results and discussion 4.1. Chemical composition Table 1 shows the chemical composition of the G4852 Micro alloy. As can be seen from this Table, all alloying elements are within the standard range of the alloy, except for silicone, which is slightly above the standard limit. Alloy G4852-Micro has small amounts of strongly carbide-forming elements (in particular Ti) in addition to Cr and Nb, which are common carbide forming elements. It is well known, that the size, volume fraction, density, and type of carbides play a key role in high temperature performance of a tube (in particular in creep resistance). 4.2. Microstructural analyses Fig. 3 shows a macro-image from the cross section of the failed tube. This Figure shows that grains are elongated from the external surface towards the center. This directional solidified structure is the typical structure of centrifugally cast tubes. Also, this figure shows a thin layer of coke, deposited on the inner surface. Coke formation during the cracking reaction is inevitable [4], and this, in
Fig. 3. Digital camera image from cross section of the Failed Tube. 218
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Fig. 4. SEM image from microstructure of as-cast specimen prior to service.
fact, can be the main reason for most of the shutdowns, planned for decoking. Fig. 4 shows microstructure of the as-cast specimen (prior to service). Fig. 4a shows typical cast structure of connected islands of primary carbides. From Fig. 4b two main types of primary carbides can be seen, lighter ones which are niobium carbide and darker ones which are primary carbides (M23C6, M = Fe,Cr). During the service, primary M23C6 carbides gradually transform to carbides with M7C3 stoichiometry due to the diffusion of carbon into the inner surface of tubes [13,16]. Figs. 5–8 show microscopic images from the inner and outer surfaces as well as from the mid-wall thickness of the reference tube and the failed tube respectively. A clear distinction between the microstructure of the reference tube and the failed tube can be seen. Due to the short service time, the failure in the reference tube has not taken place, i.e. there is no bulging, deformation, microcracking, excessive carbide coarsening, and surface oxidation. The failed tube shows remarkably different microstructures at the exposed areas (the inner and outer surfaces) and areas in the middle. The former areas typically comprise coarser phases, whereas the latter one is more similar to the cast microstructure. In addition, a black phase has appeared at exposed areas, which later is shown to be an oxide-carbide phase. Chromium oxide at the surface acts as a barrier against diffusion of oxygen and carbon [17]. However, during aging, this layer will break down, resulting in the diffusion of carbon and oxygen towards the balk of material. Carbon diffusion is obviously associated with carburization, carbide coarsening and the formation of secondary carbides [4]. Moreover, with the loss of the chromium oxide layer, chromium near the surface areas diffuses towards the inner or outer surface to form new oxide layer, which leads to the formation of Cr-depleted areas. Also, oxygen diffusion expectedly leads to the formation of oxide particles. Detailed analyses of exposed areas were performed using SEM/EDS analysis. Fig. 9 shows typical microstructure at the inner surface of the failed tube at higher magnifications. Pointed marks, shown in this Figure, were analyzed by EDS. Fig. 10 shows EDS analysis from the points. As can be seen from Fig. 10, point 1 is rich in oxygen and carbon, inferring that the black phase is a complex of oxide and carbide phases. High concentration of silicone and chromium is an indication that the oxide and carbide phases are mainly composed of silicon and chromium. Oxygen diffusion has resulted in the formation of internal large oxide particles. Oxide particles can obviously act as crack nucleation sites, due to their inherent brittleness. These oxide phases have also formed on the outer areas. Oxygen diffusion from the furnace atmosphere results in the formation of oxide phases. The formation of chromium oxides as well as diffusion of chromium towards surface depletes the matrix from chromium. This can be clearly seen in the EDS analysis of the matrix (point 2), where chromium content is significantly lower than the chromium concentration in the as-received alloy. Chromium depletion leads to the formation of Cr-depleted layer at both inner and outer surfaces, with the former being
Fig. 5. Optical microscope images from cross section of the reference tube (10,000 h' service): at a) close to the inner surface, b) close to the middle thickness, and c) close to the outer surface. 219
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Fig. 6. Optical microscope images from cross section of the Failed Tube (38,000 h' service) at: a) close to the inner surface, b) close middle thickness, and c) close outer surface.
Fig. 7. SEM images from cross section of the reference Tube (10,000 h' service) at: a) close to the inner surface, b) close to middle thickness, and c) close to the outer surface.
Fig. 8. SEM images from cross section of the Failed Tube (38,000 h' service) at: a) close to the inner surface, b) close to middle thickness, and c) close to the outer surface.
Fig. 9. Typical microstructure from cross section of the failed tube close to the inner surface of tube with EDS analysis presented at different regions. 220
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Fig. 10. EDS analyses from different regions shown in Fig. 9.
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Fig. 11. Typical micro-cracks at close to the outer surface.
approximately 170 and the latter being 120 μm (see Fig. 8). As can be seen from Fig. 8 the depletion at the outer surface is not as much as that in the inner surface. This essentially is due to the comparatively lower activity of oxygen at the pipe external surface. The inner surface has comparatively higher amounts of globular carbides. This is not surprising, because the inner surface is in contact with the deposited carbon layer, which obviously causes precipitation of secondary carbides. Point 3 contains a high percentage of chromium (68.5%) with some carbon, showing that this phase is the chromium carbide. Interestingly, chromium carbides/ matrix interface is seemingly the preferred place for the formation of chromium oxide phase. This possibly is due to the fact that there is local depletion of chromium in the vicinity of chromium carbide phase, which makes the alloy less resistant against oxidation [18]. Also, analysis at point 4 in Fig. 9 for the failed tube shows that the lighter phase is niobium carbide. In addition to obvious differences between the amount of oxide phase at the inner and outer surfaces, there is another distinct difference between the microstructure of used pipe at these two regions. While there is almost no crack at the inner surface, the outer surface comprises a considerable number of micro-cracks. A typical example is shown in Fig. 11, where a network of micro-cracks can be clearly seen. Micro-cracks are formed either by carbide defragmentation or interface de-cohesion mechanisms. At some point of service time, the micro-cracks had connected to one another and a network of macro-cracks are formed.
4.3. Tensile properties Results of tensile test for both tubes are shown in Fig. 12. As can be seen from this Figure, there is almost no plastic deformation for the reference tube and failed tube. It is clear that failed tube is much more brittle than the reference tube. Brittleness of failed tubes is due to carburization during service conditions and reduced the plastic deformation capacity of the tubes.
Fig. 12. Engineering stress-strain curve for the material in Failed Tube and the reference Tube. 222
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Fig. 13. Fracture surfaces of a) the reference tube, and b) the failed tube at low magnification.
4.4. Fractography Fig. 13 shows fracture surfaces of failed and reference tubes at low magnification. As can be seen from Fig. 13, the failed tube has a more fibrous oriented fracture alongside directionally solidified grains, as compared to the reference tube. The fracture in the failed tube has propagated along dendrites. So, the fracture in the failed tube can be characterized as an inter-dendritic fracture. The reference tube has two distinct regions. The first region, which is more at the center of the thickness and outer region (Fig. 14a), is more a brittle transgranular fracture with river marks [19], whereas the second region, which is close to the inner surface (Fig. 14b), is a typical image containing dimples, showing ductile fracture. The thickness of this ductile layer is, however, comparatively much lower than the remaining tube thickness. The fracture surface of the failed tube consisted of three regions as shown in Fig. 15a. Region 1 near the inner surface has a flat fracture which can be due to slipping at the edges. For region 2 in the failed tube, the fracture surface is entirely different. This region coincides with the depleted region that was already mentioned. There are dents and bumps, showing relatively ductile fracture has occurred by emptying this area of brittle chromium carbides. Fig. 15b shows this region at a higher magnification. Fine broken particles are visible on the surface. This is probably because of the breakdown of the brittle oxide phases in this layer. Region 3, which is close to the outer surface, has transgranular and interdendritic fractures which is an indication of fracture under abrupt loads. The dendritic structure of these tubes is due to their centrifugal casting process. Under applied stresses, micro-cracks grow from the brittle carbide phases located between dendrites and finally, interdendritic fracture occurs. Fig. 15c shows an example of these micro-cracks. 4.5. Hardness profilometry Results of micro-hardness profilometry are shown in Fig. 16. In general, the Failed Tube has higher hardness compared to the reference Tube. This is due to higher degree of carburization. The internal surface is in contact with the coke layer, which constantly supplies carbon. Carbon diffusion and carburization obviously increase the hardness of the matrix. Excess carbon promotes the formation of secondary chromium carbides, which is associated with hardness increase from the center towards both surfaces. As can be seen from Fig. 16 near both internal and external surfaces, there is a significant drop in the hardness at the chromium-depleted areas, which has to do with chromium depletion. Contrary to the failed tube, the hardness profilometry in the reference sample shows a relatively flat profile with no major change through the thickness, which is an indication that the tube has not been yet severely influenced by the service condition. Variation in the chemical composition and, therefore, the thermal expansion coefficient of the alloy through the thickness can have detrimental influence on the performance of the tube in case of temperature fluctuations and during shutdowns.
Fig. 14. Fracture surface of reference tube at: a) close to the outer surface b) close to the inner surface. 223
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Fig. 15. Fracture surface of the Failed Tube showing: a) three regions b) region 2 and c) region 3.
Fig. 16. Hardness profilometry of the tubes from inner surface towards the outer surface.
5. Thermal stress simulations In normal cooling conditions, when the coke layer is not thick enough, the internal and external temperatures of the tube are approximately equal. The gradual creation of coke on the inner wall creates a temperature gradient between the outside and inside of the tube, so the operator increases the external temperature to increase the internal temperature of the tube. Depending on the controlled or sudden shutdown conditions, there are roughly three cooling regimes: moderate, medium and severe temperature reduction, resulting in moderate, medium and severe thermal shocks within the tube. As shown in Fig. 17, it is assumed that in the moderate cooling regime, temperature on the outer surface is roughly decreased to 700 °C, whereas in the medium and severe conditions, the temperature on the external surface is approximately assumed to be reduced to 550 °C and 400 °C, respectively. Fig. 17, shows schematics of the tube thickness with different layers, starting from the coke layer at the inner surface to the chromium 224
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Fig. 17. Schematic showing cooling regimes within different layers at different service conditions.
oxide layer Cr-depleted layer, the unaffected bulk alloy, Cr-depleted layer, and chromium oxide layer at the outer surface. The thickness of each layer is an approximate value, measured from the images of failed tubes. Using the simple equation σ = Eα(T2 − T1), the amount of thermal stress induced at the tube wall at different cooling regimes can be calculated. Table 2 gives an overview of constants used for each layer in the equation [20–23]. The results are shown in Fig. 18. As shown in this figure, thermal stresses can dramatically increase in case of sudden shutdowns. Thermal shock-induced stresses at the outer surface of the tubes could become so high, ending up in the formation of surface microcracks. The SEM images presented in Fig. 15 also confirm that micro-cracks have formed at the outer surface of the tube.
6. Conclusions In this study, the failure of G4852-Micro radiant tubes in an ethylene cracking unit in a petrochemical plant was investigated. The observed failure was in the form of surface cracks, which led to the replacement of tubes after 38,000 h of service, which is much shorter than the anticipated service life (100,000 h). The recorded data from the service history showed that the tube in this unit had undergone 33 shutdowns. Amongst total number of shut times, 16 were normal shutdowns, planned for decoking operation. The remaining (17) were unexpected shutdowns. Unplanned shutdowns are in most cases associated with uncontrolled cooling, and this causes thermal shocks. The Failed tube had several microstructural features. The internal and external surfaces of the failed tube were heavily oxidized and there was a Cr-depleted area. Also, carbide particles were coarsened due to carburization. Carburization, itself, can deteriorate high-temperature ductility of the alloy. Also, a large variation of hardness was seen profilometry of the failed tube. During unexpected shutdowns, the tube should have experienced fast cooling rates and this obviously could induce thermal stresses/ strains induced by thermal shocks, cannot be accommodated at the carbide/matrix interface at the outer surface, resulting in the interface de-cohesion and formation of micro-cracks. Results showed that there were micro-cracks at the outer surface in the vicinity of carbide particles in the failed tube. Micro-cracks were formed as a result of the carbide/matrix interface de-cohesion. Carbide fragmentation was also seen, which could also result in crack initiation. Also, during shut-downs, the outer surface was cooled faster than the inner surface, which implies that the extent of thermal shock stress at the outer surface is comparatively higher than that at the inner surface. This made the outer surface more prone to crack formation. The situation in this case was even worse, since there was a large variation of hardness throughout the thickness. There was no indication of creep as the dominant failure mechanism. There were not many creep voids observed at both the reference and the failed tubes. For tubes where creep is a major degrading mechanism, creep void coalescence alongside grain boundaries should be observed. Given that thermal shock is the major cause of failure in this case, any mitigation strategy should be based on controlling the cooling regimes during shutdowns.
Acknowledgments The authors would like to thank Isfahan University of Technology, and also Aryasasol Polymer Complex for their support. Table 2 Elastic modulus and thermal expansion coefficient of Cr2O3, G4852 Micro, Coke and Cr-depleted layer (constants for Cr-depleted layer is approximated with those of a low alloy steel). Material Cr2O3 G4852 Micro Coke Low alloy steels
α (1/k)
E (Mpa) −6
10.9 × 10 16.75 × 10−6 3.6 × 10−6 11.84 × 10−6
225
273 × 103 108.5 × 103 28 × 103 209 × 103
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Fig. 18. Relationship between applied stresses at different cooling regime through the tube thickness.
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