Graphitization occurs in carbon steel (CS) reactors in high- temperature service (> 455°C). As discussed in this case history, the shell of a methyl tertiary butyl ether (MTBE) reactor became susceptible to elevated-temperature oxidation that initiated cor- rosion conditions. In addition, high-temperature fluctuations fostered stresses that promoted fatigue of the MTBE reactor shell and, ultimately, caused an emergency shutdown of the unit.
Background. Cracking in the reactor shell of an MTBE unit was detected by hot-air leaks through the reactor. This failure resulted in the emergency shutdown. Several investigation techniques were used to identify the root cause for the failure, including X-ray fluorescence (XRF), C/S analyzer, optical mi- croscopy, scanning electron microscope/energy dispersive X- ray (SEM/EDX), tensile testing and hardness testing. Results from the thorough investigation identified that the cracking of the reactor shell was attributed to corrosion fatigue (oxidation fatigue), due to thermal cycling and graphitization.
Graphitization was the root cause for the degradation of the steel mechanical properties and for significant reductions in the fracture toughness, thus facilitating fast crack growth. The recommendation from this failure and study was the practice to inspect all reactor shells for graphitization.
The rise in the shell-metal temperature may have been at- tributed to inefficient thermal insulation provided by the re- fractory lining. For this MTBE process, the metal temperature of the reactor should be kept below 427°C to prevent graphiti- zation. The condition of the reactor shells must be evaluated using metallographic techniques. Ideally, the extent of graphiti- zation can be investigated through representative sampling for metallographic examination. Another option is upgrading the construction material for the reactor shell to a chromium (Cr)- containing, low-alloy steel.
Definition of graphitization. Graphitization occurs in CS as an end result from prolonged exposure (over 40,000 hr) to high-temperature (> 455°C) process conditions. It is a process in which the pearlite decomposes into ferrite and randomly dispersed graphite. It has been reported that graphitization is accelerated by high stresses and/or temperature fluctuations.1
Two types of graphitization were observed in the CS micro- structure—random and “chain” graphitization.
In the random graphitization, the graphite nodules are scat- tered randomly across the microstructure, leading to degrada- tion in steel mechanical properties, e.g., strength and hardness. In the chain graphitization, the graphite nodules form an aligned
structure representing a plane of weakness. Chain graphitization is usually favored along planes of localized yielding or in regions that experience plastic deformation due to cold working or bending. This process leads to considerable reduction in stress- rupture strength and fatigue resistance of the steel. Furthermore, chain graphitization can cause a significant reduction in the frac- ture toughness and, consequently, facilitate fast crack growth.2,3
Case history. An emergency shutdown of an MTBE unit oc- curred due to cracking of the main reactor shell. This reactor had been in service for 15 years, and it is constructed of ASTM A515 Grade 60 CS and lined with refractory.
In the reactor, isobutene is converted into isobutylene in the presence of catalyst. In this process, hot isobutene (T ≈ 600°C) is fed to the reactor at 170 tph. Then, the reactor is purged with steam at 250°C–260°C. Finally, hot air (T ≈ 675°C) flowing at 630 tph is introduced to regenerate the catalyst. Each cycle takes about eight minutes.
Investigation. Three plate samples from the reactor shell were submitted for analysis: A, B and C. Cracks were observed on the three samples, as shown in FIGS. 1–3. The cracks appeared
to have initiated at the bolt holes (attached to the shell internal surfaces) and propagated through the shell plate. Visual exami- nation of areas near the cracks revealed the presence of multiple parallel cracks. The internal surfaces of the three samples had been covered with grayish and reddish deposits that were col-
FIG. 1. General view of reactor sample A. Note: The cracks initiated at the bolt holes attached to the shell-plate internal surface.
40MARCH 2013 | HydrocarbonProcessing.com
Corrosion Control
lected for chemical analysis. No appreciable wall thinning was noticed on the three samples.
Material deposit identification. The chemical composition of the shell material was determined using XRF spectrometry and C/S analyzer; the results are summarized in TABLE 1. These
materials conform to the chemical requirement for ASTM A515 Grade 60 CS. The chemical compositions of the deposits collected from the sample internal surfaces are listed in TABLE 2.
The deposits were composed mainly of iron oxides. Traces of Al, Si, P, Ca, Mn and Zn were also detected.
Metallographic examination. Cross sections from different areas of the collected samples were prepared for metallographic examination. Examination of the steel microstructure revealed the presence of a high concentration of graphite nodules, as
shown in FIG. 4. These graphite nodules formed aligned struc-
tures, i.e., chain graphitization, in localized areas, as shown in FIG. 5. The SEM/EDX analysis was carried out to confirm the com-
position of the graphite nodules (FIG. 6). The steel was then nital
etched to reveal the microstructure. No cementite was noticed in the microstructure, thus suggesting that a complete transfor- mation of cementite in the steel to graphite and ferrite has oc- curred. Severe, localized chain graphitization was also observed in several cross sections, as shown in FIG. 7. No creep voids and/
or micro fissures were noticed throughout the microstructure. Parallel, unbranched wedge cracks, oriented perpendicular to the surface, were observed on the cross sections removed from areas near the ruptures (FIG. 8). The cracks nucleated on
the internal surfaces of the shell plate and propagated toward the external surface. Also, the crack root walls were covered with dense oxide layers. Some of the cracks have grown through the graphite particles, suggesting that fatigue resistance was ad- versely influenced by graphitization, as depicted in FIG. 9.
Mechanical testing. Three round tensile test samples were machined from each sample. The test was conducted at room temperature on a universal testing machine. The tensile test results are summarized in TABLE 3 and FIG. 10. The three speci-
mens from each sample performed fairly consistently. The results indicated small but definite differences in strength lev- els among the three different sections of the shell. The steel strength did not meet the minimum tensile requirements for ASTM A515 Grade 60 CS. Also, the hardness test results showed that the hardness of the samples is lower than the mini- mum hardness requirement (TABLE 4).
FIG. 2. Cracks at the bolt holes were also observed on Sample B.
FIG. 3. Two through-wall cracks were observed on Sample C. The sample internal surface was covered with reddish layer.
FIG. 4. Optical photomicrograph showing the presence of nodular graphite across the microstructure, as polished.
TABLE 1. Chemical composition of the reactor shell plates, wt%
C Mn P S Si Al Cr Ni Cu Fe
ASTM A515 Grade 604 0.24 (max) 0.98 (max) 0.035 (max) 0.035 (max) 0.13–0.45 bal
Reactor shell plate 0.13 0.88 0.032 0.006 0.22 0.03 0.03 0.03 0.03 bal
TABLE 2. Chemical composition of the deposits collected from the sample internal surfaces, wt%
O Al Si P Ca Mn Fe Zn Sample A 30.4 0.2 0.3 0.1 0.2 0.6 bal 0.3
Sample B 30.2 0.2 0.4 0.1 0.3 0.6 bal 0.2
Hydrocarbon Processing | MARCH 2013 41
Corrosion Control
Discussion. The original microstructure of the reactor shell’s steel is composed of pearlite (i.e., a mixture of ferrite and ce- mentite) and ferrite. However, the metallographic examination (as well as the mechanical testing) showed that the steel under- went graphitization, and cementite appeared to have complete- ly decomposed into ferrite and graphite. Random and “chain” graphitization were observed throughout the microstructure. It is obvious that the reactor shell was exposed to temperatures greater than 455°C for extended periods. To control graphitiza- tion, the metal temperature must be kept below 427°C, where the graphitization rate is extremely low. Overheating of the reactor shell may be sourced to inefficient thermal insulation provided by the refractory lining.
The visual and metallographic examinations indicated that the cracking of the reactor shell was caused by corrosion fa- tigue (oxidation fatigue) nucleated on the shell internal surface. Oxidation fatigue is characterized by unbranched, wedge cracks oriented perpendicular to the surface, and it often appears as multiple parallel cracks. Indeed, systems that operate cyclically (e.g., an MTBE reactor) and/or subject to rapid startup and shutdown procedures are the most vulnerable to oxidation fa-
tigue. Also, the formation of chain graphitization indicated sig- nificant reduction in fatigue resistance of the reactor shell mate- rial. The oxidation fatigue process involves these steps:
• Thermal cycling causes expansion of the base metal, gener- ating high internal stresses in the oxide layer. Accordingly, the ox- ide layer could not withstand high stresses, and, thus, it began to crack, rendering the metal surface exposed to the environment.
• Oxides form on the exposed metal surface at the root of the crack, creating a notch effect. The next thermal cycle generates cracking in the oxide along the notch, thus causing the original crack to deepen.
• Repeating the listed steps results in the development of a wedge-shaped crack propagating through the shell, eventually leading to a rupture.
FIG. 5. Optical micrograph showing local concentrations of graphite nodules, i.e., chain graphitization, as polished.
FIG. 6. Backscattered electron image of the graphite nodules and EDX of the graphite nodules.
FIG. 7. Optical photomicrograph showing a microstructure composed of ferrite and graphite nodules. Chain graphitization was observed throughout the steel microstructure as nital etched.
TABLE 3. Tensile test results
Yield strength, MPa Tensile strength, MPa Elongation, %
ASTM A515 Grade 60 220 415–550 25
Sample A-1 263 361 41.7 Sample A-2 267 359 40.0 Sample A-3 263 355 40.0 Sample B-1 271 375 37.3 Sample B-2 263 370 37.3 Sample B-3 273 376 38.8 Sample C-1 242 342 41.1 Sample C-2 215 347 39.6 Sample C-3 249 344 40.3
TABLE 4. Rockwell hardness test results
Specimen Rockwell, HRB
ASTM A515 Grade 60 68–84
Sample A 58.5
Sample B 58.6
42MARCH 2013 | HydrocarbonProcessing.com
Corrosion Control
Lessons learned. With the final root cause identified in the fail- ure of the MTBR reactor, several recommendations were made: • Monitoring and estimating the shell-metal tempera-
ture is critical when predicting graphitization. The metal
temperature should be kept within the design requirements. It has been reported that exposure at temperatures below 427°C support an extremely low graphitization rate.
• Metallographic techniques should be used to evalu-
ate the condition of the reactor shell. Ideally, the extent of
graphitization can be investigated through representative sam- pling of the reactor for metallographic examination.
• Graphitization may be evaluated using metallograph-
ic field replication procedures. Remember: Damage may oc-
cur in the internal part of the shell wall, making it pointless to conduct the field replica.
• Replacement. If the condition of the reactor shell is
the same as the submitted samples, then the shell should be replaced. Cracking and ruptures facilitated by graphitization may occur suddenly and unpredictably, leading to emergency
shutdowns. If graphitization was only found in localized areas of the reactor shell, then patch repair can be conducted. When replacing the reactor shell, upgrading the shell material with Cr-containing low-alloy steel is recommended. Graphitiza- tion can be prevented by using steels containing more than 0.7% Cr.
• Inspect the refractory lining of the reactor. Shell tem-
perature rise can be linked to inefficient thermal insulation provided by the refractory lining. A refractory specialist should investigate the performance of the refractory lining inside the reactor. NOMENCLATURE Al Aluminum P Phosphorus Ca Calcium Si Silicon Fe Iron S Sulfur Mn Manganese Zn Zinc O Oxygen LITERATURE CITED
1 ASM Handbook, Vol. 1, Properties and Selection: Irons, Steels, and High Performance
Alloys, ASM International, 1993.
2 Foulds J. R., and R. Viswanathan, “Graphitisation of Steels in Elevated-Temperature
Service,” Journal of Materials Engineering and Performance, October 2001, pp. 484–492.
3 Azevedo C. R. F. and G. S. Alves, “Failure Analysis of a Heat-Exchanger Serpentine,”
Engineering Failure Analysis, December 2005, pp. 193–200.
4ASTM A515/A515M-03 (Reapproved 2007), “Standard Specification for Pressure
Vessel Plate, Carbon Steel, for Intermediate- and Higher-Temperature Service.” FIG. 8. Unbranched wedge cracks oriented perpendicular to the
surface and often appearing as multiple parallel cracks, as polished.
FIG. 9. The crack root and walls were covered with oxides. The cracking of the graphite nodule is near the crack initiation point, and the crack was transgranular, as nital etched.
FIG. 10. Tensile testing results.
ABDULAZIZ AL-MESHARI is a failure analyst at SABIC Technology Centre in Jubail, Saudi Arabia. He holds a PhD in material science and metallurgy from the University of Cambridge (UK) and an MSc degree in corrosion science and engineering from UMIST (UK). He is member of a NACE and ASM member since 2000.
SAAD AL-ENAZI is a failure analyst at SABIC Technology Centre-Jubail, Saudi Arabia. He holds an MS degree in manufacturing management and a BSc degree in mechanical engineering from the University of Toledo.
ABDELGADER ABDELGALIL is a computer simulation
specialist at SABIC Technology Centre in Jubail, Saudi Arabia. He holds a PhD in solid mechanics from the University of British Columbia, Canada, and MSc degree in solid mechanics from Benghazi University, Libya.
Hydrocarbon Processing | MARCH 2013 43
Special Report
Corrosion Control
K. RAMESH, Reliance Industries Ltd., Hazira, India