DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 181
3.34 Graphitization
3.34.1 Description of Damage
Graphitization is a change in the microstructure of certain carbon steels and ½Mo steels after long-term operation
in the 800 °F to 1100 °F (425 °C to 595 °C) range. At these temperatures, the carbide phases in these steels are
unstable and decompose into graphite nodules. This decomposition of carbides may cause a loss in strength,
ductility, and/or creep resistance.
3.34.2 Affected Materials
It involves loss of ductility but
Some grades of carbon steel and ½Mo steels. Spherodization does not have
loss of ductility
3.34.3 Critical Factors
a) The most important factors that affect graphitization are the chemical composition of the steel, stress,
temperature, and time of exposure.
b) Graphitization is not commonly observed. Some steels are much more susceptible to graphitization than
others, but exactly what causes some steels to graphitize while others are resistant is not well understood. It
was originally thought that silicon and aluminum content played a major role, but it has been shown that they
have negligible influence on graphitization.
c) Graphitization has been found in low-alloy C-Mo steels with up to 1 % Mo. The addition of about 0.7 %
chromium has been found to eliminate graphitization.
d) Temperature has an important effect on the rate of graphitization. Below 800 °F (425 °C), the rate is
extremely slow. The rate increases with increasing temperature.
e) There are two general types of graphitization.
First is random graphitization in which the graphite nodules are distributed randomly throughout the steel.
While this type of graphitization may lower the room-temperature tensile strength, it does not usually
lower the creep resistance.
The second and more damaging type of graphitization results in chains or local planes of concentrated
graphite nodules. Because of its appearance, this type is also known as “eyebrow graphitization.” This
form of graphitization can result in a significant reduction in load-bearing capability while increasing the
potential for brittle fracture along this plane. There are two forms of this type of graphitization: (1) weld
HAZ graphitization and (2) non-weld graphitization.
Multipass Weld
— Weld HAZ graphitization is found adjacent to welds in a narrow band along the low-temperature
edge of the HAZ. In multi-pass welded butt joints, these zones overlap each other. The graphite
nodules that form at the low-temperature edge of these HAZs create a band of weak graphite
extending through the entire cross section. (Figure 3-34-1 and Figure 3-34-2)
— Non-weld graphitization is a form of localized graphitization that sometimes occurs along grain
boundaries, constituent boundaries (between ferrite and pearlite), or planes of localized yielding
in steels that have experienced significant plastic deformation as the result of cold working
operations or bending. This type of graphitization also occurs in a chain-like manner.
f) The extent and degree of graphitization is usually reported in a qualitative fashion (none, slight, moderate,
severe). Although it is difficult to predict the rate at which it forms, severe HAZ graphitization can develop in
as little as 5 years at service temperatures above 1000 °F (540 °C). Very slight graphitization would be
expected to be found after 30 to 40 years at 850 °F (455 °C). Time-temperature-transformation curves for
HAZ graphitization can be found in Reference 2.
Graphite nodules act as stress concentration points or weak spots in the steel. In severe cases, especially with "eyebrow graphitization"
(chains or planes of graphite), these nodules can create brittle regions that are prone to cracking rather than deforming.
Creep is the slow, permanent deformation of a material under constant stress at high temperatures over a long period. Carbides help pin
the steel’s grain boundaries and prevent them from sliding or deforming under prolonged stress. When carbides decompose into graphite,
the microstructure loses this reinforcement. In cases of concentrated graphitization (e.g., eyebrow graphitization), the chains of graphite
nodules create weak planes that further reduce the steel’s ability to resist creep.
182 API RECOMMENDED PRACTICE 571
3.34.4 Affected Units or Equipment
a) Graphitization primarily occurs in carbon steel piping and hot-wall equipment in the FCC, catalytic reforming,
and coker units.
b) Bainitic grades are less susceptible than coarse pearlitic grades.
c) Few failures directly attributable to graphitization have been reported in the refining industry. However,
graphitization has been found where failure resulted primarily from other causes.
d) Several serious cases of graphitization have occurred in the reactors and piping of FCC units, as well as in
carbon steel furnace tubes in a thermal cracking unit. Graphitization led to the failure of seal welds at the
bottom tubesheet of a vertical waste heat boiler in an FCC. A graphitization failure was also reported in the
long seam weld of a C-½Mo catalytic reformer reactor/interheater line.
e) Where concentrated eyebrow graphitization occurs along HAZs, the creep rupture strength may be drastically
lowered. Slight to moderate amounts of graphite along the HAZs do not appear to significantly lower room-
temperature or high-temperature properties.
f) Graphitization seldom occurs on boiling surface tubing in boilers but did occur in low-alloy C-½Mo tubes and
headers during the 1940s. Economizer tubing, steam piping, and other related equipment that operates in
the temperature range of 850 °F to 1025 °F (440 °C to 550 °C) are more likely to suffer graphitization.
3.34.5 Appearance or Morphology of Damage
a) Graphitization is not visible or readily apparent and can only be observed by metallographic examination.
(Figure 3-34-2 to Figure 3-34-4)
b) At an advanced stage resulting in loss of creep strength, microvoids, microfissuring, subsurface cracking, or
surface-connected cracking may be found.
3.34.6 Prevention/Mitigation
Graphitization can be prevented by using chromium containing low-alloy steels for long-term operation above
800 °F (425 °C).
3.34.7 Inspection and Monitoring
a) Evidence of graphitization is most effectively evaluated through removal of full-thickness samples for
examination using metallographic techniques. Damage may occur mid-wall so that field replicas may be
inadequate. Samples should be taken from areas where maximum temperature limits have been exceeded
b) Advanced stages of damage related to loss in strength include surface-breaking cracks or creep deformation
that may be difficult to detect.
3.34.8 Related Mechanisms
Spheroidization (3.59) and graphitization are competing mechanisms that occur at overlapping temperature
ranges. Spheroidization tends to occur preferentially above 1025 °F (550 °C), while graphitization predominates
below this temperature.
3.34.9 References
1. H. Thielsch, Defects and Failures in Pressure Vessels and Piping, Rheinhold Publishing, New York, NY,
1965, pp. 49–83.
DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 183
2. J.R. Foulds and R. Viswanathan, “Graphitization of Steels in Elevated-temperature Service,” Proceedings of
the First International Symposium: Microstructures and Mechanical Properties of Aging Materials, November
1992.
3. R.D. Port, “Non-weld-related Graphitization Failures,” Paper No. 89248, Corrosion/89, NACE International,
Houston, TX.
4. ASM Handbook—Properties and Selection: Iron, Steels, and High-performance Alloys, Volume 1, ASM
International, Materials Park, OH.
5. D.N. French, “Microstructural Degradation,” The National Board of Boiler and Pressure Vessel Inspectors,
http://www.nationalboard.com, June 2001.
6. J.G. Wilson, Part 1: Graphitization of Steel in Petroleum Refining Equipment, WRC Bulletin 032, Shaker
Heights, OH, January 1957.
7. J.D. Dobis and L. Huang, “Assessment of Graphitized Carbon Steel Tubes in Fired Heater Service,” Paper
No. 05559, Corrosion/2005, NACE International, Houston, TX.
8. Review of the Existing Technology for the Detection of Graphitization Damage in Carbon and Carbon-
Molybdenum Steel Piping and Tubing, EPRI, Product ID: 3002003421, July 2, 2014.
9. H.J. Kerr and F. Eberle, “Graphitization of Low-carbon and Low-carbon-molybdenum Steels,” Welding
Research Supplement, February 1945.
10. J. Hau et al., “Evaluation of Aging Equipment for Continued Service,” Paper No. 05558, Corrosion/2005,
NACE International, Houston, TX.
Figure 3-34-1—A crack opened up along the low-temperature edge of the HAZ
when a graphitized piece of steel was subjected to a bending test.
The scale in the photo is in tenths of an inch. (Reference 10)
GRAPHITISATION AND SPHERODISATION
Description Appearance
Graphitisation is Microstructural changes
decomposition of a steel's involving the distribution and
carbon content into graphite form of carbon compounds
Spheroidisation is a softening
of carbon and low-alloy steels
due to dispersal carbides
agglommerating into spheres
MW
mm
Inspection: VT will not reliably show these DMs.
Metallographic (replica) examination. Tensile test
to show reduced strength/increased ductility.
Critical factors: Graphitisation affects LCS and 0.5Mo steels after
long-term exposure to 427-5933C (800-1100°F).
Spheroidisation affects LCS and a wider range of
low-alloy steels above 454°C (850~F).
FFP/Severity: Weakening causes principal stress failure (e.g.
tensile overload) similar to loss of creep strength.
Graphitisation has several forms (random-v-
concentrated graphite nodules) and FFP
assessment is unreliable.
Spheroidisation softening can cause
unpredictable deformation and bulging -again
FFP assessment is likely to be inaccurate, leaving a
risk of unexpected catastrophic failure.
References: API 571 (4.2.1-4.2.2)
FIG D23
Graphitisation and Spheroidisation
These are both essentially:
Weakening / Softening
of the mlcroHtriicture
Graphitisation
o% Spheroidisation
Both are ^
found by
metallographic
tests
Low-carbon or O.SMo steel Low-carbon and low-alloy
steels up to 9Cr-lMo
Carbides turn into weak
graphitic nodules • Ductility increases
• Ductility decreases Carbides remain as carbides
but agglomerate Into spheres
Temperature sensitivity
1025°F
Below 1025T Above I025°F
Graphitisation Spheroidisation
happens first happens first
184 API RECOMMENDED PRACTICE 571
Figure 3-34-2—A polished and etched side view of the sample in Figure 3-33-1
shows aligned graphitization along the low-temperature edge of the HAZ
as well as random graphitization in the base metal. (Reference 10)
Figure 3-34-3—High-magnification photomicrograph of a metallographic sample showing
graphite nodules. Compare to normal microstructure shown in Figure 3-33-4.
DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 185
Figure 3-34-4—High-magnification photomicrograph of metallographic
sample showing typical ferrite-pearlite structure of carbon steel.
