Transcript Serviceability of Graphitized Carbon Steel

Serviceability of Graphitized
Carbon Steel
Evan Vokes
Dr Weixing Chen
Outline
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Origin of graphitization
Microstructure development
Detection of graphite
Characterization by Creep methods
Characterization by Tensile methods
Characterization by Fracture methods
Conclusion
References
Where Graphite comes from
Solid state phase transform
Competition between formation
of cementite and carbon
g a Phase transform
Primary graphite
Secondary graphite
Cast Iron
Steels
Product of cementite Several
decomposition
mechanisms
Related to Chemistry Related to ThermoMechanical History
Secondary Graphitization mechanisms in steel
g a Phase Transform
Martensitic Transforms
Box Annealing Transforms
Result in uniform
random graphitization
in laboratory testing
Typical of higher carbon
content steels
Suspected cause of
HAZ graphitization
Often found after
spherodizing anneals
Random morphology
Time at High Temperature
Transforms
Two types of morphologies,
Random and Planar
Martensitic transforms
• Thought to be associated with high cooling
rates such as those associated with
welding
• Post weld heat treatments have effectively
reduced the occurrence of HAZ
graphitization
• Attempts have been made to re-adsorb C
into matrix by Insitu austenization but
reoccurrence is very quick
Box annealed steels
• High Carbon Content
• Held near transformation temperature for
extended periods
• Suspected result of carbon super saturation
• No data on whether graphitization is
homogeneous or heterogeneous
• Never cited as a cause of failure
High temperature steels 1
• Graphitization is not associated with welds
• Generally low carbon content
• Incident data incomplete as mixture of
plain carbon and low alloy steels
• Two known morphologies
• a) planar
• b) random
High Temperature Steels 2
• Morphology was associated with plastic
deformation of base metals
• Random morphology in base metal has
been known for over 50 years
• Planar morphology was found at same
time, often compared to weld HAZ
graphitization
• Random graphitization always associated
with planar graphite
Random
graphite
• Heterogeneous nature
• May tend to follow
banding in longitudinal
directions
Planar Graphite
• Found in two pieces of
piping
• Piping was constrained
• Random graphite present
Failure Potential from Furtado and Le May
SEM image of planes of graphite
Detection of Graphite 1
Replications and hardness tests showed that this piping
section was free of graphite
Piping was replaced on a precautionary basis of
graphitization in similar piping
Graphite was found in elbows and reducers
Piping was clean
Detection of Graphite 2
• Problem is the heterogeneous nature of
secondary graphitization
• No strong evidence that would rule out the
presence of planar graphitization if random
graphitization is found
• Need to characterize material in such a
fashion that can reveal properties we can
exploit for NDE purposes
Detection of Graphite 3
• High temperature operation on the cusp of
creep regime means we should test
elevated temperature creep properties and
mechanical properties
• Presence of a dynamic flaw shows that we
should perform fracture mechanics
High Temperature Creep Properties 1
Larson Miller A 106 B referenced to ASTM DS 11S1
4.4
4.3
Reducer 3
Flange 6
Log Stress (ksi)
4.2
4.1
4
3.9
3.8
3.7
3.6
30
31
32
33
34
LMP
35
36
37
38
High Temperature Creep Properties 2
Larson Miller A 106 B referenced to ASTM DS 11S1
4.4
4.3
Elbow 4
Elbow 1
Elbow 1 Weld
Log Stress (ksi)
4.2
4.1
4
3.9
3.8
3.7
3.6
30
31
32
33
34
LMP
35
36
37
38
High Temperature Creep Properties 3 Stress
Sensitivity
18
16
Stress Exponent (n)
14
12
10
Elbow 1
Reducer 3
Elbow 4
Flange 6
Elbow 7
new Elbow 7
API 530
8
6
4
2
0
475
500
525
550
Temperature (°C)
575
600
625
High Temperature Creep Properties 4
Ductility Relations
Creep Strain to Failure Relations
100%
Reducer 3
Elbow 1
90%
Percent Strain
80%
70%
60%
50%
40%
30%
20%
10%
0%
10.0
100.0
1000.0
Rupture time
10000.0
High Temperature Creep Properties 5
Post creep microstructure of graphitized elbows
High Temperature Creep Properties 6
Post creep microstructure near weld
High Temperature Creep Properties 7
Creep summary
• Expected life times remain reasonable for
a material on the edge of the creep regime
• Two different methods were used to
evaluate life predictions
• Some materials seemed to be stress
sensitive
• Welds do not pose a particular problem for
random graphitization
Mechanical Properties1
Tensile testing
Ultimate Tensile Strength
(MPa)
500
475
Elbow 1
Flange 6, Reducer 3
450
All other elbows
425
400
375
350
180
200
220
240
260
Yield Strength (MPa)
280
300
Mechanical Properties 2
Tensile testing
500
450
Stress (MPa)
400
350
300
250
200
150
100
50
0
0
10
20
30
% Strain
40
50
60
Mechanical Properties 3
Tensile testing
• Room temperature tensile properties show that
we have a differing of mechanical properties
consistent with degraded microstructure
• The suggested groupings show that the material
no longer offers homogeneous properties that
we would expect
• The presence of planar graphite is separated
from random graphitized SA234 materials
• The highest volume of graphite does increase
the yield strength
• Random graphite does increase the ductility
• Planar graphite limits ductility
Mechanical Properties 4
Hot Tensiletesting
Results @ 427°C
Hot Tensile
@427°C
Ultimate Tensile Strength (MPa)
450
400
350
min design API 530
DS11S1
Elbow 7
Elbow 4
Flange 6
Elbow 1
pipe
300
250
200
100
150
200
Yield Strength (MPa)
250
Mechanical Properties 5
Hot Tensile testing @427°C
• All mechanical strengths are quite good
considering the microstructure damage
• Materials tested have similar rankings as
compared to room temperature properties
Fracture properties
• An attempt to prepare a FAD using J
integrals was to be made
• Only the lowest strength poor creep
property material was investigated
• Lack of planar graphitized material did not
allow for fracture investigation of that
phenomenon
Fracture 2
Fracture 3
• Ductile tearing surface resulting from
compliance testing shows that the graphite
was not the source of fracture nucleation
• J integral values were not valid but the
critical flaw size of 0.3mm was determined
using CTOD values
• This has resulted in a detectable critical
flaw size for use with NDE
• It could not be determined if the tearing
mode was stable or not
Conclusion
• Random Graphitization has mechanical
creep and fracture properties that indicate
that it is still serviceable
• Random graphite can not be considered
benign
• Random graphite’s association with planar
graphite is known but it is not known how
one morphology becomes the other
• Planar graphite is just plain dangerous
NDE Recommendations
• The work highlights the difficulty of
determining the presence of graphitization
• Understanding where to look for the
phenomenon is important
• The challenge is to use this data to find a
useful NDE technique for the detection of
planar graphite
Thank you
• Nova Chemicals
• NSERC
• Canspec Materials Engineering
Useful References
• Furtado, H., Le May, I. (2003). "Evaluation of
Unusual Superheated Steam Pipe Failure."
Materials Characterization, 49.
• Port, R., Mack, W., Hainsworth, J. "The
Mechanisms of Chain Graphitization of Carbon
and Carbon/Molybdenum Steels. Heat Resistant
Materials." Heat Resistant Materials.
Proceedings of the First International
Conference, Fontana.
• Foulds, J., Viswanathan, R. (2001).
"Graphitization of Steels in ElevatedTemperature Service." Journal of Materials
Engineering and Performance, 10(4).