Serviceability of Graphitized Carbon Steel

1
Serviceability of Graphitized Carbon Steel

• Evan Vokes
• Dr Weixing Chen

2
Outline

• Origin of graphitization
• Microstructure development
• Detection of graphite
• Characterization by Creep methods
• Characterization by Tensile methods
• Characterization by Fracture methods
• Conclusion
• References

3
Where Graphite comes from
Solid state phase transform Competition between
formation of cementite and carbon
g? a Phase transform
Secondary graphite Steels Several
mechanisms Related to Thermo-Mechanical History
Primary graphite Cast Iron Product of cementite
decomposition Related to Chemistry
4
Secondary Graphitization mechanisms in steel
g? a Phase Transform
Martensitic Transforms Result in uniform random
graphitization in laboratory testing Suspected
cause of HAZ graphitization
Box Annealing Transforms Typical of higher carbon
content steels Often found after spherodizing
anneals Random morphology
Time at High Temperature Transforms Two types of
morphologies, Random and Planar
5
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

6
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

7
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

8
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

9
Random graphite

• Heterogeneous nature
• May tend to follow banding in longitudinal
  directions

10
Planar Graphite

• Found in two pieces of piping
• Piping was constrained
• Random graphite present

11
Failure Potential from Furtado and Le May
12
SEM image of planes of graphite
13
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
14
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

15
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

16
High Temperature Creep Properties 1
17
High Temperature Creep Properties 2
18
High Temperature Creep Properties 3 Stress
Sensitivity
19
High Temperature Creep Properties 4 Ductility
Relations
20
High Temperature Creep Properties 5 Post creep
microstructure of graphitized elbows
21
High Temperature Creep Properties 6 Post creep
microstructure near weld
22
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

23
Mechanical Properties1Tensile testing
24
Mechanical Properties 2Tensile testing
25
Mechanical Properties 3Tensile 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

26
Mechanical Properties 4Hot Tensile testing _at_427C
27
Mechanical Properties 5Hot Tensile testing _at_427C

• All mechanical strengths are quite good
  considering the microstructure damage
• Materials tested have similar rankings as
  compared to room temperature properties

28
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

29
Fracture 2
30
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

31
Conclusion

• Random Graphitization has mechanical creep and
  fracture properties that indicate that it is
  still serviceable
• Random graphite can not be considered benign
• Random graphites association with planar
  graphite is known but it is not known how one
  morphology becomes the other
• Planar graphite is just plain dangerous

32
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

33
Thank you

• Nova Chemicals
• NSERC
• Canspec Materials Engineering

34
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 Elevated-Temperature
  Service." Journal of Materials Engineering and
  Performance, 10(4).