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 Properties1 Tensile testing

24. Mechanical Properties 2 Tensile testing

25. 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

26. Mechanical Properties 4 Hot Tensile testing @427°C

27. 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

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 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

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).