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 @427°C

27. Mechanical Properties 5Hot 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).