Serviceability of Graphitized Carbon Steel

Evan Vokes

Dr Weixing Chen

Outline

• 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

Primary graphite

Cast Iron

Product of cementite decomposition

Related to Chemistry

Phase transform

Secondary graphite Steels

Several mechanisms

Related to Thermo-Mechanical History

Solid state phase transform

Competition between formation of cementite and carbon

Secondary Graphitization mechanisms in steel 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

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

3.6

3.7

3.8

3.9

4

4.1

4.2

4.3

4.4

30 31 32 33 34 35 36 37 38

LMP

Lo

g S

tres

s (k

si)

Reducer 3

Flange 6

High Temperature Creep Properties 2

Larson Miller A 106 B referenced to ASTM DS 11S1

3.6

3.7

3.8

3.9

4

4.1

4.2

4.3

4.4

30 31 32 33 34 35 36 37 38

LMP

Lo

g S

tres

s (k

si)

Elbow 4

Elbow 1

Elbow 1 Weld

High Temperature Creep Properties 3 Stress Sensitivity

0

2

4

6

8

10

12

14

16

18

475 500 525 550 575 600 625

Temperature (°C)

Str

es

s E

xp

on

en

t (n

)

Elbow 1

Reducer 3

Elbow 4

Flange 6

Elbow 7

new Elbow 7

API 530

High Temperature Creep Properties 4 Ductility Relations

Creep Strain to Failure Relations

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

10.0 100.0 1000.0 10000.0

Rupture time

Per

cen

t S

trai

n

Reducer 3Elbow 1

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 Properties1Tensile testing

350

375

400

425

450

475

500

180 200 220 240 260 280 300

Yield Strength (MPa)

Ult

imat

e T

ensi

le S

tren

gth

(M

Pa)

Elbow 1

Flange 6, Reducer 3

All other elbows

Mechanical Properties 2Tensile testing

0

50

100

150

200

250

300

350

400

450

500

0 10 20 30 40 50 60

% Strain

Str

ess

(MP

a)

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

Mechanical Properties 4Hot Tensile testing @427°C

200

250

300

350

400

450

100 150 200 250

Yield Strength (MPa)

Ulti

mat

e T

ensi

le S

tren

gth

(MP

a)

min design API 530

DS11S1

Elbow 7

Elbow 4

Flange 6

Elbow 1

pipe

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

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