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Failure Analysis of Repeat Tooth Breakage of a
40MW Steam Turbine Load Gearbox
William Liu
Industrial Services, SGS New Zealand Ltd
The Maintenance History of 40MW
Steam Turbine Load Gearbox
Commissioned in 1998.
The First Tooth Breakage
in 2003 (43,943 hours/109
cycles).
The Second Tooth
Breakage in 2010 (35,231
hours/109 cycles).
2
Heat Treatment Specifications
Steel - 17CrNiMo6
- Hardenability good enough for the gear mode
Normalizing
Then, Case Hardening
- deep case gas carburizing
Case Depth: >1.7mm
Case Hardness: 58 – 61HRC
3
Fractographic Examination
Fatigue origin A
Delaminating between
the case and core
B
FFF Crack
Fatigue origin A
The broken segment
Active flank
Non-active flank
The broken gear-tooth
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A Typical Fatigue Tooth Breakage
- Beachmarks
Stereomicroscope (7X).
5
The Final Fast Fracture (FFF) Area
The river/chevron
patterns – cleavage
(brittle) fracture.
B Pint
B point – FFF crack
origin (did not continue
from the existing
fatigue cracks).
6
The Oil Degradation Deposits
- An Auxiliary Fractographic Method
7
Hardness Tests and the Case Depth Measurement
SPECIFICATIONS
•Surface Hardness: 58 – 62 HRC
•Case depth: >1.7 mm
Conclusions:
Insufficient case hardness
Tip
56 HRC
Insufficient case depth (shallow case)
1.2 mm
0.9 mm
0.9 mm
Non-active flank
43 HRC
Core
41 HRC
Active flank
43 HRC
8
The Crack Propagation in the Subsurface
Due to the Shallow Harden Case
Sourced from: ASM Handbook, Volume 11, Failure Analysis and Prevention, Page 627.
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Metallographic Examination
Carburized Case (500X)
Core (500X)
Conclusion: Direct quenching (improper heat treatment)
10
Improper Heat Treatment vs Proper Heat Treatment
Improper Heat
Treatment
Proper Heat Treatment
11
Microstructural Alternations - “Butterfly”
Microcracks
Before etching (50x)
“Butterfly”
After etching (50x)
12
The 3 Unjustified Previous
Conclusions of Butterfly
High Hertzian
(contact) stress.
Microcrack - Fatigue
initiation.
Induced by nonmetallic inclusions,
carbides.
A Typical Butterfly (500x)
13
A Butterfly in the Gear-root
– Exclude the 3 Previous Conclusions
Transgranular Crack
Butterfly
Gear-tooth
Root
Irrelevant to Hertzian
stress.
The butterfly
microcrack did not
initiate to fatigue.
No inclusion/carbide
found in the butterfly.
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No Inclusion/Carbide Found in a Butterfly
EDS1
EDS2
EDS2
EDS1
Elemen
t
CrK
MnK
FeK
NiK
Total
Wt %
1.99
1.44
95.26
1.32
100
At %
2.13
1.46
95.16
1.25
100
Elemen
t
CrK
MnK
FeK
NiK
Total
Wt %
2.05
1.61
94.93
1.41
100
At %
2.2
1.63
94.82
1.34
100
15
The Fatigue Crack Vs the Butterfly Microcracks
Fatigue Crack
Microcracks in the
Propagation
‘Butterfly’ pattern
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The Fractoraphy of the Butterflies
The Fatigue
Microcrack Origin
in the Butterflies
Crack
Propagation
Butterfly
Fractography
Stereomicroscope (20X)
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Summary
Practical Aspect:
The root cause of repeat failures – Improper heat
treatment.
Successful solution – replaced gear from a different
manufacturer.
Academic Aspect:
Some previous conclusions regarding ‘butterfly’ are not
justified.
Reveal the fractography of butterfly.
Oil deposit - auxiliary fractographic method
18

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