steel plate investigation
TRANSCRIPT
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2. Visual examination
6 cm
- UT marking is 60 mm apart from C-seam weld.
33 mmt
R.D.
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3. Mill sheet information
1) Grade of steel: 355K2
C Si Mn P S Al Nb Ti B N
0.15 0.33 1.52 0.016 0.002 0.031 0.020 0.013 0.0008 0.0043
Y.S T.S El RA Cv (-20C)
464 583 30 - 238
2) chemical composition
3) mechanical properties
4) UT classes: S1E1, UT inspection: OK
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4. UT C-scan examination
UT C-scan imagePlate received
- Many UT defect were found at the UT making region.
- UT defects are elongated parallel to rolling direction.
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5. Macroscopic examination
1) Cutting
t
2) Macro image of cut surface
- Dark line appears near mid-thickness
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3) Cutting and milling
Macro image of specimen extracted for
microscopic examination
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6. Hardness
- Dark lines are martensite and bainite, thus high in hardness, 400 Hv maximum.
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125m 125m
- All Cracks propagate through the martensite layers that are high in hardness.
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7. SEM examination (crack #1)
ACF
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BDE
Characteristics of crack #1
(1) Branching crack within the martensite phase
(2) Crack tends to follow lath lath boundries, i.e. (110) plane
(3) No inclusions inside the crack
These characteristics demonstrate that this crack is hydrogen induced crack (HIC).
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F
Element Wt%
NK 06.45
OK 01.52
NbL 48.34
TiK 41.07
FeK 02.61
Matrix Correction
(Nb,Ti)-nitride
Nb Ti
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C
(Nb,Ti)-nitride
Nb Ti
Element Wt%
NK 05.06
SiK 00.45
NbL 85.05
TiK 05.40
MnK 00.25
FeK 03.79
Matrix Correction
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C
Martensite layer is enriched with Mn.
aElement Wt% At%
SiK 00.82 01.61
MnK 03.37 03.40
FeK 95.81 94.99
Matrix Correction ZAF
Region a
Region bb
Element Wt% At%
SiK 00.55 01.09
MnK 01.92 01.94
FeK 97.53 96.97
Matrix Correction ZAF
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8. SEM examination (crack #2)
ABF
one end of crack the other end of crack
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D G H
Characteristics of crack #2 (same as crack #1)
(1) Branching crack within the martensite phase
(2) Crack tends to follow lath lath boundries, i.e. (110) plane
(3) No inclusions inside the crack
These characteristics further demonstrate this crack is hydrogen induced crack (HIC).
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C E
MnS (Nb,Ti)N
Additional Characteristics of crack #2
(1) Crack follows elongated MnS locally formed within martensite phase.
(2) Crack follows (Nb,Ti)N inclusions locally formed within martensite phase.
These facts imply that the inclusions such as MnS and (Nb,Ti)N provide
preferential route for HIC propagation.
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9. Conclusions
1. UT defects reported in steel plates are the cracks locating in the basemetal and at near mid-thickness area where martensite structure
formed due to the alloy segregation such as Mn.
2. Even though MnS and (Nb,Ti)-nitride particles are present, these
inclusions can not be the main cause of the crack as these particles are
extremely small in proportion compared with the total length of cracks.
3. As the present cracks have all the characteristics of hydrogen
embrittlement in martensitic steel, it is concluded that these cracks are
hydrogen-induced cracks (HIC).
4. In order to prevent the occurrence of such cracks, it is highly
recommended to introduce hydrogen control practice during steel
making practice.