8/11/2019 Steel Plate Investigation

1/17

2. Visual examination

6 cm

- UT marking is 60 mm apart from C-seam weld.

33 mmt

R.D.

8/11/2019 Steel Plate Investigation

2/17

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

8/11/2019 Steel Plate Investigation

3/17

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.

8/11/2019 Steel Plate Investigation

4/17

5. Macroscopic examination

1) Cutting

t

2) Macro image of cut surface

- Dark line appears near mid-thickness

8/11/2019 Steel Plate Investigation

5/17

3) Cutting and milling

Macro image of specimen extracted for

microscopic examination

8/11/2019 Steel Plate Investigation

6/17

6. Hardness

- Dark lines are martensite and bainite, thus high in hardness, 400 Hv maximum.

8/11/2019 Steel Plate Investigation

7/17

125m 125m

- All Cracks propagate through the martensite layers that are high in hardness.

8/11/2019 Steel Plate Investigation

8/17

7. SEM examination (crack #1)

ACF

8/11/2019 Steel Plate Investigation

9/17

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).

8/11/2019 Steel Plate Investigation

10/17

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

8/11/2019 Steel Plate Investigation

11/17

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

8/11/2019 Steel Plate Investigation

12/17

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

8/11/2019 Steel Plate Investigation

13/17

8. SEM examination (crack #2)

ABF

one end of crack the other end of crack

8/11/2019 Steel Plate Investigation

14/17

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).

8/11/2019 Steel Plate Investigation

15/17

8/11/2019 Steel Plate Investigation

16/17

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.

8/11/2019 Steel Plate Investigation

17/17

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.