PSI - Issue 14

Rajwinder Singh et al. / Procedia Structural Integrity 14 (2019) 930–936 Author name / Structural Integrity Procedia 00 (2018) 000–000

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3. Results and discussion The SEM micrograph of SA508 Gr. 3 Cl. I LAS is shown in Fig. 2. The microstructure of the subject LAS is tempered bainitic composed of bainite-ferrite matrix and randomly distributed M/A islands. The fine carbides are distributed within the prior austenite grains whereas coarse long-rod type carbides are distributed along the PAGBs. The average prior austenite grain size of the materials is 20 µm.

M/A island

PAGBs

fine carbides

long-rod carbides

Fig. 2. SEM micrograph of microstructure of SA508 Gr. 3 Cl. I LAS

The comparison of the stress-strain curve of the hydrogen charged and un-charged SA508 Gr. 3 Cl. I LAS is shown in Fig. 3. The yield of the material increased from 523.3 MPa to 540.8 MPa after hydrogen charging. This is due to the pinning of the dislocations caused by the hydrogen present in the material (Wu and Kim (2003)). The Luders bands are formed in both un-charged and hydrogen charged conditions as shown in the inset of Fig. 3. The high dislocation density in the Luders band region will act as trapping site for hydrogen and this trapped hydrogen will then assist the dislocation motion in the Luders bands region. This will cause localized strain in the Luders band region and thus will cause micro-void formation at lower value of applied strain (Wu and Kim (2003)). This phenomenon has reduced the ultimate tensile strength from 663.03 MPa to 642.38 MPa and percentage elongation (ductility) from 28.48% to 21.85% of the subject LAS after hydrogen charging.

700

charged un-charged

600

500

400

300

200 stress (MPa)

100

0

0

5

10

15

20 25

30

strain (%)

Fig. 3. Comparison of stress-strain curve of hydrogen charged and un-charged SA508 Gr. 3 Cl. I LAS