PSI - Issue 54

Daria Pałgan et al. / Procedia Structural Integrity 54 (2024) 322 –331

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Daria Pałgan et al./ Structural Integrity Procedia 00 (2023) 000 – 000

for A516 and austenitic grains with delta ferrite bands for 304L and 316L steels. Note that the delta ferrite amount was somewhat higher in 304L as compared to the 316L steel. Coupons with size of 25 mm x 10 mm x 3 mm were extracted from the delivered plates of the steels. Same size of samples was used for charging all steel samples with hydrogen and by using both charging methods. All surfaces of the samples were mechanically ground in steps down to 1200P SiC abrasive paper and prior gaseous charging all samples were again reground to make sure that the oxide was removed.

Table 1. Chemical composition of investigated steels.

Element wt. % A516 Grade 70

C

Mn

P

Si

S

N

Cr

Ni

Mo

Cu 0.4

Fe

0.28 0.03 0.03

0.8-1.3

-

0.1-0.5

-

-

-

0.4

- -

Bal. Bal. Bal.

304L (UNS S30403) 316L (UNS S31603)

2.00 2.00

0.045 0.045

0.75 0.75

0.03 0.03

0.1 18.00-20.00

8.00-12.00

- -

0.1 16.00-18.00 10.00-14.00 2.00-3.00

Figure 1. Microstructure of investigated steels a) A516; (b) 304L; (c) 316L. Rolling direction (RD) and the direction normal to RD (ND) is indicated in the images.

Two electrode setup was used to charge the samples with hydrogen in aqueous electrolyte. The sample was cathode while a Pt wire was used as anode. A Pt wire was welded to the samples surface to ensure electrical contact. A potentiostat was used to impose cathodic current on the samples. Table 2 summarizes the electrolytes and different cathodic charging conditions including different charging parameters used to charge the steel samples. Duplicate samples per condition were charged. After charging the samples were removed from the electrolyte rinsed with deionized water and ethanol, dried with cool air, and stored in liquid nitrogen until hydrogen analysis.

Table 2. Cathodic hydrogen charging conditions used to charge the steel samples with hydrogen in this work.

Material

Electrolyte

Current density (mA/cm 2 )

Charging time (h)

Temperature (°C)

1 and 10

24, 48 and 72

25, 50 and 80

3.5 wt.% NaCl + 0.3 wt.% NH 4 SCN 0.1M NaOH + 0.3 wt.% NH 4 SCN

A516 Grade 70

304L (UNS S30403) 316L (UNS S31603)

20 and 100

24, 48 and 72

25, 50 and 80

The gaseous hydrogen charging of the samples was carried out in an autoclave at a pressure of 200 bar of pure H 2 gas. Duplicate samples of each steel type (total 6 samples) were inserted into the autoclave and charged simultaneously for the same time and temperature condition. To ensure an oxygen-free environment in the autoclave the autoclave was purged repeatedly with pure H 2 gas up and down to 200 bars. Once a stable pressure of 200 bar was achieved, after 5 times of purging, the autoclave was inserted into a tube furnace set to a predetermined temperature. A thermocouple was attached to the wall of the autoclave and the temperature was monitored during the entire charging time. Two temperatures 180 °C and 360 °C and two times 24 and 72 hours were used for the gaseous hydrogen charging. After charging the autoclave was removed from the tube furnace and left to cool down to room temperature