PSI - Issue 42

Simon Vander Vennet et al. / Procedia Structural Integrity 42 (2022) 813–820 S. Vander Vennet / Structur Integrity Procedia 00 (2019) 000–00

816 4

TRIP Q&P

1000

800

600

400

200

Engineering Stress (MPa)

0

0

5

10 15 20 25 30 Engineering Strain (%)

Fig. 2: Engineering stress-strain graphs for TRIP and Q&P

2.4. Hydrogen / material interaction

In order to determine the trapping sites present in the material, TDS was used on a Bruker Galileo G8 ON / H. For TDS, a sample charged with hydrogen was heated in an infrared furnace to 850 ◦ C at a constant heating rate of 1200 ◦ C / h. Upon heating, the hydrogen is released from the sample as gaseous H 2 and measured by a quadrupole mass spectrometer (IPI-ESD 100) using a N 2 carrier gas.

3. Results

3.1. Characterisation

Before performing electrochemical hydrogen permeation at a constant load, the YS of each material was determined. Figure 2 shows the engineering stress versus strain curves of TRIP and Q&P. Both the yield strength and ultimate tensile strength measured for Q&P are significantly higher than for TRIP. Generally, this is due to the presence of martensite in Q&P (Edmonds et al. (2006); Thomas et al. (2011)) and in industrial steels this e ff ect is sometimes enhanced due to precipitation hardening by microalloying with Nb, Ti, Mo, V in order to form nano-sized carbides (Soleimani et al. (2020)). Using the 0.2% strain o ff set method, the YS of TRIP and Q&P is determined at 415MPa and 700MPa, respectively. For TRIP and Q&P, the RA fraction is characterised by XRD. The full scanned spectrum is presented in Figure 3a, showing the respective BCC and FCC peaks utilised for the determination of the RA fraction. The volume fraction of RA is determined at 13.7 % for TRIP and 11.0 % for Q&P. More importantly, however, is the peak shape of RA since it contains additional information about the austenite phase. First of all, the RA peak of Q&P is shifted towards a higher di ff raction angle, indicating the presence of a compressive stress on the RA in Q&P, which was already demonstrated by Allain et al. (2018) using high energy XRD. Additionally, the RA peak in Q&P is broadened compared to TRIP, which is a characteristic for microstresses present in the RA (Maeder et al. (1981)).

3.2. Electrochemical hydrogen permeation

In order to compare the change in di ff usivity in all three materials, electrochemical hydrogen permeation was performed at increasingly higher constant loads. The obtained D app for each condition tested is summarized in Figure 4. In the unstressed condition (0% YS), the apparent di ff usivity of hydrogen is the highest in DP and quite

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