PSI - Issue 54

Magdalena Eškinja et al. / Procedia Structural Integrity 54 (2024) 123–134 M. Eškinja et al./ Structural Integrity Procedia 00 (2019) 000 – 000

128

6

3. Results and discussion 3.1. Hydrogen absorption and mechanical behaviour

TDS spectra revealed presence of one low-temperature peak below 300 °C in case of both steels, as seen in Figure 3. It is common to refer to hydrogen trapped below that temperature as diffusible hydrogen [13]. Diffusible hydrogen can move easily through interstitial sites in crystal lattice; while trapped hydrogen is fostered around crystal imperfections. Therefore, diffusible hydrogen is known as the origin of deterioration of mechanical properties in steel and it is principal to control this content to diminish sensitivity of the materials to HE. High Mo content steel clearly showed higher desorption rate and increased uptake of hydrogen when compared to steel with lower Mo content. In addition, center of the peak is shifted to slightly higher temperature for HMoS, indicating presence of relatively stronger hydrogen traps in this material. Deconvolution of desorption spectra was not performed since in lath martensite microstructure there are several reversible hydrogen traps with low activation energies (15-35 kJ/mol) such as dislocations, laths, grain boundaries, what consequently makes it complex to distinguish between them with very close desorption energies. It is rather postulated in literature that activation energy at low temperature is average value of desorption from these sites together, including also interstitial hydrogen in lattice [14].

Fig. 3. Desorption profiles for (a) HMoS and (b) LMoS after electrochemical hydrogen charging at heating rates of 200, 400, 800 and 1200 K/h.

The stress strain curves for H-uncharged and H-charged specimens are shown in Figure 4. The strength of uncharged HMoS is higher compared to LMoS, while elongation is similar in both cases. Once specimens are subjected to hydrogen environment, elongation decreased significantly and there is a notable reduction of plastic region. Hydrogen induced strengthening is observable as well. To establish the loss of mechanical properties instigated by hydrogen, hydrogen embrittlement susceptibility index ( I HES ) was calculated by the following expression (2): = − (2) where δ air is total elongation in air and δ H total elongation in hydrogen environment. Calculation of susceptibility index pointed out higher susceptibility (59.6%) for HMoS, whereby the susceptibility for LMoS was 57.5%. The latter, corroborates good the results obtained by TDS measurements suggesting higher diffusible hydrogen uptake for HMoS and thus slightly elevated deterioration of mechanical properties.