PSI - Issue 13

A. Laureys et al. / Procedia Structural Integrity 13 (2018) 1330–1335 Author name / Structural Integrity Procedia 00 (2018) 000–000

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combinations of charging times, ranging from 2 minutes to 1 hour and different applied current densities (0.8, 5, 20 mA/cm²). These variable charging conditions allowed to introduce variable amounts of blisters into the material. The material surface was ground prior to electrochemical hydrogen charging. 2.3. Melt extraction Melt extraction is used to determine the total amount of hydrogen inside the material. The hydrogen content is determined with a GALILEO G8 ON/H (Bruker) device. Small rectangular samples were machined with dimensions of 6 x 8 x 1.7 mm³. The specimens were electrochemically charged and subsequently, introduced into a pulse furnace where they are quickly heated to 1550 °C. Upon heating, H 2 -gas is released and taken up by a N 2 -flow, which is sent to a thermal conductivity measuring cell. The hydrogen concentration of the sample is calculated based on the thermal TDS is commonly used to assess the hydrogen trapping characteristics of a material microstructure. Oval shaped samples with a major axis of 20 mm and a minor axis of 15 mm and a thickness of 1.7 mm were machined. The major axis of the samples coincides with the rolling direction of the plate. Hydrogen was introduced in the material by electrochemical charging. Spectra of the hydrogen desorption rate as a function of the heating temperature are generated, at a fixed heating rate of 1200 °C/s. Analysis of such spectra allow to identify several peaks corresponding to different trapping sites in the material releasing their trapped hydrogen. 3. Results and discussion The material was subjected to a variety of hydrogen charging conditions in order to obtain variable amounts of hydrogen induced damage. Samples were evaluated with melt extraction and TDS to assess if blisters and internal damage manifested during such measurements. First, specific charging conditions were selected in order to obtain samples with equal amounts of hydrogen, but variable amounts of damage. Furthermore, two additional types of test conditions were performed by TDS in order to even better assess the effect of hydrogen induced damage on the TDS spectra. These two test conditions were (i) applying shorter charging times and (ii) waiting 48 h after hydrogen charging prior to starting the TDS measurement. 3.1. Effect charging conditions on hydrogen induced damage The material was subjected to three different charging current densities, i.e. 0.8, 5 and 20 mA/cm² for various times. Curves displaying the hydrogen amount for a certain charging time (cf. Fig. 1) were generated through melt extraction. conductivity variation of the gas flow. 2.4. Thermal desorption spectroscopy

0 1 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 60 70 80 H content (wppm) Charging time (min) 0.8 mA/cm² 5 mA/cm² 20 mA/cm²

Figure 1: Charging time vs. wppm H for cold deformed ULC steel charged at 0.8, 5 and 20 mA/cm².