PSI - Issue 13

Evy De Bruycker et al. / Procedia Structural Integrity 13 (2018) 226–231 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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 Simulation of a two step Post Weld Heat Treated heat affected zone: T24 PWHT (20' at 1200°C, water quenching, 1h at 600°C, air cooling, 1h at 750°C, air cooling) The corresponding average hardness values for these different test material conditions are summarized in Table 2.

Table 2. Average Vickers HV10 hardness values of the tested materials. Material condition Hardness ( HV10 ) T12 BM <170 T12 HAZ HH 309 T12 HAZ 247 T24 BM 210 T24 HAZ HH 385 T24 PWHT 280

2.2. Electrochemical charging with hydrogen In order to investigate the interaction of the materials with hydrogen, all samples were electrochemically charged with hydrogen. The conditions used for this charging were:  Electrolyte: H 2 SO 4 (0.5 M) + Thiourea (10 mg/l)  1 mA/cm² Optimization of the charging conditions and time was done on T12 BM. After 2h no further hydrogen pick-up was observed. 2.3. Total and diffusible hydrogen content The total hydrogen and diffusible hydrogen content measurements were measured using the G8 GALILEO equipment from the company Bruker. For the total hydrogen measurement, the sample is heated as quickly as possible up to 1600°C in a nitrogen atmosphere. At this temperature, the metallic sample liquefies and releases its hydrogen as gaseous H 2 . The latter is taken up by a nitrogen flow and the mixture (N 2 -H 2 ) is sent to the thermal conductivity measuring cell. The thermal conductivity of the mixture depends on the H 2 -concentration because of the significant difference in conductivity of H 2 and N 2 . The software calculates the hydrogen concentration of the sample based on the thermal conductivity variation. For the total hydrogen measurements, the calibration is done by using reference samples certified and provided by the company Bruker. The measurement uncertainty can be calculated at a maximum of 0.05 ppm. The diffusible hydrogen is defined as the quantity of the gas which effuses from the sample when held at a temperature of 300°C. The hydrogen which is not trapped at this temperature effuses from the sample in a few minutes. An external furnace is connected to the G8 GALILEO in order to enable the heating of the sample as well as the collection of the released hydrogen which is conveyed with a carrier gas (N 2 ) to the thermal conductivity detector of the equipment. Prior to the test, the equipment is calibrated using an integrated gas calibration unit, the furnace being at the test temperature (300°C). 2.4. Thermal desorption spectrometry (TDS) On selected materials (T12 BM, T24 BM and T24 HAZ HH) TDS was used in order to gain a more in-depth understanding of the interaction of hydrogen with the materials. TDS consists in heating a sample with a constant heating rate increase and measuring the hydrogen quantity that is released as a function of time. If diffusion is not rate limiting, then the release is uniquely due to the detrapping of hydrogen, a phenomenon which is thermally activated.