PSI - Issue 68

Adam Ståhlkrantz et al. / Procedia Structural Integrity 68 (2025) 1051–1058 Author name / Structural Integrity Procedia 00 (2025) 000–000

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samples were exposed to a load corresponding to 75% of the yield strength. The electrolyte used consisted of 0.5 M H₂SO₄ with 1 g/L thiourea. The current density used was -1 mA/cm² for a total time of 1 hour. Thermal desorption mass spectroscopy (TDMS) was used to identify the trapping and type of trapped hydrogen in a representative industrially produced material with 6-7% RA. The testing was conducted in a Bruker (Galileo G8 model) coupled with mass spectrometry from In Process Instruments (TDMS) with an infrared quartz tube furnace from Bruker (IR07 model). The instrument was calibrated prior to testing using pure H₂ gas with three defined volumes. The samples were then tested by heating from room temperature up to 900 °C, with a heating rate of 0.33 °C/s. 3. Results and discussion The different heat treatment cycles resulted in five complex microstructures with different combinations of ferrite, bainite and martensite. The types of heat treatment cycles are shown in Table 2. The aim with the different heat treatment cycles was to obtain different amounts of RA which then could be charged and used to understand the interaction between RA and hydrogen. To ensure that strongly trapped hydrogen was present in the specimens, TDMS was conducted. Figure 1 shows representative TDMS curves of non-charged (blue curve) and charged samples (red curve). Constant tensile load corresponding to 75% of the steel yield strength was applied during the charging to achieve deep trapping. The red curve shows that by utilizing this hydrogen charging procedure diffusible and trapped hydrogen were present in the specimen.

Figure 1: TDMS curves of non-charged (blue curve) and charged sample (red curve) from a representative sample used.

The microstructures for the five different heat treatments were investigated with XRD and EBSD. XRD is a better quantitative method to measure the amount of RA, and the etching before with Piranha removed any mechanically deformation in the surface region, hence allowing for a more accurate measurement. The amount of detected RA in the samples is presented in Table 2. It can be noted that sample A has a very low amount of RA, which is most likely just background, and no detectable amount of RA was found. However, the same settings for the Rietveld refinement were used for all five samples in the analysis. Figure 4 presents two sets of images for each of the heat treatment cycles. One set is the band contrast images (BC) obtained from the EBSD, showing the microstructure, and the second set shows the successfully indexed pixels colored according to indexed phase. The phase fractions of RA measured in EBSD are clearly lower than the amount of RA measured with the XRD. While EBSD gives an indication of the distribution of the RA from the phase fractions it is not a good quantitative method for measurement of RA volume fraction. This is due to the resolution limitations of EBSD, as well as the manner in which the samples are prepared, where the changed stress state resulting from opening the cross-section surface has resulted in the transformation of the least stable austenite into fresh martensite. This is also seen in the BC images, where the dark areas most likely correspond to fresh martensite. This is because these freshly transformed areas contain many more dislocations and grain boundaries, resulting in more noise and correspondingly lower Kikuchi pattern contrast in the EBSD, translating as darker colors in the BC map.