PSI - Issue 42

Lisa Claeys et al. / Procedia Structural Integrity 42 (2022) 390–397 Claeys et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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performed in air. For the hydrogen condition, the tensile tests were performed after precharging for seven days. A comparable strain rate was used for both materials, i.e. 5E-5 s -1 for the TWIP steel and 3.3E-5 s -1 for 304L ASS. The gauge dimensions of the dogbone-shaped tensile specimens for both materials were 10x4x0.7 mm³, the thickness being in accordance with the melt extraction specimens. The surfaces were also polished up to 1 µm. Electron backscatter diffraction (EBSD) was used for the characterization of the microstructure and the active deformation mechanisms on the intermediately tensile tested specimens. A FEI Quanta field emission gun 450 scanning electron microscope (SEM), equipped with an EBSD detector, was used for this purpose. The SEM was operated with an accelerating voltage of 20 kV and a spot size of 5 nm. The specimen was tilted to 70° for EBSD. The specimens were mechanically polished up to 0.04 µm (OP-U, colloidal silica) to get high-quality EBSD measurements, which occurred after tensile testing for the evaluation of the active deformation mechanisms. Post processing of the EBSD measurements was done with TSL-OIM Data analysis V7.3 software. Points with a confidence index (CI) below 0.1 were excluded from the measurements (Field (1997)). Deformation twin boundaries were defined with the OIM software based on their orientation relationship, i.e. a Σ 3 character (60 degrees @ <111>). Secondary electron (SE) imaging was additionally used to characterize hydrogen-assisted cracking (HAC). 3. Results and discussion Fig. 1 shows normal direction (ND) inverse pole figure (IPF) maps of the initial microstructure created through post-processing of EBSD measurements. Images of the transverse direction (TD) – rolling direction (RD) plane are presented. Both materials showed a well-annealed microstructure with polygonal grains and a significant amount of annealing twins. For 304L ASS, the fraction of delta ferrite was restricted to <1%. The TWIP steel showed a fully austenitic microstructure as well. The texture can be regarded as random. Please note that the scale bars are different for both images, meaning that the average grain size of the TWIP steels (17.4 ± 8.9 µm) is about 2.5 times larger than the average grain size of the 304L ASS (7.33 ± 1.77 µm).

a

b

//ND

TD

RD

60 µm

25 µm

Fig. 1: ND IPF maps of (a) 304L ASS; (b) 18Mn-0.6C TWIP steel

Representative engineering stress/strain curves for the two materials are shown in Fig. 2. The engineering stress was calculated from the force applied to the specimen divided by the initial cross-sectional area. The engineering strain was evaluated by the crosshead displacement divided by the initial length of the gauge section. The figure includes the reference condition as well as the seven days hydrogen-charged condition. Both austenitic steel types were clearly characterized by a relatively low yield strength. Due to a high level of work hardening, both materials eventually reached a high ultimate tensile strength. Moreover, large strain values were obtained. The TWIP steel showed the most attractive combination of strength and ductility reaching higher values for both parameters. In austenitic steels, this outstanding combination of properties is obtained by the action of alternative deformation mechanisms. The preferred mechanism depends on the chemical stability and the stacking fault energy of the material. In the present work, the active deformation mechanisms were evaluated through EBSD on the ND plane of specimens