PSI - Issue 42

Margot Pinson et al. / Procedia Structural Integrity 42 (2022) 471–479 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1.1C steel. As dislocations are effective H trapping sites, the 100Cr6 material can store more H (Depover & Verbeken, 2018). Secondly, the 100Cr6 steel has a lower PAG size. The smaller the grains, the more HAGBs and thus the more H can be trapped inside the material. Finally, the difference in carbide fraction also plays an important role. Depover et al. (Depover et al., 2015; Depover & Verbeken, 2016) showed that Cr-based carbides are effective H traps and thus the higher volume fraction of carbides in the 100Cr6 material will increase the amount of H trapping sites. Since more H can be trapped at the dislocations, HAGBs and carbides in the 100Cr6 material as compared to the Fe-8Al-1.1C material, the H diffusion coefficient decreases as the movement of H is slowed down by effective H trapping sites.

Figure 4: H saturation curves of the two bearing materials, measured by melt extraction. The 2 nd law of Fick (shown as lines) is fitted to the experimental data (shown as individual points with their experimental deviation).

3.3

HE mechanisms of bearing steels

To determine the degree of HE, bending tests both in air and in an H rich environment (saturated in-situ) are performed and the results are given in Figure 5. All bending tests are performed at a crosshead deformation speed of 1 mm/min. The fracture stress in air is lower for the lightweight Fe-8Al-1.1C steel and is most likely linked to the quench cracks. However, an optimized heat treatment could circumvent this issue. Nevertheless, the Fe-8Al-1.1C material can attain higher values of flexure stresses before final fracture, when tested in H. By comparing the amount of extension in air and in H, the embrittlement index (EI) can be calculated. The EI of Fe-8Al-1.1C and 100Cr6 is 55 % and 85 %, respectively. The lower EI of the innovative Fe-8AL-1.1C steel implies that it has a better resistance to HE than 100Cr6.

Figure 5: Bending curves of the two bearing materials both tested in air and in a H rich environment after H saturation. The percentages show the degree of HE.