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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propagation. Even though a similar fracture surface is detected, fracture in an H environment occurs at smaller extension values because H diffuses to these susceptible boundaries where it can cause accelerated decohesion (according to the HEDE mechanism). Figure 9 shows the EBSD scan of a crack on the ND plane of a bending sample tested in H, which indicates that the H-assisted crack propagates in a transgranular way, through the martensitic blocks.

Figure 8: SEM fracture surface analysis of the lightweight Fe-8Al-1.1C bearing material. Both the samples tested in air and in a H rich environment show a cleavage type of fracture behavior.

Figure 9: (A) SEM image and corresponding (B) IPF map of an H induced crack on the ND surface of an H charged Fe-8Al-1.1C sample after being subjected to the in-situ bending test.

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Conclusions

This paper compares the resistance to HE of two bearing steels: an industrial steel grade (100Cr6) and a newly developed lightweight alternative (Fe-8Al-1.1C). Microstructural characterization with SEM indicates that the 100Cr6 material contains a high amount of Cr-carbides whereas the Fe-8Al-1.1C material shows very small kappa carbides and quench cracks along the PAGBs. Although the two bearing steels demonstrate a similar hardness value around 800HV, their interaction with H is completely different. The large Cr-based carbides in the industrial bearing steel act as H traps thus increasing the H saturation limit and lowering the H diffusion coefficient, as compared with the lightweight steel. However, these carbides also act as fracture initiation sites when mechanically tested in an H rich environment. Fracture surface analysis shows a transition from a mixed type of fracture dominated by the presence of the carbides in air to an intergranular fracture behavior in H for the 100Cr6 steel, meaning that the cohesive forces of the PAGBs are weakened by H. This effect is even more accelerated by the hydrogen enhanced decohesion of the interface of the Cr-carbides located at these boundaries. In contrast, the fracture behavior of the Fe-8Al-1.1C material is similar in air and in an H containing environment, indicating that H does not influence the fracture behaviour of the