PSI - Issue 42

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

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The fracture surfaces both in air and in H are analysed using SEM in order to investigate the responsible HE mechanisms. Figure 6 shows the difference in fracture surface for the samples tested in air and in a H rich environment. In air, a mixed fracture surface is obtained which shows both intergranular features (indicated by white arrows) and zones dominated by the presence of carbides (under the orange dotted line). However, the fracture behavior changes to a high extent when tested in an H rich environment since a transition to a purely intergranular fracture surface is detected. Thus, H embrittles the PAGBs which can be explained by the hydrogen enhanced decohesion (HEDE) mechanism. HEDE is the most common embrittlement mechanism for martensitic steels, meaning that H trapped at the HAGBs weakens the cohesive forces at these boundaries which leads to premature fracture (Oriani, 1972; Troiano, 2016). Moreover, carbides (indicated by red arrows) and detached carbide holes (indicated by red circles) are clearly visible on the top of the intergranular facets. As a clear decohesion between the martensitic matrix and the carbides is detected in the H charged samples, the HEDE mechanism is also active on the carbide interface. Moreover, the detached Cr-carbides are located at the PAGBs and thus these carbides with their weakened interface can enhance crack initiation and propagation along the PAGBs. In order to further validate this statement, EBSD analysis of an H assisted crack on the ND plane of a bending sample is performed. The EBSD scan in Figure 7 shows indeed that the crack propagation path is highly influenced by the presence of the Cr carbides.

Figure 6: SEM fracture surface analysis of the industrial 100Cr6 bearing material. In air, the fracture surface shows both intergranular features (indicated by white arrows) and fracture dominated by the presence of carbides (separated by the yellow dashed line). The sample fractured in H shows an intergranular fracture surface with clear Cr carbides (red arrows) or detached carbides (red circles) on the PAGBs.

Figure 7: (A) SEM image and corresponding (B) phase map and (C) IPF map of a H induced crack on the ND surface of a H charged 100Cr6 sample after being subjected to the in-situ bending test.

For the Fe-8Al-1.1C materials, the fracture surfaces in Figure 8 of the samples tested in air and of those in H are very similar; a cleavage type of fracture dominates both fracture surfaces. This observation indicates that the martensitic substructure, more specifically the martensitic blocks and packets boundaries, are the preferential path for fracture