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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treatment to control the carbide formation prior to the quenching treatment. For the industrial bearing steel, an optimized soft annealing route as outlined by Luzginova et al. (Luzginova et al., 2008) is applied. Afterwards, the 100Cr6 samples are austenitized at 860°C for 25 minutes and quenched in water to obtain a martensitic microstructure. A final tempering procedure at 160°C for 90 min is performed to partly relieve the high stresses imposed by the quenching treatment. The heat treatment of the Fe-8Al-1.1C samples is designed based on the study of Springer et al. (Springer et al., 2019). Soft annealing takes place at 700°C for 16h in order to form kappa carbides. Afterwards, the samples are fully austenized at 1150°C for 10min and quenched to form a martensitic structure. Finally, the samples are tempered at 250°C for 90min. The microstructure of both alloys is investigated by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) by a FEI Quanta 450 with field emission gun operated at an acceleration voltage of 20 kV and a spot size of 5 nm. Therefore, all samples are ground and polished up to and 1 µm diamond paste. For SEM, the samples are etched with 2% nital for the industrial bearing material and Vilella’s etchant for the lightweight material. For EBSD analysis, an extra polishing step with colloidal silica (OPS) is performed. The scan step size for the EBSD measurements is set to 0.1 µm on a hexagonal scan grid and a working distance of 16mm is used. When a more detailed microstructural characterization is needed, transmission electron microscopy (TEM) is applied. Therefore, thin foil samples are prepared by grinding and polishing the samples to a thickness below 100 µm. Subsequently, the thin foils are electropolished using a TenuPol-5 electropolishing unit in a 10% perchloric acid and 90% acetic acid solution. Moreover, X-ray diffraction (XRD) is performed on polished samples with a Siemens D5000 diffractometer using MoKa radiation at 40kV and 50mA in order to quantify the amount of retained austenite. The bulk hardness of the materials is measured by Vickers indents generated under a load of 3kg. The prior austenitic grain (PAG) size is calculated by adopting the linear intercept procedure on optical microscopy images (according to the standardized ASTM E112M method) and verified by the EBSD scans. H is introduced in the samples by electrochemical galvanostatic charging with a current density of 0.8mA/cm 2 in a 0.5M H 2 SO 4 solution with 1g/l thiourea. H saturation curves are determined by melt extraction, that measures the amount of H after different charging times by heating the samples (8mmx6mmx1mm) to 1550°C and measuring the flow of released H 2 with a thermal conductivity detector. To evaluate the susceptibility to HE, bending tests with the possibility of simultaneous H charging are performed according to the recently developed methodology of the same authors (Pinson et al., 2020). Rectangular sheet specimens with the dimensions of 80mmx20mmx1mm are tested both in air and H charged prior to and during testing (saturated in-situ).

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Results and discussion

3.1 Microstructural characterization A SEM image and EBSD maps of the final microstructure of the tempered industrial steel are found in Figure 1. The phase map indicates that no retained austenite is present inside the martensitic matrix, which is confirmed by XRD measurements. The Cr-based carbides are detected as cementite particles due to the similar orthorhombic crystal structure (Schmuecker et al., 2017). The PAG size is 15.7 µm and the final hardness of the samples is 819 HV, which is very close to the hardness of 848 HV as described in the ASTM standard for bearing materials (He, n.d.).

Figure 1: (A) SEM image, (B) phase map and (C) inverse pole figure map measured by EBSD of the tempered 100Cr6 material.