PSI - Issue 13

Frank Tioguem Teagho et al. / Procedia Structural Integrity 13 (2018) 763–768 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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2.2. Mechanical tests Vickers micro-hardness measurements, uniaxial tension and Charpy impact tests were carried out for mechanical characterization. The hardness was measured with a load of 300 g and measurement time of 10 s by filiation. Fifty filiations were made for each material along the radial direction of the bar. Tensile tests were realized along the bar axis, at room temperature on smooth specimens according to ISO 6892-1: 2016 standard. Specimens of 6 mm of diameter were used and the prescribed elongation rate was 1  10 -3 s -1 . A MTS extensometer of 30 mm in gauge length was used to measure longitudinal elongation. Charpy V-notch impact specimens with dimensions of 10  10  55 mm 3 were prepared according to ISO 148 – 1: 2017 standard. The loading direction and crack propagation direction were the bar axis and the radial direction of the bar, respectively (X-Z according ISO 3785:2006). Charpy impact tests were performed on a 300 J instrumented pendulum, at temperatures from -100°C to + 60°C. The fracture surfaces were observed by using the same Sigma 300 electron microscope in the in-lens secondary electron imaging mode. Images at magnification 20,000 were acquired for statistical analysis of dimples.

3. Results and discussion 3.1. Microstructural characterization

All three microstructures were fully martensitic (Fig. 1). Coarser carbides were mainly observed at laths and prior austenite grain boundaries while elongated fines carbides were observed both within the laths and at lath boundaries. Fine and elongated carbides were in higher density in Material A than Material B, they were not observed in Material C. Complementary transmission electron microscopy observations showed that all carbides were of the M 3 C type. They were some clusters of carbides in replica micrographs, probably due to some artefacts during experiments; they were not considered in further analysis. The equivalent diameter of coarser M 3 C carbides was estimated by using ImageJ software. More than 2.10 3 carbides were considered for each material.

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Fig. 1. (a,c,e) bulk sample and (b,d,f) replica micrographs: (a, b) Material A; (c, d) Material B; (e, f) Material C. Same magnification.

The mean equivalent circle diameter of coarser carbides was respectively 0.080 µm, 0.095 µm and 0.122 µm for Materials A, B, and C. Further tempering of material A probably dissolved fine and elongated carbides while allowing coarsening of coarser carbides. Inter-carbide spacing distributions of Fig. 2 were obtained by applying a Voronoi tiling algorithm on replica micrographs. M 3 C carbides were shown to be closer to each other in Material A, than in Materials B and C. The mean inter-carbide spacing was respectively 0.226 µm, 0.293 µm and 0.516 µm.