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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strength. The presence of dimples was obvious on the fractured surfaces of the Charpy specimens for Materials A, B and C (Fig. 4). There was at least one M 3 C carbide inside any given dimple, suggesting that dimples had nucleated around these carbides, before growing and linking together to form the overall crack during the impact test. Thus, M 3 C carbides are proposed as the microstructural constituent controlling the ductile fracture process of the investigated quenched and tempered martensitic steel microstructures. Statistical analysis on fracture surfaces was performed to get reliable and quantitative information on the fracture mechanism. To this aim, four images were taken at different locations (but outside final shear fracture close to the specimen side surface) to be as representative as possible. Statistical analysis of dimples showed that the difference in average dimple size between uniaxial tension specimens tested at room temperature and Charpy impact specimens tested in the USE temperature range was inferior to 2%. An obvious increase in both the USE and the mean dimple size with the inter-carbide spacing is shown in Fig. 3b. This suggests that higher absorbed energy of Material C well correlates with higher mean dimple size and larger inter-carbide spacing. From Fig. 5, the initiation energy of Material A was significantly lower than that of Materials B and C, yet coarse carbides of Material C tend to accelerated fracture initiation compared to Material B. Higher is the strength of the steel, lower is the propagation energy. This last contribution accounts for the major part of the evolution of USE with the microstructure. Further work is in progress in order to more deeply understand this effect.

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Fig.3. (a) Ductile-to-brittle transition curves; (b) Link between the upper self-energy, dimple size and inter-carbide spacing in Material C.

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200 nm

Fig.4. Typical fracture surfaces of Charpy specimens in the USE domain: (a) Material A, (b) Material B and (c) Material C. Same magnification.

4. Conclusions

 In quenched and tempered martensitic steel, both the size and the interspacing of M 3 C cementite increased with tempering temperature. These carbides were shown to control the ductile fracture mechanism.  In the upper shelf energy domain, the increase in absorbed energy was mainly due to the contribution of the propagation process of fracture. The propagation energy increased with the intercarbide spacing.