PSI - Issue 13

Nový F. et al. / Procedia Structural Integrity 13 (2018) 2170–2173 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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a) b) Fig. 2 a) Microstructure of the examined draw hook – coarse grain pearlitic-ferritic microstructure. b) Proper fine grain microstructure of the same material (different draw hook). 3. Results and discussion The results of the chemical analysis (Table 1) revealed, that examined draw hook was manufactured from the 47Mn steel grade. The chemical composition of the draw hook meets the requirements for that material prescribed in the TNŽ 28 2612 standard, without any significant deviations from the prescribed chemical composition. One of the most important drawbacks of the TNŽ 28 2612 standard is that for the steel grade 47Mn it prescribes only the maximal amounts of chemical elements, what is not sufficient to cover all the requirements locomotive draw hooks material. The light microscopy observation revealed coarse pearlitic-ferritic microstructure (Fig. 2a), which is not desired for this application, as it could exhibit lower ductility, Gomes et al. (1997), Qiu et al. (2014). The appropriate microstructure of the same material (from different draw hook) is shown in the Fig. 2b. Note the significant difference in the average grain size. Microstructure also contains a higher amount of large MnS inclusions. The tensile mechanical properties obtained from the tensile tests are shown in Table. 2. Experimentally obtained values of the ultimate tensile strength do not meet requirements prescribed in the TNŽ 28 2612 standard, where the minimum required values of UTS are 735 MPa, while the UTS obtained from examined draw hook are 725 and 719 MPa. From the detailed observation an absence of any plastic deformation in the area of fracture is clearly evident and this suggest that the tensile overloading was not the cause of failure. Based on those facts, it can be stated that despite the finding that the UTS of examined draw hook does not meet requirements prescribed in the TNŽ 28 2612 standard, this was not the cause of the fatal failure. Charpy impact toughness test revealed insufficient toughness of the draw hook (Fig. 3). The TNŽ 28 2612 standard requires a minimal value of K U = 29 J at the ambient temperature of +20 °C. The average experimentally examined value was 15.5 J, what is much lower than the minimal allowed value. Even more dramatic results were obtained by testing at lower temperatures. At the temperature of 0 °C the average impact toughness drops to 6.5 J and at -20 °C to only 5 J. Both mentioned temperatures (0 °C and -20 °C) are nothing exceptional during the winter days in Central Europe.

Fig. 3 Results of the impact toughness tests for different temperatures.