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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2

TN Ž 28 2612 standard, is K U = 29 J. This is relatively low value, considering dynamic loading, to which the draw hook can be exposed during the operation, Holzman and Klesnil (1972) , Skočovký et al. (2000). Even more unsettling is the fact, that the standard does not prescribe impact toughness values for lower temperatures than +20° C. During average winter in Central Europe, the temperature often drops to -20 °C and lower, thus the sufficient toughness properties at lower temperatures are essential for the reliable operation. In the presented paper, the cause analysis of the locomotive draw hook fracture is examined. The failure occurred when the locomotive accelerated with a heavy freight train in the winter, at the ambient temperature of -2 °C. The fracture exhibits fully brittle character, without the presence of any fatigue cracks or ductile fracture. 2. Experimental part The detailed analysis of the broken draw hook revealed fully brittle character (Fig. 1). There were no signs of the presence of any fatigue cracks or any region of the ductile fracture. Based on those findings, the material was found to be a cause of the examined failure. Considering material as a primary cause of failure, the next steps were focused on the characterization of the material and its behavior under different operation conditions.

a)

b) Fig. 1. Broken locomotive draw hook, a) overall view, b) view of the fracture surface.

To reveal the cause of the fatal failure, the mechanical properties of the draw hook were evaluated. Considering the brittle character of the fracture, the emphasis was set on the impact toughness and ductility. Primarily, the chemical composition of examined material was evaluated, using the Arc-spark OES. Three analyses were carried out and the average values of chemical composition are shown in Table 1. The tensile properties were evaluated using the standard tensile test according to EN 10002-1. The results are shown in Table. 2. Two specimens with the round cross section were used and the tests were carried out at ambient temperature of 23 ± 2 °C. The impact toughness tests were carried out using the standard Charpy testing device, with nominal energy of 300 J. Standard specimens were used, with the square cross-section (10 x 10 mm) with machined standardized U-type notch. To evaluate the transition behavior of the draw hook material and monitor the decrease of the impact toughness with decreasing temperature, the tests were carried out at several temperatures, in the range from -60 °C to +20 °C. The microstructure of the draw hook was examined using the light microcopy. Specimens were prepared by standard techniques for the light microscopy and etched by 1% Nital. Typical microstructure of the examined draw hook is shown in Fig. 2.

Table 1 Chemical composition of the examined draw hook, steel grade 47 Mn Max. content of the alloying elements in wt.% C Si Mn P S Cr Cu Mo Ni

V

P+S

Standard TNŽ 28 2612 Examined draw hook

0.60

0.60

1.50

0.050

0.050

0.50

0.25

0.10

0.50

0.10

0.090

0.428 0.548

1.3 0.0156 0.0299 0.0628 0.0392 0.014 0.0197 0.0027 0.0455

Table 2 Mechanical properties of examined draw hook compared with the values prescribed in the TNŽ 28 2612 standard. UTS [MPa] YS 0.2 offset [MPa] Elongation [%] Reduction of area [%] Specimen No. 1 725.3 418.1 17.2 46.9 Specimen No. 2 719 405.1 14.25 48.3 standard TN Ž 28 2612 735-845 --------- 11 ---------