Analysis of causes of a locomotive draw hook brittle fracture

Abstract

The traction force of locomotives is constantly increasing due to the pulling of heavier trains, while the material of the draw hooks remained the same as 50 years ago. The use of the current UIC standards and the relevant national standards defining the methodology for testing the mechanical properties of the hooks is very problematic. Despite the fact that all material properties of the draw hooks, tested according to UIC 825 and other related standards (In Czech and Slovak republic TNŽ 28 2612 standard) fully comply with the minimum values set by the standards, in operation the unexpected failure occurs frequently. This article deals with causes of brittle fractures of locomotive draw hooks operated by Slovak Railways. An exemplary study uses a traction hook of a locomotive that broke when a freight train accelerated after the stop at signal between the two stations in the winter period at ambient air temperature of -2 °C. The performed analysis showed that the brittle fracture of the draw hook was caused by usage of the low-toughness steel with the coarsened microstructure. All experimentally measured material properties of a broken hook met the limits specified in UIC 825 and TNŽ 28 2612 standards as the material testing at temperatures below +20 °C is not required in those standards. This study shows that the only technical solution to the long-term persistent problem of brittle fractures of locomotive draw hooks is to redefine their acceptance conditions and change the prescription for testing of the impact strength of the steels, from which the draw hooks are made.