Failure analysis of automatic coupler SA-3 in railway carriages
Introduction
Automatic couplers play an important role in railway transportations, as they are used to connect two wagons as well as to transfer the axial loads (compressive and tensile) and to damp the impacts received by each wagon. They are also designed to keep the wagons and locomotives at the proper distance. Therefore, the safety, transportation intensity and economic aspects as well as the amount of employed load (goods or passengers) and the permissible speed are directly dependent on the mechanical properties and the proper function of this part. Any increase in freight tonnage and train’s speed causes the acting forces to be increased which in its part decreases the lifetime of automatic coupler and increases damage intensity of the coupler components [1].
Fig. 1 shows the schematic of interaction mechanism between two couplers during their operation. As the wagons start moving, these parts are interlocked which leads to an established link or interaction between the wagons. In this way, the traction force is transformed from one wagon to another.
These couplers made of cast steel, GS20Mn5, are normalized after casting. As far as mechanical issues are concerned, the strength and impact resistance are among the most important factors contributing to the fracture behavior of these kind of parts. However, as mechanical and metallurgical aspects are interrelated, for the proper function of this part, these aspects must be considered simultaneously. In general, the best mechanical properties can be provided by a defect free structure [2]. For instance, the existence of casting defects can readily reduce the load-bearing capacity of any part. A homogenization treatment, for example, should be done on the casting parts in order to diminish the concentration gradients (segregations) and to relieve the residual stresses as well. Therefore, the manufacturing defects normally determine the final mechanical properties of a part. It is found that mis-run (an important casting defect) can be a major cause of failure in parts with the same application as automatic couplers [3].
The present study was therefore carried out so as to figure out the failure causes of an automatic coupler, considering both the metallurgical and mechanical aspects.
Section snippets
Investigation method
An automatic coupler, which had fractured during service, was received two months after the occurrence of fracture and due to this fact its fracture surfaces was corroded. After collecting the data regarding its service conditions, the fractured coupler was first subjected to visual examination and macro-fractography to determine direction of crack growth and the origins of fracture and to find out the external casting defects such as hot crack, mis-run and cold lap. The fracture surfaces were
The metallurgical analysis of failure
The chemical compositions of the standard [4] and the broken part are summarized in Table 1. Microstructure of the specimen near the fracture surface is shown in Fig. 2. In terms of microstructure, the matrix composed of almost 35% pearlite and 65% ferrite. The microstructure shows that normalization has been done properly.
Fig. 3 shows the whole picture of the fracture surface investigated here. The standard dimensions of the automatic coupler (according to UIC 522 [4]) and the location of
Conclusions
- 1.
Molten metal fluidity decreased due to thickness reduction of two stiffeners which led to incomplete filling and poor molten metal flowing, hence mis-runs and cold laps in the stiffeners were formed.
- 2.
Fatigue is the dominant failure mechanism. Fatigue cracks initiated from two adjacent mis-runs in the stiffeners. These mis-runs act as stress raiser.
- 3.
Hot tear resulted from the high strength of the core and chemical deviation is observed.
- 4.
While decreasing Mn and Si contents, high density of
Acknowledgements
The authors wish to thank the Research Center of Islamic Republic of Iran Railway for providing funds for the research work. Special thanks are given to Hosein Yazdani tabaii, Mansor Mortazavi and Reza Ebrahimi.
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