Strengthening Mechanisms of Rails’ Surface in Ultra Long-Term Operation

The studies of structural phase state and defect substructure of head surface of 100-meter differentially quenched rails after extreme long-term service (passed tonnage of 1770 million gross tons) on experimental ring of the Russian Railways are carried out by means of the methods of transmission electron diffraction microscopy (TEM). The quantitative comparative analysis of strengthening mechanisms of tread surface and surface of working fillet is performed. It is shown that steel strength is a multifactor value and is determined by combined action of a number of physical mechanisms. The contributions caused by matrix lattice friction, internal stress fields, solid solution strengthening and pearlite constituent of steel structure are estimated. It is detected that for tread surface the main strengthening is provided by long-range stress fields and for fillet the main mechanism is a substructural one. A comparison is made between dependences of combined yield point on a distance to head surface and steel data before operation and after passed tonnage of 691.8 and 1411 million gross tons. The detected decrease in yield point at passed tonnage of 1770 million tons can be related to formation of fragmentary structure in surface layers due to transformation of subgrain low angle boundaries in large angle boundaries in extremely long operation.