Physical nature of rail surface hardening during long-term operation

A comparative quantitative analysis of the physical mechanisms of hardening of rails surface layers after extremely long­term operation has been performed. The method is based on previously established patterns of formation of structural-phase states and mechanical properties of differentially hardened long­length rails produced by JSC “EVRAZ ZSMK” at a depth of up to 10 mm in the cutting of rails along the central axis and cutting out after the missed tonnage of 1411 million tons. The calculations took into account the volume fractions and characteristics of a particular type of substructure. Increase in microhardness and hardness of the surface layers of the rails subjected to ultra­long operation on the experimental ring of the Russian Railways is multifactorial and is determined by superposition of a number of physical mechanisms. The contributions are estimated due to friction of the matrix lattice, internal phase boundaries, dislocation substructure, presence of carbide particles, internal stress fields, solid hardening, and pearlitic component of the steel structure. Regardless of the analysis direction (along the central axis of the head or along the axis of symmetry of the chip), strength of the rails metal depends on the distance to the surface: it increases as it approaches the top of the head. The most significant physical mechanisms have been established, which provide high strength properties of the metal of the rail head subjected to extremely long­term operation. In the subsurface layer (located at a depth of 2 – 10 mm) of the rail head, the most significant physical mechanisms are dislocation mechanism, due to the interaction of moving dislocations with stationary dislocations (dislocations of the “forest”); in the surface layer of the rail head, substructural mechanism, due to the interaction of dislocations with small­angle boundaries of fragments and subgrains of a nanometer–sized polygon. A comparison with the quantitative values of the rail hardening mechanisms after the missed tonnage of 691.8 million tons was carried out. It is shown that an increase in the missed tonnage in the range of 691.8 – 1411 million tons leads to a significant (1.5 – 2.0 times) increase in strength.

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