Stages and Fracture Mechanisms of Lamellar Pearlite of 100-m-Long Differentially Hardened Rails Under Long-Term Operation Conditions

doi:10.1007/s40195-018-0810-9

Abstract

Abstract:

Using the methods of transmission electron microscopy, the carbide phase evolution in surface layers of the differentially quenched rails is studied after the passed tonnage of 691.8 million tons at the depth up to 10 mm along the central axis and fillet of rail head. The action of two mutual supplement mechanisms of steel carbide phase transformation in surface layers at rail operation is established: (1) cutting mechanism of cementite particles with the following departure in the volume of ferrite grains or plates (in pearlite structure); (2) cutting mechanism and following dissolution of cementite particles, transfer of carbon atoms on dislocations (in Cottrell atmospheres and dislocation cores), transfer of carbon atoms by moving dislocations into ferrite grains volume (or plates) with the following repeated formation of nanosized cementite particles. The first mechanism is accompanied by the change in linear sizes and morphology of carbide particles. Cementite element composition change is not essential. Carbide structure change can take place during the second mechanism.

Key words: Cementite, Pearlite, Fracture, Rails, Mechanisms, Operation

A. A. Yuriev, V. E. Gromov, V. A. Grishunin, Yu. F. Ivanov, R. S. Qin, A. P. Semin. Stages and Fracture Mechanisms of Lamellar Pearlite of 100-m-Long Differentially Hardened Rails Under Long-Term Operation Conditions[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(12): 1356-1361.

Figures/Tables 6

Table 1 Chemical composition of rails (wt%, Fe the balance)

C	Mn	Si	Cr	Ni	Al	V	N	S	P
0.77	0.87	0.34	0.08	0.06	0.08	0.08	0.01	0.022	0.081

Fig. 1 Diagram of rail sample preparation when studying its structure by methods of optical and electron diffraction microscopy. The solid lines designate the directions along the central axis 1 and the fillet 2; the dotted lines designate the sites of location of metal layers used for foils preparation (0, 2 and 10 mm from the surface)

Table 2 Mechanical properties of rails after the passed tonnage of 691.8 million tons

Material	Yield point (MPa)	Ultimate strength (MPa)	Elongation unit per length (%)	Contraction ratio (%)	Impact toughness at temperature +?20 °C (J/cm²)
DT 350	820	1270	11.5	40	34
Requirements of Russian standard R 51685-2013 for DT 350 category rails not less	800	1180	9.0	25.0	15.0

Fig. 2 TEM images of rail structure: a lamellar pearlite grains, b grains of ferrite-pearlite mixture, c grains of structurally free pearlite

Fig. 3 Electron microscopy image of tread surface structure: a light field, b dark field obtained in reflection [012] Fe3C, c the corresponding selected-area electron diffraction pattern, the arrow in c designates the reflection of obtaining of dark field b; arrows in b designate cementite particles

Fig. 4 TEM image of stages of transformation process of cementite plate of pearlite colony: a first stage, the arrows designate the fragments in cementite plates; b second stage being realized by mechanisms “at the site”; c third stage, the arrows designate the nanodimensional particles of carbide phase in the structure of cementite plates

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