Temperature dependence of mechanical characteristics of wheel steels
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TEMPERATURE DEPENDENCE OF MECHANICAL CHARACTERISTICS OF WHEEL STEELS І. М. Andreiko1 and V. V. Kulyk1, 2
539.424:669.018.294
We investigate the influence of high temperatures (to 800°C) on the mechanical characteristics of medium- and high-strength wheel steels. It is shown that the lower the austenization temperature and the sharper the increase in the plasticity, which is caused by the dissolution of vanadium carbides and carbonitrides in a high-strength steel, the more favorable the conditions of formation of flat-type defects on the rolling surface of high-strength wheels (of KP-T type) against those for medium-strength wheels (of KP-2 type). Keywords: railroad wheel steel, high temperature, strength, plasticity.
In the last several years, the tonnage and speed of transportation in railroad transport have increased, but the quality of wheels has not changes, which led to an increase in the number of cases of fracture of wheels without wear and fatigue damages [1]. The operation of high-strength railroad wheels (of KP-T type) recently introduced in Ukraine instead of medium-strength ones (of KP-2 type) shows that their durability in terms of the wear-resistance criterion is higher [1]. However, according to data of the Ukrainian Railways, in a number of cases, high-strength wheels were taken out of service prematurely because of the formation of flat- or spallingtype defects [3] as a result of the thermal influence and contact loading, caused predominantly by braking, on a rolling surface [4, 5]. The increase in the load on an axle from 16 to 24 tons causes an increase in the maximum pressure in a contact spot during rolling of a wheel by a rail from 1010 to 1240 MPa. In this case, the contact area increases from 155 to 192 mm 2 [6]. In the case of slip between a wheel and a rail under high contact stresses, a substantial local thermal heating occurs. It was shown [4] that, in braking of a train at a velocity of 0.36 km/h (0.1 m/sec) under a load on an axle of 14 t, the peak of temperature (920°C) in the contact zone exceeds the austenization temperature and is attained in a time of 0.1 sec. Thus, on the rolling surface of the wheel during the slip between it and the rail, an austenitic layer is generated, which is crushed as a result of decrease in the mechanical properties of the steel, and a flat is formed. The aim of the work is to compare the mechanical characteristics of steels of high- and medium-strength wheels depending on the test temperature. Material and Test Method We investigated specimens of an experimental-commercial high-strength wheel steel produced according to TU U 35.2-23365425-600 of chemical composition (mass %) 0.66 С, 0.79 Mn, 0.33 Si, 0.2 Cr, 0.105 V, 0.018 P, 0.20 Cu, 0.12 Ni, 0.006 Ti, 0.018 Al, and 0.009 S cut off from a KP-T-type wheel [2]. For comparison, we tested specimens of a medium-strength steel (0.58% С, 0.64% Mn, and 0.34% Si), cut off from a KP-2 type wheel, 1 2
Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. Corresponding aut
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