PSI - Issue 20

Valeriy Lepov et al. / Procedia Structural Integrity 20 (2019) 57–62 Valeriy Lepov et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The question of ensuring the reliability of railway transport acquires a special relevance taking into account the input of the new sites of railroad in the regions of Central Yakutia with the prospect of the advance to regions of the Arctic and the Subarctic regions. During the rolling stock using the difficult interaction of loading mechanisms of knots and details leads to their damages and the failures takes place. Along with the fatigue nature of fracture, there are the damage accumulation processes take place in the material of members of railway equipment induced by the joint action of shock and contact loading during the interaction between a wheel and a rail, for example. It is known that during the passing of rail joints there are considerable dynamic impact loads on a wheel which can significantly affect its working capacity. So the vertical accelerations reaching to 50g and the striking powers exceeding 400 kN at contact of the locomotive wheel with the rail joint are observed, as shown by Verigo (1984). Due to the increase in hardness of materials of rails and massive use of more powerful P65 and P75 type rails and with increase in rigidity of a railway into the winter time the nature of shock interaction of a wheel when passing rail joints is significantly changed. Particularly it is established that the contact durability increases due to increase in content of carbon and hardness in steel, thereby it is claimed that material of railway wheels has to have high durability and hardness for ensuring the required resistance to contact and fatigue damage. However the increase in hardness only by means of increase in content of carbon in material leads to improvement of strength characteristics of steel, but at the same time the impact toughness which characterizes the material’s ability to resist brittle fracture is decreased, absorbing the energy of shock load. The impact toughness value considerably depends on operating temperature so. At the temperature drop the impact strength of samples made from the same material can significantly decrease. The temperature range in which the value of impact toughness sharply changes is called as temperature range of brittleness. Than more the temperature range of brittleness is shifted towards the lower temperatures, the less material is sensitive to temperature at impact loads and is more reliable. 2. Materials and equipment In frames of the study of the locomotive wheel operated in the range of climatic temperatures from positive (20 °C) to deeply under zero (at –20 °C, –40 °C and –60 °C) was investigated. For determination of the impact toughness value the test pieces on a shock bend with a V-shaped cut were made from the studied railway wheel. Drop-weight test specimens were tested according to Russian standard GOST 9454-78 (2003) on Zwick Royell impact pendulum-type testing machine in IPTPN SB RAS Center of collective usage in Yakutsk. Cooling of samples in the cooling LAUDA master camera was made for carrying out tests at low temperatures. Process of cooling took not less than 30 minutes, and then samples were set on testing machine and immediately subjected to the test shock. The test results are presented on fig.1. In the range of temperatures from 20 º С to –20 º С the insignificant decrease in impact toughness has been observed. But in the range from -20 º С to –60º С the sharp drop in the value of impact toughness was recorded. Thus the experimental results showed that the impact toughness of locomotive wheel steel at –60 °C test temperature has decrease three times, and in spite of the fact that steel has higher mechanical characteristics, the temperature drop leads to lost the plasticity of material. Thereby, as a result of low-temperature ductile-brittle transition, the shock resistance of steel dramatically decreases. It is caused by sharp drop of the mobility of main carriers of plastic deformation – the dislocations which energy of activation falls below a potential barrier of plastic

shear in bcc-metal structure. 3. Discussion and results

Apart from the V-notched samples impact bending test for assessment of cold resistance and completeness of a micromechanism of fracture it is expedient to apply to samples breaks the optical and electron fractography method. Usually distinguish fragile (light) and ductile (opaque) types of a break visual appearance. The fragile break is