Formation of Ultrafine Microstructure during Uniaxial Warm-Compressive Deformation in an Fe-0.67%C Steel for Railway Whe
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1225-HH03-06
Formation of Ultrafine Microstructure during Uniaxial Warm-Compressive Deformation in an Fe-0.67%C Steel for Railway Wheels Yuji YASUMOTO1,2, Kazuyuki HANDA1,2,3, Yoshisato KIMURA1 and Yosinao MISHIMA1 1 Tokyo Institute of Technology, Interdisciplinary Graduate School of Science and Engineering, Department of Materials science and Engineering, 4259-G3-23 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. 2 Graduate student 3 Railway Technical Research Institute, Materials Technology Division, 2-8-38 Hikari-cho, Kokubunji, Tokyo 185-8540, Japan.
ABSTRACT The evolution of microstructure in an Fe-0.67%C steel used for railway wheels has been investigated. To elucidate the mechanism of the ultrafine microstructure which is formed on the railway wheels tread surface, we have experimentally reproduced the same microstructure using uniaxial compressive deformation and subsequent annealing at 873 K. The deformation conditions required for ultrafine microstructure formation are the initial strain rate of 1 (=100) s-1 and total strain of 0.7. The mechanism of microstructural refinement is not the primary recrystallization but continuous recrystallization, i.e., the recovery process associated with the rearrangement of accumulated dislocations during deformation and annealing at temperatures between 773 K and 873 K. The recovery process never occurs at temperatures lower than 773 K. Primary recrystallization which involves the nucleation and growth of new ferrite grains takes place at temperatures higher than 873 K.
INTRODUCTION A high carbon steel with 0.67%C having a ferrite-pearlite microstructure is used for railway wheels because it provides enough strength and toughness, which ensure safe transportation.[1] However, a few problems concerning safety remain unsolved, including so-called tread thermal cracks generated on the tread surface under the actual service conditions.[2] The cracks have no effect for safety as fine cracks, but we have to remove them by cutting so that fine cracks never grow into harmful ladder cracks. In the previous work, we clarified that the cause of these tread thermal cracks is both heavy rolling contact with a rail and cyclic frictional heat from braking.[3,4] Since these conditions applied on the tread surface can be regarded as thermomechanical processes, we have focused on the relationship between crack generation and microstructural change near the tread surface. As a result, we elucidated that ultrafine microstructure was formed as the surface affected layer in the region about 100 µm depth below the contact surface. This ultrafine microstructure consists of ferrite grains with the average size less than 1 µm and spheroidized cementite particles having uniform distribution. Since the growth of fine cracks seems to be interrupted in the affected layer, it is interesting to understand how microstructure change affects crack generation. To
clarify the relationship between the cracks and the microstructure on the surface layer, it is necessary to investigate mechanical propertie
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