Effects of Heating Rate on Microstructure and Fracture Toughness of Railway Wheel Steel
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TRODUCTION
IT has been one of the primary objectives of modern research in the field of mechanical metallurgy to determine the correlation of the processing, microstructure and mechanical properties of structural materials. Heat treatments are widely used to alter the mechanical properties of steels by changing the solid-state phase, precipitation, and internal stress. The mechanism and quantitative relation of the microstructure to the heat treatment process have been widely investigated and are thoroughly understood for several steels.[1,2] The heat treatment parameters can be carefully selected to attain desired microstructures. Middle or high carbon low-alloy steels are generally used for railway wheels because of their well-integrated mechanical properties i.e., their wear resistance, fatigue property, facture toughness, and cost-effective price. With increasing running speed, wheel steels with better mechanical properties are needed for safety purposes. The fracture toughness is one of the most important mechanical properties to avoid fracture of wheels with undetected fatigue cracks.[3,4] For low-alloy ferrite-pearlite steels, the quantitative correlation of the strength, plasticity, and ductile–brittle transition temperature (DBTT) to the microstructure and alloy element concentration has been well established in the past several decades,[5–8] e.g., the wellknown Hall–Petch relation. However, the effect of the
XUECHONG REN and LEI WEN, Associate Researchers, and JI QI and JIANYU GAO, Master Students, are with the National Center for Materials Service Safety, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mails: [email protected], [email protected] BO JIANG, GANG CHEN, and HAI ZHAO, Senior Engineers, are with the Technical Center of Ma’anshan Steel & Iron Corporation, Ma’anshan 243000, China. Manuscript submitted April 27, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
grain size on the fracture toughness of polycrystalline metals and alloys is still not clear. In the literature, the grain size dependence of fracture is complex due to various fracture modes. The plain strain fracture toughness of two 4340 steels with different grain sizes showed that the particle spacing was of primary importance in controlling the toughness in the ductile fracture mode.[9] For 12 pct Cr steel,[10] variation in the grain size has a significant effect on the fracture toughness. When the fracture mode is intergranular, the fracture toughness decreases significantly with increasing grain size. The toughness associated with quasi-cleavage fracture appears to be relatively insensitive to changes in the grain size. Dimpled rupture appears to be more sensitive to precipitation in the matrix than to differences in the grain size. Many other investigations on tempered martensite steels or maraging steels showed similar results.[11–13] It appears that the fracture toughness mainly relies on the second-phase particles in the ductile fracture mode and the fracture toughness relies more on the grain size in the
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