Abstract
51CrV4 steel is extensively used in large-size damping springs for trains and vehicles. Quenching conditions play an important role in performance enhancement. The present work investigated the effects of various oil-bath temperatures and out-of-oil temperatures on the microstructure and the mechanical properties of this steel. The morphological examination focused on both the quenched martensite and the tempered troostite. Tensile and hardness tests were carried out to evaluate the mechanical properties. The results showed that a coarsening of the martensite occurred at a high oil-bath temperature. In addition, the size and fraction of bainite islands also increased with the increase of oil-bath temperature. In contrast, the carbide size and the intercarbide spacing both increased with the increase of oil-bath temperature. Thus, the tensile strength and the hardness both decreased with increasing oil-bath temperature in accordance with the Hall-Petch relationship. Correspondingly, the ductility increased as the oil-bath temperature increased. At a relatively high out-of-oil temperature, the martensite underwent an auto-tempering process, which led to the precipitation of many tiny carbide particles in the as-quenched martensite laths. This auto-tempering effect enhanced the width of large-sized carbides and reduced their length in the final microstructure. The intercarbide spacings increased with increasing out-of-oil temperature. As the oil-bath temperature increased, the tensile strength and hardness decreased, and the ductility increased. The fracture morphology was examined to explain the results of mechanical properties.
Highlights
The development of high-speed railway trains imposes rigid requirements on the mechanical properties of coil springs used in freight car bogies
Manganese, and silicon are added to these steels [1]. 51CrV4 steel has high strength and fatigue performance due to the addition of Cr and V, which is extensively used in large-size damping springs for trains and vehicles [2]
The surface temperature of the workpiece was measured during the quenching process, and the workpieces were taken out of the oil bath at various temperatures (60 ◦ C, 90 ◦ C, and 120 ◦ C), which was defined as the out-of-oil temperature in this paper
Summary
The development of high-speed railway trains imposes rigid requirements on the mechanical properties of coil springs used in freight car bogies. Most coil springs for railway applications are made of quenched and tempered high-strength steels. Elements such as chromium, manganese, and silicon are added to these steels [1]. The recommended properties of spring steel include high ductility and toughness at operating temperatures and good hardenability that provides required mechanical properties even at large dimensions [3]. Thermodynamic calculation was used to identify the effects of element change on the phase fraction and transformation temperature in the soaking
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