Unified Kinetics Modeling of Isothermal Bainite Transformation in 60Si2CrA Steel
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INTRODUCTION
SPRING steel 60Si2CrA can be used in helical rolling of grinding balls, and the post-rolling heat treatment method is closely related to the wear resistance of steel balls. Extensive studies have been done on heat treatment of medium- to high-carbon steel. Chen et al.[1] quenched and then tempered 60Si2CrA steel at low temperature ranging from [433 K (160 °C)] to [513 K (240 °C)], and the final microstructure is tempered martensite with high hardness. Wang et al.[2] obtained ausferritic microstructure, which contains carbide-free bainitic ferrite and retained austenite in 60Si2CrVA, and the ausferritic microstructure is known as carbide-free bainite (CFB). Leiro et al.[3] concluded that the higher dry rolling/sliding wear performance of 60SiCr7 spring steel, which has a CFB microstructure austempered at lower temperature, is due to the finer microstructure. It has been found that martensite/bainite duplex microstructure, especially martensite/CFB, shows superior matching for strength and toughness.[4] Therefore, a bainite study of 60Si2CrA steel is important for wear-resistant steel balls.
HE HUANG, BAOYU WANG, JUNLING LI, and LONGFEI LIN are with the School of Mechanical Engineering, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing 100083, China and also with the Beijing Key Laboratory of Metalforming Lightweight, Beijing 100083, China. Contact e-mail: [email protected] Manuscript submitted February 27, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
There has been an everlasting argument on bainite transformation mechanism whether the transformation is diffusion type or shear type. For example, Hsu et al.[5] found that the habit plane of bainitic ferrite in low-carbon Cr-Ni steels is different from that in martensite, and the calculated driving force at Bs temperature shows that none support to the shear mechanism is given by the experimental results and thermodynamic calculation. Kazuyuki et al.[6] came to the conclusion that the bainitic transformation is the diffusion-controlled type based on the high-resolution electron microscopy observation of the atomic arrangement of the growing martensite and bainite plates. On the contrary, Sandvik[7,8] studied the bainite primary and secondary stages of Fe-Si-C alloys, and results, such as the displacements of austenite twins and measured austenite carbon content, indicate strongly that the bainitic ferrite and carbide are formed in a shear type. Moritani et al.[9] investigated the lath bainitic ferrite in Fe-0.6C-2Si-1Mn and lath martensite in Fe-20Ni-5.5Mn, and the boundary structures of both cases are similar. Caballero et al.[10] concluded that partitioning of substitutional elements in isothermal CFB transformation of the investigated high-carbon high-silicon steel does not happen by employing a local atom-probe tomography, and the results were explained using the shear growth of bainite sub-units.[11] Therefore, bainite transformation mechanism is still not a completely resolved scientific problem, which pro
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