Thermal, structural, and microstructural characterization of eutectoid steel at different heat treatments
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ojas-Rodrígueza) Universidad Tecnológica de Querétaro, Querétaro, Qro., C. P. 76148, México
César J. Ortiz-Echeverri División de Posgrado, Facultad de Informática, Universidad Autónoma de Querétaro, Querétaro, Qro., C.P. 76230, México
M. Robles-Agudo Universidad Tecnológica de Querétaro-Conacyt, Querétaro, Qro., C. P. 76148, México
M.E. Rodríguez-García Departamento de Nanotecnología, Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Querétaro, Qro., C.P. 76230, México (Received 6 September 2016; accepted 17 January 2017)
A eutectoid carbon steel was studied at three different annealing heat treatment cycles: spheroidizing, isothermal annealing, and normalization (air cooling). The aim of this study was to determine the correlation among thermal, structural, and metallurgical properties, as a result of the annealing heat treatment. Microstructure differences were produced by the heat treatment cooling rate with significant effects on Vickers nanohardness, thermal properties, and crystallinity. It was reflected in photothermal radiometry (PTR) images as in thermal conductivity and diffusivity. The amplitude signal increased as the cooling rate increased. It means that as the cooling rate increased, crystallinity, thermal diffusivity, and conductivity decreased. The cooling rate affected the metallurgical structure directly, and consequently, the nanohardness which decreased due to the solid solution formation and decomposition of the pearlite phase. As the cooling rate increased, the nanohardness increased modifying structural properties and the steel crystallinity. As the cooling rate decreased, the crystallinity increased. I. INTRODUCTION
The eutectoid point is a characteristic of iron carbon alloys. The eutectoid point corresponds to 0.80% wt C; the crystallographic structure is a body-centered cubic (bcc), and the metallurgical structure at the Fe–C equilibrium diagram is 100% pearlite. Steel mechanical and metallurgical properties depend on microstructural changes as a result of the heat treatment. The steel metallurgical microstructure after the softening heat treatment consists of ferrite (Fea) and pearlite (Fea 1 Fe3C). The pearlite amount depends on the carbon content. As the carbon content increases, hardness, ductility, strength, and toughness properties change. Pearlite can be considered as a composite of ferrite and cementite. Pearlite can be transformed to coarse perlite, fine pearlite, or spheroidized, depending on the cooling rate during the heat treatment.1–4
Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.47
Furthermore, thermal properties as thermal diffusivity and conductivity of carbon steels are modified by structural changes induced by the heat treatment and carbon content. There are different techniques as micro and nanohardness test, tensile test, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) to evaluate mechanical and metallurgical pr
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