Simulation of Heating Cycles for Large Steel Ingots
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Simulation of Heating Cycles for Large Steel Ingots L.F. Romano Acosta1, O. Zapata1, I. Álvarez1, R. Cerda2 and L. Leduc Lezama1 1 Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Pedro de Alba s/n, San Nicolás de los Garza, Nuevo León, C.P. 66450, México. 2 Frisa S.A. de C.V. Santa Catarina, Nuevo León, C.P. 66150, México. ABSTRACT A simulation model is presented, where temperature, phases and internal stresses can be predicted as a function of time during the heating of large steel ingots for forging. Heating cycle measurements and computer simulations are compared for an A105 steel grade 34-Ton tapered ingot. A study of the heat transfer inside a natural gas-fired furnace was carried out to make an estimation of internal stresses due to thermal expansion and phase transformation from α ferrite and pearlite to γ austenite during heating. The model was validated with a second test of an AISI 4330 steel grade 35.4-Ton ingot. The simulation model described can calculate internal stresses in any ingot in order to optimize its heating cycle without compromising ingot internal quality, reducing energy consumption and increasing productivity of the furnace. INTRODUCTION An important role during open forging of large workpieces is to design the correct forging strategy so that the desired microstructure can be achieved and the casting defects are reduced from the core of the workpiece with the minimum energy consumption and heating time. It is fairly common to design heating curves prior to forging large steel ingots by the finite element method due to energy and time savings. The ideal heating condition is when the center and the surface temperatures are equal to the forging temperature, resulting in the dissolution of all carbides, but this is not cheap because the ingot stays inside the furnace for a long time. Therefore, the time when the temperature difference between the center and the surface of the ingot is adequate for the forging operation must be calculated. Time and heating rate are the most important factors defining the heating cycle; they depend on the chemical composition, shape and size of the ingot, i.e. for each steel grade there is an austenization temperature, which is determined by its composition. It is profitable to know how the ingot is heated because in each stage of the heating cycle there will be a temperature gradient between the center and the surface of the ingot that causes internal stresses that could crack the material. Temperature distribution, microstructure and stress and/or strain distribution are well known to change in a complex manner during the heating cycle due to coupling effects between them. Figure 1 is a schematic representation of the phenomena induced during the heat treatments. When the temperature in the steel raises, the phase transformation (1) of the metallic structure occurs with generation of latent heat, affecting the temperature distribution (2). The phase transformation also generates a volume change; these local changes of temperat
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