Microstructural Evolution of High Dimension Rings of Steel AISI 4140
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Microstructural Evolution of High Dimension Rings of Steel AISI 4140 E. Martínez1, C. Márquez1,2, J. Rodríguez2, F. Fernández2, J. Cárdenas2 and A. Pérez1. 1
Universidad Autónoma de Nuevo León, FIME, Ave. Universidad s/n, Cd. Universitaria, San Nicolás de los Garza, N.L. 66450 México. Email: [email protected] 2 Frisa Forjados, S.A. de C.V. P.O. Box 1273, Monterrey, N.L. 64000 México
ABSTRACT This paper presents the results of the analytical study of the microstructural evolution of forged and rolled rings with a diameter greater than 2 meters, since the raw material, set-up forgings, ring rolling and heat treatments, including normalizing, quenching and tempering. Also, great care will be taken on the chemical micro-segregation. Four rings, were instrumented with 20 thermocouples each one in order to follow the thermal profile during heating and quenching. Cooling rates will be used to predict the microstructure developed under the different heat treatments and it will be compared at different positions inside the rings. It can be seen that the cooling curves obtained show no presence of soft phases, as shown in the microstructures in the figure shown below. With the results of microstructure obtained, which show martensitic structure to a greater extent, it meets the desirable mechanical properties, which are shown below. Keywords: Steel, Microstructure, Stress, Polymer, Structural.
INTRODUCTION Manufacturing of high dimension steel rings impose several technological challenges. These rings up to 7 m external diameter with very slim transversal section are pieces used as bearings cases, structural flanges for wind mill electrical generators, and structural pieces. One of the challenges to overcome is the dimensional stability and the residual stresses. It is common to find pieces that during their final machining, they show high dimensional distortions [1]. Some of the distortion is accounted by the liberation of internal stresses during the machining operations. One source of the high level of internal stresses is the heat treatment. The rings, after the rolling-forging conformation process undergo a normalizing treatment before the quenching and tempering treatments. Quenching is performed in liquid polymer. Polymer quenchants are used to control distortion and to prevent cracking of steel during the induction hardening process [2]. Those rings are manufactured in structural steel 1%Cr, 0.20% Mo, 0.40% C, similar to AISI 4140. To achieve optimum mechanical properties, these alloys are usually heat treated by quenching in an aqueous polyalkylene glycol (PAG) [3, 4, 5] polymer solution from the hardening temperature, then tempered. This produces a microstructure with high strength and excellent toughness, ideal for applications involving heavy loading and cyclic fatigue [6-10]. During hardening, internal stresses created from the thermal shock of direct polymer quenching
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almost always cause some degree of distortion in the heat treated component. A final machining allowance is often left on criti
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