Influence of Prior Warm Deformation on Cementite Spheroidization Process in a Low-Alloy Medium Carbon Steel
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IT is well known that the lamellar morphology of the pearlite formed after conventional bar rolling of medium carbon steels limits up to certain point its cold formability.[1,2] In the industrial procedure, low cooling rates are usually imposed just after rolling, in order to provide a coarse pearlite microstructure which is desirable from the point of view of softness and ductility. When high deformation processes are required, such as cold headability or extrusion, the as-rolled condition is not appropriate enough and soft annealing treatments need to be applied. During these treatments, the ferrite matrix softens and the lamellar cementite evolves into a globularized morphology with improved ductility. As annealing treatments can be both time and energy consuming, different new processes have been proposed to reduce treatment duration, some of them related to the conditioning of the initial microstructure prior to annealing treatment and others modifying the process by applying severe deformation or by changing the treating cycle.[3–7] The initial pearlite features become a critical aspect in order to reduce annealing production costs.[8–10] The initial microstructure conditioning, either by modifying the cooling rates applied after rolling or even by changing the rolling schedule, may lead to significant reduction of the treatment time and consequently to a J. ARRUABARRENA, Ph.D. Student, and B. LO´PEZ and JOSE M. RODRIGUEZ-IBABE, Professors, are with the CEIT and Tecnun, University of Navarra, P de Manuel Lardizabal 15, 20018 San Sebastian, Basque Country, Spain. Contact e-mail: jmribabe@ ceit.es Manuscript submitted June 13, 2013. Article published online October 19, 2013 1470—VOLUME 45A, MARCH 2014
reduction in the manufacturing costs.[2,3] Some research done on pearlite deformation-assisted technologies, such as severe plastic deformation (SPD) and heavy warm deformation (HWD) techniques, has shown that high deformation applications lead to an ultrafine grained microstructure development, in addition to significantly enhancing pearlite spheroidization rates.[11–16] It is necessary to take into account that in conventional soft annealing the morphological changes on the cementite are mainly generated through the migration of defects present in the cementite lamella and through the break-up related to the Rayleigh instability phenomena.[17–20] Crystalline defects such as lamella terminations and holes provide high curvature areas. According to the Gibbs-Thompson effect, the solute concentration in the vicinity of these places is higher than in the matrix adjacent to the flat areas of the lamella. This difference establishes a constant solute flux away from the tips constituted by such defects, so the defect migrates while the nearby flat areas, either in the same lamella or in the neighboring lamella, coarsen. The continuous solute transference may give rise to the formation of a ridge along both the edge of the lamella and the cementite edges surrounding a hole. As the ridge grows the structure along its l
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