Transformation characteristics of functionally graded steels produced by electroslag remelting
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I. INTRODUCTION
SINCE the introduction of electroslag remelting (ESR) by Hopkins[1] in 1935, this process has been used extensively to refine and produce clean steels. The effect of the process variables on microstructures, impurity, and correlated mechanical properties has been the subject of numerous investigations.[2–5] In this work, a new method has been introduced to make laminated composites, in the form of functionally graded steels (FGSs), from stainless and carbon steels, using “ESR.” Generally, there is a good deal of interest in making materials that are graded in some respect. The gradient may be of the chemical composition, changes in microstructure, or second-phase particles in a matrix. Instead of having a step function in composition at an interface, a gradually varying composition from one component to the other prevails.[6,7] Generally, it seems that the utility of laminated materials with the metallic base is yet to be investigated. II. EXPERIMENTAL PROCEDURE To fabricate FGSs, a miniature ESR apparatus was employed. The slag used was a mixture of 20 pct CaO, 20 pct Al2O3, and 60 pct CaF2, and consumable electrodes were made by slices of AISI 1020 and 316 steels named alpha and gamma in this work, with the chemical composition shown in Table I. The heights of slices were 25 mm in fourpiece electrodes, 25 and 37.5 mm in three-piece electrodes, and 50 mm in two-piece electrodes, and those were spot welded together in a manner that is shown in Figure 1. Experiments were carried out with a constant power supply of 16 KVA. After remelting, the composite ingot were upset and finally hot rolled into plates of multilayered composites. Rolling and forging operations were carried out at 980 °C. The plates were air cooled and some of those, for J. AGHAZADEH MOHANDESI, Associate Professor, is with the Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran. Contact e-mail: [email protected] M.H. SHAHOSSEINIE, Assistant Professor, is with the Metallurgical Engineering Department, Faculty of Engineering, University of Tehran, Tehran, Iran. Manuscript submitted March 14, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
the purpose of heat treatment, were austenitized at 950 °C, oil quenched, and tempered at 350 °C. For metallographic examinations, the plates were sliced, ground, polished, and etched in a “Kalling” solution and 1 pct “Nital.” Vickers hardness tests were carried out using 50 Kgf weight. Point and line analyses were made with a scanning electron microscope equipped with data processing facilities. III. RESULTS AND DISCUSSION During the process of remelting, functionally graded phases were formed via diffusion of alloying elements. As the alloying elements diffuse, alternative regions with different transformation characteristics are created. Thus, it was possible to obtain composites with various compositions of ferrite, bainite, martensite, and austenite; when the primary electrode contains two slices of gamma and alpha steels, the morphology of the r
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