Tree-ring formation during vacuum arc remelting of INCONEL 718: Part I. Experimental investigation
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INTRODUCTION
WROUGHT nickel-based superalloys (e.g., INCONEL* *INCONEL is a trademark of Special Metals Corporation, Huntington, WV.
718) have been continuously developed and applied in forged turbine discs for decades. There has always been a strong motivation to enhance the performance and efficiency of the engines that use such discs, and one option is to extend the disc stress and temperature capabilities. This requires disc alloys with higher yield strengths and stress rupture performance, together with improved fatigue resistance. Increasing levels of alloying elements have been found to be beneficial in achieving these goals, but these additions make it more difficult to produce an ingot of the starting material for disc production that is homogenous in both microstructure and chemical composition and has an acceptable macrostructure and microstructure. For example, the nickel-based superalloy INCONEL 718 contains approximately 50 pct alloying elements of which aluminum and titanium are very reactive, and niobium, in particular, is prone to segregation and to the formation of low melting-point eutectics and carbides during solidification. The final wrought-grain structure is dependent in some aspects upon the as-cast ingot grain structure as well as on the recrystallization during cogging and forging. Therefore, it is essential to control the macrostructural and microstructural development appropriately during the melting and solidification of the ingot in addition to subsequent thermomechanical processing.
X. XU, Research Associate, is with IMMPETUS, The University of Sheffield, Sheffield SW1 3JD, United Kingdom. R.M. WARD, Research Fellow, and M.H. JACOBS, Deputy Head, are with the IRC in Materials Processing, University of Birmingham, Birmingham B15 2TT, United Kingdom. P.D. LEE, Senior Lecturer, and M. McLEAN, Professor, are with the Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, United Kingdom. Contact e-mail: [email protected] Manuscript submitted April 4, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
Vacuum arc remelting (VAR) is the principal, secondary melting process used to produce ingots of nickel-based alloys, such as INCONEL 718. A schematic of the VAR process is shown in Figure 1. A cylindrical electrode made by vacuum induction melting is loaded into the top of the water-cooled copper crucible; the furnace is evacuated; and a direct-current arc is struck between the electrode and starting material at the bottom of the crucible. The arc heats both the starting material and the electrode, melting both near the interface, and the molten alloy from the electrode drips down forming a melt pool. As the cooling water extracts heat from the crucible walls, the molten alloy next to the wall solidifies first, and solidification then proceeds inward. At some distance below the molten-pool surface, the alloy completely solidifies, yielding a fully dense ingot and producing a shrinkage gap between the ingot and the crucible wall. Low-pressure helium is
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