Solidification Map of a Nickel-Base Alloy
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erformance nickel-base alloys are widely used in industry[1–5] because of their superior structural stability, desirable mechanical properties, and high resistance to stress corrosion cracking. For example, InconelÒ Alloy 690, a high chromium content nickelbase alloy, is widely used in installation and repair of the steam generator tubing and pressurized water reactor components.[6] Fabrication and maintenance of these alloy parts require an understanding of their solidification behavior because of its significant impact on the mechanical properties. Unlike well established alloy systems, such as stainless steels,[7] a quantitative understanding of Alloy 690 solidification behavior is not currently available. Previous studies on the solidification behavior of Alloy 690 have provided useful knowledge about the morphology of the solidification structure. Cellular and columnar dendritic structures were observed in both arc and laser welded fusion zones.[8–13] Characterization of the solidification structures showed that cell spacings and the secondary dendrite arm spacings varied with heat input per unit length during the fabrication process. The scale of the solidification structures were correlated with heat input because the heat input could be accurately determined. However, it is now well established that the heat input does not uniquely define the solidification structure because the same heat input can result in significantly different thermal conditions depending on the fabrication speed and the power used.[14] J.J. BLECHER, Ph.D. Candidate, T. DEBROY, Professor, and T.A. PALMER, Associate Professor, are with the Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Contact e-mail: [email protected] Manuscript submitted August 12, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
The scale of the cells or dendrites depends primarily on the fundamental solidification parameters, such as the temperature gradient (G) and the solidification growth rate (R). The cooling rate can be directly related to the scale of the solidification structures, regardless of the heat input or other attributes of a fabrication process.[15] A morphological map showing solidification structures as a function of fundamental solidification parameters can provide significant benefits to the construction of new power plants and the refurbishment of existing plants. This enhanced understanding of the solidification mechanisms will lead to improved fabrication and performance of Alloy 690 in high temperature applications. Realistic calculations of the solidification parameters during welding have been enabled by recent advances in numerical modeling. For example, Zhang et al.[16] utilized a heat transfer and fluid flow model and showed that calculated thermal cycles during cooling after arc spot welding agreed well with the corresponding experimental thermal cycles. Rai et al.[17–19] calculated G, R, GR, and the morphology parameter (G/R) at the trailing edge of the weld pool during laser welding of a wide ran
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