Role of Titanium, Carbon, Boron, and Zirconium in Carbide and Porosity Formation during Equiaxed Solidification of Nicke
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Role of Titanium, Carbon, Boron, and Zirconium in Carbide and Porosity Formation during Equiaxed Solidification of Nickel-Based Superalloys Eduardo Cortes, Arnoldo Bedolla-Jacuinde, Mark Rainforth, Ignacio Mejia, Alberto Ruiz, Noemi Ortiz, and Jorge Zuno (Submitted September 23, 2018; in revised form May 18, 2019; published online July 2, 2019) This work analyses the effect of Ti in the MC carbide formation and also in the c/c eutectic promotion in two nickel-based alloys melted under vacuum conditions and solidified into ceramic molds. The role of boron, carbon and zirconium on the porosity level after solidification of both alloys was also investigated; since the alloys were vacuum melted, it is assumed that the observed porosity is due to micro-shrinkage. The chemical compositions for two alloys were: 20.2Cr-13.7Co-4.78Mo-1.19Al-0.69Ti-0.08C-2.34Fe for Alloy 1, and 20.5Cr-15.6Co5.03Mo-1.57Al-3.42Ti-0.15C-1.64Fe-0.076Zr-0.01%B for Alloy 2. Results showed a higher volume fraction of MC carbides and the c/c eutectic for the Alloy 2 with higher titanium content since this element is a carbide and c/c eutectic promoter. The porosity level in the Alloy 2, with higher amounts of carbon, boron and zirconium was considerably lower than that for the Alloy 1. These results are discussed in terms of different phenomena caused by the segregating effects of boron, carbon and zirconium towards the interdendritic regions and the formation of zirconium carbides that produce an expansion in volume when they are formed at the end of the solidification process. Keywords
boron, carbon, casting and solidification, nickel alloy, porosity, segregation, zirconium
1. Introduction In most fossil power plants the steam temperatures will achieve 700 C in a few years, and the use of such a steam temperature conditions implies technological developments as described by Viswanathan and Bakker (Ref 1). These developments, according to Nomoto et al. (Ref 2) are first of all, high-temperature material development, cooling design, steam path development, casing design, and so on. On the other hand, Ro¨sler et al. (Ref 3) and Yamamoto et al. (Ref 4) affirm that the development of a new steam turbine generation for use in advanced coal fired plants with prospective operating temperatures beyond 700 C and a projected thermodynamic efficiency of about 55% requires, amongst other innovations, the partial substitution of steels parts by Ni-base superalloys. Although most parts of thermal and fossil steam turbines are made of several kinds of stainless steels due to the economy aspects, the need for better materials is increasing the use of Nibase alloys to increase thermal efficiency and service life.
Eduardo Cortes, Arnoldo Bedolla-Jacuinde, Ignacio Mejia, Alberto Ruiz, and Noemi Ortiz, Universidad Michoacana de San Nicola´s de Hidalgo, Morelia, Mexico; Mark Rainforth, The University of Sheffield, Sheffield, UK; and Jorge Zuno, Universidad Auto´noma del Estado de Hidalgo, Pachuca, Mexico. Contact e-mails: lalocc_22@hotma
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