Journal of Materials Processing Technology 186 (2007) 200–206 Structure of the continuously cast Ni-based superalloy GMR 235 F. Zupaniˇ c a,∗ , T. Bonˇ cina a , G. Lojen a , B. Markoli b , S. Spai´ c b a University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia b University of Ljubljana, Faculty of Natural Sciences and Engineering, Aˇ skerˇ ceva 12, SI-1000 Ljubljana, Slovenia Received 8 August 2006; accepted 14 December 2006 Abstract In this work we characterized the structure of continuously cast small cross-section rods (Ø10 mm) of the Ni-based superalloy GMR 235. In the microstructure prevailed dendritic columnar -grains with  ′ -precipitates. In the interdendritic regions MC-carbide and M 3 B 2 -boride were identified. The inverse macrosegregation was very faint, except at the secondary witness marks and natural corrugations. It was found that the alternating drawing mode had much greater influence on microstructure than other casting parameters. Special attention was given to explanation of processes leading to formation of surface marks (primary and secondary witness marks and natural corrugations). Formation of hot tears and appearance of inverse segregation is also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Ni-based superalloy; Solidification; Microstructure; Continuous casting; Surface marks 1. Introduction The alloy GMR 235 is a precipitation hardening Ni-based superalloy [1,2]. It is used mainly at elevated temperatures as a hot-end turbocharger wheel in the automotive industry. The manufacturing route of this alloy comprises vacuum induction melting (VIM) and casting into cylindrical steel moulds to pro- duce remelting ingots. These ingots are melted in foundries and investment cast into final products (i.e. turbocharger wheels). Conventional casting of remelting ingots has many disadvan- tages: it is labour intensive with high energy and material consumption, the yield of material is low, etc. and conse- quently, it is also cost intensive. Continuous casting represents a viable alternative to the current conventional casting of remelting ingots because it has a potential of increasing produc- tivity and quality of cast products, and of reducing production costs. Since there was no information in the available literature regarding the ability of GMR 235 of being continuously cast, it was necessary to study its behaviour on continuous casting using laboratory scale equipment under a broad range of casting parameters. In this work we present the structure of continu- ∗ Corresponding author. Tel.: +386 2 220 7863; fax: +386 2 220 7990. E-mail address: franc.zupanic@uni-mb.si (F. Zupaniˇ c). ously cast rods and the special focus is given to the explanation of processes taking place during solidification on the basis of metallographic analysis. 2. Experimental procedure The chemical composition of GMR 235 used in this work, given in mass percents, was: 63.3% Ni, 14.7% Cr, 10.7% Fe, 3.6% Al, 4.6% Mo, 2.0% Ti, 0.3% Mn, 0.5% Si, 0.15% C and 0.05% B. Continuous casting experiments were carried out using a pilot-scale set up consisting of a Leybold Heraeus vacuum induction melting furnace and a Technica Guss vertical continuous caster. Approximately 14 kg of the alloy was melted in an alumina crucible under a vacuum of 10 -3 to 10 -2 mbar, and heated to temperatures between 1793 K and 1821 K. Continuous casting took place under a protective argon atmosphere. The alloy solidified in a water-cooled copper-alloy mould (∅10 mm). The rod was extracted out of the mould using the “alternating drawing mode” consisting of the drawing stroke, the resting period and the reverse stroke. This type was chosen because it is also typical for similar commercial continuous casting machines. The “cycle length” was defined as the difference between the length of the drawing stroke and length of the reverse stroke. The casting conditions are given in Table 1. Sample preparation, light microscopy (LM), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and transmission electron microscopy (TEM) were carried out as described elsewhere [3]. In order to reveal macrosegregation tendencies, EDS was performed on areas of ∼15,000 m 2 at different distances from the rod surface (first at 150 m from the rod surface and then every 500 m up to the rod centre). Additional composition profiles were made up to 400 m from the rod surface, and the probed areas were ∼1000 m 2 . 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.12.035