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STRUCTURAL ANALYSIS OF EN – GJL 300 CASTINGS BY METALLOGRAPHIC TECHNIQUES Viera Majerová, Radomila Konečná and Gianni Nicoletto^ Dept. of Materials Engineering, University of Žilina, Univerzitná 1, 010 26 Žilina, Slovakia ^ Dept. of Industrial Engineering, University of Parma, 43100 Parma, Italy e-mail: [email protected] Introduction. Castings used in the production of hydraulic gear pumps and made of gray cast iron (EN-GJL 300) are analyzed in this paper. Castings with the same geometry were produced for the Italian company Casappa SpA by three different Italian foundries. Values of mechanical properties (Rp0.2, Rm, σc) were obtained using specimens extracted from the castings. The macrostructure and microstructure were analyzed in detail with the goal of a microstructure-based explanation of the variability of the mechanical properties. Materials and experimental procedures The experimental materials were produced by three foundries designated as A, B and C. Castings were made of pearlitic gray cast iron GJL 300, according to the norm EN 1561. The chemical composition of the materials is presented in Tab. 1. Table 1 Chemical composition [%] Casting C Si Mn P 3.20 1.92 0.64 0.031 A 3.29 1.52 0.79 0.055 B 3.15 1.98 0.85 0.086 C Casting S Cr Sn Mo 0.068 0.29 0.11 0.005 A 0.030 0.15 0.009 0.006 B 0.086 0.08 0.018 0.012 C The microstructure was evaluated according to the norm STN 42 0461. The distribution and the size of lamellar graphite were evaluated on unetched specimens. Shape and dispersion of pearlite and shape, size and distribution of phosphide eutectic (PE) were determined after etching with 3 % nitric acid for 5 s. The structural evaluation focused on eutectic cells (EC) revealed by color etching with sodium picrate [2]. The size of EC was characterized by the average diameter D . The distribution and size of PE network were evaluated according to the norm STN 42 0461 after etching with 3 % nitric acid for 30 s. The content of PE was evaluated using a test grid estimate method [3] after etching with picric acid. Results and discussion A summary of mechanical properties and structural parameters obtained the study of the castings is given in Tab. 2. The influence of the foundry practice on the

material performance is documented by different responses for castings, where the highest tensile strength was found for casting A and the lowest for casting C. Table 2 Rm Rp0.2 A Casting [MPa] [MPa] [%] 270 236 0.66 A 253 208 0.8 B 230 196 0.55 C EC size PE σc Casting [%] [MPa] D [%] 39.7 3.9 0.944 A 40.5 3.7 0.842 B 62.8 8.2 0.631 C The structure of gray cast iron depends on chemical composition and, especially, cooling conditions [1, 4]. The gray cast iron in all the present castings was characterized by a fully pearlitic matrix with a typical structure shown in Fig. 1a. The pearlite had a lamellar structure with characteristic lamella spacing from 1.3 to 1.6 μm in the central part of castings and from 0.8 to 1.3 μm near the casting surface. The length of graphite lamellas was from 120 to 250 μm and, less frequently, from 60 to 120 μm. In all castings the distribution of graphite lamellas was mainly of type D, that is characterized by interdendritic segregation and random orientation with a minor part of type A, that is uniform and random orientation [1]. The solidification process of cast iron containing phosphorus (from 0.5 to 0.8 % in dependence of carbon content) does not terminate with the eutectic reaction because a ternary phosphide eutectic (steadite) develops according to the system Fe-Fe3C-Fe3P at the end of eutectic reaction [4]. The PE was found at the boundaries of the EC as fine–grained and concave triangular in shape (Fig. 1b) in all castings. The

distribution of PE was according to a single configuration in casting A, irregular in casting B (i.e. a continuous network in the central part, a discontinuous network near edges of casting and a single orientation elsewhere) and as a continuous network in casting C.

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graphite lamellas (Fig. 2b). EC have always been emphasized in relationship with mechanical properties in cast iron but it was also shown in [5] that the effect of the primary structure may exceed that of EC. The highest tensile strength is obtained in the presence of large primary dendrites as in Casting B (Fig. 3a) and the lowest strength when dendrites are short and globular as in casting C (Fig. 3b).

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Fig. 1 Casting A - a) microstructure, etched with 3 % nitric acid, b) shape of phosphide eutectic, etched with picric acid

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Fig. 3 Dendrites a) casting B, b) casting C, etched with sodium picrate On the other hand in agreement with [4] the EC size of C was smaller than those of casting A and B (Tab. 2) and the fatigue limit of casting C was higher than the fatigue limit of casting A and B.

Fig. 2 a) Micro shrinkages, etched with picric acid, b) Eutectic cells, etched with sodium picrate The average size of the PE network was from 500 to 750µm in all castings. The results of the quantitative evaluation of PE content are presented in Tab. 2. The highest content of the PE was found in the casting C, which also showed the lowest tensile strength (Tab.2). Contents of PE are almost equal and lower in casting A and B, which have higher strength. Ref. [4] states that the presence of PE decreases the tensile strength, increases hardness and fatigue life of gray cast iron. If PE is assumed to have a damaging effect, then PE content (i.e. increasing from A to C) correlates with the tensile strength (i.e. decreasing from A to C). Significant micro shrinkages were found in presence of PE in all castings (Fig. 2a). The present gray cast irons are all hypoeutectic (Carbon Equivalent is ~ 0.86 % in all castings). Therefore the primary crystallization is characterized by formation of austenite dendrites [1, 4] and the formation of EC (during the eutectic reaction there is the continuous and simultaneous creation of the graphite and austenite phases [5]) can only take place in the space left by the primary phase [5]. The EC size (Tab. 2) depends mainly on the nucleation count of the melt [5]. The EC identified by color etching have an approximately circular shape with radially directed

Conclusions Castings made of gray cast iron (EN-GJL 300) by three different Italian foundries were analyzed structurally to obtain a microstructure-based explanation of the variability of the mechanical properties. The smallest dimension of primary dendrites and the highest content of phosphide eutectic were found at boundaries of eutectic cells in casting C, which showed the lowest tensile strength. The smallest average size of the eutectic cells was found in the casting C that is also characterized by the highest fatigue limit. References [1] DAVIS, J. R.: Cast Irons/Metallurgy and Properties of Ductile Irons. ASM Specialty Handbook, the Materials Information Society, USA 1996. [2] SKOČOVSKÝ, P. – PODRÁBSKÝ, T.: Farebná metalografia zliatin železa/Colour metallography of ferrous alloys. 1. vyd., EDIS, Žilina 2001. [3] SKOČOVSKÝ, P. – MATEJKA, M.: Mikroštruktúra liatin. ES VŠDS, Žilina 1994. [4] SKOČOVSKÝ, P. – ŠIMAN, I.: Štruktúrna analýza liatin. 1. vyd., ALFA, Bratislava 1989. [5] VELDE, C.: (December 4, 1999), The Solidification of Gray Cast Iron. Found 4/14/2004, http://81://.lycos.nl/ cvdv/eutcelpart3.html.

Acknowledgements. This work was done as a part of the SK/IT project 10/NT and of the grant No.1/0027/03 of the Scientific Grant Agency of Ministry of Education of Slovak Republic and Slovak Academy of Sciences. The partial and interest of Casappa SpA is acknowledged.