Effect of the ratio Mo/Cr in the precipitation and distribution of carbides in alloyed nodular iron
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Effect of the ratio Mo/Cr in the precipitation and distribution of carbides in alloyed nodular iron H.D. Rivero1, José .A. García1, E. Cándido Atlatenco1, Alejandro D. Basso2, J. Sicora2 1
Departamento de Ingeniería Metalúrgica, Facultad de Química, Universidad Nacional Autónoma de México. 2 INTEMA-Universidad Nacional de Mar del Plata, Argentina ABSTRACT This investigation deals with the effect of 2/0, 1/1 and 1/0.5 Cr/Mo ratios on the local fraction, distribution and the comparative size of carbides precipitated in cast nodular iron. "Y" block castings with a thickness of 1.5 cm are cast in green sand molds. Two samples are cut from each casting, one located on the center and another on the wall. The carbide volume fraction is evaluated by a digital analysis system. Each sample is analyzed in three zones: bottom, middle and top. Carbide mappings are generated according to the average local carbide fraction in order to get the distribution of carbides on the casting. Results show that higher volume fractions of carbides precipitate for the ratio 2/0 of Cr/Mo with values between 28.5 and 19.5%. The lowest fraction of carbides is presented in nodular iron alloyed with a Cr/Mo ratio of 1/1 between 6.5 and 4.6%. Also a very heterogeneous distribution of the carbides is observed in the three alloys and massive carbides are observed in the last freezing zone of the castings. INTRODUCTION The manufacturing of conventional nodular iron with carbides is a topic associated to its negative effect on the mechanical properties. However recently it has been more intensely investigated because Carbide Ductile Iron (CDI) is important for the manufacturing of Carbide Austempering Ductile Iron (CADI) in components with high wear and abrasion resistance. These characteristics are associated to the residual fraction of carbides in the ausferrite matrix. In CADI materials the carbides work as reinforcements of the matrix. According to its microstructure and properties CADI materials have important applications in mining, construction, agricultural and other industries. CDI can be produced by casting processes controlling the chemical composition (alloying elements or C-Si balance) and cooling rate during solidification. Some alloying reduces the difference between the eutectic temperature of the Fe-C stable and metastable diagrams [1] that promote the precipitation of carbides and graphite. Whitening elements such as Cr, V, Mo and B are strong carbide promoters in nodular cast irons, however they can cause a heterogeneous distribution of carbides and strong segregation patterns in the Last Freezing Zones (LFZ) [2, 3]. The aim of this work is to determine the effect of three different Cr/Mo ratios on the average local fraction carbide, carbide mapping (distribution), total carbides % in sample located in the wall with a high cooling rate and in the center with a slow cooling rate. EXPERIMENT Nodular irons whit three Cr/Mo ratios i.e., 2/0, 1/1 and 1|/0.5, are manufactured in an induction furnace with a 120 Kg of capacity. Raw materials
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