Effect of cooling rate on the formation and distribution of carbides in nodular iron alloyed with Cr
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Effect of cooling rate on the formation and distribution of carbides in nodular iron alloyed with Cr Ramses Zenil1, Jose A. García1, Gerardo A. Ruiz1, Alejandro D. Basso2, Jorge. Sicora2 Departamento de Ingeniería Metalúrgica, Facultad de Química, Universidad Nacional Autónoma de México. 2 INTEMA-Universidad Nacional de Mar del Plata, Argentina 1
ABSTRACT This investigation deals with the effect of cooling rate on the formation of carbide nodular iron alloyed with 2.2 % Cr and equivalent carbon near 4.3%. In the experimental stage three Y-blocks of 1.5, 3 and 5.5 cm in thickness are poured in green sand molds. Castings are sectioned in two positions (wall and center samples) in order to determine the characteristics of the precipitated carbides (fraction, distribution and relative size) from surface to center and from the bottom to the top of the castings applying quantitative analysis of images. The obtained results show the presence of carbides in all of the castings. Finer carbides are obtained in the thinnest casting but with a high variation between the samples located in the wall and center. All castings present massive carbides in the last freezing zone (LFZ). Therefore the cooling rate associated with casting thick has an important effect on the fraction and distribution of carbides. INTRODUCTION Carbide nodular iron is a recent variation of nodular iron, nowadays it is studied to get Austenpered Carbide Ductile Iron (CADI) with high wear and abrasion resistance associated to presence of carbides in an ausferrite matrix. CADI´s materials have important applications in mining, construction, agricultural or in components with a high demand for abrasion and wear use. The Carbide Ductile Iron (CDI) is produced by casting processes that require controlling the chemical composition and cooling rate during solidification [1, 2]. Nodular iron produced with high cooling rate promotes a high undercooling and the material phase decomposition follows the Fe-carbide system and the precipitation of graphite is reduced. Some alloying elements decrease the difference between the eutectic temperature of the Fe-C diagram under stable and metastable conditions [3] promoting the precipitation of carbides and graphite at the same time. The presence of whitening elements such as Cr, V, Mo and B affect the microstructure, they are strong carbide promoters, however they can also cause a heterogeneous distribution of carbides and strong microsegregation patterns in the Last Freezing Zones (LFZ) [4]. Thermal stability of as cast carbides is important during the austenitizing stage of the CADI´s isothermal heat treatment. Thus the objective of this work is to determine the effect of three cooling rates on the average local carbide fraction, carbide mapping (distribution), overall carbides % in each sample and the comparative size in nodular irons cast in green sand moulds. EXPERIMENT The nodular irons are manufactured in an induction furnace with a 120 Kg of capacity, raw materials are steel scrap with high manganese and scrap of cas
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