Microstructures and Mechanical Properties of a Wear-Resistant Alloyed Ductile Iron Austempered at Various Temperatures
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TRODUCTION
IN recent years, bainitic ductile cast iron has been widely used in machinery components and parts due to its low fabrication cost, excellent mechanical properties, and abrasive wear resistance. Usually, the bainite phase in ductile cast iron can be obtained by adding alloy elements and controlling cooling rate through centrifugal casting or sand casting. In some fields of application, e.g., large roller mill in mining industry, requirement for heavy section bainite ductile iron castings has been growing significantly. Unfortunately, as the section size of the casting increases so does the difficulty in producing satisfactory microstructures.[1] On one hand, increasing the section size slows down the cooling rate which results in a low nodule count.[2] On the other hand, heavy section ductile iron needs significant amount of alloying elements to provide the adequate austemperability in order to fully austemper the whole casting and improve the wear
JUNJUN CUI, PhD Candidate, is with the State Key Laboratory of Rolling and Automation, Northeastern University, 3-11 Wenhua Road, Shenyang 110819, P.R. China, and also Lecturer with the College of Mechanical and Vehicle Engineering, Shenyang Institute of Technology, 1 Binhe Road, Fushun 113122, P.R. China. LIQING CHEN, Professor, is with the State Key Laboratory of Rolling and Automation, Northeastern University. Contact e-mail: lqchen@mail. neu.edu.cn Manuscript submitted December 1, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
resistance of the ductile iron.[3] Hence, it is important to use suitable and optimum amounts of the alloying elements to obtain the required austemperability and at the same time eliminate or minimize the segregation of the elements. In order to prevent the above mentioned casting problem, it is necessary to control the casting parameters such as molten metal treatment for production of ductile iron, inoculation, pouring temperature, centrifugal casting parameters.[4] Apart from mentioned above, austempering parameters including austempering time and temperature play important roles in the resultant microstructure, mechanical properties, and wear resistance of the alloyed ductile iron for heavy section wear-resistant alloyed ductile iron.[5–8] Austempered ductile cast irons (ADI) contain highcarbon austenite and ferrite (often referred to as ausferrite microstructure) as matrix, where the dual-phase microstructure confers a high strength.[8] Ductile cast iron can possess aus-ferrite structure by alloying addition such as Ni, Mo, and Cu followed by subsequent heat treatment. A two-stage heat treatment, i.e., austenitization and austempering, is generally applied to ADI. Austempering process consists of austenitizing the casting in the temperature range of 1123 K to 1255 K (850 C to 982 C) for sufficient time to get a fully austenite (c) matrix, and then quenching it to an intermediate temperature range of 533 K to 673 K (260 C to 400 C). After that, the casting is maintained at this temperature for 2 to 4 hours. During austemper-
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