Improvement in Abrasion Wear Resistance and Microstructural Changes with Deep Cryogenic Treatment of Austempered Ductile
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ED ductile cast iron (ADI) has become an important engineering material in recent years due to its interesting combination of properties, such as very good strength-to-weight ratio, good toughness, high wear resistance, and excellent fatigue strength in combination with the low cost, design flexibility, and good machinability. As a result of these properties, which are a consequence of the ADI’s specific microstructure that consists of needle-like bainitic ferrite (aB) and high-carbon austenite (cHC),[1–5] ADI is increasingly used in the automotive industry as a substitute for steel and aluminum parts. Due to the similarity of the heat-treatment procedure and the microstructure when compared with austempered steels, it has wrongly been believed for some time that the microstructure of ADI consists of bainite. This bainite is a low-temperature eutectoid that consists of needle-like ferrite and carbides, while correctly heattreated ADI has no carbides present in the microstructure. Also, the presence of austenite at room
SANJA SˇOLIC´ and CˇRTOMIR DONIK, Senior Research Associates, and MATJAZˇ GODEC, Director, are with the Institute for Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia. Contact e-mail: [email protected] ZDRAVKO SCHAUPERL, Full Professor, is with the Faculty for Mechanical Engineering and Naval Architecture, University of Zagreb, I. Lucˇic´a 5, 10000 Zagreb, Croatia. Manuscript submitted October 8, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature leads to the wrong conclusions, i.e., it is the residual austenite that is equal to the unstable residual austenite present in the steel microstructure after an incomplete heat treatment. The austenite present in the microstructure of ADI is stabilized with carbon enrichment during the heat-treatment process and will, according to some authors, begrudgingly transform to martensite even at subzero temperatures.[6–8] The heat treatment that results in the ADI’s specific ausferritic microstructure involves austenitizing in order to achieve a full austenitic matrix, followed by rapid cooling or quenching to the austempering temperature in order to avoid the formation of pearlite and other high-temperature phase transformations. The austempering temperature, which is in the range between 523 K and 673 K (250 °C and 400 °C), is the most important parameter in the heat-treatment procedure for ADI, because it determines the final microstructure and the properties of the material. With different combinations of the time and the temperature of austenitizing and austempering, it is possible to obtain different combinations of properties that can be specifically designed and adjusted for a certain purpose. Austempering in the lower-temperature range results in a microstructure of fine-grained, needle-like, or acicular-shaped ferrite and austenite (lower ausferrite) with a higher strength and a lower impact toughness and ductility. Whereas, the higher austempering temperatures result in coarse, feathery ferrite, and austenite (upper ausferrite), which lowe
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