Effect of spheroidization of cementite in ductile cast iron
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Effect of spheroidization of cementite in ductile cast iron Basavaraj, Pavankumar R. Sondar, and Subray R. Hegde Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka (NITK), Surathkal 575025, India (Received: 16 January 2020; revised: 10 March 2020; accepted: 12 March 2020)
Abstract: The research aims to provide an alternative to austempering treatment of ductile cast iron with a simple and cost-effective heat-treatment process. This goal was accomplished by applying a simple one-step spheroidization heat treatment to the as-cast ductile iron, which would normally possess a coarse pearlitic microstructure to a significant extent. Spheroidization experiments involving isothermal holding below the lower critical temperature (A1) were conducted followed by standard mechanical testing and microstructural characterization for an experimental ductile iron. After improving the spheroidization holding time at a given temperature, the work shows that the ductility and toughness of an as-cast ductile iron can be improved by 90% and 40%, respectively, at the cost of reducing the tensile strength by 8%. Controlled discretization of the continuous cementite network in pearlitic matrix of the ductile iron is deemed responsible for the improved properties. The work also shows that prolonged holding time during spheroidization heat treatment leads to degradation of mechanical properties due to the inhomogenous microstructure formation caused by heterogeneous decomposition and cementite clustering in the material. The main outcome of this work is the demonstration of ductile cast iron’s necking behavior due to spheroidization heat treatment. Keywords: spheroidization; cementite; ductile iron; toughness; heat treatment
1. Introduction Cast iron is an important class of structural materials that find wide applications for various industries, particularly in bulk component forms including automotive, machine tools, piping, and electrical machinery. In terms of tonnage use, cast irons may be more like that of structural steels. Typical components made from cast irons include engine blocks, valves, pistons, clutch plates, liners, brake drums, motors, and generators housings [1]. Characteristic advantages of cast iron are castability, machinability, wear/abrasion resistance, damping capacity, compressive strength, corrosion resistance, and low cost over steels. Low cost and castability of the cast iron are extremely significant, so it may replace certain steels especially for making bulky and intricate engineering components [2]. From the microstructural point of view, typical cast iron should have graphite embedded in 100% pearlitic matrix in different morphologies [3]. The impact resistance or toughness of the cast iron is substantially lower compared to structural steels, thus limiting their use for fatigue and impact loading conditions. To put it simply, cast iron is significantly brittle and lacks the toughness needed for use in special applications [1,4–5]. Throughout the years, the to
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