Effect of Nickel Content on Microstructural Evolution in Austempered Solution-Strengthened Ferritic Ductile Cast Iron
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JMEPEG (2019) 28:4034–4040 https://doi.org/10.1007/s11665-019-04184-y
Effect of Nickel Content on Microstructural Evolution in Austempered Solution-Strengthened Ferritic Ductile Cast Iron Tatsuya Tokunaga, Yeong-Jin Kim, and Hidenori Era (Submitted October 17, 2018; in revised form April 10, 2019; published online July 5, 2019) The effects of Ni content and austempering temperature on microstructural evolution during austempering in solution-strengthened ferritic ductile cast iron with Ni contents between 0 and 1.1 mass% were investigated. The microstructure observation for the samples austempered at temperatures between 375 and 325 °C using an infrared gold image furnace was carried out with optical microscopy, x-ray diffraction measurements, and electron back-scattered diffraction pattern (EBSP) analysis. In addition, the Vickers hardness of both the austempered and as-quenched samples was measured under a load of 0.9807 N. According to the microstructure observations, the matrix of the samples austempered at temperatures between 375 and 325 °C exhibited finer acicular ferrite microstructure with increasing Ni content. Optical microscopy and EBSP analysis revealed that the amount of high-carbon austenite phase for the austempered samples increased with increasing Ni content and austempering temperature. Although samples with higher Ni content were found to contain more high-carbon austenite phase in the matrix, the Vickers hardness of the samples tended to increase with increasing Ni content at the same austempering temperature. The reason for this may be the higher carbon content in the matrix of the samples with higher Ni contents. Keywords
acicular ferrite, austempering, high-carbon austenite, solution-strengthened ferritic ductile cast iron, transformation
1. Introduction Austempered ductile cast iron (ADI) has emerged as an important engineering material because of its excellent properties (Ref 1) such as high strength with good ductility (Ref 24), good fracture toughness (Ref 5-9), and good wear resistance (Ref 10). These superior properties of ADI are attributed to its microstructure, which consists of acicular ferrite and highcarbon austenite with graphite nodules. ADI can be obtained from heat treatment that consists of austenitizing at around 950 °C followed by quenching and isothermal holding in the bainitic transformation temperature range. Further improvement of the mechanical properties of ADI has been achieved by the addition of alloying elements and optimization of heat This article is an invited submission to JMEP selected from presentations at the 73rd World Foundry Congress and has been expanded from the original presentation. 73WFC was held in Krakow, Poland, September 23-27, 2018, and was organized by the World Foundry Organization and Polish FoundrymenÕs Association. Tatsuya Tokunaga and Hidenori Era, Department of Materials Science and Engineering, Kyushu Institute of Technology, 1-1, Sensuicho, Tobata-ku, Kitakyushu 804-8550, Japan; Yeong-Jin Kim, Graduate School of Engineering, Kyushu In
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