In Situ Study of the Influence of Nickel on the Phase Transformation Kinetics in Austempered Ductile Iron
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AUSTEMPERED ductile iron (ADI) is a very attractive material for applications where high strength, good ductility, wear resistance, and fatigue strength are required. The excellent mechanical properties of ADI compared to the initial nodular cast iron result from its specific microstructure, which is created by a heat treatment consisting of austenitizing, quenching, and isothermal austempering.[1–4] In order to reach the desired mechanical properties, the heat treatment parameters, however, have to be designed properly. The heat treatment process of ADI starts with heating to an austenitizing temperature typically in the range of 1123 K to 1223 K (850 C to 950 C). During austenitizing the initial ferritic to pearlitic matrix transforms to austenite and carbon diffuses into the austenite. When the carbon enrichment reaches the equilibrium, the material is quenched to austempering temperatures usually between 523 K and 723 K (250 C and 450 C). The cooling rate is a limiting factor since it PATRICK SAAL and LEOPOLD MEIER, Scientists, and WOLFRAM VOLK, Professor, are with the Institute of Metal Forming and Casting, Technische Universita¨t Mu¨nchen, Walther-Meißner-Str. 4, 85747 Garching, Germany. XIAOHU LI, Scientist, and MICHAEL HOFMANN and MARKUS HOELZEL, Instrument Scientists, are with the Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universita¨t Mu¨nchen, Lichtenbergstr. 1, 85747 Garching, Germany. Contact e-mail: [email protected] JULIA N. WAGNER, Scientist, is with the Karlsruher Nano Micro Facility and AM-WK, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Manuscript submitted July 15, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
has to exceed the critical cooling rate when pearlite starts to form in the temperature range of 973 K to 773 K (700 C to 500 C). Subsequently, the material is austempered isothermally. Other non-isothermal austempering strategies are also widely reported,[5–7] however, will not be considered within the present work. With the beginning of isothermal austempering, ferrite platelets grow into the surrounding retained austenite (referred to as stage I reaction). As the solubility of carbon within ferrite is low, additional carbon is transferred into the retained austenite, and accordingly, the austenite gets stabilized against the transformation to martensite when cooled to room temperature. The presence of silicon suppresses the formation of carbides during stage I reaction, nevertheless, after long austempering times austenite eventually decomposes to ferrite and carbides. This undesired transformation is the so-called stage II reaction. Alloying elements such as copper, nickel, manganese, and molybdenum are used as alloying elements in order to suppress the pearlite formation during quenching, particularly useful for heat treatment of heavy-section casting.[8–11] Earlier we reported on the processes of phase transformation and carbon redistribution during the heat treatment in unalloyed ADI.[12] Withi
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