Browsing by Subject "austempering"

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Access status: Open Access ,
Determination of the Heat Transfer Coefficient in the Isothermal Quenching Process of ADI Cast Iron Cooled with Water Mist
(Akademia Górniczo-Hutnicza, 2026) Burbelko, Andriy A.; Stręk, Piotr
Isothermal quenching of austempering cast iron (ADI) castings requires that, after austenitization the casting must be rapidly cooled to the temperature of isothermal austenite decomposition. The cooling rate throughout the entire volume of the heat-treated product must be high enough to prevent pearlitic transformation. At the same time, the temperature of the cooled surface must not decrease below the martensitic transformation start temperature Ms. The cooling rate of the casting surface is determined by factors such as the temperature difference between the surface of the cooled casting and the cooling medium, the thermal conductivity of cast iron, the heat transfer coefficient, and the wall thickness of the treated casting. In the case of cooling with water mist, the heat transfer coefficient depends on the temperature of the cooled surface. To control the cooling process of castings using water mist, information about this relationship for the temperature range of 200–800°C is needed. Available scientific publications on this subject contain contradictory data. Therefore, a measuring station was built with the ability to set the temperature of the cooled surface. The station includes a measuring system that allows the measurement of the heat flux flowing from the heating element into the environment. The result of the research is the measurement of the relationship between the temperature of the cooled surface and the heat transfer coefficient. The values obtained will be used in the future to build a numerical model of ADI castings heat treatment. This work may contribute to the future replacement of salt baths (currently used for fast cooling and austempering) with water mist spraying. In this case, the low-temperature operation of austenite decomposition can be performed without the use of salt a bath.
Access status: Open Access ,
Optimization of isothermal transformation period for austempered ductile iron
(Wydawnictwa AGH, 2017) Parhad, Prashant; Likhite, Ajay; Bhatt, Jatin; Peshwe, Dilip
The present paper examines and compares the influence of austempering parameters such as temperature and time on the isothermal transformation and microstructural changes of ductile iron. To identify the compositional and structural changes during an isothermal transformation, a very wide austempering period is chosen at a transformation temperature for the precise determination of the process window. XRD, optical, and scanning electron microscopic techniques are exploited to identify and analyze the changes in the austempered structure, at austempering temperatures of 250°C and 400°C. The various structural parameters like austenite volume fraction (Vg, its carbon content (Cg), lattice parameter, and the average cell size of the ferrite are ascertained. Electron backscattered diffraction (EBSD) analysis is used to identify the carbide precipitation obtained due to the austempering Stage-II reaction. It is noticed that, at the end of the austempering Stage-II reaction, there is a significant reduction in the volume fraction of stabilized austenite and it's carbon content, as the microstructure at this stage not only contains ausferrite but also additional precipitated iron carbides. With an increase in austempering time, the austenite and ferrite volume fraction increase until the austenite becomes stabilized with sufficient carbon. The increase in the lattice parameter of the austenite during austempering corresponds to the rise in carbon content within the austenite. A rise in the austempering temperature leads to a reduction in the volume fraction of the ferrite and an increase in the stabilized austenite volume fraction. The optimum isothermal transformation period for austempered ductile iron is established, based on the period during which the maximum content of the austenite volume fraction, its carbon, the lattice parameter, and the average cell size of the ferrite are maintained.