Heat treatment is usually used in strengthening and toughening treatment of ductile iron. The heating temperature of most heat treatment processes is higher than the upper limit of critical eutectoid temperature (AC upper limit) of nodular iron heating, which involves the austenization process of nodular iron matrix structure, which indicates that austenitizing treatment is an important link to improve the mechanical properties of ductile iron parts. However, the amount of carbon content in high temperature austenite after austenitizing will directly affect the structure transformation reaction in the subsequent heat treatment stage, especially for the preparation of austempered ductile iron, the carbon content in high temperature austenite and retained austenite will have a significant impact on the subsequent isothermal transformation and mechanical properties of the alloy, but the solubility of carbon atoms in austenite It will be different with different matrix structure and austenitizing process. At the same time, many excellent properties of ADI are closely related to its unique microstructure.
The typical matrix structure of austempered ductile iron is composed of acicular ferrite, which contains the mixed structure of acicular ferrite (α b) and high carbon austenite (γ HC) strengthened by carbon and silicon solution. This is the key to the high strength and high plasticity toughness of austempered ductile iron.
However, up to now, there are few reports on the carbon content in as cast and heat-treated ductile iron matrix at home and abroad, especially the research on the correlation between the as cast microstructure and the carbon content in austenite during austenitizing process. At the same time, ADI under different austempering temperatures is observed and analyzed deeply at high rate There are few studies on Microstructure and morphology of medium austenite ferrite, especially the fine structure of fine ferrite needle treated at low austempering temperature, and the systematic study of process factors (as cast microstructure, austempering process and casting wall thickness) on the mechanical properties of austempered ductile iron is relatively less.
To a certain extent, it limits the popularization and application of austempered ductile iron. Therefore, it is necessary to study the carbon content in as cast and heat-treated ductile iron matrix systematically and deeply, and to explore the influence of austenitizing process on carbon content in austenite of as cast ductile iron with different matrix in order to provide the optimal austenitizing process parameters for the preparation of austempered ductile iron. At the same time, the fine structure of austempered ductile iron was observed and analyzed, the internal relationship between austempering temperature and fine structure was explored, and the influence of process factors on mechanical properties of austempered ductile iron was studied. This is also the key to be solved in the development of isothermally quenched ductile iron.