The excellent properties of austemperedare 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. Up to now, there are many macroscopic descriptions about the change of matrix morphology with austempering temperature in Austempered Ductile Iron at home and abroad, but the description of microstructure of austenite in Austempered Ductile Iron under different austempering temperatures, especially the fine structure of acicular ferrite, is rarely studied. Therefore, it is necessary to group the microstructure of austempered ductile iron To further refine the research. Therefore, this experiment intends to control the change of austempering temperature to study the effect of Austempering Temperature on the microstructure of austempered ductile iron matrix, and to explore the fine structure of acicular ferrite, so as to provide theoretical basis for optimizing the mechanical properties of austempered ductile iron.
(1) C, Si, Mn, Ni, Cu and other elements will be dissolved in austenite and ferrite in varying degrees during the isothermal transformation of ductile iron, resulting in the change of lattice and lattice constant. At the same time, with the increase of isothermal transformation temperature, the amount of austenite in the matrix of austempered ductile iron increases gradually, and the carbon content in austenite increases.
(2) The matrix structure of ADI is composed of graphite ball and ferrite after being treated at 920 ℃ / 2H + 280 ℃ / 1.5h with different as cast microstructure (MS = 0.25cm, Ms = 0.50cm, Ms = 0.75cm, Ms = 1.00cm, Ms = 1.25cm). At the same time, the austempered ductile iron with MS = 0.25cm has the shortest austempered ductile iron matrix, the ADI obtained by Ms = 0.50cm ductile iron is the second, and the ADI needle of ADI produced by Ms = 0.75cm, Ms = 1.00cm and Ms = 1.25cm is longer.
(3) The matrix structures of austempered ductile iron treated at 280 ℃, 330 ℃ and 380 ℃ are respectively austenite, austenite + strip austenite, and austenite + strip austenite + massive austenite. With the increase of austenite holding time, the amount of austenite increases and the amount of ferrite increases with the increase of austenite holding time.
(4) The results show that the microstructure of austenite and ferrite is composed of high carbon austenite and ferrite which are parallel in orientation or 20 ° to 25 ° in orientation. With the decrease of isothermal transformation temperature, high carbon austenite and ferrite sheets are refined to a certain extent. At the same time, in the matrix of austempered ductile iron obtained by isothermal transformation at a lower temperature (280 ℃, 330 ℃), there are clusters of ultra-fine austenite (about 36-57 nm thick) and nano high-carbon ferrite (24-29 nm) interlaced in the matrix of austempered ductile iron The amount of austenite increases with the decrease of isothermal transformation temperature, and there is an austenite structure with orientation angle of about 20 ° to 25 ° between two adjacent clusters of ultra-fine ferrite.
(5) The content of carbon is higher in austenite on both sides of ferrite sheet in matrix structure of austempered ductile iron. At the same time, the carbon content in austenite increases with the decrease of distance from ferrite / austenite grain boundary along the direction perpendicular to ferrite needle growth. In addition, the distribution of carbon content in massive austenite is U-shaped.