ADI (Austeniteductileiron) is the abbreviation for austempered ductile iron. The ideal microstructure of austempered ductile iron is composed of acicular ferrite and stable high-carbon austenite. The international standard ISO17804:2005 refers to austempered ductile iron as Ausferriticspheroidalgraphite castirons. Compared with steel parts, austempered ductile iron parts have many advantages such as low density, high strength, high plasticity, good overall performance, good wear resistance, good sound and shock absorption performance, simple manufacturing process, and low cost.

 The conventional austempering process of ductile iron mainly includes two stages. First, the casting is fully austenitized in the temperature range of 850~950℃ to obtain austenite with saturated carbon content, and then the casting is quickly quenched to 239~400oC In the cooling medium of the temperature range, and keep the temperature for 0.5～4h, and finally air-cooled to room temperature. At present, foreign ADI processes have turned to research on new austempering processes such as two-step ADI, dual-phase ADI (DualPhaseAustemperedDuctileIron), CADI (CarbidicAustemperedDuctileIron, that is, austempered ductile iron containing carbides); at the same time, ADI failure analysis and crack growth have been carried out. Research on the mechanism, the influence of alloying element doping on the performance of ADI, and the numerical simulation of ADI; some research has also been done on ADI materials with high fatigue and corrosion resistance. Among them, the two-step method ADI is to complete austenitization of ductile iron and then carry out low temperature (about 280℃) heat preservation nucleation + high temperature (340℃～390℃) quenching two-step austempering, so that the matrix structure has a relatively high carbon content. High retained austenite and refined structure to improve the overall mechanical properties of the workpiece. Two-phase ADI refers to heating the workpiece to A before austempering. Between S and Az, partial austenitization is carried out to use the part of the proeutectoid ferrite present in the structure to improve the plastic toughness of ADI, reduce the hardness, and improve the processing performance, but it also resists tensile strength, yield strength and bending fatigue The intensity reduction is not large. K. Aslantas et al. used finite element analysis to simulate the growth and propagation process of cracks under the surface of sheet ADI. J. H. Yang et al. studied the fracture properties of the two-step ADI and the traditional one-step ADI and the crack growth rate near the threshold value, and found that the crack growth rate is related to the austenite carbon content, austenite grain size, and ferrite The size is related. While obtaining high strength and high hardness, the two-step ADI has a slower crack growth rate than traditional ADI.

 The research work of ADI in China mainly focuses on the heat treatment process parameters, structure and performance of austempered ductile iron, as well as the application of ADI and heat treatment equipment. At the same time, new isothermal heat treatment processes such as dual-phase ADI and CADI have also been studied. The microstructure and properties of austempered ductile iron obtained under the two-step method and the traditional single-step method austempering process conditions were compared and studied. The two-step austempering method of "285℃ heat preservation nucleation + 340℃ or 370℃ austempering" Compared with the traditional 340℃ or 370℃ single-step austempering process, the ADI obtained by the two-step method is more refined than the ADI obtained by the single-step method, the hardness is significantly improved, and the toughness is also significantly improved. The mechanical properties of dual-phase ADI at different isothermal temperatures and different isothermal times have been tested. When the isothermal temperature is between 250 and 390 ℃, as the austempering temperature increases, the tensile strength decreases and the elongation gradually increases. The hardness first decreases and then increases, and the impact toughness first increases and then decreases; when the isothermal time is 3o～120min, with the extension of the austempering time, the tensile strength increases. After 90min, the tensile strength is slightly There is a decrease; when the austempering time is 60min, the impact toughness reaches the maximum. The influence of austempering temperature on the structure, hardness and impact toughness of CADI. The structure after austempering is bainite, retained austenite and carbide. The austempering temperature is in the range of 230～290℃, the hardness gradually decreases with the increase of temperature, while the impact toughness gradually increases, and the bainite needles in the microstructure gradually become thicker.