austempered ductile iron (ADI) is considered as a new kind of engineering material and exhibits excellent combinations of high strength, ductility, toughness, fatigue strength and wear resistance.The attractive properties of Austempered ductile iron (ADI) are related to the unique microstructure that consists of high carbon austenite and acicular bainitic ferrite with graphite nodules dispersed in the matrix. In recent years, it has been applied to many engineering components such as gears and crankshafts. Considerable works have also been conducted on microstructural characteristics as well as mechanical properties and the influences of factors such as austempering temperature, austempering time and alloying elements on microstructure and mechanical properties have been investigated.
With the development of Austempered ductile iron (ADI), standards for Austempered ductile iron (ADI) have also been published. In ASTM standard 897M-06, there are 6 grades of Austempered ductile iron (ADI) according to the ultimate tensile strength. In later years, the research and application of Austempered ductile iron (ADI) have developed rapidly in China and the standard GB/T 24733-2009 has been published in 2009. However, researches on Austempered ductile iron (ADI) are related primarily to the austempering process parameters rather than to the standard Austempered ductile iron (ADI) strength grade, which can be confusing to the designer of Austempered ductile iron (ADI) structure. So the microstructure and properties of Austempered ductile iron (ADI) with different strength grades should be studied in detail to supply useful information for the Austempered ductile iron (ADI) makers and component designers. Austempered ductile iron (ADI) with three strength grades were produced; the microstructure, tensile tests and impact tests were conducted; the influences of microstructure on mechanical properties and fracture behavior during impact testing were discussed.
Austempered ductile iron (ADI) with three strength grades were produced successfully and the strengths of them satisfy the requirement of ASTM standard 897M-06 grade 900/650/09, grade 1050/750/07 and grade 1200/850/04. Meanwhile, the ductile and impact toughness of the Austempered ductile iron (ADI) are larger than those required by the ASTM standard.
The austempering temperature has significant influence on microstructures of Austempered ductile iron (ADI). When the austempering temperature decreases from 380 ??C to 300 ??C, the morphology of bainitic ferrite changes from upper to lower and the volume fraction of retained austenite decreases.
As the austempering temperature decreases from 380 ??C to 300 ??C, the yield strength and tensile strength of the Austempered ductile iron (ADI) increase, while both the impact energy and elongation of them decrease.