Krzy ń ska a analysis by transmission electron microscopy (TEM) shows that the ferrite in acicular ferrite in ADI is composed of many micrometer thin ferrite sheets, which have the same orientation. At the same time, two adjacent thin ferrite sheets are separated by grain boundary or nano scale austenite thin films. In addition, lamellar ferrite and austenite film have a crystallographic habitual plane (110) α / / (111) γ. During the isothermal transformation of austenite, the interface energy produced by ferrite formed by this habit plane is the lowest. This kind of high carbon thin film austenite also contains supersaturated carbon, which is stable austenite.
Marrow TJ used electron backscatter diffraction (EBSD) to detect the grain boundary, size and orientation of the original austenite before ADI isothermal treatment, and observed the microstructure around the crack. It is found that the short fatigue crack in Austempered Ductile Iron begins at the boundary of graphite ball / original austenite or at the micro defect (micro shrinkage porosity or shrinkage cavity), and ends at the boundary of austenite bundle or original austenite in the original austenite grain.
Komatsu s, Masud L, Martinez R A and druschitz AP have successively studied the environmental embrittlement of ADI. It is found that when the surface of Austempered Ductile Iron contacts with water and other liquids or some gases, the environment induced embrittlement will occur, and the strength and toughness of ADI will decrease obviously. Environmental embrittlement is not a unique phenomenon of austempered ductile iron. High strength ductile iron, including pearlite ductile iron, martensite ductile iron and even steel, has environmental embrittlement phenomenon. However, a large number of test results show that the anti rust lubricant WD-40 can effectively prevent moisture or moisture from invading into the metal interior, so as to ensure that the initial crack will not further expand, and the embrittlement problem caused by environment can be eliminated.
In recent years, countries all over the world are competing to develop new production technology of isothermally quenched ductile iron, and have successively developed a variety of isothermally quenched ductile iron materials and a variety of new processes for the preparation of austempered ductile iron, including non austenitic austempered ductile iron, double equal temperature quenched ductile iron, carbide containing isothermally quenched ductile iron and low carbon equivalent austempering Ductile iron, as cast isothermally quenched ductile iron, two-step austempered ductile iron, deformation isothermally quenched ductile iron, extrusion isothermally quenched ductile iron, etc., have greatly improved the diversity of ADI products and accelerated the pace of ADI industrialization.
