Austempered Ductile Iron material standard
American, European, Japanese and Chinese Austempered Ductile Iron grades and performance standards. The publication and implementation of Austempered Ductile Iron standards in various countries indicate that ADI materials have entered the stage of mass production from the stage of experimental development. In this international situation, Austempered Ductile Iron products all over the world have developed rapidly in recent years with an annual output of about 20%.
Microstructure of Austempered Ductile Iron
Figure 1 shows the microstructure of as cast ductile iron and ADI. The as cast microstructure of ductile iron is composed of pearlite + bovine eye ferrite + graphite ball, while the isothermal transformation structure of ductile iron is composed of acicular ferrite + high carbon austenite + graphite ball. As the unique matrix of ADI, the mixed structure of carbon rich austenite and ferrite without carbide inclusion makes ADI material have good mechanical properties and structure consistency at room temperature and low temperature. Fig. 2 is the SEM image of acicular ferrite in ADI matrix. It can be seen that the acicular ferrite in ADI microstructure observed by conventional optical microscope is actually composed of thin ferrite sheets and thin strip austenite which are in parallel with each other. At the same time, there are small angle grain boundaries between the thin ferrite sheet and the thin strip austenite. Each thin ferrite sheet has the same crystal orientation, and the strip austenite also contains supersaturated carbon.
Mechanical properties of Austempered Ductile Iron
Figure 3 shows the performance comparison of ADI with ordinary carbon steel, alloy steel and traditional ductile iron. It can be seen from the figure that the tensile strength of ADI is much higher than that of traditional ductile iron, but close to or slightly higher than that of carbon steel and alloy steel. However, the elongation of both traditional ductile iron and Adi is lower than that of steel, which is mainly due to the fact that the steel sample can continue to deform after the first crack appears.
The mechanical properties of Austempered Ductile Iron at different isothermal transformation temperatures. It can be seen that:
(1) The decrease of isothermal transformation temperature can improve the strength and hardness of Austempered Ductile Iron, but decrease its ductility, which is mainly related to the coarsening of grains in Austempered Ductile Iron matrix and the increase of the amount of untransformed austenite in Austempered Ductile Iron matrix;
(2) The KIC of ADI decreases with the decrease of isothermal transformation temperature, which is mainly related to the decrease of carbon content in untransformed austenite in Austempered Ductile Iron matrix.
In addition, during the isothermal transformation of nodular iron matrix, due to the incomplete γ – α B transformation reaction, ADI after isothermal transformation is caused There are always a certain amount of untransformed γ in the matrix of ADI. Due to the low carbon content in some of the untransformed γ, work hardening will occur in different degrees under tensile stress or compressive stress. This unique hardening characteristic combined with high strength and toughness promotes ADI material to have good fatigue performance, and increases with the increase of tensile strength.